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OPERATION AND SERVICE MANUAL

1. Generally choose a capture rate higher than the time constant bandwidth The lock in time constant and filter slope should be chosen to attenuate outputs at frequencies higher than 1 2 the capture rate as much as possible SR865 DSP Lock in Amplifier 136 Programming Chapter 4 CAPTURELEN n The CAPTURELEN command sets or queries the capture buffer length The value n is the buffer length in kilobytes 256 total data points For example 32 k points of X and Y is a total of 64 k data points 256 kbytes of buffer at 4 bytes point and a length of n 256 32 k points of X Y R and 0 requires twice as much or a length of n 512 Since the internal blocks that make up the buffer are 2 kbytes each the value of n must be even and is limited to 1 lt n lt 4096 If n is odd then the buffer length will be set to n 1 Example CAPTURELEN 256 Sets the capture buffer length to 256 kbytes or 64 k total data points CAPTURELEN Returns the buffer length n kbytes CAPTURECFG X XY RT XYRT i The CAPTURECFG i command sets the capture configuration to X i 0 X and Y i 1 R and 0 1 2 or X Y R and 0 i 3 For a given buffer length total data points capturing more parameters results in fewer points of each parameter and a shorter total capture time The capture rate is unaffected by CAPTURECFG Example CAPTURECFG 1 Capture X and Y CAPTURECFG XY CAPTURECFG Returns the capture configuration i CAPTURERATEMAX
2. HARM i The FREQINT f command sets the internal frequency to f The value of f will be rounded to 6 digits or 0 1 mHz whichever is greater The value of f is limited to 1 mHz lt f lt 2 5 MHz The query form FREQINT always returns the internal reference frequency Example FREQINT 12 34E3 Set the internal frequency to 12 34 kHz FREQINT 12 34 KHZ FREQINT 12340 FREQINT Returns the internal frequency in Hz The FREQEXT query returns the external reference frequency Example FREQEXT Returns the external frequency in Hz The FREQDET query returns the actual detection frequency This is helpful in dual reference mode or harmonic detection Otherwise the detection frequency is either the internal or external reference frequency Example FREQDET Returns the detection frequency in Hz The HARM i command will set the lock in to detect at the i harmonic of the reference frequency The value of i is limited to 1 lt i lt 99 Example HARM 2 Set the harmonic detect to 2 HARM Returns the harmonic number i HARMDUAL i ASRS The HARMDUAL i command will set the lock in to detect at the i harmonic of the external frequency in dual reference mode The value of i is limited to 1 lt i lt 99 The actual detection frequency when operating in dual reference mode is x x int HARMDUAL ext Tia ra Laer SR865 DSP Lock in Amplifier 108 Programming Chapter 4 Example HARMDUAL 2 Set the dual e
3. Key Front panel keys are referred to in square brackets Some keys have a second italicized label Press and hold these keys for 2 seconds to invoke the italicized function lt Knob gt Knobs are referred to in lt angle gt brackets Knobs are used to adjust parameters which have a wide range of values Some knobs have a push button function Some also have a second italicized label Press and hold these knobs for 2 seconds to invoke the italicized function Touch Touchscreen buttons and icons are referred to in curly brackets Touchscreen buttons are used to adjust the data display as well as change certain lock in parameters SR865 DSP Lock in Amplifier 2 Getting Started Chapter 1 SR865 Front Panel The SR865 s buttons and knobs are mainly used to configure the lock in measurement while the touchscreen is mainly for data display The touchscreen display is also used for keypad entry Display functions Auto functions Signal path settings Reference settings XorR output Yor output Signal Path The signal path settings configure input BNC s input gain time constant and filters and the sensitivity Unlike previous generation lock ins the SR865 does not have a dynamic reserve setting The input range setting is simply the largest input signal before overload It is best to decrease the input range setting as much as possible without overload This increases the gain and utilizes more of the A D c
4. Since the synchronous filter follows the phase sensitive detectors the time constant filters and output scaling any change in the signal amplitude reference frequency phase time constant slope or sensitivity will cause the outputs to settle for one period of the filter These transients are because the synchronous filter provides a steady output only if its input is repetitive from period to period The transient response also depends upon the time constants of the regular filters Very short time constants lt lt period have little effect on the transient response Longer time constants lt period can magnify the amplitude of a transient Much longer time constants gt period will increase the settling time far beyond a period Sensitivity SRS The Sensitivity setting of the SR865 determines what input signal corresponds to 10 V full scale output for X Y and R For example a Sensitivity of 100 mV means that a signal at f of 100 mVrms will result in a 10 V output of R X and Y will reach a maximum of 10 V depending upon the phase of the signal The Sensitivity also sets the scale for the displayed bar graphs and numeric readouts Note that this is a numerical output conversion Output overloads do not generally affect the actual measurement results They only indicate that the output value exceeds 100 of the chosen Sensitivity and the output BNC the bar graph and the displayed numerical readout will be pinned at the
5. The value of j 0 3 corresponds to the DAT1 green DAT2 blue DAT3 yellow and DAT4 orange data channels Example GSCL DAT2 0 1 Set the vertical scale for data channel 2 blue to 0 1 div GSCL 1 0 09 GSCL 1 0 12 GSCL 1 Returns the vertical scale for data channel 2 blue GSCL DAT2 GOFF DAT1 DAT2 DAT3 DAT4 j x The GOFF j x command sets the vertical offset of data channel j to x The value of j 0 3 corresponds to the DAT1 green DAT2 blue DAT3 yellow and DAT4 orange data channels Example GOFF DATS 0 1 Set the vertical offset for data channel 3 yellow to 0 1 GOFF 2 0 1 GOFF 2 Returns the vertical offset for data channel 3 yellow GOFF DAT3 SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 121 GAUT DAT1 DAT2 DAT3 DAT4 j The GAUT j command performs an Auto Scale on data channel j This command is the same as pressing the A button in the scale palette The value of j 0 3 corresponds to the DAT1 green DAT2 blue DAT3 yellow and DAT4 orange data channels Example GAUT DAT3 Auto Scale data channel 3 yellow GAUT 2 GACT DAT1 DAT2 DAT3 DAT4 j The GACT j command performs an Auto Scale Keeping zero at the center on data channel j This command is the same as pressing the D button in the scale palette The value of j 0 3 corresponds to the DAT1 green DAT2 blue DAT3 yellow and DAT4 orange data channels
6. fret Any signal at f appears at the lock in output at dc On the FFT display these signals are shown at their true input frequencies Note that the peak amplitude is diminished from its value at 100 000 kHz The readout is about 36 dB This is because the lock in time constant acts like a bandpass filter in the spectrum centered at fref center Appendix B The FFT Display 167 Press the Filter Slope key once to increase the The peak amplitude at 99 000 kHz is filter slope to 12 dB oct diminished even further This is the effect of the S000 dey 490000 kelam 4 A 7 y steeper time constant filter At the lock in output the 1 kHz output is attenuated further by this filter 51 89 dB 98 998642 kHz 98 479 KHZ Src post Fiter dB div 2 4 Hz div 3052 Avgs This example has provided a quick overview of the FFT display in the SR865 A sRs SR865 DSP Lock in Amplifier 168 The FFT Display Appendix B SR865 DSP Lock in Amplifier ASRS Using the Webserver 169 Appendix C Using the Webserver The SR865 webserver provides a quick start to interfacing via ethernet Make sure to set the IP address subnet mask and gateway address for your network in the system ethernet menu Be sure to enable the webserver in the VXI 11 amp Web settings Press and hold Calc system to access the system menu To bring up the unit s webpage simply type the unit s IP address into the browser bar of a computer which ca
7. 180 to 180 regardless of the sensitivity or range When CH2 outputs a voltage proportional to 0 the output scale is 18 Volt or 180 10 V The phase bar graph and numeric readout scales are also unaffected by the Sensitivity X Y and R Output Offset Ratio and Expand The SR865 has the ability to offset the X Y and R outputs This is useful when measuring deviations in the signal around some nominal value The offset can be set so that the output is offset to zero Changes in the output can then be read directly from the display or output voltages The offset is specified as a percentage of the Sensitivity and the percentage does not change when the Sensitivity is changed Offsets up to 999 can be programmed The X Y and R outputs may also be rescaled with a ratio function This allows signals to be normalized to some experimental parameter being monitored with an auxiliary input voltage Whenever enabled ratio is indicated by the RATIO LED and Ratio labels on the display The X Y and R outputs may also be expanded This simply takes the output minus its offset and multiplies by an expansion factor Thus a signal which is only 10 of sensitivity can be expanded to provide 10 V of output rather than only 1 V The normal use for expand is to expand the measurement resolution around some value which is not zero For example suppose a signal has a nominal value of 0 9 mV and we want to measure small deviations say 10 uV or so
8. 20 88 dB 100 098 kHz y 0 0 Hz Src paw Hzd aooe Avos 4 Touch the Avgs tile along the bottom to display the averaging up down buttons Touch A twice to increase the number of averages to 10 1 Ampl Phase 0 000 deg E 100 000 khz 20 88 dB 100 098 kHz p 0 Hz Sre Raw ADC dB div 2 4 Hz div 3906k ANO 10 Touch v twice to decrease the number of averages to 1 5 Touch the Live tile along the bottom to pause the FFT display Touch Paused to return to a live display Press the Time Constant Down key multiple times to set the time constant to 1 ms SRS always baseband i e the frequency span starts at 0 Hz or dc The cursor readout now reads 100 098 kHz because each frequency bin has been reduced to 0 6 kHz FFT graphs can be averaged to reduce fluctuations Averaging is exponential in the number of averages Turn the averaging off to speed up the display The FFT may be paused When the FFT source is set to the PSD output the maximum frequency span is set by the time constant The shorter the time constant the higher the maximum frequency span This is because the PSD output is frequency limited by SR8 amp 65 DSP Lock in Amplifier 166 The FFT Display Appendix B 6 Touch the Src tile along the bottom to display the source up down buttons Touch Al repeatedly to change the source to Post Filter 100 000 khz M 4 Amol Phase 0 000 deg Fint 19 94 dB 10
9. SVDT Interface Commands RST IDN TST OPC LOCL i OVRM OFF ON i ASRS 137 137 137 138 138 138 page 140 141 141 141 141 142 142 142 page 143 143 143 143 143 144 144 144 144 144 145 145 145 page 146 146 146 146 146 147 Start capture OneShot or Cont HW trigger offfon Stop capture Query the capture buffer state Query the length kB of captured data after stop Query the nt sample 1 2 or 4 values after stop Download binary capture buffer description Configure streaming to X XY RO or XYRO 0 3 Query the maximum streaming rate Set the streaming rate to max rate 2 Set the streaming format to float32 0 or int16 1 Set packet size to 1024 512 256 or 128 bytes 0 3 Sets the Ethernet port to i 1024 65535 Sets big little endianness and integrity checking Turn streaming offfon description Set time Set date Set the 10 MHz timebase Query the current 10 MHz timebase ext 0 or int 1 Select the BlazeX output Turn sounds on or off Set screen shot mode Set data file type Set file name prefix Set file name suffix Query next file name Screen shot Save data description Reset the unit to its default configuration Query the unit identification string No op returns 0 Operation Complete Set LOCAL 0 REMOTE 1 or LOCKOUT 2 Set GPIB Overide Remote off 0 or on 1 SR865 DSP Lock in Amplifier xiv Commands Status C
10. The FFT of the signal input is displayed There is only a single quantity shown The left and right edge frequencies are labelled at the bottom of the graph The cursor readout is at the right The display is adjusted with the tiles across the bottom Src selects the source data for the FFT dB and Hz adjust the vertical and horizontal scales Avgs sets the amount of averages and Live toggles to Paused Use the lt Cursor gt knob to move the cursor The half screen FFT display behaves the same as the full screen version The only difference is that the numeric and bar graphs are full sized The full numeric screen adds readouts and bar graphs for the 4 aux inputs on the rear panel The 4 aux outputs are shown in tiles across the bottom Touching an output tile displays a keypad to set the aux output Chapter 1 Getting Started 19 18 Press Screen Layout again to cycle back to the Use Screen Layout to cycle through the trend graph various display screens This concludes the measurement example You should have a feeling for the basic operation of the display screens A sRs SR865 DSP Lock in Amplifier 20 Getting Started Chapter 1 Sensitivity Offset and Expand This measurement is designed to use the internal oscillator to explore some of the basic lock in outputs You will need BNC cables and a digital voltmeter DVM Specifically you will measure the amplitude of the Sine Out and provide anal
11. 9 10 11 12 13 14 15 16 17 18 19 Data X 0 Y 0 R O 0 0 X 1 SR865 DSP Lock in Amplifier ASRS Appendix D Data Streaming and Capture 173 If the packet contains integer XY data Data bytes Byte 0O 1 2 3 4 5 6 7 8j 9 10 11 12 13 14 15 16 17 18 19 Data X O Y O XC YC X 2 Y 2 X 3 Y 3 X 4 Y 4 The maximum sample rate is 1 25MHz If floating point XYR data is streamed then the data rate can exceed 1 25 MHz x 4 channels x 4 bytes 20 MBytes s This data rate requires Gigabit ethernet to work Fast 100Mb s ethernet is not sufficient to support the maximum data rate At high data rates we recommend putting the SR865 and the receiving computer on the same dedicated network This reduces traffic congestion for other users and reduces the chance of dropped packets A sRs SR865 DSP Lock in Amplifier Dual Reference Detection 175 Appendix E Dual Reference Detection In a typical lock in experiment the signal of interest is at a known frequency ig The lock in is set up to detect signals at only this frequency by multiplying the input signal by Sin iet using either internal or external reference mode If the signal is Avg sin t then the result of the lock in multiplication is wa Age i 20 1 sig SIN X sin 1 wi cos 2 1 and the time constant l
12. 9 90 V 5 000 5 05 V 4 95 V 0 000 0 010 V 0 010 V 5 000 4 95 V 5 05 V 10 000 9 90 V 10 10 V Input Voltage Lower Limit Reading Upper Limit AUX IN 3 10 000 10 10 V 9 90 V 5 000 5 05 V 4 95 V 0 000 0 010 V 0 010 V 5 000 4 95 V 5 05 V 10 000 9 90 V 10 10 V Input Voltage Lower Limit Reading Upper Limit AUX IN 4 10 000 10 10 V 9 90 V 5 000 5 05 V 4 95 V 0 000 0 010 V 0 010 V 5 000 4 95 V 5 05 V 10 000 9 90 V 10 10 V Input Noise Frequency Input Range Sensitivity Reading Upper Limit 1 kHz 100 nV 100 nV 5 6 nV noise limit corresponds to 5 nV VHz with time constant filter ENBW 1 25 Hz Test Record sheet 4 of 4 SR865 DSP Lock in Amplifier ASRS Circuit Description 201 Appendix H Circuit Description This appendix provides a brief orientation to the signal interface and power supply portions of the SR865 electronics The FPGA based motherboard is beyond the scope of this documentation and servicing or other detailed hardware questions should be referred to the factory Signal Input Amplifier The SR865 signal input module contains both the differential voltage preamplifier and the current input preamplifier Steering relays set the input configuration including grounding ac coupling and differential or single ended mode Most relays are of the latching type requiring only a short pulse to switch states Relays K102 and K106 are single side stable however and are separately energized to select the vo
13. External Noise Sources 55 Noise Measurements 58 Chapter 3 Operation 61 Introduction 61 Standard Settings 64 Signal Input 65 CH1 and CH2 Outputs Offset Ratio and Expand 71 Reference 74 Display 81 Cursor 83 A sRs SR865 DSP Lock in Amplifier vi Contents Functions 85 Interface and Data 87 Setup 88 Rear Panel 96 Chapter 4 Programming 101 Introduction 101 Command Syntax 103 Reference Commands 106 Signal Commands 111 CH1 CH2 Output Commands 114 Aux Input and Output Commands 116 Auto Function Commands 117 Display Commands 118 Strip Chart Commands 120 FFT Screen Commands 125 Scan Commands 128 Data Transfer Commands 132 Data Capture Commands 134 Data Streaming Commands 140 System Commands 143 Interface Commands 146 Status Reporting Commands 148 Status Byte Definitions 151 Appendix A Advanced Filters 155 Appendix B The FFT Display 161 Appendix C Using the Webserver 169 Appendix D Data Streaming and Capture 171 Appendix E Dual Reference Detection 175 Appendix F Fuse Installation and ac Line Select 177 Appendix G Performance Tests 181 Appendix H Circuit Description 201 SR865 DSP Lock in Amplifier ASRS Specifications vii SR865 Specifications Signal Channel Voltage Inputs Sensitivity Output Scale Input Impedance Input Range Gain Accuracy Input Noise CMRR Harmonic Distortion Dynamic Reserve Current Input Ranges Reference Channel Frequency Range Timebase Ext TTL Refe
14. The CAPTURERATEMAX query returns the maximum allowed capture rate at the current time constant in Hz This is a query only command The actual capture rate can be this maximum rate or this rate divided by powers of 2 Note that this maximum rate is dependent on the time constant and sync filter settings Example CAPTURERATEMAX Returns the maximum capture rate in Hz CAPTURERATE n The CAPTURERATE n command sets the capture rate to the maximum rate divided by 2 The value of n is limited to 0 lt n lt 20 Set n 0 for the maximum capture rate For example if CAPTURERATEMAX returns 78125 then CAPTURERATE 4 will set the capture rate to 78125 2 4882 8125 Hz Note that if the time constant is modified during a capture the sampling rate will change mid capture The CAPTURERATE query returns the actual capture rate in Hz not the value n SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 137 Example CAPTURERATE 4 Sets the capture rate to the max rate divided by 2 CAPTURERATE Returns the actual capture rate in Hz CAPTURESTART ONEshot CONTinuous i OFF ON j The CAPTURESTART i j command starts data capture in either OneShot i 0 or Continiuous i 1 mode The parameter j selects whether capture starts immediately hardware trigger OFF j 0 or waits for a trigger hardware trigger ON j 1 The hardware trigger is a TTL input on the rear panel A falling edge triggers the beginning of c
15. This tests how accurately the lock in measures a signal smaller than full scale This test requires the preceding Sine Output Amplitude Accuracy and Flatness test results to verify specified performance Setup We will use the internal sine output to provide an accurate sine wave for testing the signal input function This test requires the preceding Sine Output Amplitude Accuracy and Flatness test results to verify specified performance Connect the Sine Out to the Voltage A input of the SR865 using a 1 meter BNC cable with the precision 50Q terminator at the A input BNC This test only uses ONE Sine Out BNC This means that each measurement will nominally be 50 of the programmed sine amplitude Procedure 1 PRESET press Save Recall then touch Recall default Touch Confirm 2 Use the front panel to make the following adjustments Touch Fint to display the internal frequency keypad Enter a frequency of 1 0 kHz Press Filter Slope adv multiple times Select 24 dB oct 3 For each of the amplitudes listed below perform steps 3a through 3b Sine Out Amplitude Sensitivity 1 0000 Vrms 1V 100 000 mVrms 100 mV 10 000 mVrms 10 mV 1 000 mVrms 1 mV a Use the front panel to make the following adjustments Touch Ampl to display the amplitude keypad Enter the amplitude from the table Press Sensitivity Down Select the sensitivity from the table b Wait for the R reading to stabilize Record the value of R 4
16. Using a heavy ground bus to reduce the resistance of ground connections 3 Removing sources of large ground currents from the ground bus used for small signals 4 Using differential excitation and signal sensing Microphonics Not all sources of noise are electrical in origin Mechanical noise can be translated into electrical noise by microphonic effects Physical changes in the experiment or cables due to vibrations for example can result in electrical noise over the entire frequency range of the lock in For example consider a coaxial cable connecting a detector to a lock in The capacitance of the cable is a function of its geometry Mechanical vibrations in the cable translate into a capacitance that varies in time typically at the vibration frequency Since the cable is governed by Q CV taking the derivative we have Ci gy E dt dt dt Mechanical vibrations in the cable which cause a dC dt will give rise to a current in the cable This current affects the detector and the measured signal Some ways to minimize microphonic signals are 1 Eliminate mechanical vibrations near the experiment 2 Tie down cables carrying sensitive signals so they do not move 3 Use a low noise cable that is designed to reduce microphonic effects Thermocouple effects The emf created by junctions between dissimilar metals can give rise to many microvolts of slowly varying potentials This source of noise is typically at very low frequenc
17. and the resolution for the numeric readouts for X Y and R Sensitivity is also the signal reading corresponding to 10 V on the CH1 and CH2 outputs for X Y and R By decreasing the scale value the bar graphs and numeric readings display much more resolution for the smaller signal The Time Constant is indicated with 3 LEDs In SR865 DSP Lock in Amplifier 12 Getting Started Chapter 1 select 300 ps 12 Press Slope to select 12 dB oct Press Slope twice more to select 24 dB oct Press Slope again to select 6 dB oct 13 Press Sync to turn on synchronous filtering SR865 DSP Lock in Amplifier this case the 3 x100 and us The output values become noisy This is because the 2f component of the output at 2 kHz is no longer attenuated completely by the low pass filter The red Output Overload LED for CH1 will light indicating that the output voltage is clipping The 50 mV signal outputs 10 V when the sensitivity is 50 mV The large additional 2f component will cause the output to try and exceed 10 V and results in an output overload Output overload does not affect the actual displayed value it just indicates that the CH1 or CH2 output is not following the measured value Parameters which have only a few values such as Filter Slope and External Source have only a single key which cycles through all available options Press the key until the desired option is indicated by an LED The output
18. if not removed by the ac coupling filter will multiply with the reference sine wave and produce a PSD output at the reference frequency sometimes referred to as 1f This signal is not normally present and needs to be removed by the low pass filter If the dc component of the signal is large then this output will be large and require a long time constant or synchronous filter to remove ac coupling removes the dc component of the signal without any sacrifice in signal as long as the frequency is above 160 mHz Current Input I The current input on the SR865 uses a separate input BNC The current input has a gain of either 10 or 10 Volts Amp 1 uA or 10 nA range The input burden resistance is either 100 Q 1 pA range or 1 KQ 10 nA range Currents from 3 pA down to 1 fA full scale can be measured The impedance of the signal source is the most important factor to consider in deciding between voltage and current measurements For high source impedances greater than 1 MQ 1 uA range or 100 MQ 10 nA range and small currents use the current input Its relatively low input impedance greatly reduces the amplitude and phase errors caused by the cable capacitance source impedance time constant The cable capacitance should still be kept small to minimize the high frequency noise gain of the current preamplifier For moderate to low source impedances or larger currents the voltage input is preferred A small value resistor may be used t
19. in the signal The Sensitivity of the lock in is set to 1 mV to accommodate the nominal signal If the offset is set to 90 of full scale then the nominal 0 9 mV signal will result in a zero output The 10 uV deviations in the signal only provide 100 mV of dc output If the output is expanded by 10 these small deviations are magnified by 10 and provide outputs of 1 VDC The SR865 can expand the output by 10 or 100 provided the expanded output does not exceed 10 V In the above example the 10 uV deviations can be expanded by 100 times before they exceed full scale at 1 mV Sensitivity The analog output with offset ratio and expand is Ziar Offset Output A x Expand x 10V Ratio In 1 000 V where Offset is a fraction of 1 50 0 5 Expand is 1 10 or 100 and the output cannot exceed 10 V When enabled Ratio In is the voltage on either Aux 3 for X or Y or Aux 4 for R if disabled the Ratio In factor is simply 1 In the above example without ratio Output 0 91mV 1mV 0 9 x 10 x 10V 1V for a signal which is 10 uV greater than the 0 9 mV nominal Offset 0 9 and expand 10 SR865 DSP Lock in Amplifier 48 Basics Chapter 2 The X and Y offset ratio and expand functions in the SR865 are output functions and do not affect the calculation of R or 0 R has its own output offset ratio and expand 8 has no offset or ratio or expand capability To offset 6 simply use Auto Phase X Y and R Display Offset and R
20. 0 7 Reading this byte has no effect on its value SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 149 PSC i Example STB Returns the decimal value of the serial poll status byte STB 3 Returns bit 3 of the standard event status byte The PSC i command sets the value of the power on status clear bit If i 1 the power on status clear bit is set and all status registers and enable registers are cleared on power up If i 0 the bit is cleared and the status enable registers maintain their values at power down This allows a service request to be generated at power up PSC queries the value of the power on status clear bit Example PSC 1 Set the power on status clear bit PSC Returns power on status clear bit ERRE j i ERRS j The ERRE i command sets the error status enable register to the decimal value i 0 255 The ERRE j i command sets bit j 0 7 to i 0 or 1 The ERRE command queries the value 0 255 of the error status enable register The ERRE j command queries the value 0 or 1 of bit j Example ERRE 48 Set bits 4 and 5 in the error status enable register ERRE 4 1 Set bit 4 in the error status enable register ERRE Returns decimal value of the error status enable register ERRE 4 Returns bit 4 of the error status enable register The ERRS command queries the value of the error status byte The value is returned as a decimal number from 0 to 255 The ERRS j command queries the
21. 3e Output Voltages 10 000 5 000 0 000 5 000 10 000 d Use the keypad to enter the Aux Output level from the table e Record the DVM reading Use the front panel to make the following adjustments Press Aux Output to display the Aux Output keypad Touch Outl button to display Aux Output 1 in the keypad Press Screen Layout multiple times Set the screen to show 8 big numerical bar graphs Press Config Touch Datal Display to highlight the green data source Touch In 1 to choose Aux input 1 for Data1 Repeat this to assign In2 In4 to Data2 Data4 For Aux Inputs 1 2 3 and 4 repeat steps 5a through 5e a Connect Aux Out 1 to the Aux Input under test with a BNC cable b For each output voltage in table 3c above repeat steps 5c and 5d c Use the front panel to make the following adjustments Press Aux Output to display the Aux Output keypad Use the keypad to enter the Aux Output 1 level from the table d Record the Aux Input 1 2 3 or 4 value This completes the dc outputs and inputs test Enter the results of this test in the test record at the end of this section SR865 DSP Lock in Amplifier 196 Performance Tests Appendix G Input Noise This test measures the lock in input noise Setup Connect a 50Q termination to the A input This grounds the input so the lock in s own noise is measured Procedure 1 PRESET press Save Recall then touch Recall default Touch C
22. CH2 outputs when set to X and Y These outputs are affected by the sensitivity and the X and Y offsets and expands Signal Monitor Output This BNC provides a buffered output from the signal amplifier This is the signal just before the anti aliasing filter A D converter and PSD The amplifier gain is determined by the Input Range voltage input or the Current Range current input The output is about 1 V for a maximum input SR865 DSP Lock in Amplifier 98 Operation Chapter 3 This output is useful for determining the cause of input overloads not necessarily for viewing the signal of interest If the f signal is small relative to the input range it may not be amplified enough to be viewed at the monitor output Trig Input This TTL input is used to start data capture to the internal buffer This buffer is only accessible via the computer interface The remote interface sets the capture start mode to triggered The Trig Input then starts data capture into the data buffer The internal buffer is 32 MB and can be configured to record X X and Y R and 8 or X Y R and 8 at various sample rates up to 1 25 MHz See the Remote Programming section for more information Sync BlazeX Output This output can be either the reference Sync output or the BlazeX output as configured in the system menu hold the Calc system key Sync The Sync output is a 2V or a 0 2V square wave whose edges are linked to the sine output zero
23. DSP Lock in Amplifier ASRS Appendix B The FFT Display 163 Using the FFT Display This example is designed to use the SR865 internal oscillator and an external signal source to explore some the FFT display You will need a synthesized function generator capable of providing a 100 mVrms sine wave at 100 000 kHz BNC cables and a terminator appropriate for the generator function output You will display the FFT when measuring a signal close to but not equal to the internal reference frequency This setup will illustrate the input and output FFT s Do This Disconnect all cables from the lock in Turn the power on while holding down the Local key The power switch is on the power entry module on the rear panel Turn on the function generator set the frequency to 100 000 kHz exactly and the amplitude to 100 mVrms Connect the function output sine wave from the synthesized function generator to the A input using a BNC cable and appropriate terminator 100 000 kiz M 4 Ae Phase 0 000 deg Fint 15 Press Screen Layout multiple times to change the display to the full screen FFT Explanation When the power is turned on with Local pressed the lock in returns to its standard default settings See the Standard Settings list in the Operation section for a complete listing of the settings The input impedance of the lock in is 10 MQ The generator may require a terminator Many generators have either a 50Q or 600Q
24. Float 1 pA 1V 100 ms 6 dB Off Off 1V 0 00 None 0 000 V X Graph On Y Graph On R Graph On 0 Graph On Trend No SR865 DSP Lock in Amplifier System General 10 Mhz Timebase Sync BlazeX Output Sounds System Files Prefix Suffix Print Mode Data Format Auto Sync On SR865 Screen CSV Operation 65 Signal Input Input Select Slope adv SENSITIVITY r FILTER aV pA 4 gt Advanced 5 x100 mV nA 6dB a2 x10 V pA ot 12dB axl anv fA he 18 dB a 24 dB F aa o s A x100 ms 3 x10 ams EER 1 x mus V INPUT RANGE __ INPUT m Overload 1V ZN Voltage 2 300 mV Current 100 mV aaa iy amp 10mV Tna VOLTAGE CURRENT sA AC Float a1 pA A B DC Ground 10nA e Ground A B l 10 M0 25 pF AC gt 1Hz Chassis Ground The Input Select key selects between the voltage and current preamps The voltage inputs A and B have a 10 MQ 25 pF input impedance Their connector shields are isolated from the chassis by either 10 Q Ground or 10 KQ Float Do not apply more than 10 V to either input The shields should never exceed 1 V The current input uses the I connector The input burden resistance is 100 Q 1 uA range or 1 KQ 10 nA range to a virtual ground The largest allowable current before overload is around 3 uA 1 uA range or 30 nA 10 nA range No current larger than 10
25. Set the external reference input to 50 Q REFZ 500HMS REFZ 0 REFZ 1M Set the external reference input to 1 MO REFZ 1MEG REFZ Returns the external reference input i SR865 DSP Lock in Amplifier 110 Programming Chapter 4 PSTF j f HZ KHZ MHZ The PSTF j f command sets a frequency preset to f These preset buttons are available whenever the internal frequency keypad is displayed The parameter j 0 3 selects a preset F1 F4 The value of f will be rounded to 6 digits or 0 1 mHz whichever is greater The value of f is limited to 1 mHz lt f lt 2 5 MHz Example PSTF 1 12 34 KHZ Set frequency preset F2 to 12 34 kHz PSTF 1 12340 PSTF 1 Returns the frequency preset F2 in Hz PSTA j v NV UV MV V The PSTA j v command sets a sine out amplitude preset to v These preset buttons are available whenever the sine out amplitude keypad is displayed The parameter j 0 3 selects a preset Al A4 The value of v will be rounded to 3 digits or 1 nV whichever is greater The value of v is limited to 1 nV lt v lt 2 0 V Example PSTA 2 12 3 MV Set amplitude preset A3 to 12 3 mV PSTA 2 0 0123 PSTA 2 Returns the amplitude preset A3 in Volts PSTL j v NV UV MV V The PSTL j v command sets a sine out dc level preset to v These preset buttons are available whenever the sine out dc level keypad is displayed The parameter j 0 3 selects a preset L1 L4 The value of v will be rounded to 3 digi
26. The SR865 does not require that one of the frequencies be much greater than the other The two frequencies can be very close together The only requirement is that the time constant remove all output components other than dc The SR865 does require that one of the frequencies be the internal reference and the other the external reference and the difference between them must be less than 2 5 MHz SR865 DSP Lock in Amplifier ASRS Fuse Installation and ac Line Select 177 Appendix F Fuse Installation and ac Line Select The SR865 operates from 100 V 120 V 220 V or 240 V nominal ac power source having a line frequency of 50 or 60 Hz and accommodates both North American single fuse or metric dual fuse operation This appendix provides detailed instructions for modifying the input voltage selection and replacing the line fuse Power Entry Module The line cord receptacle power switch fuse holder and line voltage selector are all part of the power entry module located on the rear panel of the SR865 Detailed instructions for changing the line voltage selection and fuse replacement follow Housing Voltage Selector Indicator Pin Fuse Holder Cover hinged SR865 DSP Lock in Amplifier 178 Fuse Installation and ac Line Select Appendix F ac Voltage Selector The SR865 line voltage selection is indicated by the white pin visible at the right hand edge of the power entry module In the image below the line volt
27. This completes the amplitude linearity test Enter the results of this test in the test record at the end of this section SR865 DSP Lock in Amplifier ASRS Appendix G Performance Tests 191 Frequency Accuracy This test measures the frequency accuracy of the lock in This tests the accuracy of the frequency counter inside the unit The counter is used only in external reference mode The internal oscillator frequency is set by a crystal and has 25 ppm frequency accuracy Setup We will use the frequency synthesizer to provide the reference signal Connect the TTL SYNC output of the frequency synthesizer to the Reference input of the lock in Procedure 1 PRESET press Save Recall then touch Recall default Touch Confirm 2 Set the frequency synthesizer to a frequency of 9 9990 kHz 3 Use the front panel to make the following adjustments Press Ref chop Select External reference mode Press External Trig Select Pos TTL Press Config Touch Datal Display to highlight the green data source Touch Fext to choose external reference frequency for Datal Touch Close Press Screen Layout multiple times to set the screen to show half screen strip chart Fy Touch the green vertical tile for Fe and touch autoscale Mto expand the trace Touch the white horizontal scale tile Time to display the horizontal scale palette and touch B repeatedly to zoom out to 10 s div Touch Cursor MinMaxMean to set t
28. VHz x V2 5Hz or 4 nVrms For Gaussian noise the peak to peak noise is about 5 times the rms noise Thus the output will have about 20 nV pk pk of noise Remember that the SR865 reports its measurements in Volts referred to the input BNC In this case the SR865 will appear to have 20 nV pk pk of noise at f at the input with its input grounded and no signal even applied If this noise floor is too large for your experiment then you need to add more filter stages or increase the time constant to decrease the ENBW All of this assumes that the signal input is being driven from a low impedance source Remember resistors have Johnson noise equal to 0 13x VR nVrms VHz Even a 500 resistor has almost 1 nVrms VHz of noise A signal source impedance of 400 Q will have a Johnson noise greater than the SR865 s input noise To determine the overall noise of multiple noise sources take the square root of the sum of the squares of the individual noise figures For example if a 400 Q source impedance is used its Johnson noise will be 2 6 nVrms VHz The overall noise at the SR865 input will be 2 6 2 5 or 3 6 nVrms VHz We ll talk more about noise sources later in this section At lower gains Input Ranges above 10 mV there is not enough gain to amplify the input noise to a level greater than the noise of the A D converter In these cases the apparent input noise increases with the Input Range For example the configuration above will
29. adjusted to remove the 2 moa component leaving a dc signal proportional to AcaAmoa as desired SR865 Dual Reference Mode The SR865 Dual Reference mode makes this measurement in a single lock in One of the frequencies is the external reference and the other is the internal reference In Dual Reference mode the SR865 detects at Oext Ointl ie the difference frequency It doesn t matter which is the carrier and which is the modulation It also doesn t matter which frequency is greater or by how much The detection frequency is the difference between the two frequencies and will always be lower than the greater frequency The original experimental signal contains components at the sum and difference frequencies Aenea coso Z O mod I zz COS O cay F mod p In Dual Reference mode the SR865 multiplies by cos car Mmoa t in a single step resulting in the output Aar Amoa 1 cos 2 Opoa ft cos 2 t COSCO poat The SR865 time constant is adjusted to remove all components at 21Qcar Omoal 2Mcar and 2 moa leaving a de signal proportional to AcarAmoa as desired It should be noted the lock in reports the measured amplitude at Ocar moa Which is AcarAmoa 2 Correcting for rms the displayed result will be A carAmoa 2V2 The Dual Reference mode in the SR865 does not care about the order of Oext and int The expermental signal could be a fast modulation of a slow carrier or vice versa
30. and Y Rear Panel Outputs The X and Y rear panel outputs are the outputs from the two phase sensitive detectors with low pass filtering offset and expand These outputs are the traditional outputs of an analog lock in The X and Y outputs update at 2 0 MHz CH1 and CH2 Front Panel Outputs The two front panel outputs can be configured to output voltages proportional to X or R and Y or 0 These outputs update at 2 0 MHz If the outputs are set to X or Y these outputs duplicate the rear panel outputs X Y R and 8 Output Scales The Sensitivity setting of the SR865 determines what input signal corresponds to 10 V full scale output for X Y and R For example a Sensitivity of 100 mV means that a signal at fie of 100 mVrms will result in a 10 V output of R X and Y will reach a maximum of 10 V and a minimum of 10 V depending upon the phase of the signal The Sensitivity also sets the scale for the displayed bar graphs and numeric readouts If the signal input exceeds the sensitivity the outputs will overload The actual measurement is typically unaffected since it is done in floating point and has no overload The data displayed in the chart will still be accurate provided the Sync filter is not overloaded Lock in amplifiers are designed to measure the RMS value of the ac input signal All values of X Y and R outputs and displays are RMS values SR865 DSP Lock in Amplifier ASRS Chapter 2 Basics 47 SRS O ranges from
31. appear to have 30 nV pk pk noise at the input when the Input Range is set to 100 mV and 250 nV pk pk when the Input Range is 1 V This means that even with 1 V of interfering noise the SR865 can measure f signals below 1 uV by increasing the amount of low pass filtering SR865 DSP Lock in Amplifier ASRS Chapter 2 Basics 51 Anti aliasing Filter Between the input amplifier and A D converter there is an anti aliasing filter This filter is required by the signal digitization process According to the Nyquist criterion signals must be sampled at a frequency at least twice the highest signal frequency In this case the highest signal frequency is 2 5 MHz and the sampling frequency is 10 MHz so things are OK However no signals above 5 MHz can be allowed to reach the A D converter These signals would violate the Nyquist criterion and be undersampled The result of this undersampling is aliasing these higher frequency signals appear as lower frequencies in the digital data stream Thus a signal at 9 MHz would appear as 1 MHz in the digital data stream and be detectable by the digital PSD This would be a problem To avoid this aliasing the analog signal is filtered to remove any signals above 2 5 MHz This filter has a flat pass band from dc to 3 MHz so as not to affect measurements in the operating range of the lock in The filter rolls off rapidly from 3 MHz to 8 MHz Amplitude variations and phase shifts due to this filter are
32. are listed Use the 1 and buttons in this section to scroll the list of subdirectories Touch a subdirectory to select it as the destination Touch to go up one directory level Repeat until the desired directory is selected Prefix and Suffix File names are generated from the Prefix string and the Suffix number The Next File Name is shown The suffix number is automatically incremented after every file save The extension is BMP for screenshots and either CSV or MAT for data files File names must conform to the 8 3 format so the length of Prefix plus the length of the Suffix is limited to an 8 character string Enter an alphanumeric prefix string and a numeric suffix Print Mode Pressing Screen Shot saves a screen shot to the memory stick as a BMP file Set the Print Mode to Screen Print or Monochrome Screen is an exact screen shot Print replaces the black background with white and Monochrome is black and white Format Determines the format of the data files that are produced when the Save button is pressed Has no effect on screenshots Format can be set to CSV comma delimited ASCII text file or MAT Matlab binary file SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 93 SRS Ethernet Settings T General Files Static 172 25 98 Subnet Mask 255 255 Ethernet Gateway GPIB 172 25 Choose either Static or DHCP for the TCP IP address mode Highlight an address value to
33. be referred to as PRESET Front Panel Test To test the front panel press and hold Local and then Auto Range at the same time Turn any knob to run through all of the LEDs Make sure all of the knobs and all of the LEDs work Touch and drag on the touchscreen to display a little trail The bright end signifies the lift location This verifies the touch functionality Press each key to toggle its position green gray on the screen Keys with a second press and hold function need to be held to toggle The knobs also have a press and hold feature Check to see that all keys and knobs function The Screen Layout key exits this screen Keys Knobs and Touch Buttons These test procedures require the unit to be adjusted from the front panel The test procedures will follow this convention ASRS SR865 DSP Lock in Amplifier 182 Performance Tests Appendix G Key Front panel keys are referred to in square brackets Some keys have a second italicized label Press and hold these keys for 2 seconds to invoke the italicized function lt Knob gt Knobs are referred to in lt angle gt brackets Knobs are used to adjust parameters which have a wide range of values Some knobs have a push button function Some also have a second italicized label Press and hold these knobs for 2 seconds to invoke the italicized function Touch Touchscreen buttons and icons are referred to in curly brackets Touchscreen buttons are used to adju
34. calibrated out at the factory and do not affect measurements This filter is transparent to the user Input Impedance The input impedance of the SR865 is 10 MQ If a higher input impedance is desired then a preamplifier such as the SR550 must be used The SR550 has an input impedance of 100 MQ and is ac coupled from 1 Hz to 100 kHz Input Connections SRS In order to achieve the best accuracy for a given measurement care must be taken to minimize the various noise sources that can be found in the laboratory With intrinsic noise Johnson noise 1 f noise or input noise the experiment or detector must be designed with these noise sources in mind These noise sources are present regardless of the input connections The effect of noise sources in the laboratory such as motors signal generators etc and the problem of differential grounds between the detector and the lock in can be minimized by careful input connections There are two basic methods for connecting a voltage signal to the lock in The single ended connection is more convenient while the differential connection eliminates spurious pick up more effectively Single Ended Voltage Connection A In the first method the lock in uses the A input in a single ended mode The lock in detects the signal as the voltage between the center and outer conductors of the A input only The lock in does not force the shield of the A cable to ground rather it is internally connected to th
35. close to the actual generator frequency The X green and Y blue displays should read values which change slowly The lock in and the generator are not phase locked but they are at the same frequency with some slowly changing 0 orange The signal magnitude R yellow is phase independent and does not change Parameters which may be scanned are Finternali Amplitude DC Level Aux Out 1 and 2 Scans can be linear or logarithmic repeat repeat up and down or run once and pause The Scan Duration is the total time to move from the Begin Value to the End Value The Parameter Update Interval is the time spent at each scan step along the way The shorter the update time the smaller the steps and the smoother the scan The longer the update time the fewer steps With experiments that take time to settle after a parameter change it can be beneficial to set the update time long enough to accommodate the settling Chapter 1 Getting Started 31 SRS Let s leave the Scan Parameter at internal reference frequency Touch Begin Value to highlight the start frequency Enter 99 990 kHz Touch End Value to highlight the stop frequency Enter 100 010 kHz Begin Value 99 9900 End Value 400 010 Touch End Mode once to select Repeat Touch Close to return to the Trend Graphs Press Scan setup briefly don t hold it to turn on the scan Press Play Pause reset briefly don t hold it to st
36. crossings This is useful when the sine output amplitude is small and a synchronous trigger is required to a scope for example This output is active even when the SR865 is locked to an external reference BlazeX The BlazeX output is a 2V low latency version of the X output It s intended for experiments that require the fastest possible output response such as feedback loops that employ the SR865 within the loop The BlazeX output is always an RC filter 1 to 4 poles regardless of the front panel Advanced Filter setting and its maximum time constant is 10 ms If a longer time constant is required the normal X output should be used instead The BlazeX output is modified by offset and expand but is not affected by the ratio function Preamp Connector This 9 pin connector provides power to external preamplifiers such as the SR550 and SR552 The power connections are described below ax in Voltage 18 5 V 5 V 18 5 V Signal Ground Ground E Using SRS Preamps When using an SRS preamp connect the power cable standard male to male 9 pin D connectors from the preamp to the rear panel preamp connector on the SR865 Use BNC SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 99 cables to connect the A output from the preamp to the A input of the SR865 and the B output from the preamp preamp ground to the B input of the SR865 Use A B as the input configuration Be sure to twist the A and B cables so that the
37. dc level to v The value of v will be rounded to 3 digits or 0 1 mV whichever is greater The levels may be programmed from 5 00 V to 5 00 V Example SCNDC BEGIN 1 23 Set the begin reference dc level to 1 23 V SCNDC 0 1230 MV SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 131 SCNDC 0 Returns the begin reference dc level SCNAUX1 BEGin END j v NV UV MV V The SCNAUX1 j v command sets the begin j 0 or end G 1 AuxOut1 value to v The voltage v can be set from 10 5 V to 10 5 V with 1 mV resolution Example SCNAUX1 BEG 4 567 Set the begin AuxOut1 value to 4 567 V SCNAUX1 0 4567 MV SCNAUX1 0 Returns the begin AuxOut1 value SCNAUX2 BEGin END j v NV UV MV V SRS The SCNAUX2 j v command sets the begin j 0 or end G 1 AuxOut2 value to v The voltage v can be set from 10 5 V to 10 5 V with 1 mV resolution Example SCNAUX2 END 4 567 Set the end AuxOut2 value to 4 567 V SCNAUX2 1 4567 MV SCNAUX2 1 Returns the end AuxOut2 value SR865 DSP Lock in Amplifier 132 Programming Chapter 4 Data Transfer Commands OUTR DAT1 DAT2 DAT3 DAT4 j The OUTR j query returns the value of data channel j The value of j 0 3 corresponds to the DAT1 green DAT2 blue DAT3 yellow and DAT4 orange data channels Example OUTR DAT2 Returns the value of data channel 2 blue OUTR 1 OUTP j The OUTP j query returns the value of a single lock in par
38. displays will read 0 for X Y and R 0 will be just noise The lock in parameters shown in the info bar at the top of the screen may be entered using a numeric keypad simply by touching them The Sine Out amplitude is specified for differential output Sine Sine In this case each BNC has an amplitude of 250 mV rms with a 50Q output The lock in input is high impedance so the output of each BNC is doubled and the lock in measures 500 mV Since the phase shift of the sine output is very close to zero X green should read about 0 5000 and Y blue should read close to 0 0000 V Automatically adjust the reference phase shift to eliminate any residual phase error This should set the values of Y and 9 to zero SR8 amp 65 DSP Lock in Amplifier 10 Getting Started Chapter 1 5 Press the 90 key This adds 90 to the reference phase shift The value of X drops to zero and Y becomes minus the magnitude 0 5000 V Use the lt Phase gt knob to adjust the phase shift The lock in parameters shown in the info bar back to zero press and hold the lt Phase gt knob Phase Reference Frequency Detected inward as a short cut The Phase shift is Harmonic Sine Amplitude and Offset can all displayed in the info bar at the top of the screen be adjusted from the front panel as well as via a touchscreen keypad 6 Touch Fint in the info bar Display the numeric entry screen for internal Sc Gee CT E reference frequency Th
39. i The value of i is limited to 0 99 Example DATE YEAR 21 Set system clock year to 20 21 DATE 2 21 DATE YEAR Returns the system clock year last 2 digits TBMODE AUTO INternal i The TBMODE i command sets the external 10 MHz timebase mode to auto i 0 or internal i 1 Example TBMODE AUTO Set the timebase mode to auto TBMODE 0 TBMODE Returns the timebase mode i TBSTAT The TBSTAT query returns the current 10 MHz timebase source either external 0 or internal 1 Example TBSTAT Returns the current 10 MHz timebase BLAZEX BLazex Blsync UNIsync i The BLAZEX i command sets the rear panel BlazeX output to blazex i 0 bipolar sync i 1 or unipolar positive sync 1 2 Example BLAZEX UNI Set the BlazeX output to unipolar sync BLAZEX 2 BLAZEX Returns the BlazeX output selection i SR865 DSP Lock in Amplifier SRS 144 Programming Chapter 4 KEYC ON MUte i The KEYC i command turns sounds on i 0 or off mute i 1 This is primarily the key and knob clicks but also includes various system sounds Example KEYC MUTE Turn system sounds off KEYC 1 KEYC Returns the system sounds state i PRMD SCReen PRNt MONOchrome i The PRMD i command sets the screen shot mode to screen i 0 print i 1 or monochrome i 2 Example PRMD SCR Set the screen shot mode to screen PRMD 0 PRMD Returns the screen shot mode i SDFM CSV MATfile i
40. in this mode Press and hold the Ref chop key to display the Chopper configuration screen Phase 427 775 deg 105000 ai 11 ATP 400 my 434 v Choper Blade Slot Count Set the SR540 REFERNCE MODE to finner f up connect the SR540 CONTROL VOLTAGE to Aux Out 4 6 5 and connect the SR540 f output to Ref In Please set the SR540 MAX FREQ to 400 Hz center 90 0 Touch the Chopper Blade Slot Count setting to specify whether the 6 5 or 30 25 slot blade is mounted on the SR540 chopper head Follow the directions to set the MAX FREQ and REFERENCE MODE switches on the SR540 Connect the SR540 f fowr output right hand BNC to the SR865 Ref In BNC Connect the SR865 Aux Out 4 rear panel BNC to the SR540 CONTROL VOLTAGE input BNC SR865 DSP Lock in Amplifier SRS Chapter 3 Operation 77 Make sure that the SR865 External Trig is set to Pos TTL and the Input is 1 MQ Adjust the SR865 internal frequency to a value that the SR540 can reach with the selected blade The Phase setting in this screen is the blade phase This is the phase of the blade opening relative to the optical detector at the base of the chopper head When a single chopper is being used this is not important since the demodulator reference phase of the SR865 can be adjusted to maximize the signal When using multiple choppers this blade phase can be used to align multiple chopped beams to arrive in phase together at the experiment This would o
41. input before turning the PMT on Furnished Accessories e Power Cord e Operating Manual Environmental Conditions Operating Temperature 10 C to 40 C Specifications apply over 18 C to 28 C Relative Humidity lt 90 Non condensing Non Operating Temperature 25 C to 65 C Humidity lt 95 Non condensing SR865 DSP Lock in Amplifier ASRS Safety and Preparation For Use Symbols you may Find on SRS Products na Alternating current Caution risk of electric shock ra Frame or chassis terminal a Caution refer to accompanying documents Earth ground terminal pt fem o stew o ASRS SR865 DSP Lock in Amplifier iv Safety and Preparation For Use SR865 DSP Lock in Amplifier Contents v Contents Safety and Preparation for Use i Contents v SR865 Specifications vii SR865 Command List x SR865 Status Bytes xv Chapter 1 Getting Started 1 Introduction 1 SR865 Front Panel 2 SR865 Touchscreen 4 The Basic Lock in 9 Using Displays 13 Sensitivity Offset and Expand 20 Saving and Recalling Setups 25 Aux Outputs and Inputs 28 Scanning 30 Chapter 2 Lock in Amplifier Basics 37 What is a Lock in Amplifier 37 What Does a Lock in Measure 40 Block diagram 41 The Reference Oscillator 42 The Phase Sensitive Detectors 43 Time Constants and Sensitivity 44 Outputs and Scales 46 What is Dynamic Reserve Really 48 The Input Amplifier 50 Input Connections 51 Intrinsic Random Noise Sources 54
42. is a square wave at frequency fet This might be the sync output from a function generator If the sine output from the function generator is used to excite the experiment the response might be the signal waveform shown below The signal is V sigSiN Oreft Osig Where Oret 27fe and Vsig is the signal amplitude SR865 DSP Lock in Amplifier 38 Basics Chapter 2 The SR865 generates its own sine wave at frequency f shown as the lock in reference below The lock in reference is sin t Oret where 27f External Reference Lock in Reference The SR865 amplifies the signal and then multiplies it by the lock in reference using a phase sensitive detector PSD or multiplier The output of the PSD is simply the product of two sine waves V psa V sig sin t Osig sin rt F Oiee 1 2 Vay cos O Ort Osig Bret 1 2 Vis cos O a olt X Que Bret The PSD output is two ac signals one at the difference frequency and the other at the sum frequency If the PSD output is passed through a low pass filter the ac signals are removed What will be left In the general case nothing However if equals the difference frequency component will be a dc signal In this case the filtered PSD output will be V psa 1 2 Vsig COS Ocig Oier This is a very nice signal it is a dc signal proportional to the signal amplitude Narrow band detection Now suppose the input is made up of si
43. is returned is a decimal number from 0 to 4095 where the bits are defined below Bit Weight Definition 0 1 CH1 output scale 1 2 CH2 output scale 2 4 unused 3 8 External reference unlocked 4 16 Input range 5 32 unused 6 64 unused 7 128 unused 8 256 Data Channel scale 9 512 Data Channel 2 scale 10 1024 Data Channel 3 scale 11 2048 Data Channel 4 scale Example CUROVLDSTAT Returns decimal value of the overload status word SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 151 Status Byte Definitions The SR865 reports on its status by means of four status bytes the Serial Poll Status byte the Standard Event Status byte the LIA Status byte and the Error Status byte The status bits are set to 1 when the event or state described in the tables below has occurred or is present Serial Poll Status Byte SRS bit name usage 0 unused 1 unused 2 ERR An enabled bit in the error status byte has been set 3 LIA An enabled bit in the LIA status byte has been set 4 MAV The interface output buffer is non empty 5 ESB An enabled bit in the standard status byte has been set 6 SRQ SRQ service request has occurred 7 unused The ERR LIA and ESB bits are set whenever any bit in BOTH their respective status bytes AND enable registers is set Use the SRE ESE ERRE and LIAE commands to set enable register bits The ERR LIA and ESB bits are not cleared until ALL ena
44. mode to on CRAT 0 1 CRAT 0 Returns the X ratio mode CRAT X SR865 DSP Lock in Amplifier 116 Programming Chapter 4 Aux Input and Output Commands OAUX j The OAUX j query returns an aux input voltage The parameter j 0 3 selects aux input 1 4 on the rear panel Example OAUX 0 Returns the aux in1 voltage v OAUX 1 Returns the aux in2 voltage v AUXV j v NV UV MV V The AUXV j v command sets an aux output to voltage v The parameter j 0 3 selects aux output 1 4 on the rear panel The voltage v can be set from 10 5 V to 10 5 V Example AUXV 2 678 9 MV Set aux out3 to 678 9 mV AUXV 2 6 789E 1 AUXV 2 0 6789 AUXV 2 Returns the aux out3 voltage setting in Volts SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 117 Auto Function Commands APHS ARNG ASCL SRS The APHS command performs the Auto Phase function This command is the same as pressing the Auto Phase key The outputs may take many time constants to reach their new values Do not send the command again without waiting the appropriate amount of time The ARNG command performs the Auto Range function This command is the same as pressing the Auto Range key The outputs may take many time constants to return to their steady state values The ASCL command performs the Auto Scale function This command is the same as pressing the Auto Scale key This automatically sets the sensitivity Measurements with the s
45. of X green is a line 10 mV above zero center This is because the offset affects the value of X unlike the sensitivity The offset for CH1 can be turned on and off without changing the offset value Notice how the trace of X changes when the offset is turned off Leave the X offset on for now Expand x10 effectively decreases the sensitivity by 10 after the offset is applied CH1 orse x Expand x 10V Sensitivity Now X 100 mV Sensitivity 100 mV the offset is 90 and the expand is x10 Thus the DVM reads 10 V The X bar graph is now at 100 and the numeric readout has added resolution The X display has an Expd indication that the displayed quantity is affected by a non unity expand Expand increases the resolution of the X display and CH1 output Note that the trace of X is unaffected by expand This is because expand does not change the value of X SR865 DSP Lock in Amplifier 24 Getting Started Chapter 1 14 Press CH1 Expand once to select x100 15 Press CH1 Expand once to turn off expand Press the lt CH1 Offset gt knob briefly once to turn the X offset off 16 Touch the green scale tile X to display its scale palette Touch BEY to auto scale the green X trace Phase 5 588 deg 100 000 kHz m 4 AM 194 py A z x SR865 DSP Lock in Amplifier The red output Overload LED lights and a Scale overload is indicated in the X display This is because C
46. oscillator frequency In this case the lock in detects signals at Nx fe which are synchronous with the reference The SINE OUT frequency is not affected The SR865 can detect at any harmonic up to N 99 as long as N x f e does not exceed 2 5 MHz The Phase Sensitive Detectors SRS The amplified input signal is converted to digital form using A D converter sampling at 10 MHz The SR865 then multiplies the signal with the reference sine wave digitally The dynamic reserve is limited by the quality of the A D conversion Once the input signal is digitized no further errors are introduced Certainly the accuracy of the multiplication does not depend on the size of the numbers The A D converter used in the SR865 is extremely linear meaning that the presence of large noise signals does not impair its ability to correctly digitize a small signal In fact the dynamic reserve of the SR865 can exceed 120 dB without any problems We ll talk more about dynamic reserve a little later We ve discussed how the digital signal processor in the SR865 computes the internal oscillator and two reference sine waves and handles both phase sensitive detectors In the SR865 DSP Lock in Amplifier 44 Basics Chapter 2 next section we ll see how the SR865 performs the low pass filtering and de amplification required at the output of the PSD s Time Constants and Sensitivity Remember the output of the PSD contains many signals Most of the output signals h
47. output impedance Use the appropriate feedthrough or T termination if necessary In general not using a terminator means that the function output amplitude will not agree with the generator setting The SR865 is set to an internal reference of 100 000 kHz This should be very close to the external generator frequency The X green and Y blue displays should read values which change slowly The lock in and the generator are not phase locked but they are at the same frequency with a slowly changing 0 orange The signal magnitude R yellow is phase independent and does not change You can adjust the generator frequency slightly to get closer to the SR865 The FFT of the signal input is displayed There is only a single quantity shown The left and right edge frequencies are labelled at the bottom of the graph The cursor readout is at the right The display is adjusted with the tiles across the bottom Src selects the source data for the FFT dB and Hz adjust the vertical and horizontal scales Avgs sets the amount of averages and Live toggles to Paused SR8 amp 65 DSP Lock in Amplifier 164 The FFT Display Appendix B 100 000 kz M Amel Phase 0 000 deg Fint 92 45 mdB 97 6562 kHz 0 0 Hz Sre Rawan dB dv ig 4 Hz dv ego Avos 1 Touch the dB div scale tile at the bottom to display its scale palette Touch BA to auto scale the FFT Touch and drag in the graph area to move the graph
48. pressure temperature wavelength etc Chapter 1 Getting Started 29 5 SRS Touch Screen Layout multiple times to show the full numeric display screen 1 Ampl Phase 0 000 deg 100 000 kiz Out 1 5 000 Y Out2 0 001 V Guta ee l 0 000 V Out4 l Note the 4 Aux Output values are displayed in white tiles across the bottom Touching an output tile will display the Aux Output keypad Disconnect the DVM from Aux Out 1 Connect Aux Out 1 to Aux In 1 on the rear panel 0 000 V This screen displays the 4 Aux Input readings along with the 4 lock in data channels The Aux Inputs are always scaled to 10 V The Aux Inputs can read 4 analog voltages These inputs are useful for monitoring and measuring other parameters in an experiment such as pressure temperature position etc We ll use Aux Out 1 to provide an analog voltage to measure Aux In 1 should now read 5 000 V The Aux Inputs can be assigned to a data channel and graphed on the strip chart alongside lock in outputs Use the Config key to change a data channel SR8 amp 65 DSP Lock in Amplifier 30 Getting Started Chapter 1 Scanning This measurement is designed to use the internal oscillator and an external signal source to explore some of the display types You will need a synthesized function generator capable of providing a 500 mVrms sine wave at 100 000 kHz BNC cables and a terminator appropriate for the generator f
49. requested In this example the returned string might be 0 951359 0 0253297 1 234 The first value is X the second is Y and the third is Aux In1 Not all parameters are measured at the same rate within the SR865 The external frequency for example is only measured every reference period at low frequencies The most recent measurements for these parameters are returned by SNAP A sRs SR865 DSP Lock in Amplifier 134 Programming Chapter 4 Data Capture Commands Overview The SR865 can capture data points in an internal capture buffer This can be up to 1 Mpoints of X 512 kpoints of either X Y or R 0 or 256 kpoints of X Y R 0 Data points are stored as floating point values 4 bytes per data point The capture buffer is made of 2 kbyte blocks 512 total data points per block but specified in terms of its overall length in kbytes which must therefore always be an even number of kbytes The buffer can be configured between 2 kbytes 512 total points and 4 Mbytes 1 048 576 total points The capture buffer is not retained when the power is turned off Data Format The captured data is stored and read in 32 bit floating point format Data is stored and transferred in little endian byte order Capture Rate The capture rate sets how often data points are sampled and added to the capture buffer All parameters are sampled simultaneously at the same rate The maximum capture rate is determined by the time constant of t
50. signal is detected at the timebase input If the external timebase signal drifts out of range either in amplitude or frequency this setting will revert to Internal The Ext 10 MHz LED is on when the unit is locked to an external timebase The SR865 can also output its own 10 MHz timebase to another unit Sounds Sounds can be toggled from On to Mute These are primarily the key and knob clicks but also includes various other notification sounds SR865 DSP Lock in Amplifier 92 Operation Chapter 3 Sync BlazeX The rear panel BlazeX output BNC can be configured to output a reference frequency sync signal either bipolar or unipolar square sync or a low latency X output for closed loop applications at short time constants File Settings Prefix Suffix t General SR865_ a ft Files Next File Name AESSHO 1 ACEBIT SR865_3 Ethernet TRASHE 1 Print Mode Format GPIB 4 SPOTL Screen CSV 4 Q WERT YUI O P Data and screenshots can be saved to a USB memory stick inserted into the front panel Press the Save button to save all the data values currently plotted on the strip chart Press the Screen Shot button to save an image of the screen The memory stick must be formatted as FAT32 and the saved files can be read by Windows and Macintosh computers Folder Navigate to the destination folder on the memory stick New files will be saved in the destination folder The subdirectories in the current destination folder
51. state all front panel operation is locked out including the LOCAL key The REMOTE indicator is directly above the LOCAL key and is on when the unit is in REMOTE or LOCAL LOCKOUT The Overide Remote mode must be set to Off in order for the front panel to be locked out If Overide Remote is On then the front panel is active even in the REMOTE state Example LOCL 1 Set the unit to REMOTE SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 147 LOCL Returns the local remote state i OVRM OFF ON i SRS The OVRM command sets the GPIB Overide Remote to Off i 0 or On Gi 1 When Overide Remote is On then the front panel is NOT locked out when the unit is in the REMOTE state The REMOTE indicator will still be on and the LOCAL key will still return the unit to the Local state The default mode is Overide Remote On To lock out the front panel use the OVRM 0 command before local lock out Example OVRM 0 Set Overide Remote to No OVRM Returns the Overide Remote state i SR865 DSP Lock in Amplifier 148 Programming Chapter 4 Status Reporting Commands CLS The CLS command clears all status registers The status enable registers are NOT cleared ESE j i ESR j The ESE i command sets the standard event enable register to the decimal value i 0 255 The ESE j i command sets bit j 0 7 toi 0 or 1 The ESE command queries the value 0 255 of the standard event enable r
52. stop values for the scanned parameter The Scan Time is the total time to move from the Begin Value to the End Value The Scan Parameter Update is the time spent at each scan step along the way The shorter the time the more steps in the scan and the smoother the parameter varies The longer the time the fewer steps With experiments that take time to settle after a parameter change it is often beneficial to set the update time long enough to accommodate the settling The scan End Mode can be Repeat repeat Up Down or Once pause at the end When the End Mode is set to repeat the scan repeats over and over Often when scanning it is convenient to pause the strip chart at the end of the scan to review the results Set the End Mode to Once and the scan stops at the end value and the strip chart is automatically paused SR865 DSP Lock in Amplifier SRS Chapter 3 Operation 89 To Run a Scan Press Scan setup briefly don t hold it to turn on the scan to ready This key turns scanning on and off The Ready LED turns on indicating that the scan is ready at the begin value At this time the scanning parameter if it is being displayed on screen is shown in orange indicating that it is under scan control Press Play Pause reset briefly to start the scan The Run LED turns on indicating that the scan is running The scanning parameter starts from its begin value to its end value Press Play Pause reser briefly to pause and r
53. the front panel is locked out by a computer interface No front panel adjustments may be made Local When a host computer places the unit in the REMOTE state the keys knobs and touchscreen are locked out The Remote indicator is on above the Local key To return to front panel operation press the Local key Press and hold Local while turning the power on to reset the unit to its default settings This does not affect the computer interface settings Save The SR865 can save data files to a USB memory stick inserted into the front panel USB slot Press Save to save the data points in the current strip chart to the memory stick This file can be CSV or MAT file as selected in the system menu The Busy LED indicates that the USB stick is busy and should not be removed A sRs SR865 DSP Lock in Amplifier 88 Operation Chapter 3 Setup Calc system Ready Run a Done m Pause a eee setup Scan setup and Play Pause Press and hold Scan setup to display the scan setup screen Scan Parameter Scan Time Scan Paramaeter Update 01 40 0 469 S Scan Type Begin Value t J 400 000 End Mode End Value 200 000 7 8 4 5 Fu F3 1 00000 kHz 1 00000 MHz 1 2 0 F2 F4 50 0000 Hz 100 010 kHz The Scan Parameter can be Finterna Sine Amplitude Sine DC Level Aux Out 1 or Aux Out 2 The Scan Type can be Linear or Logarithmic The Begin and End Values are the start and
54. the trigger is received Once capture is started bits O and 1 both become 1 giving a return value of 3 Bit 0 remains 1 while capture is recording data After receiving the CAPTURESTOP command bit 0 may remain 1 until the current 2kbyte block of data is filled If the capture buffer wraps around then bit 2 becomes 1 this occurs at the end of a OneShot capture or when a Continuous capture begins overwriting old data Bits 1 and 2 once set will remain set until cleared by resetting the capture system with a CAPTURECFG or CAPTURELEN command Example CAPTURESTAT Returns the capture buffer state CAPTUREPROG The CAPTUREPROG query returns the number of kilobytes of data that were written during the most recent capture acquisition Capture must be stopped before performing this query If the acquisition wrapped during operating in Continuous mode CAPTUREPROG will simply return the value of CAPTURELEN Example CAPTUREPROG Returns the amount of data captured in kilobytes CAPTUREVAL n The CAPTUREVAL query returns data from the capture buffer in plain text ASCII format Data is returned as comma separated values of one two or four floating point numbers based on the value of CAPTURECFG Query with n 0 returns the oldest data from the buffer Example CAPTUREVAL 3 returns the data from position 3 CAPTUREGET i j The CAPTUREGET query returns all or part of the capture buffer contents as a binary block The
55. then start a new one by cycling the Scan setup button the Ready LED goes off and then on SR865 DSP Lock in Amplifier 34 Getting Started Chapter 1 17 Press Play Pause reset briefly don t hold it to The Run LED turns on indicating that the scan re start the scan is running 18 Touch the white scale tile Time to display the This scan is 100 s in length so set the graph to horizontal scale palette bottom right of screen 10 divisions of 10 s to show a complete scan repeatedly to zoom in to 10 S div Touch the highlighted white scale tile again to dismiss the scale palette Wait for the scan to finish When the scan finishes the Done LED turns on 19 Use the lt Cursor gt knob to move the cursor to the When the scan in progress reaches the end peak of R yellow value the strip chart pauses Pa 0 000 aeg The frequency will hold at the end value 100 010 kHz as displayed in the info bar The cursor is active when the strip chart is paused 20 Touch the white scale tile Time to display the Notice that data collection continued while the horizontal scale palette bottom right of screen chart was paused Restarting the chart realigns the time history so the current time is the right Touch Dl start the strip chart again edge again Touch the highlighted white scale tile again to In this case the frequency has been constant at dismiss the scale palette 100 010 kHz the entire time we have been examining the pau
56. up and down or use the and buttons Use the lt Cursor gt knob to move the marker onto the signal peak 1 Ampl Phase T deg a 21 03 dB 97 6562 kHz p 0 Hz Sre Raw ADC oe dB div 2 ERAC eae 1 Touch to change the width of the cursor In the FFT display the wide cursors always find the peak within the cursor region This makes reading an FFT much easier Touch the highlighted dB div tile again to dismiss the scale palette Touch the Hz div tile along the bottom to display the span up down buttons SR865 DSP Lock in Amplifier Use the lt Cursor gt knob to move the cursor The vertical axis is always logarthmic in the FFT display This allows a large dynamic range to be shown in a single graph The amplitude is not calibrated Notice that the cursor readout at the upper right reads the peak amplitude and frequency This is the signal from the external generator The peak is about 20 dB or 0 1 Vrms The cursor reads 97 6562 kHz This is because at this full span the FFT frequency bins are 9 76 kHz apart and the 100 00 kHz signal falls mostly in the 97 6562 kHz bin The FFT frequency span can be reduced to provide better frequency resolution When the FFT source is Raw ADC the FFT display is ASRS Appendix B The FFT Display 165 Touch v 3 times to decrease the frequency scale to 39 06 kHz div Move the cursor back to the peak 100 000 kz M 4 Aol Phase 0 000 deg Fint
57. using the GPIB interface serial polling may be used to check the Interface Ready bit in the Serial Poll Byte while an operation is in progress After the Interface Ready bit becomes set signaling the completion of the command then the ERR or ESB bit may be checked to verify successful completion of the command If an interface other than GPIB is used then serial polling is not available The STB ESR ERRS and LIAS status query commands may be used to query the Status Bytes Since the SR865 processes one command at a time the status query will not be processed until the previous operation is finished Thus a response to the status query in itself signals that the previous command is finished The query response may then be checked for various errors SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 103 Command Syntax SRS A command to the SR865 consists of a command mnemonic a space followed by arguments if necessary and a command terminator There MUST be at least one space between the command and the first argument Multiple arguments are separated by commas Some commands have no arguments The command terminator must be a linefeed lt If gt on RS 232 or a linefeed lt If gt or EOI on GPIB No command processing occurs until a command terminator is received Commands function identically on all interfaces whenever possible Command mnemonics beginning with an asterisk are IEEE 488 2 defined common comm
58. 0 000 mVrms 30 mV 50 mV 10 000 mVrms 10 mV 10 mV SR865 DSP Lock in Amplifier ASRS Appendix G Performance Tests 189 a Use the front panel to make the following adjustments Touch Ampl to display the amplitude keypad Enter the amplitude from the table Press Input Range Down Select the input range from the table Press Sensitivity Down Select the sensitivity from the table b Wait for the R reading to stabilize Record the value of R yellow for each sensitivity 4 Frequency response is checked at frequencies above 1 kHz The test frequencies are listed below Test Frequencies 1 0 kHz 10 kHz 100 kHz 1 0 MHz a Use the front panel to make the following adjustments Touch Ampl to display the amplitude keypad Enter the 100 mV Press Input Range Up Down Set the input range to 100 mV Press Sensitivity Up Down Set the sensitivity to 100 mV b Set the sine output to the frequency in the table Touch Fint to display the internal frequency keypad Enter the frequency from the table c Wait for the R reading to stabilize Record the value of R yellow d Repeat steps 4b and 4c for all of the frequencies listed 5 This completes the amplitude accuracy and frequency response test Enter the results of this test in the test record at the end of this section ASRS SR865 DSP Lock in Amplifier 190 Performance Tests Appendix G Amplitude Linearity This test measures the amplitude linearity
59. 0 00000 kHz 8 479 KHz Src post Fiter CPO oe 7 Touch the Hz div tile along the bottom to display the span up down buttons Use the A and v buttons to set the Hz div to 305 2 Hz The total span is about 1 kHz 8 Increase the external generator frequency to 101 000 kHz Move the cursor to the peak Decrease the external generator frequency to 99 000 kHz Phase 0 000 as Fint 1 Ampl 100 000 khz 7 36 31 dB 98 998642 kHz 98 479 kHz Src post Fiter dB div 9 Hz div 3052 Avgs 9 Leave the external generator at 99 000 kHz Move the cursor to the signal peak SR865 DSP Lock in Amplifier the time constant filters The FFT of the PSD filtered output is displayed This display is presented as a zoomed spectrum the same as a spectrum analyzer The center of the span is the lock in reference frequency Signals to the right of center are above the fier signals to the left are below fet The center frequency is the signal at fret Note that this is not the same as connecting the lock in output BNC to a spectrum analyzer At a 1 ms time constant the maximum span is about 50 kHz 4 883 kHz div At a 1 s time constant the maximum span is 1 6 kHz 152 6 Hz div The signal peak moves to the right of center and the cursor reads 101 00 kHz The signal peak moves to the left of center and the cursor reads 99 00 kHz Both of these signal frequencies will appear at the lock in output at 1 00 kHz lfsig
60. 360000 lt p lt 360000 and will be wrapped around at 180 For example the PHAS 541 0 command will set the phase to 179 00 541 360 181 181 360 179 Phase may be specified in degrees default or millidegrees microdegrees radians milliradians or microradians Example PHAS 12 34 DEG Set the reference phase to 12 34 deg PHAS 1 234E1 PHAS 12340 MDEG PHAS Returns the reference phase in degrees The APHS command performs the Auto Phase function This command is the same as pressing the Auto Phase key The outputs will take many time constants to reach their new values Do not send the command again without waiting the appropriate amount of time FREQ f HZ KHZ MHZ The FREQ f command sets the internal frequency to f The value of f will be rounded to 6 digits or 0 1 mHz whichever is greater The value of f is limited to 1 mHz lt f lt 2 5 MHz The query form FREQ returns the internal reference frequency whenever the reference mode is either Internal Dual or Chop The query returns the external frequency when operating in External mode This behavior mirrors the value displayed in the info bar at the top of the display SR865 DSP Lock in Amplifier SRS Chapter 4 Programming 107 Example FREQ 12 34E3 Set the internal frequency to 12 34 kHz FREQ 12 34 KHZ FREQ 12340 FREQ Returns the internal or external frequency in Hz FREQINT f HZ KHZ MHZ FREQEXT FREQDET
61. 49 0mV G 51 0 mV 100 kHz 49 0mV H 51 0 mV 1 0 MHz 49 0mV J 51 0 mV Note the readings recorded as A through J will be used to determine lower and upper limits for the Amplitude Accuracy and Flatness and Amplitude Linearity both of which are on the next page Test Record sheet 1 of 4 Jes SR865 DSP Lock in Amplifier 198 Performance Test Record Appendix G Amplitude Accuracy and Flatness Input Range Sine Out Ampl Lower Limit Reading Upper Limit 1V 1 0000 Vrms 0 99x A 1 01x A 300 mV 300 00 mVrms 0 99x B 1 01x B 100 mV 100 000 mVrms 0 99x C 1 01x C 30 mV 30 000 mVrms 0 99x D 1 01x D 10 mV 10 000 mVrms 0 99x E 1 01x E Input Range Frequency Lower Limit Reading Upper Limit 100 mV 1 0 kHz 0 99x C 1 01x C 100 mV 10 kHz 0 99x G 1 01x F 100 mV 100 kHz 0 99x H 1 01x G 100 mV 1 0 MHz 0 98x J 1 02x H Amplitude Linearity Input Range Sine Out Ampl Lower Limit Reading Upper Limit 1V 1 0000 Vrms 0 99x A 1 01x A 100 00 mVrms 0 99x C 1 01x C 10 000 mVrms 0 99x E 1 01x E 1 0000 mVrms 0 99x F 1 01x F Frequency Accuracy Input Frequency Lower Limit Reading Upper Limit 9 9990 kHz 9 9987 kHz 9 9992 kHz Phase Accuracy Frequency Lower Limit Reading Upper Limit 10 Hz 1 0 deg 1 0 deg 100 Hz 1 0 deg 1 0 deg 1 kHz 1 0 deg 1 0 deg 10 kHz 1 0 deg 1 0 deg 100 kHz 1 0 deg 1 0 deg 1 0 MHz 2 5 deg 2 5 deg dc Outputs and Inputs Output Offset Lowe
62. BUG Returns the cursor readout mode i FCRW Line NARrow Wide i The FCRW i command sets the cursor width to line 0 narrow i 1 or wide i 2 This command is the same as pressing in the horizontal scale palette Example FCRW NAR Set the cursor to narrow FCRW 1 FCRW Returns the cursor width i SCRY DAT1 DAT2 DAT3 DAT4 STATus j The SCRY j query returns the strip chart cursor value of data channel j The value of j 0 3 corresponds to the DAT1 green DAT2 blue DAT3 yellow and DAT4 orange data channels Querying SCRY STAT j 4 returns 0 if there is no overload or unlock at the cursor positon The cursor is only available when the strip chart is paused The status response encodes three distinct sources that cause the violet error mark to appear at the bottom of the strip chart display Bit Weight Definition 0 1 Timebase error 1 2 External reference unlock 2 4 Signal overload 3 8 Sync filter error Example SCRY DAT2 Returns the cursor value of data channel 2 blue SCRY STATUS Returns the error status for the current cursor position CURDATTIM SRS The CURDATTIM query returns the strip chart cursor horizontal date and time as a string This string is in the format 28Apr14 14 25 35 96 In this example the date is 28 April 2014 and the time is 14 25 35 96 2 25 PM Performing the CURDATTIM query when the display is not paused generates a parameter ra
63. CNENBL 1 SCNENBL Returns the scanning off on state i The SCNRUN command starts or resumes the scan This command has no effect if the scan is already running The SCNPAUSE command pauses the scan This command has no effect if the scan is already paused The SCNRST command resets the scan regardless of its current state running or paused This sets the scan parameter to its begin value but does not start a scan The SCNSTATE query returns the current state of the scan off or disabled 0 reset 1 running 2 paused 3 or done 4 This is a query only command SCNFREQ BEGin END j f HZ KHZ MHZ The SCNFREQ j f command sets the begin G 0 or end j 1 frequency to f The value of f will be rounded to 6 digits or 0 1 mHz whichever is greater The value of f is limited to 1 mHz lt f lt 2 5 MHz Example SCNFREQ BEG 1234 Set the begin frequency to 1 234 kHz SCNFREQ 0 1 234 KHZ SCNFREQ 0 Returns the begin frequency SCNAMP BEGin END j v NV UV MV V The SCNAMP j v command sets the begin j 0 or end j 1 ref amplitude to v The value of v will be rounded to 3 digits or 1 nV whichever is greater The value of v is limited to 1 nV lt v lt 2 0 V Example SCNAMP END 1 23 Set the end reference amplitude to 1 23 V SCNAMP 1 1230 MV SCNAMP 1 Returns the end reference amplitude SCNDC BEGin END j v NV UV MV V The SCNDC j v command sets the begin j 0 or end G 1 ref
64. Example GACT DAT1 Auto Scale zero center data channel 1 green GACT 0 GAUF DAT1 DAT2 DAT3 DAT4 j The GAUF j e s an Auto Find on data channel j This command is the same as pressing the button in the scale palette The value of j 0 3 corresponds to the DAT1 green DAT2 blue DAT3 yellow and DAT4 orange data channels Example GAUF DAT4 Auto Find data channel 4 orange GAUF 3 CGRF DAT1 DAT2 DAT3 DAT4 j OFF ON i SRS The CGRF j i command turns the graph of data channel j off i 0 or on G 1 The value of j 0 3 corresponds to the DAT1 green DAT2 blue DAT3 yellow and DAT4 orange data channels Example CGRF DAT2 ON Turn graphing on for data channel 2 blue CGRF 1 1 CGRF 0 OFF Turn graphing off for data channel 1 green CGRF 1 Returns data channel 2 blue graphing state i CGRF DAT2 SR865 DSP Lock in Amplifier 122 Programming Chapter 4 GLIV OFF ON i The GLIV i command pauses i 0 or resumes i 1 the strip chart Example GLIV OFF Pause the strip chart GLIV 0 GLIV Returns the strip chart state i PCUR i The PCUR i command sets the strip chart cursor position i The right edge is i 0 and the left edge is i 639 The cursor is visible only when the chart is paused Example PCUR 319 Set the strip chart cursor to the middle of the chart PCUR Returns the strip cursor position i CURREL OFF ON i The CURREL i comma
65. Floating point streaming uses twice the bandwidth Data is sent over the ethernet interface in UDP packets containing 132 260 516 or 1028 payload bytes Missed packets are NOT resent It is important that the receiving computer be able to keep up with the data rate to prevent data loss and interruption Streaming Rate The streaming rate sets how often data points are sampled and added to the stream All parameters are sampled at the same rate and at the same times The maximum allowed streaming rate is determined by the time constant of the SR865 Shorter time constants allow faster streaming rates up to a limit of 1 25 MHz The actual streaming rate can be set to the maximum allowed rate divided by factors of 2 Starting and Stopping The STREAM ON OFF command turns streaming on and off NOTE Changes to streaming parameters only take effect when streaming is turned back on So if you want to change e g the kind data being streamed you need to turn streaming off change the data that is sent and then turn streaming back on Aliasing Effects In any sampled data stream it is possible to sample a high frequency signal such that it will appear to be a much lower frequency This is called aliasing Aliasing occurs whenever the signal being sampled contains signals at frequencies greater than 1 2 the sample rate The effect is most noticeable when trying to sample an output frequency at an integer multiple of the sample rate Generally c
66. Fuse Installation and ac Line Select 179 SRS Fuse Installation Two options are available for fusing the SR865 North Americal style single fuse and metric dual fuse installation The fuse holder is reversible to accommodate both styles The following steps describe how to install or replace the fuse s 1 Disconnect and remove the power cord 2 USE ONLY WITH 250V FUSES DISCONNECT POWER BEFORE REPLACING FUSES Section A A 3 Gently lift the entire door back away from the rear panel of the SR865 Lift away approximately 0 25 6 mm Once lifted the door will pivot on its hinges to expose the fuse holder Lift Up 5 4 When the fuse holder is installed in the single fuse North American position apply a screwdriver or small needle nose plyers as shown and gently lift out Use a tool as shown do not use fingers SR865 DSP Lock in Amplifier 180 Fuse Installation and ac Line Select Appendix F NOT here nsert Here When the fuse holder is installed in the dual metric position it will normally release as soon as the door is opened 5 Install one 1 AG size 0 25 x 1 25 fuse or two 2 metric size 5x20 mm fuses as shown Install fuses on one side only do not install both AG and metric together at the same time North American Metric single fuse installation dual fuse installation Also note that when installing the North American AG fuse the fuse is not centered_on the
67. G LIN LOG i The SCNLOG i command sets the scan type to linear i 0 or logarithmic i 1 Example SCNLOG LIN Set the scan type to linear SCNLOG 0 SCNLOG Returns the scan type i SCNEND ONce REpeat UPdown i The SCNEND i command sets the scan end mode to once i 0 repeat G 1 or up down i 2 Example SCNEND REPEAT Set the scan end mode to repeat SCNLOG 1 SCNLOG Returns the scan end mode i SCNSEC x The SCNSEC x command sets the scan time to x seconds The scan time is limited to 20 days 1728000 seconds Example SCNSEC 1234 Set the scan time to 1234 seconds 20 34 SCNSEC 1 234E3 SCNSEC Returns the scan time in seconds SCNAMPATTN i The SCNAMPATTN i command sets the operating mode for the output attenuators when scanning sine out amplitude to automatic i 0 or fixed i 1 Normally the sine out hardware will automatically switch one or more balanced attenuators into the output signal path to optimize performace for the commanded amplitude During scans however it can be helpful to force the attenuators to remain in a fixed configuration inhibiting the relay switching that would otherwise introduce transients in the sine output SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 129 Example SCNAMPATTIN 1 Set the scan amplitude attenuator mode to fixed SCNAMPATTN Returns the mode for amplitude output attenuators SCNDCATTN i The SCNDCATTN i command set
68. G30 AVG100 i The FFTA i command sets the FFT averaging to 1 i 0 3 G 1 10 G 2 30 i 3 or 100 i 4 Example FFTA AVG3 Set the FFT averaging to 3 FFTA 1 FFTA Returns the FFT averaging FFTL OFF ON i The FFTL i command pauses i 0 or resumes i 1 the FFT graphing Example FFTL OFF Pause the FFT graph FFTL 0O FFTL Returns the FFT graphing i FCRW Line NARrow Wide i The FCRW i command sets the cursor width to line i 0 narrow 1 1 or wide i 2 Example FCRW NAR Set the cursor to narrow FCRW 1 FCRW Returns the cursor width i SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 127 FCRX FCRY ASRS The FCRX query returns the frequency value of the FFT cursor readout in Hz FCRX is a query only command Example FCRX Returns the FFT cursor frequency value The FCRY query returns the amplitude value of the FFT cursor readout in dB FCRY is a query only command Note that FCRY only returns valid data when the SR865 display is set to FFT mode Example FCRY Returns the FFT cursor amplitude value SR865 DSP Lock in Amplifier 128 Programming Chapter 4 Scan Commands SCNPAR Fint REFAmp REFDc OUT1 OUT2 i The SCNPAR i command sets the scan parameter to Fint i 0 Ref Ampl i 1 Ref dc i 2 Out 1 3 or Out2 i 4 Example SCNPAR F Set the scan parameter to Fint SCNPAR 0 SCNPAR Returns the scan parameter i SCNLO
69. H1 is trying to reach 100 V 10 times the previous DVM reading Since the CH1 output is limited to 10 V the output is overloaded This has no affect on the value of X and the trace is unchanged The bar graph and displayed value are pinned however The X display returns to 100 mV 100 bar graph and 10 V CH1 output The X graph is a line at 100 mV 2 divisions above center With offset and expand the output voltage gain and offset can be programmed to provide control of feedback signals with the proper bias and gain for a variety of situations Offsets add and subtract from the values of X Y and R Expand increases the resolution of the displays and analog outputs but does not change the values of X Y and R When using the strip chart graph exclusively there is no need to use offset or expand to zoom in on the data Simply auto scale the data channel to graphically offset and expand the chart data See the Outputs and Scales discussion in the next chapter for more detailed information on output scaling Chapter 1 Getting Started 25 Saving and Recalling Setups The SR865 can store 8 complete instrument setups in non volatile memory SRS Do This Disconnect all cables from the lock in Turn the power on while holding down the Local key The power switch is on the power entry module on the rear panel Press and hold Calc system to display the system menu General t Files Ethernet 1OMHz Tim
70. H2 output scale overload unused External reference or Chop unlock detected Input range overload detected Sync filter frequency out of range Sync filter overload Set when data storage is triggered Data Channel 1 scale overload Data Channel 2 scale overload Data Channel 3 scale overload Data Channel 4 scale overload Display capture to USB stick completed Scan started Scan completed The LIA Status bits stay set until cleared by reading or by the CLS command Error Status Byte bit name 0 CLK 1 BACKUP 2 3 4 VXI 5 GPIB 6 USBDEV 7 USBHOST usage External 10 MHz clock input error Battery backup failed unused unused VXI 11 error GPIB fast data transfer mode aborted USB device error interface error USB host error memory stick error The Error Status bits stay set until cleared by reading or by the CLS command SR865 DSP Lock in Amplifier SRS Advanced Filters 155 Appendix A Advanced Filters Traditionally analog lock in amplifier time constant filters were cascaded single pole RC sections These are easy to construct require minimal tuning and have familiar time and frequency domain characteristics But with the advent of digital lock in amplifiers anything is possible as far as filters are concerned So let us consider what is possible what is optimal and what a lock in user would want in digital filter Different filters have different time domain characteristics such as
71. LS ESE j ESR j SRE j STB j PSC i Reporting Commands tt Hi ERRE j Hi ERRS j LIAE j LIAS j i CUROVLDSTAT SR865 DSP Lock in Amplifier page description 148 148 148 148 148 149 149 149 149 150 150 Clear all status bytes Set the standard event enable register Query the standard event status byte Set the serial poll enable register Query the serial poll status byte Set the Power On Status Clear bit Set the error status enable register Query the error status byte Set LIA status enable register Query the LIA status word Query the present overload states Commands XV SR865 Status Bytes Serial Poll Status Byte bit DD e w NIe 7 name ERR LIA MAV ESB SRQ usage unused unused An enabled bit in the error status byte has been set An enabled bit in the LIA status byte has been set The interface output buffer is non empty An enabled bit in the standard status byte has been set SRQ service request has occurred unused Standard Event Status Byte bit SYD S WIN oO LIA Status Word WOO AD WT HS WN PF oO tT gt a FP WwW NY FF ASRS name OPC INP QRY EXE CMD URQ PON name CH10V CH20V UNLK RANGE SYNCF SYNCOV TRIG DAT1OV DAT20V DAT30V DAT40V DCAPFIN SCNST SCNFIN usage Operation complete Input queue overflow unused Output queue overflow A command cannot execu
72. Operation Manual SR865 2 MHz DSP Lock in Amplifier y S RS Stanford Research Systems Revision 1 26 Certification Stanford Research Systems certifies that this product met its published specifications at the time of shipment Warranty This Stanford Research Systems product is warranted against defects in materials and workmanship for a period of one 1 year from the date of shipment Service For warranty service or repair this product must be returned to a Stanford Research Systems authorized service facility Contact Stanford Research Systems or an authorized representative before returning this product for repair Information in this document is subject to change without notice Copyright Stanford Research Systems Inc 2015 All rights reserved Stanford Research Systems Inc 1290 C Reamwood Avenue Sunnyvale California 94089 www thinkSRS com Printed in U S A Document number 9 01707 903 SR865 DSP Lock in Amplifier ASRS Safety and Preparation For Use i Safety and Preparation for Use Warning Dangerous voltages capable of causing injury or death are present in this instrument Use extreme caution whenever the instrument covers are removed Do not remove the covers while the unit is plugged into a live outlet Line Voltage Selection Caution This instrument may be damaged if operated with the LINE VOLTAGE SELECTOR set for the wrong ac line voltage or if the wrong fuse is installed The SR865 o
73. R Note that this is a numerical output conversion Output overloads do not affect the actual measurement results They only indicate that the output value exceeds 100 of the chosen Sensitivity and the output BNC the bar graph and the displayed numerical readout will be pinned at their maximums The results displayed on the strip charts or available over the computer interfaces are the floating point outputs and are unaffected by output overloads The Sensitivity is chosen to conveniently and accurately display the measurement results on the output BNC the bar graph and the numerical readout The Sensitivity however must be chosen appropriately when using synchronous filters see below Synchronous Filters Even if the input signal has no noise the PSD output always contains a component at 2f sum frequency of signal and reference whose amplitude equals or exceeds the desired dc output depending upon the phase At low frequencies the time constant required to attenuate the 2f component can be quite long For example at 1 Hz the 2f output is at 2 Hz and to attenuate the 2 Hz by 60 dB in two stages requires a time constant of 3 seconds A synchronous filter on the other hand operates totally differently The PSD output is averaged over a complete cycle of the reference frequency The result is that all components at multiples of the reference 2f included are notched out completely In the case of a clean signal almost no additional filte
74. Rs SR865 DSP Lock in Amplifier 146 Programming Chapter 4 Interface Commands RST IDN TST OPC LOCL i The RST command resets the SR865 to its default configurations The communications setup is not changed All other modes and settings are set to their default conditions and values This command takes some time to complete This command resets any data scan in progress Data stored in the buffers will be lost See page 64 for a complete list of setting values that result from RST The IDN query returns the SR865 s device identification string This string is in the format Stanford_Research_Systems SR865 000111 v1 23 In this example the serial number is 000111 and the firmware version is 1 23 The TST query always returns 0 The OPC command sets the Operation Complete bit within the Standard Event Status register see page 152 Querying OPC Always returns a 1 but does not affect the Standard Event Status register The LOCL command sets the local remote function If i 0 the SR865 is set to LOCAL if i 1 the SR865 will go REMOTE and if i 2 the SR865 will go into LOCAL LOCKOUT The states duplicate the GPIB local remote states In the LOCAL state both command execution and keyboard input are allowed In the REMOTE state command execution is allowed but the keyboard and knob are locked out except for the LOCAL key which returns the SR865 to the LOCAL state In the LOCAL LOCKOUT
75. SLVL 12 3 MV Set the sine out amplitude to 12 34 mV SLVL 1 23E 2 SLVL 0 0123 SLVL Returns the sine out amplitude in Volts SOFF v NV UV MV V The SOFF v command sets the sine out dc level to v The value of v will be rounded to 3 digits or 0 1 mV whichever is greater The level may be programmed from 5 00 V to 5 00 V SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 109 Example SOFF 34 5 MV Set the sine out dc level to 34 5 mV SOFF 3 45E 2 SOFF 0 0345 SOFF Returns the sine out dc level in Volts REFM COMmon DiFference i The REFM i command sets the sine out dc mode to common i 0 or difference 1 Example REFM DIF Set the dc mode to difference REFM DIFFERENCE REFM 1 REFM Returns the dc mode i RSRC INT EXT DUAL CHOP i The RSRC i command sets the reference source to internal i 0 external G 1 dual i 2 or chop i 3 Example RSRC EXT Set the reference mode to external RSRC 1 RSRC Returns the reference mode i RTRG SIN POSitl NEGttl i The RTRG i command sets the external reference trigger mode to sine i 0 positive TTL Gi 1 or negative TTL i 2 Example RTRG POS Set the external reference trigger to positive TTL RTRG POSTTL RTRG 1 RTRG Returns the external reference trigger mode i REFZ 50ohms 1Meg i SRS The REFZ i command sets the external reference trigger input to 50 Q i 0 or 1 MQ i 1 Example REFZ 50
76. STF j f HZ KHZ MHZ PSTA j v NV UV MV V PSTL j v NV UV MV V Signal Commands IVMD VOLTage CURRent i ISRC A A B i ICPL AC DC i IGND FLOat GROund i IRNG 1Volt 300Mvolt 100Mvolt 30Mvolt 10Mvolt i ICUR 1MEG 100MEG i ILVL SCAL i OFLT i OFSL i SYNC OFF ON i ADVFILT OFF ON i ENBW SR865 DSP Lock in Amplifier page 106 106 106 106 106 107 107 107 107 107 108 108 108 108 109 109 109 109 110 110 110 page 111 111 111 111 111 112 112 112 113 113 113 113 113 description Set the 10 MHz timebase Query the current 10 MHz timebase ext 0 or int 1 Set the reference phase to p Auto Phase Set the reference frequency to f Set the internal reference frequency to f Query the external reference frequency Query the detection frequency Set harmonic detect to i Set harmonic for dual reference mode to Set the chopper blade number of slots Set the chopper blade phase to p Set sine output amplitude to v Set sine output dc level to v Set sine output dc mode Set reference mode Set external reference trigger Set external reference input impedance Set frequency preset j to f Set sine amplitude preset j to v Set sine dc level preset j to v description Set input to voltage current Set voltage input configuration Set voltage input coupling Set voltage i
77. Status byte AND Enable register is set an SRQ is generated Bit 6 SRQ in the Serial Poll Status byte is set Further reference unlocks will not generate another SRQ until the UNLK status bit is cleared The UNLK status bit is cleared by reading the LIA Status byte with LIAS Presumably the controller is alerted to the unlock via the SRQ performs a serial poll to clear the SRQ does something to try to remedy the situation change frequency experimental parameters etc and then clears the UNLK status bit by reading the LIA status register A subsequent UNLK overload will then generate another SRQ Standard Event Status Byte bit name usage 0 OPC Operation complete set by the OPC command 1 INP Input queue overflow too many commands received at once queues cleared 2 unused QRY Output queue overflow too many responses waiting to be transmitted queues cleared 4 EXE A command cannot execute correctly or a parameter is out of range 5 CMD An illegal command is received 6 URQ Set by any user front panel action 7 PON Set by power on The bits in this register remain set until cleared by reading them or by the CLS command SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 153 Lia Status Word bit name CHIOV CH20V UNLK RANGE SYNCF SYNCOV TRIG DAT10V DAT20V DAT30V DAT40V DCAPFIN SCNST SCNFIN O OmANNHNDNN PWN KY CO pet et ao A WwW N usage CH1 output scale overload C
78. The SDFM i command sets the data file format to csv G 0 or Matlab i 1 Example SDFM CSV Set the data file format to csv SDFM 0 SDFM Returns the data file format i FBAS s The FBAS s command sets the file name prefix to the string s The string s must be within quotes All letters will be converted to upper case The string s is limited to 7 characters and may only contain characters which are allowed in DOS file names Example FBAS F65 Set the file name prefix to F65 FBAS f65 FBAS Returns the file name prefix FNUM i The FNUM i command sets the file name suffix to value i If the value of i would make the filename exceed 8 characters the suffix will be set to 0 For example if the file name prefix is SR865_ 6 characters then FNUM i can set the suffix to 0 99 limited to 2 characters Example FNUM 12 Set the file name suffix to 12 FNUM Returns the file name suffix SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 145 FNXT The FNXT command queries the next file name Example FNXT Returns the next file name DCAP The DCAP command is the same as pressing the Screen Shot key The file save will take some time to complete A USB memory stick must already be in the front panel USB port SVDT The SVDT command is the same as pressing the Data Save key The file save will take some time to complete A USB memory stick must already be in the front panel USB port A s
79. These filters can provide digital filtering that has no analog filter counterpart and can settle up to twice as quickly as corresponding RC filters If the input signal is changing rapidly or if there is significant noise away from the reference frequency then the advanced filters often provide better signal to noise with faster output response See Appendix A for more information Floating Point Math in the SR865 The output points of the digital PSD in the SR865 are converted into floating point numbers These numbers reflect the actual analog signal gain preceding the analog to digital converter and are simply the signal input voltages at the input BNC s All digital filtering except for the Sync filter described below is performed using floating point math The SR865 plots these floating point outputs in the strip chart displays in units of Volts or Amps referred to the signal inputs These values have no real limit in size either too small or too large and do not overload Sensitivity So how does the SR865 provide an analog output proportional to the signal when the result is a floating point value that can range between 10 to 10 while the analog output can only range between 10V and 10V The answer is that the user must set a Sensitivity which sets the output voltage corresponding to full scale 10 V at the output BNC The Sensitivity also sets the scale for the displayed bar graphs and numerical readouts of X Y and
80. Use the front panel to make the following adjustments Touch Fint to display the internal frequency keypad Enter a frequency of 1 0 kHz Touch Ampl to display the sine out amplitude keypad Enter an amplitude of 100 mV Press Filter Slope adv multiple times Select 24 dB oct Press Time Constant Up Select 300 ms Press Input Range Down Select 300 mV Press Couple Select DC coupling 3 The value of R yellow should be 0 050 V 2 and the value of 0 orange should 0 1 4 Phase accuracy is checked at various frequencies The test frequencies are listed below Test Frequencies 10 Hz 100 Hz 1 kHz 10 kHz 100 kHz 1 0 MHz SR865 DSP Lock in Amplifier ASRS Appendix G Performance Tests 193 a Use the front panel to make the following adjustments Touch Fint to display the internal frequency keypad Enter the frequency from the table b Wait for the readings to stabilize Record the value of 0 orange c Repeat steps 4a and 4b for all frequencies in the table 5 This completes the phase accuracy test Enter the results of this test in the test record at the end of this section A sRs SR865 DSP Lock in Amplifier 194 Performance Tests Appendix G dc Outputs and Inputs This test measures the dc accuracy of the dc outputs and inputs of the lock in Setup We will use the digital voltmeter DVM to measure the dc outputs of the lock in Then we will use one of the outputs to ge
81. VXI 11 and telnet and interfaces easily with LabVIEW NI VISA and Matlab In addition SR865 has a built in webserver Data streaming over ethernet is also supported Before attempting to communicate with the SR865 over the ethernet interface the instrument IP address must be set in the system menu hold Calc system Additional settings for ethernet operation must also be set Status Indicators To assist in programming the SR865 has 3 interface status indicators The Activity indicator flashes whenever any interface is actively sending or receiving The Error indicator flashes when an error such as an illegal command or parameter out of range is detected The Remote indicator is on whenever the SR865 is in a remote state front panel locked out SR865 DSP Lock in Amplifier 102 Programming Chapter 4 GPIB Interface Ready And Status The Interface Ready bit bit 1 in the Serial Poll Status Byte signals that the SR865 is ready to receive and execute a command When a command is received this bit is cleared indicating that an operation is in progress While the operation is in progress no other commands will be received Only GPIB serial polling will generate a response while a command is in progress When the command execution terminates the Interface Ready bit is set again and new commands can be received Since most commands execute very quickly the host computer does not need to continually check the Interface Ready bit When
82. ag dt aC stray noise where is 27 times the noise frequency V noise is the noise amplitude and Cstray is the stray capacitance For example if the noise source is a power circuit then f 60 Hz and V noise 120 V Ctray Can be estimated using a parallel plate equivalent capacitor If the capacitance is roughly an area of 1 cm separated by 10 cm then Cstray is 0 009 pF The resulting noise current will be 400 pA at 60 Hz This small noise current can be thousands of times larger than the signal current If the noise source is at a higher frequency the coupled noise will be even greater If the noise source is at the reference frequency then the problem is much worse The lock in rejects noise at other frequencies but pick up at the reference frequency appears as signal Cures for capacitive noise coupling include 1 Removing or turning off the noise source 2 Keeping the noise source far from the experiment reducing Cstray Do not bring the signal cables close to the noise source 3 Designing the experiment to measure voltages with low impedance noise current generates very little voltage 4 Installing capacitive shielding by placing both the experiment and detector in a grounded metal box SR865 DSP Lock in Amplifier ASRS Chapter 2 Basics 57 SRS Inductive coupling An ac current in a nearby piece of apparatus can couple to the experiment via a magnetic field A changing current in a nearby
83. age selection is showing 120 V Note that on some units as pictured below the 220 V selection can be labeled 230 V that position is appropriate for both 220 V and 230 V nominal ac line voltages The following steps describe how to change the line voltage selection 1 2 Disconnect and remove the power cord Open the fuse cover see instructions below for fuse installation for detailed illustrations Pull the voltage selector card straight out of the housing using small needle nose plyers to grip the plastic indicator pin body Orient the selector card so that the desired line voltage is readable at the bottom Z 4 9 N m N is i 1 0 0 2 0 2 4 9 100V 120V 230V amp 220V 240V Orient indicator pin to point up when desired line voltage is readable at the bottom note that when indicator pin is fixed successive voltages are selected by rotating the card 90 clockwise Insert the voltage selector card into housing with the printed side of the card facing the fuse and IEC connector Orient the card so the text showing the desired voltage is inserted first Replace the fuse cover and snap into position verifying the indicator pin is showing at the desired line voltage If necessary replace the fuse for the appropriate rating based on line voltage SR865 DSP Lock in Amplifier SRS Appendix F
84. al External and Chop Reference When the reference source is internal external or chop the synchronous filter period is 1 fep regardless of the harmonic detect number The output is averaged over N periods of the detect frequency where N is the harmonic detect number Synchronous filtering is only active when the reference frequency is 4 8 kHz or below If the reference frequency is above 4 8 kHz and synchronous filtering is on then a Sync error is displayed with displays of X Y R or indicating that the synchronous filter is not working as expected Dual Reference When the reference source is dual the synchronous filter period is a single period of the actual detect frequency faua NiXfin Noxfex where N and N are the internal harmonic and dual reference external harmonic detect numbers respectively Synchronous filtering is only active when the actual detect frequency is 4 8 kHz or below If the detect frequency is above 4 8 kHz and synchronous filtering is on then a Sync error is displayed with displays of X Y R or 8 indicating that the synchronous filter is not working as expected Settling Time When the synchronous filter is active the phase sensitive detectors PSD s are followed by the specified low pass filtering time constant filter and then the synchronous filter This removes non harmonic noise before the synchronous filter The settling time of the synchronous filter is one period of the filter usually 1 f
85. al value more than twice as fast as the RC filter The FIR filter also attenuates signals above fgg much more The FIR filter is ideal when the experiment is sweeping a parameter and faster response time translates to faster sweeping better peak definition and peak symmetry What s the drawback The FIR fzag is 50 greater than the RC This means signals near dc might not be attenuated as much as the RC filter The SR865 implements multiple poles of FIR filtering by simply adding identical poles in succession Because the Gaussian filter is computationally intensive it is not available for time constants longer than 3s Linear Phase Filter For time constants of 10 s through 30000 s the advanced filter is a Linear Phase IIR filter with same attenuation slope as the RC filters of the same time constant and number of poles In other words the Linear Phase filter s stop band is aligned with the corresponding RC filter For 1 pole of filtering the Linear Phase and RC filter are identical For a given stop band attenuation the 2 pole Linear Phase filter reaches 99 of its final value almost twice as fast as the RC filter There is slight penalty in ENBW and overshoot but at these longer time constants the long wait times of the RC filter can make the experiment exceptionally arduous In Real Life In an actual experimental situation the signal is never simple If the goal is to read a static output value then longer time constants wi
86. allowed Example CEXP Y X100 Set Y expand to X100 CEXP 1 2 CEXP 1 Returns the Y expand mode i CEXP Y COFA X Y R j OFF ON i The COFA j i command turns the output offset for X j 0 Y G 1 or R G 2 to off i 0 or on i 1 Setting an offset for phase is not allowed Example COFA X ON Set X output offset to on COFA 0 1 COFA 0 Returns the X output offset state i COFA X COFP X Y RIj x The COFP j x command sets the output offset percentage for X 0 Y G 1 or R G 2 to x percent The value of x is limited to 999 99 to 999 99 with 0 01 resolution Example COFP X 12 34 Set X output offset to 12 34 COFA 0 12 34 COFP 0 Returns the X output offset percentage x COFP X OAUT X Y R j The OAUT j command auto offsets X G 0 Y G 1 or R G 2 This is the same as Auto Offset in the offset keypad display SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 115 CRAT X Y R j OFF ON i SRS The CRAT j i command turns the ratio function for X j 0 Y G 1 or R G 2 to off i 0 or on i 1 Setting an ratio for phase is not allowed The ratio function for X and Y are tied to aux input voltage 3 following the equation X orY Sensitivity Aux In3 1 000 V The ratio function for R is similar but is tied to aux input voltage 4 Offset Output x Expand x 10V R Sensi Offset Output eeen x Expand x 10V Aux In4 1 000 V Example CRAT X ON Set X ratio
87. ameter The argument j selects the parameter according to the table below The enumeration strings may be used instead of the integer value j The parameter list is j enumeration parameter j enumeration parameter 0 X X output 9 OUT2 Aux Out2 1 Y Youtput 10 XNOise Xnoise 2 R R output 11 YNOise Ynoise 3 THETa 8 output 12 PHAse Reference Phase 4 IN1 Aux In 13 SAMp Sine Out Amplitude 5 IN2 Aux In2 14 LEVel DC Level 6 IN3 Aux In3 15 FINT Int Ref Frequency 7 IN4 Aux In4 16 FEXT Ext Ref Frequency 8 OUTI Aux Outl Example OUTP 1 Returns the value of Y OUTP Y SNAP j k 1 The SNAP query returns the values of 2 or 3 parameters at a single instant For example SNAP is a way to query values of X and Y or R and 8 which are taken at the same time This is important when the time constant is very short Using the OUTP or OUTR commands will result in time delays which may be greater than the time constant between reading X and Y The SNAP command requires at least two arguments and at most three The arguments j k l select the lock in parameters as listed above see OUTP The enumeration strings may be used instead of the integer values for j k 1 Example SNAP 0 1 4 Returns the values of X Y and Aux In1 SNAP X Y IN1 SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 133 The requested values are returned in a single string with the values separated by commas and in the order in which they were
88. amplitude attenuator mode for scanning 129 Setthe dc attenuator mode for scanning 129 Setthe scan parameter update interval 129 Turn the scan offfon 130 Startor resume the scan 130 Pause the scan 130 Reset the scan 130 Query the scan off reset run pause or done 0 4 130 Set the begin end frequency 130 Setthe begin end reference amplitude 130 Set the begin end reference dc level 131 Setthe begin end AuxOutl value 131 Setthe begin end AuxOut2 value page description 132 Query data channel j 132 Query lock in parameter j 132 Query multiple lock in parameters at once page description 136 Set the buffer length to n 1 kbyte blocks 136 Configure capture to X XY RO or XYR 0 3 136 Query the maximum capture rate 136 Set the capture rate to max rate 2 Commands xiii CAPTURESTART ONEshot CONTinuous i OFF ON j CAPTURESTOP CAPTURESTAT CAPTUREPROG CAPTUREVAL n CAPTUREGET i j Data Streaming Commands STREAMCH X XY RT XYRT i STREAMRATEMAX STREAMRATE n STREAMFMT i STREAMPCKT i STREAMPORT i STREAMOPTION i STREAM OFF ON i System Commands TIME SEConds MINutes HOUrs j1 i DATE DAY MONth YEAr j i TBMODE AUTO INternal i TBSTAT BLAZEX BLazex Blsync UNIsync i KEYC ON MUte i PRMD SCReen PRNt MONOchrome i SDFM CSV MATfile i FBAS s FNUM i FNXT DCAP
89. ands These commands also function identically on the other interfaces Multiple commands may be sent on one command line by separating them with semicolons The present value of a particular parameter may be determined by querying the SR865 for its value A query is formed by appending a question mark to the command mnemonic and omitting the desired parameter s from the command No space is allowed between the command and the Values returned by the SR865 are sent as a string of ASCII characters If multiple queries are sent on one command line separated by semicolons of course the responses will be returned as semicolon separated strings Command Conventions Commands and arguments may be in either UPPER or lower case Arguments shown in are optional or may be queried Arguments in are required A list of allowable enumerated values is shown as A B C The argument can be either A or B or C Commands that may be queried have a question mark in parentheses after the mnemonic Commands that may ONLY be queried have a after the mnemonic without parentheses NO SPACE is allowed between the mnemonic and the Commands that MAY NOT be queried have no Do NOT send or or or as part of the command A space is required between a command and its arguments Variables i j k 1m n integers real number f frequency real number p phase real number v voltage real number s string In
90. annel s scale tile to display a palette of scale functions 0 000 deg F 100 000 kz 4 AmI Chart vertical scale palette SR865 DSP Lock in Amplifier SRS Chapter 1 Getting Started 7 Use the palette functions to scale the selected data channel s graph Touch the scale tile again to dismiss the palette Resume Autoscale live scroll zero center Zoom in faster scroll Zoom in about center Vertical Scale Palette Horizontal Scale Palette Vertical Scale Palette Vertical scale changes are applied to each data channel separately Autoscale adjusts the scale and center so the graph occupies as much of the screen as possible Autoscale Zero Center forces the center of the graph to be zero and then sets the scale to show the data The location of zero is indicated by the small triangle on the right edge It points left where zero is It points up or down if zero is above or below the graph Zoom In and Zoom Out change the scale about the center Use Center Newest Point to bring the current point to the center of the graph before zooming in or out Move Up and Move Down simply move the graph up and down on the screen The graph can also be moved simply by touching and dragging on the screen while the vertical scale palette is displayed Each graph can also be turned off Touch the scale tile to turn the graph back on All changes to the graphs are non destructive They simply change the way data is v
91. apture When operating in CAPTURESTART 1 1 mode Continuous capture and hardware trigger the capture will be stopped when the TTL input returns high or by receiving the CAPTURESTOP command Both parameters i and j are required in this command This command clears any previously captured data and starts a new capture When operating in CAPTURESTART 1 1 mode Continuous capture and hardware trigger the capture will be stopped when the TTL input returns high or by receiving the CAPTURESTOP command Example CAPTURESTART ONE OFF Starts OneShot capture immediately CAPTURESTART 0 0 CAPTURESTOP The CAPTURESTOP command stops data capture If the capture is waiting for a hardware trigger then the capture is aborted and the trigger will be ignored If capture is in progress then capture is halted Any data already captured is preserved and may be read Capture is stopped at the next 2 kbyte boundary 512 total data points Example CAPTURESTOP Stop data capture at the next 2 kB boundary CAPTURESTAT SRS The CAPTURESTAT query returns the data capture state This is a query only command The returned integer is a 3 bit binary encoded word Bit Weight Definition 1 Capture currently in progress 1 2 Capture triggered either by TTL or remote command 2 4 Capture wrapped SR865 DSP Lock in Amplifier 138 Programming Chapter 4 If capture is started with hardware trigger On then all 3 bits remain 0 until
92. art the scan Press Screen Layout once to show the full screen strip chart Touch the white scale tile Time to display the horizontal scale palette bottom right of screen D repeatedly to zoom out to 10 S div Let s scan a 20 Hz span around the signal frequency 100 000 kHz This setup will increase the internal frequency from 99 990 kHz to 100 010 kHz To reverse the scan simply reverse the begin and end values This will repeat the scan over and over jumping from the end value back to the begin value at the end of each scan The lock in frequency has not been changed we have only configured a scan The scan has not been started The Ready LED turns on indicating that the scan is ready at the start value In this case the internal frequency in the info bar at the top is shown in orange indicating that it is under scan control At this time the frequency is the start value or 99 9900 kHz The Run LED turns on indicating that the scan is running The internal frequency starts scanning up The current value is shown in orange in the info bar When the end value is reached the scan resets to the beginning value and repeats The trend graphs show a resonance as the internal frequency passes through 100 000 kHz This is because the difference between the fsignal and finternat gets Slower as the finterna1 approaches fsigna and then gets faster after it passes fsignal The time constant attenuates this
93. as a separate ground network all of these grounds are loosely tied to chassis ground through resistors R31 R36 10 KQ each to avoid any supplies drifting away during testing before connection to their loads Probing of the output voltage levels is best accomplished from test point array TP1 TP20 these test points are protected through 4 7 KQ resistors to avoid arcing if accidentally shorted SR865 DSP Lock in Amplifier 204 Circuit Description Appendix H Partial schematics follow this page SR865 DSP Lock in Amplifier
94. at the bits within each byte are NOT reversed Each UDP packet sent consists of a 32 bit 4 byte header and 128 1024 bytes of data as shown below The endianness of the data and packet integrity checking are controlled by the STREAMOPTION command If integrity checking is enabled it is handled automatically by Ethernet hardware if an error is detected then the entire packet is dropped The amount of data sent in each packet is controlled by the STREAMPCKT command You might want to send fewer data bytes per packet if the sample rate is low and you want packets to arrive with less latency Otherwise larger data sizes are recommended as they have less overhead and are more bandwidth efficient Header Data 4 bytes 128 1024 bytes Big endian Big or little endian ASRS SR865 DSP Lock in Amplifier 172 Data Streaming and Capture Appendix D The header word consists of the following fields Header bits 31 24 MSB 23 16 15 12 11 8 7 0 LSB Status Sample rate Packet length Packet content Packet counter Bit 24 Overload 0 1 25 MHz 0 1024 bytes 32 bit floats 0 255 Bit 25 Error 1 625 kHz 1 512 bytes 0 X Bit 28 Little end 2 312 5 kHz 2 256 bytes 1 XY Bit 29 Checksum s 3 128 bytes 2 RO N 1 25MHz 2 3 XYRO ae 16 bit integers 31 0 582 mHz 4 X 5 XY 6 RO 7 XYRO Packet counter allows the receiving computer to detect lost or dropped packets Each UDP packet has a packe
95. atio Output offsets and ratios are reflected in the displays For example when the X output is offset to zero the displayed value will drop to zero also This means that the bar graph and numeric readout both drop to zero In addition if X is being charted on the graph its graph will drop to zero Any display which is showing a quantity which is affected by a non zero offset will display a highlighted Offset indicator within its display Similarly any display which is showing a quantity affected by a non unity ratio factor will display a highlighted Ratio indicator within its display Remote queries of offset and or ratioed quantities are also affected by the offset or ratio X Y and R Display Expand Output expands do not increase the displayed numeric values of X Y or R Expand increases the resolution of the displayed X Y or R numeric value This is because the expand function increases the resolution of the output not the size of the input signal The displayed value will show an increased resolution but will continue to display the original value minus the offset Any display which is showing a quantity which is affected by a non unity expand will display a highlighted Expand indicator within its display Output expands affect the bar graphs The bar graphs are simply a visualization of the BNC outputs and as such are expanded to provide more visual resolution Output expands do not affect the strip charts The values being charted are a
96. ation 85 Functions Auto Auto 2 ise Save Auto Recall Scale Auto Phase Auto Phase adjusts the reference phase shift so that the measured signal phase is 0 This is done by subtracting the measured value of 8 from the reference phase shift It will take several time constants for the outputs to reach their new values during which time 0 will move towards 0 Do not press Auto Phase again until the outputs have stabilized When the measurement is noisy or if the outputs are changing Auto Phase may not result in a zero phase Auto Range Auto Range will adjust the Input Range so that the input signal does not overload the input amplifier and that there is sufficient gain to drive the A D converter Remember that the signal input is both the signal at f plus noise which may be much larger The largest component of the signal will determine the resulting input range When the input is at very low frequency Auto Range may not function well In this case it is better to watch the signal strength indicators and change the input range manually Auto Scale Auto Scale will adjust the Sensitivity so that the displays and outputs of X Y and R are optimized This results in numeric and bar graph displays with the best resolution possible Note that the Sensitivity will affect analog outputs proportional to X Y and R Do not use Auto Scale if changes to the analo
97. ave frequencies which are either the sum or difference between an input signal frequency and the reference frequency Only the component of the input signal whose frequency is exactly equal to the reference frequency will result in a dc output The low pass filter at the PSD output removes all of the unwanted ac signals both the 2f sum of the signal and the reference and the noise components This filter is what makes the lock in such a narrow band detector RC filters Traditionally the time constant setting of a lock in amplifier determines the bandwidth of an RC lowpass filter The time constant is simply 1 2nf where f is the 3 dB frequency of the filter The low pass filters are simple 6 dB octave roll off RC type filters A 1 second time constant referred to a filter whose 3 dB point occurred at 0 16 Hz and rolled off at 6 dB octave beyond 0 16 Hz Typically there are two successive filters so that the overall filter can roll off at either 6 dB or 12 dB per octave The time constant referred to the 3 dB point of each filter alone not the combined filter The notion of time constant arises from the fact that the actual output is supposed to be a dc signal In fact when there is noise at the input there is noise on the output By increasing the time constant the output becomes more steady and easier to measure reliably The tradeoff comes when real changes in the input signal take many time constants to be reflected at the outpu
98. binary block returns j kbytes of capture buffer contents beginning with an offset of i kbytes If i j is longer than CAPTURELEN then CAPTUREGET returns wrapped data The maximum length j is 64 corresponding to 64 kbytes Both parameters i and j must be provided Capture must be stopped when executing CAPTUREGET or a range error is generated The binary block query response is in the following format NCCCCXXXXXXX SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 139 ASRS Where is the hash character character code 35 n is a single digit specifying the number of digits to follow in the block length count cccc is the n digit integer size of the binary block to follow and Xxxxxxxx is the cccc byte binary data Data is packed as 4 byte 32 bit single precision floating point binary in little endian format Example CAPTUREGET 0 32 returns a binary block with 32 kbytes of capture data SR865 DSP Lock in Amplifier 140 Programming Chapter 4 Data Streaming Commands Overview See Appendix D for more detailed information about data streaming Data Streaming The SR865 can stream data points continuously in real time over its ethernet interface This stream can be X values XY or RO values or XYR8 values Streaming more values uses more ethernet bandwidth Data points are streamed as either floating point values 4 bytes per data point or integers 2 bytes per data point
99. bled status bits in the Error LIA and Standard Event status bytes are cleared by reading the status bytes using CLS or by clearing the enable register bits Using STB command to read the Serial Poll Status Byte A bit in the Serial Poll status byte is NOT cleared by reading the status byte using STB The bit stays set as long as the status condition exists This is true even for SRQ SRQ will be set whenever the same bit in the serial poll status byte AND enable register is set This is independent of whether a serial poll has occurred to clear the service request Using GPIB Serial Poll Except for SRQ a bit in the Serial Poll status byte is NOT cleared by GPIB serial polling the status byte When reading the status byte using a GPIB serial poll the SRQ bit signals that the SR865 is requesting service The SRQ bit will be set 1 the first time the SR865 is polled following a service request The GPIB serial poll automatically clears the service request SRQ Subsequent GPIB serial polls will return SRQ cleared 0 until another service request occurs Polling the status byte and reading it with STB can return different values for SRQ When polled SRQ indicates a service request has occurred When read SRQ indicates that an enabled status bit is set GPIB Service Requests SRQ A GPIB service request SRQ will be generated whenever a bit in both the Serial Poll Status byte AND Serial Poll Enable register is set Use SRE to set bi
100. bly dominated by a single frequency component What the FFT does is represent the time domain signal by its component frequencies Why Look at the Spectrum A lock in amplifier measures a single frequency component of the input signal at fy With time constant filtering all other frequencies are ignored and filtered away Sometimes it can be useful to see what s going on at other frequencies even if only with a broad overview This can help identify sources of interference or noise Other times it may be useful for estimating the signal at other frequencies without making multiple measurements each at different reference frequencies and time constants Take harmonic distortion Instead of making separate measurements at fiet 2Xfrep 3Xfep etc the FFT can display the harmonic frequencies and amplitudes with amazing clarity in a single measurement Another example is noise analysis In an FFT the noise as a function of frequency is displayed The FFT Analyzer The lock in amplifier digitizes the input signal and multiplies by f in a phase sensitive detector yielding a very narrow band measurement at fier An FFT analyzer works in an entirely different way A time record of samples 1024 samples in the SR865 is mathematically transformed into a frequency spectrum using an algorithm known as the Fast Fourier Transform or FFT The FFT is simply a clever set of operations which implements Fourier s basic theorem The resulting spectrum show
101. case the reference square wave is provided to the lock in The PLL in the lock in locks the internal reference oscillator to this external reference resulting in a reference sine wave at with a fixed phase shift of Oet Since the PLL actively tracks the external reference changes in the external reference frequency do not affect the measurement All lock in measurements require a reference signal In this case the reference is provided by the excitation source the function generator This is called an external reference source In many situations the lock in s internal oscillator may be used instead The internal oscillator is just like a function generator with variable sine output and a TTL sync which is always phase locked to the reference oscillator Magnitude and phase Remember that the PSD output is proportional to V ijgcos where 0 Osig Oret O is the phase difference between the signal and the lock in reference oscillator By adjusting Oef we can make equal to zero in which case we can measure V ig cos0 1 Conversely if 0 is 90 there will be no output at all A lock in with a single PSD is called a single phase lock in and its output is V i cos0 This phase dependency can be eliminated by adding a second PSD If the second PSD multiplies the signal with the reference oscillator shifted by 90 i e sin t Oef 90 its low pass filtered output will be V psd2 1 2 Vsig sin Osig Oier V
102. change it using the numeric keypad GPIB and RS 232 Settings GPIB Primary Address RS232 Baud Rate 1152k f RS232 Transmit Terminator RS232 Parity None Highlight the GPIB Address to change it using the numeric keypad Use the RS 232 Baud Rate ft and buttons to select a baud rate Select a terminator and parity VXI 11 and Web Settings t Files Ethernet Enabled Disabled C GPB The SR865 allows computers to connect via VXI 11 from a range of Trusted IP addresses or Disabled altogether Set the Trusted IP address using as a wildcard This allows any computer on a specified subnet VXI 11 access The SR865 has a webserver which allows computer access thru a webpage at the SR865 s IP address This access can be Enabled or Disabled The webserver allows instrument settings to be changed and the outputs to be monitored SR865 DSP Lock in Amplifier 94 Operation Chapter 3 Telnet Settings Telnet t Ethernet Enabled Echo Off GPIB Receive Terminator Val 11 amp Web Transmit Terminator Telnet The SR865 supports telnet connections This can be Enabled or Disabled Choose Echo On Off and the Receive Transmit Terminators depending upon your host software requirements Streaming Settings X Float 32 bit Maximum Sample Ra Native f Packet Size 1024 t No Host The SR865 can stream output values continuously over the ethernet interface to a host computer It is the host c
103. circuit gives rise to a changing magnetic field which induces an emf d dt in the loop connecting the detector to the experiment This is like a transformer with the experiment detector loop as the secondary winding Cures for inductively coupled noise include 1 Removing or turning off the interfering noise source 2 Reduce the area of the pick up loop by using twisted pairs or coaxial cables or even twisting the 2 coaxial cables used in differential connections 3 Using magnetic shielding to prevent the magnetic field from crossing the area of the experiment 4 Measuring currents not voltages from high impedance detectors Resistive coupling or ground loops Currents flowing through the ground connections can give rise to noise voltages This is especially a problem with reference frequency ground currents In this illustration the detector is measuring the signal relative to a ground far from the rest of the experiment The experiment senses the detector signal plus the voltage due to the noise source s ground return current passing through the finite resistance of the ground between the experiment and the detector The detector and the experiment are grounded at different places which in this case are at different potentials Experiment Detector Noise Source Cures for ground loop problems include 1 Grounding everything to the same physical point SR865 DSP Lock in Amplifier 58 Basics Chapter 2 2
104. ck to RC filters and turns off the Advanced LED Gaussian FIR Filter When the time constant is 3 s or shorter an advanced Gaussian FIR filter pole of equivalent noise bandwidth ENBW is substituted for an RC filter pole The Gaussian filter has a faster settling time than the RC filter since it lacks the RC filter s agonizing exponential tail The one stage Gaussian filter gets to within 1 of its final value twice as fast as the RC filter The Gaussian filter has better rejection of high frequencies than the RC filter The Gaussian filter has a constant 11 6 dB more rejection of high frequencies per stage than SR865 DSP Lock in Amplifier 156 Advanced Filters Appendix A the RC filter Thus a four stage Gaussian filter rejects high frequencies 4 11 6 dB 46 dB better than a four stage RC filter The Gaussian filter has a flatter pass band than the RC filter The 3dB bandwidth of the Gaussian filter is 1 5 times greater than that of the RC filter One disadvantage of the Gaussian filter is the increased latency as more stages are added When 4 stages of Gaussian filtering are used there is seemingly no response for several time constants before the output quickly rises Lock in users have long complained that traditional RC filters are slow i e for a given width in the frequency domain they take the longest time to reach a given percentage of their final value Notice that the FIR filter reaches 99 of its fin
105. dB oct The advanced filters always settle in less time than the RC filters Gaussian filters have increasing latency with more stages Linear Phase filters have lt 1 overshoot 1 0 12dB oct RC 0 8 H Gaussian FIR H Linear Phase 0 6 Time to reach 1 of final value 2 RC 6 6 t S Gaussian 4 0 t 3 Linear Phase 4 4 t x 0 4 ff 0 2 F 7 Fi 0 0 pe 4 0 1 5 6 7 Time t 1 0 a 24dB oct RC 0 8 F Gaussian FIR Linear Phase 0 6 Z Time to reach 1 of final value a RC 10t c 2 Gaussian 7 1 t o Linear Phase 5 4 t 0 4 0 2 F Gaussian suffers Z from latency 0 0 4 4 Time t SR8 amp 65 DSP Lock in Amplifier 160 Advanced Filters Appendix A SR865 DSP Lock in Amplifier ASRS The FFT Display 161 Appendix B The FFT Display The FFT display takes a time varying signal like you would see on an oscilloscope trace and computes its frequency spectrum In the SR865 this signal is either the output of the input amplifier Raw ADC or the output of the PSD Post Mixer or Post Filter Fourier s basic theorem states that any waveform in the time domain can be represented by the weighted sum of pure sine waves of all frequencies If the signal in the time domain as viewed on an oscilloscope is periodic then its spectrum is proba
106. ddition the default setup can be recalled Recall default 6 Touch the Save button next to the large tile Setups are numbered 1 through 8 Setups should labelled 1 be named so they are easily distinguished Save to Location 1 Cancel Confirm My New Setup 7 z Enter a name for this setup using the keypad Touch Confirm to commit the current setup to location 1 7 Now change the Sensitivity Time Constant and Change the lock in setup before recalling the Filter Slope to new settings saved settings 8 Press Save Recall to display the Save Recall Note that the Location 1 tile displays the setup screen again name and the time and date it was created This makes it easier to recall the correct setup Recall Save default Recall SR865 DSP Lock in Amplifier ASRS Chapter 1 Getting Started 27 9 Touch Recall for Location 1 Recall from Location 1 Confirm My First Setup Voltage In 1 V A AC FloatCurrent In Touch Confirm to recall the setup and dismiss this screen SRS A summary of settings which will change upon recall is shown Simply touch Cancel to skip recalling this setup The Sensitivity Time Constant and Filter Slope should all return to the saved settings SR865 DSP Lock in Amplifier 28 Getting Started Chapter 1 Aux Outputs and Inputs This measurement is designed to illustrate the use of the Aux Outputs and Inputs on the rear panel You wi
107. difference frequency when it is large It is much easier to visualize a scan using the strip charts since the chart time scale can be matched to the scan time This scan takes 1m40s or 100 s so a chart span of 100 s will display an entire scan SR8 amp 65 DSP Lock in Amplifier 32 Getting Started Chapter 1 Phase 0 000 deg Fint 10 Touch the green scale tile X to display its Let s clean up the chart by dismissing the X and scale palette Touch x to dismiss the X trace Y traces Touch the blue scale tile Y to display its scale palette Touch Ix to dismiss the Y trace 11 Touch the yellow scale tile R to display its The magnitude trace R shows the lock in scale palette Touch D to auto scale the R SPONSE a gt nie iieii medueney ee through the signal frequency at this time trace constant and filter Touch th le til to display it ee ae a h P S Reduce the phase trace to see the resonance at scale palette Touch ZH to repeatedly to change 100 000 kHz the scale to 200 deg div 12 Press Config to change the assignments of the It would be nice to show the frequency on the data channels Touch Data 2 Display the blue display to highlight the channel 2 data source Touch Fint from the keypad below X noise Y noise Out 1 Out 2 Touch Close to return to the strip chart SR865 DSP Lock in Amplifier graph The 4 data channels can be assigned to different parameters in the Config scre
108. display a little trail The bright end signifies the lift location This verifies the touch functionality Press each key to toggle its position green gray on the screen Keys with a secondary press and hold function need to be held to toggle The knobs also have a press and hold feature Check to see that all keys and knobs function The Screen Layout key exits this screen A sRs SR865 DSP Lock in Amplifier 64 Operation Chapter 3 Standard Settings If the Local key is held down when the power is turned on the lock in settings will be set to the defaults shown below rather than the settings that were in effect when the power was last turned off The default settings may also be recalled using the RST command over the computer interface or by pressing Save Recall and selecting Recall default In this case the communications parameters and status registers are not changed Reference Phase Reference Source Internal Frequency Harmonic Sine Amplitude DC Level Ext Reference Trigger Ext Reference Impedance Signal Input A B Couple Ground Current Range Input Range Time Constant Filter Slope Advanced Filter Synchronous Sensitivity Outputs CH1 Output CH2 Output All Offsets All Expands All Ratios All Aux Outputs Display Green Channel Blue Channel Yellow Channel Orange Channel Screen Layout Blank 0 000 Internal 100 000 kHz 1 0 00 Vrms 0 0 V Sine 50 Q Voltage A AC
109. e Select key the lt CH1 Offset gt knob and Expand key above the CH1 BNC set the X offset and expand The CH1 Select key determines which quantity X or R is offset ratioed or expanded and output on the BNC The ratio function is described later in this manual in the Operation chapter Here we will explore offset and expand Auto Offset automatically adjusts the X offset or Y or R such that X or Y or R becomes zero In this case X is offset to zero The offset should be about 100 Offsets are set as a ASRS Chapter 1 Getting Started 23 11 12 13 SRS Touch 9 0 Enter to set the offset to 90 Press the lt CH1 Offset gt knob briefly once to turn the X offset off Press it again to turn it back on Press CH1 Expand once to select x10 percentage of the Sensitivity up to 999 10x Offsets are useful for making relative measurements Offsets can also be set using the keypad or lt Offset gt knob The offset affects the value of X and any outputs or displays of X The DVM voltage should be zero in this case The Offset indicator turns on next to the lt CH1 Offset gt knob The X display on the screen has an Ofst indication that the displayed quantity is affected by an offset In this case the bar graph and numeric value are both zero The X output 10 mV is now 10 of the sensitivity 100 mV The bar graph is at 10 and the DVM reads 1 V Notice that the trace
110. e 4 buttons labelled F1 thru F4 are frequency presets Press and hold them to memorize new frequencies me i 100 000 Touch 1 0 kHz to enter a new frequency Change the frequency to 10 kHz Use the lt Frequency gt knob to adjust the The knob is very useful for making small frequency to 1 00000 kHz adjustments or optimizing a setting Large changes are better left to the numeric keypad The measured signal amplitude X and R should stay within 1 of 500 mV and Y and should stay close to zero 7 Use the lt Amplitude gt knob to adjust the sine out As the amplitude is changed the values of X to 5 0 mV The Amplitude is displayed in the and R change to follow mo bar The yellow LED in the Input Range section should light The Input Range is the largest input signal before overload The lower the range the higher the gain The signal strength indicates how much of the A D converter range is being used When the yellow indicator lights it means that more gain should be used Since the signal has just been reduced by a factor of 100 the input range should be adjusted as well 8 Press Auto Range The Auto Range function changes the Input SR865 DSP Lock in Amplifier Range to maximize the signal at the A D ASRS Chapter 1 Getting Started 11 9 Use the lt Amplitude gt knob to increase the sine out to 50 0 mV The Amplitude is displayed in the info bar 10 Press Auto Range Press Auto Range again 10 P
111. e filters back to RC filters and turns off the Advanced LED For two filters with the same noise bandwidth i e whose outputs would be equally noisy if the input was white noise the advanced filters have faster transient response less overshoot and higher stopband attenuation to ensure greater suppression of out of band spurs When the time constant is 3 s or faster an advanced FIR filter pole of equivalent noise bandwidth ENBW is substituted for each RC filter pole For time constants of 10 s through 1000 s the advanced filter is a Linear Phase LP filter with same attenuation slope as the RC filters of the same time constant and number of poles In other words the Linear Phase filter s stop band is aligned with the corresponding RC filter For time constants greater than 1000 s the advanced filter is the same as the RC filter In an actual experimental situation the signal is seldom simple If the goal is to read a static output value then longer time constants will achieve that If the output value is changing because of parameter sweeping or a signal turning on and off then the advanced filters can be of great help In practice it is simple to try these filters at various time constants and number of poles in comparison with RC filters They often yield better results in less time See Appendix A for more information about the advanced filters Filter Sync Pressing this key turns synchronous filtering on or off S
112. e frequency This setup ensures changing outputs which are more illustrative than steady outputs SRS Do This Disconnect all cables from the lock in Turn the power on while holding down the Local key The power switch is on the power entry module on the rear panel Turn on the function generator set the frequency to 100 000 kHz exactly and the amplitude to 500 mVrms Connect the function output sine wave from the synthesized function generator to the A input using a BNC cable and appropriate terminator 100 000 kz 7 14 Phase 0 000 deg i Explanation When the power is turned on with Local pressed the lock in returns to its standard default settings See the Standard Settings list in the Operation section for a complete listing of the settings The input impedance of the lock in is 10 MQ The generator may require a terminator Many generators have either a 50Q or 600 output impedance Use the appropriate feedthrough or T termination if necessary In general not using a terminator means that the function output amplitude will not agree with the generator setting The default screen is the Trend Graph Four data channels are displayed as values bar and trend graphs The trend graph is the recent history of each data channel with continuous auto scaling In this case R yellow auto scales to show the tiny amount of noise in the signal magnitude Trend graphs have no adjustments and are most useful whe
113. e lock in s ground via a resistor The value of this resistor is selected by the user Float uses 10 kQ and Ground uses 10Q This avoids ground loop problems between the experiment and the lock in due to differing ground potentials The lock in lets the shield quasi float in order to sense the experiment ground However noise pickup on the shield will appear as noise to the lock in This is bad since the input amplifier cannot reject this noise Common mode noise which appears on both the center and shield is rejected by the common mode rejection of the lock in input but noise on only the shield is not rejected at all SR865 DSP Lock in Amplifier 52 Basics Chapter 2 Experiment Lock in Signal Source Grounds may be at different potentials Differential Voltage Connection A B The second method of connection is the differential mode The lock in measures the voltage difference between the center conductors of the A and B inputs Both of the signal connections are shielded from spurious pick up Noise pickup on the shields does not translate into signal noise since the shields are ignored The shields of A and B are connected together and can set to Float or Ground as above Experiment Lock in Signal Source Saar Loop Area et Grounds may be at different potentials When using two cables it is important that both cables travel the same path between the experiment and the lock in In particular the
114. e output can never exceed full scale 410 V when expanded An output expanded beyond full scale will be output overloaded SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 73 Output lt Offset gt Press this knob briefly to toggle the X or R offset CH1 or the Y offset CH2 on and off Use the Select key to select either X or R CH1 and Y or 0 CH2 The value of 0 cannot be offset Use Auto Phase to offset phase to zero This allows the offset to be turned on and off without adjusting the actual offset percentage The OFFSET LED is on when the offset is being applied Press and hold the lt Offset gt knob to display an offset keypad as shown below Phase 0 000 deg t 151270 kiz 1 Ratio Auto Touch Auto Offset to automatically adjust the X offset or Y or R such that X or Y or R becomes zero Offsets can also be set using the keypad or by turning the lt Offset gt knob Turning the knob ALWAYS adjusts the offset even if the offset is toggled off and the keypad is not shown Offsets are set as a percentage of the Sensitivity up to 999 10x The offset percentage is not changed with the sensitivity it is an output function Touch Close to dismiss the keypad Touch Ratio to cycle between Off no ratio and an Aux Input The RATIO LED is on when a ratio is being applied Output Overload Outputs which would exceed 10 V generate an Output Overload and the red Overload LED will light An
115. e signal is amplified by U101 and optionally U102 before being launched through R109 R116 as a 100Q impedance differential signal Programmable attenuators are switched in or out using steering relays K102 K109 The dc level control is generated by DAC U105 and amplifier U106 and then driven with 100Q impedance either directly or through an attenuator divider selected by K110 The ac signal is then wire added to the 100 impedance dc level to create 50Q impedance signals at the output BNCs Chip selects clock gating and relay control is implemented with U201 U711 All power rails for the circuit board are re regulated from the main power supply 16V to 15V and 5V with U212 U214 linear regulators Auxiliary I O The rear panel auxiliary I O circuit supports the 4 AUX OUT and 4 AUX IN analog signals Outputs are received differentially referenced to system ground from the motherboard on header J1 through difference amplifier circuits U1A U1D Analog inputs are buffered with amplifiers U1 A U1D and presenting 1 MQ input impedances The TTL compatible TRIG IN signal is also on this board but separately interconnected to the motherboard via J11 Linear Power Supply The SR865 uses a low noise linear power supply A multi tap universal primary toroidal transformer can be configured at the power entry module for either 1OOVAC 120VAC 220VAC or 240VAC Multiple secondary windings most of which are center tapped provide ded
116. e sync output either thru 50 Q 4 BNC D A outputs 10 5 V thru 50 Q 1 mV resolution 4 BNC A D inputs 10 5 V 1 mV resolution 1 MQ input TTL input triggers storage into the internal capture buffer Analog output of the signal amplifier Video output to external monitor or TV 640x480 60 Hz 1 Vrms 10 MHz clock to synchronize internal reference frequency to other units SR865 DSP Lock in Amplifier ASRS Specifications ix General Interfaces IEEE 488 RS 232 USB device Test and Measurement Class and Ethernet VXI 11 and telnet USB Flash Front panel slot for USB flash storage of screen shots and data and firmware upgrades Preamp Power 9 pin D connector to power SRS preamps Power 60 Watts 100 120 220 240 VAC 50 60 Hz Dimensions 17 W x5 25 H x17 D Weight 22 Ibs Warranty One year parts and labor on materials and workmanship A sRs SR865 DSP Lock in Amplifier xX Commands SR865 Command List Reference Commands TBMODE AUTO INternal i TBSTAT PHAS p UDEG MDEG DEG URAD MRAD RAD APHS FREQ f HHZ KHZ MHZ FREQINT f HZ KHZ MHZ FREQEXT FREQDET HARM i HARMDUAL i BLADESLOTS SLT6 SLT30 i BLADEPHASE p UDEG MDEG DEG URAD MRAD RAD SLVL v NV UV MV V SOFF v NV UV MV V REFM COMmon DIF ference i RSRC INT EXT DUAL CHOP i RTRG SIN POStt NEGtt i REFZ 50ohms 1Meg i P
117. ebase Blazex Sounds GPIB amp RS232 Auto BlazeX Touch the h button in the Time section to highlight the hours setting Use the keypad to set the hour of day in 24 hour format The time date will highlight in orange indicating that the displayed time is not the current time but rather the time to be set Ethernet Continue to enter the minutes seconds then touch Time set to commit the time to the internal clock Set the date in the same manner touching Date set to commit the date Press Sensitivity Down 3 times to select 100 mV Press Time Constant Up twice to select 1 s Explanation When the power is turned on with Local pressed the lock in returns to its standard default settings See the Standard Settings list in the Operation section for a complete listing of the settings First let s set the SR865 clock The system menu is where instrument parameters not measurement parameters are set This includes file numbering interface settings and software updates The time and date are used to label data files screen shots and saved settings Let s change the lock in setup so that we have a non default setup to save Change the Sensitivity Time Constant and Filter Slope SR8 amp 65 DSP Lock in Amplifier 26 Getting Started Chapter 1 Press Filter Slope once to select 12 dB oct 5 Press Save Recall to display the Save Recall The SR865 can store 8 complete setups In screen a
118. eference Unlock indicates that the SR540 chopper is unlocked This can be because the switches on the SR540 are incorrectly configured or the blade is not turning freely SR865 DSP Lock in Amplifier 78 Operation Chapter 3 Timebase The SR865 has a 10 MHz TIMEBASE input and output on the rear panel Apply a 10 MHz sine signal 1 Vrms to the TIMEBASE input to lock the SR865 s timebase to an external 10 MHz timebase The internal reference frequency of the SR865 is derived from this timebase When multiple SR865 s are locked to the same 10 MHz timebase then harmonically related internal frequencies on different units will stay in phase indefinitely The Ext 10 MHz LED is on when the unit is locked to an external timebase The SR865 can also output its own 10 MHz timebase to another unit Note that external function generators even DDS locked to the same 10 MHz timebase will not stay in perfect phase with the SR865 This is because the resolution of the frequency tuning word in the external synthesizer differs from the SR865 lt Phase gt This knob may be used to adjust the phase The phase shift ranges from 180 to 180 with 0 001 resolution Press and hold this knob to set the phase to zero The phase value is shown in the info bar at the top of the screen Touch this Phase tile to show the numeric entry screen When using an external reference the reference phase shift is the phase between the external refere
119. eference frequency reference phase shift sine output amplitude sine dc level offsets and Aux Output levels The knobs also have secondary key press functions labeled in italics below the knobs these functions are accessed by pressing the knob inward This manual will refer to knobs in angled brackets such as lt Knob gt ASRS SR865 DSP Lock in Amplifier 62 Operation Chapter 3 Touchscreen Buttons Touchscreen buttons on the display allow for direct keypad entry text entry calculator functions graph scaling and much more This manual will refer to touchscreen buttons with curly brackets such as Button Reset To reset the unit hold down the Local key while the power is turned on The unit will use the standard settings A similar reset is available without cycling power by pressing the Save Recall key and selecting Recall default Serial Number and Firmware Version Press and hold Local and then Auto Range at the same time to display the unit serial number and version numbers Press Screen Layout to dismiss the informational screen Local Lockout If a computer interface has placed the unit in the Local Lockout state indicated by the Remote LED then the keys knobs and touchscreen are disabled Attempts to change the settings from the front panel will display a message indicating local control is locked out by the interface Reference Input The reference input can be a sine wave rising zero crossing detec
120. eference is used the phase locked loop contributes a little phase jitter The internal oscillator is supposed to be locked with zero phase shift relative to the external reference Phase jitter means that the average phase shift is zero but the instantaneous phase shift has a few microdegrees or millidegrees of noise This shows up at the output as noise in phase or quadrature measurements Phase noise can also cause noise to appear at the X and Y outputs This is because a reference oscillator with a lot of phase noise is the same as a reference whose frequency spectrum is spread out That is the reference is not a single frequency but a distribution of frequencies about the true reference frequency These spurious frequencies are attenuated quite a bit but can still cause problems The spurious reference frequencies result in signals close to the reference being detected Noise at nearby frequencies now appears near dc and affects the lock in output Phase noise in the SR865 is very low and generally causes no problems In applications requiring no phase jitter the internal reference mode should be used Since there is no PLL the internal oscillator and the reference sine waves are directly linked and there is no jitter in the measured phase Actually the phase jitter is the phase noise of a crystal oscillator and is very very small Harmonic Detection It is possible to compute the two PSD reference sine waves at a multiple of the internal
121. egister The ESE j command queries the value 0 or 1 of bit j Example ESE 17 Set bits 0 and 4 in the standard event enable register ESE 4 1 Set bit 4 in the standard event enable register ESE Returns decimal value of the standard event enable register ESE 4 Returns bit 4 of the standard event enable register The ESR command queries the value of the standard event status byte The value is returned as a decimal number from 0 to 255 The ESR j command queries the value 0 or 1 of bit j 0 7 Reading the entire byte will clear it while reading bit j will clear just bit j Example ESR Returns decimal value of the standard event status byte ESR 4 Returns bit 4 of the standard event status byte SRE j i STB j The SRE i command sets the serial poll enable register to the decimal value i 0 255 The SRE j i command sets bit j 0 7 toi 0 or 1 The SRE command queries the value 0 255 of the serial poll enable register The SRE j command queries the value 0 or 1 of bit j Example SRE 12 Set bits 0 and 3 in the serial poll enable register SRE 3 1 Set bit 3 in the serial poll enable register SRE Returns decimal value of the serial poll enable register SRE 3 Returns bit 3 of the serial poll enable register The STB command queries the value of the serial poll status byte The value is returned as a decimal number from 0 to 255 The STB j command queries the value 0 or 1 of bit j
122. en Any data channel can be assigned any of the sources in the keypad When the Config screen is closed the strip chart does not display a blue trace This is because we dismissed it previously Chapter 1 Getting Started 33 13 14 15 16 SRS Touch the blue scale tile Fi to display its scale palette Touch Sa to auto scale the blue F trace Phase 0 000 deg Fint CEORKE Touch the white scale tile Time to display the horizontal scale palette bottom right of screen Iz repeatedly to zoom out a few times 0 000 deg E Press and hold Scan setup to display the scan menu again Touch End Mode multiple times to select Once End Mode Once Touch Close to return to the strip chart Press Scan setup briefly don t hold it to turn the scan off then press Scan setup briefly again to re arm Simply touching the blue scale tile turns the blue trace back on and displays its scale palette The blue frequency trace shows the upward scan of the frequency spanning 4 divisions or 20 Hz Since the scan End Mode is set to repeat we see the scan repeat over and over in the history When scanning it is convenient to pause the strip chart at the end of the scan to review the results When the End Mode is Once then the scan stops at the end value and the strip chart is paused Changes to the scan setup do not affect a currently running scan We stop the scan and
123. end up less than 1 mV Prase 0 000 E E ET Ar A D Touch BEA to auto scale a trace Touch auto scale while keeping zero in the center The phase is ramping from 180 to 180 so the resulting scale is 50 div for a graph of 200 Touch the orange scale tile again to dismiss the scale palette Selecting a scale tile automatically dismisses any other palette Touch a highlighted scale tile to simply dismiss its palette leaving all palettes off The X and Y outputs are 0 2 Hz sine waves with 500 mV amplitudes The magnitude R is phase independent and is a straight flat line at about 500 mV By centering the trace the zoom function will expand the trace to reveal noise on R The little triangles along the right edge indicate the zero for each data channel Zooming in on R moves the zero for R below the graph as indicated by the downward facing yellow triangle at the bottom right edge SR8 amp 65 DSP Lock in Amplifier 16 Getting Started Chapter 1 9 Touch and drag in the graph area while the When a trace is selected by displaying its scale yellow scale palette is displayed to move the palette touching anywhere inside the graph yellow trace up and down area and not a scale button drags the trace up and down The IA and v buttons also move the trace up and down 10 Touch the highlighted yellow scale tile R to Touching within the graph area when no trace is dismiss its scale pale
124. entry screen to adjust either harmonic number The limits described above apply The Sine Out will still be at fint lt Amplitude gt SRS This knob adjusts the sine out amplitude The amplitude is shown in the info bar at the top of the screen Touch the Ampl tile to show the numeric entry screen The amplitude has 3 digits or 1 nV resolution whichever is larger The amplitude can range from 1 nVrms to 2 00 Vrms Press and hold this knob to set the amplitude to zero The Sine Out amplitude is specified for differential output Sine Sine Each output BNC has the specified rms amplitude through a 50Q output impedance If an output is SR865 DSP Lock in Amplifier 80 Operation Chapter 3 terminated in a high impedance the amplitude will be the specified value If an output is terminated in 50Q the amplitude will be half the specified value The outputs are 180 out of phase If both outputs are used differentially the amplitude will be twice the output of a single BNC In general if single output is used as the A input the measured signal will be the specified amplitude a single output terminated in a high impedance Using differential signals can be helpful when the sine amplitude is very small When the reference source is Internal or Dual this is the excitation source provided by the SR865 When an external reference is used or in Chop mode this sine output provides a sine wave phase locked to the ext
125. entually you end up with a very long measurement of a very narrow spectrum much like a lock in amplifier At this point you lose the FFT advantange and should indeed rely on the lock in measurement FFT s in the SR865 The SR865 FFT display is not a replacement for a full FFT spectrum analyzer instrument It is intended to give the user an overview of the spectrum of the input signal or the output of the PSD and time constant filters The length of the time record is always 1024 points the maximum span of an output FFT is determined by the time constant the window is Kaiser and the averaging is rms The amplitude is always dB Vrms and is uncalibrated only accurate to a few dB None of this can be changed eee Facts About SR865 FFT s The acquisition time of a spectrum averaging off is equal to the time record Narrow frequency spans means long time records The time record is about 500 span Hz Changing the measurement in narrow spans means waiting for at least a time record for a spectrum 2 The lock in time constant determines the maximum span for post filter FFT s Using a long time constant means having a long FFT time record 3 The frequency scale is labelled with respect to the signal s frequency at the input BNC Thus a signal at f which appears at dc at the lock in output will be labelled as fet in the FFT 4 Amplitudes are uncalibrated Amplitude measurements are generally accurate to a few dB SR865
126. ere are a variety of external noise sources within the laboratory Most of these noise sources are asynchronous i e they are not related to the reference and do not occur at the reference frequency or its harmonics Examples include lighting fixtures motors cooling units radios computer screens etc These noise sources affect the measurement by increasing the required dynamic reserve or lengthening the time constant Some noise sources however are related to the reference and if picked up in the signal will add or subtract from the actual signal and cause errors in the measurement Typical sources of synchronous noise are ground loops between the experiment detector and lock in and electronic pick up from the reference oscillator or experimental apparatus SR865 DSP Lock in Amplifier 56 Basics Chapter 2 Many of these noise sources can be minimized with good laboratory practice and experiment design There are several ways in which noise sources are coupled into the signal path Capacitive coupling An ac voltage from a nearby piece of apparatus can couple to a detector via a stray capacitance Although Cstray may be very small the coupled noise may still be larger than a weak experimental signal This is especially troublesome if the coupled noise is synchronous at the reference frequency Stray Capacitance Experiment Noise ny We can estimate the noise current caused by a stray capacitance by dV i C
127. ernal reference The rear panel BlazeX BNC output can be configured in the System menu hold Calc system to provide a square wave sync at the reference frequency This square wave is generated by discriminating the zero crossings of the sine output This signal can provide a trigger or sync signal to the experiment when the internal reference source is used This signal is also available when the reference is externally provided In this case the sync output is phase locked to the external reference lt DC Level gt This knob adjusts the sine out dc level The dc level is shown in the info bar at the top of the screen Touch the DC tile to show the numeric entry screen The dc level has 3 digits or 0 1 mV resolution whichever is larger The dc level ranges from 5 00 V to 5 00 V Press and hold this knob to set the dc level to zero The dc level adds an offset to the sine output Each output BNC has the specified dc level through a 50Q output impedance If an output is terminated in a high impedance the dc level will be the specified value If an output is terminated in 50Q the dc level will be half the specified value The dc level is added to the outputs in Difference mode opposite sign dc level on each BNC or in Common mode same sign dc level on both BNC s Choose the mode with the Mode key DC Level Mode This key selects the dc level mode The dc level is added to the sine outputs in Difference mode opposi
128. ertical scale dB div according to the table below i dB diy i dB div 8 2m 0 10 7 5m 1 20 6 10m 2 50 5 20m 3 10 4 50m 4 20 3 100m 5 50 2 200m 6 100 1 500m 7 200 The argument i is limited to 20 lt i lt 20 Vertical scale changes preserve the amplitude of the top edge of the graph Example FFTS 4 Set the FFT vertical scale to 20 dB div FFTS Returns the FFT vertical scale i The FFTO x command sets the FFT vertical offset x dB This is the amplitude associated with the top edge of the graph Example FFTO 20 0 Set the FFT vertical offset to 20 0 dB FFTO Returns the FFT vertical offset SR865 DSP Lock in Amplifier 126 Programming Chapter 4 FAUT The FAUT command performs an Auto Scale on the FFT display This command is the same as pressing the EZ button in the FFT vertical scale palette FFTMAXSPAN The FFTMAXSPAN query returns the maximum allowed FFT span Hz div for the current FFT source and lock in configuration time constant and reference frequency Example FFTMAXSPAN Returns the maximum allowed FFT span in Hz div FFTSPAN x The FFTSPAN x command sets the FFT span to x Hz div The value of x may not exceed the maximum allowed span as returned by FFTMAXSPAN The FFT span will be set to the nearest allowed span max span divided by powers of 2 Example FFTSPAN 305 Set the FFT span to 305 2 Hz div FFTSPAN Returns the FFT span in Hz div FFTA AVG1 AVG3 AVG10 AV
129. es Ethernet Touch a category to show its settings SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 91 SRS To change a setting touch the setting shown within a dark gray tile to either toggle the selection or highlight it for keypad entry The Enter 1 button advances to the next entry tile in a section For more information about using the computer interfaces see the Remote Programming chapter General Settings a t General Files Ethernet 1OMHz Timebase Blazex Sounds 4 GPIB amp RS232 Auto BlazeX Time and Date Touch the h button in the Time section to highlight the hours setting Use the keypad or ft and buttons to set the hour of day in 24 hour format The time will highlight in orange indicating that the display is not the current time but rather the time that will be set Continue to enter the minutes and seconds then touch set to commit the time to the internal clock Repeat this process for the date The time and date are used to label data files and screen shots as well as saved settings 10MHz Timebase The SR865 can accept an external 10 MHz timebase input on the rear panel This allows the internal reference frequency of the SR865 to remain frequency and phase stable with other SR865 units on the same timebase When set to Internal the external timebase input is ignored When set to Auto the SR865 will automatically lock to an external timebase whenever a 1 Vrms 10 MHz
130. esume the scan The Pause LED turns on indicating a scan is paused When the End Mode is set to Repeat or Up Down the scan repeats over and over When the End Mode is Once the scan stops at the end value the strip chart is paused and the Done LED turns on Note that data collection continues while the chart is paused Restarting the chart realigns the time history so the current time is the right edge again Press and hold Play Pause reset to reset the scan back to the Begin Value This leaves the Ready LED on and turns the Done LED off Press Play Pause reset briefly to start the scan again To Stop Scanning Press Scan setup briefly don t hold it to turn off the scan This key turns scanning on and off The Play Pause reset has no effect unless scanning is on and the Ready LED is lit Turning the scan off reverts the scanned parameter back to its original value from before scanning and its display back to white under front panel control Changes to Scan Parameters Once the scan is turned on to ready by briefly pressing Scan setup the scan configuration is loaded into the scanning system for use Edits to scan parameters such as Scan Time End Mode etc can be entered from the scan setup screen but will not affect the current scan To put any changes into action Scan setup must be briefly pressed to turn off the Ready LED and then briefly pressed again to re enable scans and turn the Ready LED back on Alternat
131. example if STREAMRATEMAX returns 78125 then STREAMRATE 4 will set the streaming rate to 78125 2 4882 8125 Hz The STREAMRATE query returns the value n Example STREAMRATE 4 Sets the streaming rate to the max rate divided by 24 STREAMRATE Returns the streaming rate n STREAMFMT i The STREAMFMT i command sets the stream data format to float32 G 0 or int16 i 1 Float32 requires 4 bytes per data point and int16 requires 2 bytes per point Float32 will use twice the ethernet bandwidth at the same streaming rate as int16 Int16 values range from 32768 to 32767 where the sensitivity divided by the expand is the value 31000 Example STREAMFMT 0 Sets the streaming format to float32 4 bytes per point STREAMFMT Returns the streaming format i STREAMPCKT i The STREAMPCKT i command sets the ethernet stream packet size to 1024 i 0 512 i 1 256 G 2 or 128 i 3 bytes Example STREAMPCKT 0 Sets the ethernet streaming packet size to 1024 bytes A sRs SR865 DSP Lock in Amplifier 142 Programming Chapter 4 STREAMPCKT Returns the streaming packet size i STREAMPORT i The STREAMPORT i command sets the ethernet streaming to stream to port i where i is between 1024 and 65535 inclusive The port should be free and not used by another application on the receiving computer By default the port is 1865 Example STREAMPORT 12345 Sets the ethernet streaming port to port 12345 STREAMPORT Returns t
132. f the harmonic number is greater than 1 In this case the upper limit is 2 5 MHz N where N is the harmonic number Press and hold this knob to set the harmonic detect to 1 fundamental Frequency Harm Up and Harm Down Pressing these keys increments and decrements the harmonic detect number from 1 to 99 The harmonic number is shown in the info bar at the top of the screen Touch this Harm tile to show the numeric entry screen The SR865 can detect signals at harmonics of the reference frequency The SR865 multiplies the input signal with digital sine waves at Nxf Only signals at this harmonic will be detected Signals at the original reference frequency are not detected and are attenuated as if they were noise Always check the harmonic detect number before making any measurements The Sine Out is not affected by the harmonic detection Its frequency is always the fundamental reference frequency Internal and Chop Reference When the reference mode is Internal or Chop these keys change the internal harmonic number N and detection is at NinXfin and the internal reference frequency is limited to 2 5 MHZ Nint External Reference When the reference mode is External these keys change the external harmonic number Nex and detection is at NexXfext The SR865 will always track the external reference Dual Reference When the reference mode is Dual detection is at faual NintXfint NextXfext Use the harmonic numeric
133. fer to the first page of this manual for instructions on selecting the correct line voltage and fuse GPIB Port The 24 pin GPIB IEEE 488 port allows a computer to control the SR865 via the IEEE 488 instrument bus The address of the instrument is set in the system menu hold the Calc system key RS 232 Port The RS 232 interface port is configured as a DCE The baud rate and parity are set in the system menu hold the Calc system key The number of data bits is always 8 USB Port The USB port allows a computer to control the SR865 via USB The SR865 is a Test and Measurement Class instrument Ethernet Port The Ethernet port allows a computer to control the SR865 via an ethernet network connection In addition the SR865 can serve web pages to the network and stream data over the network The network addresses and permissions are configured in the system menu hold the Calc system key The green LED indicates that the unit is connected The yellow LED is on for a gigabit connection and off for a 10 100 connection SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 97 SRS Timebase In Out Apply a 10 MHz sine signal 1 Vrms to the TIMEBASE input to lock the SR865 s timebase to an external timebase The internal reference frequency of the SR865 is derived from this timebase When multiple SR865 s are locked to the same 10 MHz timebase then harmonically related internal frequencies on different units will s
134. fier ASRS Chapter 3 Operation 63 CH1 amp CH2 Outputs The Channel 1 and Channel 2 outputs can be configured to output a voltage from 10 V to 10 V proportional to X or R CH1 and Y or 0 CH2 The sensitivity setting determines full scale 10 V Signal Inputs The voltage input mode may be single ended A or differential A B The A and B inputs are voltage inputs with 10 MQ 25 pF input impedance Their connector shields are isolated from the chassis by 10 Q Ground or 1 KQ Float Do not apply more than 10 V to either input The shields should never exceed 1 V The current input is configured for either 1 uA or 10 nA current range The input burden resistance is 100 Q 1 uA or 1 KQ 10 nA to a virtual ground Key Click On Off Press and hold the Calc system key to display the system menu Touch Sounds to toggle the key click on and off Display Off Operation Press the Blank key to operate with the front panel display and LEDs off The SR865 is still operating the outputs are active data collection continues and the unit responds to interface commands To change a setting press Blank to return to normal operation change the desired parameter then press Blank again Front Panel Test To test the front panel press and hold Local and Auto Range at the same time Turn any knob to run through all of the LEDs Make sure all of the knobs and all of the LEDs work Touch and drag on the touchscreen to
135. g output scales cannot be tolerated in the experiment Save Recall Press Save Recall to display the Save Recall screen A sRs SR865 DSP Lock in Amplifier 86 Operation Chapter 3 Recall default The SR865 can store 8 complete setups In addition the default setup can be recalled Touch one of the 8 Save buttons to save a setup Enter a name for this setup using the keypad Touch Confirm to commit the current setup or Cancel to quit the operation Locations which already contain a saved setup will show the setup name and the time and date it was saved Recall Save 5 default Unused Recall us Touch Recall for a location with a previously saved setup and a summary of settings which will change upon recall is shown Recall from Location 1 Confirm My First Setup Voltage In 1 VA AL yatCurrent In Touch Cancel to skip recalling this setup or touch Confirm to recall the setup and dismiss this screen SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 87 Interface and Data m Error Remote Busy won Trigger L Error The Error indicator flashes whenever there is a computer interface error such as an illegal command or out of range parameter is received Activity The Activity indicator flashes when there is command activity on a computer interface Trigger The Trigger indicator flashes for triggered data acquisition Remote The Remote indicator is on when
136. ger Input on the rear panel or immediately with a command SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 135 SRS Starting and Stopping Capture The CAPTURESTART and CAPTURESTOP commands control data capture CAPTURESTART specifies the capture mode and either starts capture immediately or arms the SR865 to wait for a hardware trigger The CAPTURESTOP command stops capture this is required for Continuous mode but can also be used to terminate a OneShot capture early Note that the capture system will continue recording data until the current 2 kbyte block is filled Once capture has stopped either in OneShot mode or by the CAPTURESTOP command data in the buffer can be read over the interfaces The CAPTURESTAT query returns the state of the capture Hardware Triggering In many situations it is important to synchronize the beginning of data acquisition to an external event The SR865 supports this with the rear panel TRIG IN input BNC When configured for hardware trigger by the CAPTURESTART command the lock in will wait for a falling edge at the BNC input before beginning the capture The TRIG IN BNC has a 10 KQO internal pull up resistor to 3 3 V giving users flexibility around interfacing When operating in OneShot mode with hardware trigger the SR865 begins capturing data upon the first falling edge of TRIG IN after the CAPTURESTART command is executed When the capture is finished either by recording the set nu
137. gnal plus noise The PSD and low pass filter only detect signals whose frequencies are very close to the lock in reference frequency Noise signals at frequencies far from the reference are attenuated at the PSD output by the low pass filter neither noise Oref NOT Onoise Oret are close to dc Noise at frequencies very close to the reference frequency will result in very low frequency ac outputs from the PSD poise Oretl is small Their attenuation depends upon the low pass filter bandwidth and roll off A narrower bandwidth will remove noise sources very close to the reference frequency a wider bandwidth allows these signals to pass The low pass filter bandwidth determines the bandwidth of detection Only the signal at the reference frequency will SR865 DSP Lock in Amplifier ASRS Chapter 2 Basics 39 SRS result in a true dc output and be unaffected by the low pass filter This is the signal we want to measure Where does the lock in reference come from We need to make the lock in reference the same as the signal frequency i e Not only do the frequencies have to be the same the phase between the signals cannot change with time otherwise cos Osig Oret will change and Vpsa will not be a dc signal In other words the lock in reference needs to be phase locked to the signal reference Lock in amplifiers use a phase locked loop PLL to generate the reference signal An external reference signal in this
138. good low noise amplifier may have about 5 nV VHZ of input noise If the amplifier bandwidth is 100 kHz and the gain is 1000 then we can expect our output to be 10 uV of signal 10 nV x 1000 and 1 6 mV of broadband noise 5 nV VHz x V100 kHz x 1000 We won t have much luck measuring the output signal unless we single out the frequency of interest If we follow the amplifier with a band pass filter with a Q 100 a very good filter centered at 10 kHz any signal in a 100 Hz bandwidth will be detected 10 kHz Q The noise in the filter pass band will be 50 uV 5 nV VHz x V100 Hz x 1000 and the signal will still be 10 uV The output noise is still much greater than the signal and an accurate measurement cannot be made Further gain will not help the signal to noise problem Now try following the amplifier with a phase sensitive detector PSD The PSD can detect the signal at 10 kHz with a bandwidth as narrow as 0 01 Hz In this case the noise in the detection bandwidth will be only 0 5 uV 5 nV VHz x V 01 Hz x 1000 while the signal is still 10 uV The signal to noise ratio is now 20 and an accurate measurement of the signal is possible What is phase sensitive detection Lock in measurements require a frequency reference Typically an experiment is excited at a fixed frequency from an oscillator or function generator and the lock in detects the response from the experiment at the reference frequency In the diagram below the reference signal
139. he SR865 Shorter time constants allow faster capture rates up to a limit of 1 25 MHz The actual capture rate can be set to the maximum allowed rate divided by factors of 2 Note that if the time constant is modified during a capture the sampling rate will change mid capture This will likely create confusing results and should be avoided Capture Time The entire capture time is the buffer length number of data points of each parameter divided by the capture rate For example a 4 MByte buffer holds 1 M total data points If X and Y are captured the buffer holds 512 kpoints of X Y pairs captured together The total capture time is thus 512 kpoints 1 25 MHz 0 419 seconds Capture Mode When the capture buffer becomes full data capture can either stop or continue The first case is called OneShot data points are captured for a single buffer length When the buffer fills data capture stops The second case is called Continuous In this case data capture continues at the end of the buffer The buffer will fill and start at the beginning again overwriting the oldest data Only the most recent points will be contained in the buffer Data is recorded until explicitly stopped using the CAPTURESTOP command or by hardware trigger This can be useful in situations when an external event determines the end of when data should be captured 1 e post triggering In both cases capture can be started with the hardware Trig
140. he cursor mode EJ and touch Cursor Width to select wide mode HHI 4 The lock in should be locked to the external reference Wait at least 45 seconds locked to the external synthesizer Then touch Pause m and move the cursor to 30 seconds from the right hand end of the chart Record the average value of Fe external frequency from the cursor display 5 This completes the frequency accuracy test Enter the results of this test in the test record at the end of this section A sRs SR865 DSP Lock in Amplifier 192 Performance Tests Appendix G Phase Accuracy This test measures the phase accuracy of the lock in The phase accuracy is determined by measuring the phase of the internal oscillator Sine Out Setup Connect the Sine Out to the Voltage A input of the SR865 using a 24 inch BNC cable with the precision 50Q feedthrough terminator at the A input BNC This test only uses ONE Sine Out BNC Please note that the phase calibration is performed at the factory relative to a 24 inch RG 58 BNC cable with approximately 3 ns of propagation delay Results will be show a significant dependence on cable length For example using a 2 m long cable instead of the specified 24 inch cable will introduce an additional 6 9 ns of propagation delay between sine output and signal input at 2 MHz this corresponds to 5 of additional phase delay Procedure 1 PRESET press Save Recall then touch Recall default Touch Confirm 2
141. he streaming port i STREAMOPTION i The STREAMOPTION i command sets certain advanced features of ethernet streaming Setting bit 0 true sends data in little endian format while setting bit 0 false sends data in big endian format Setting bit 1 true enables data integrity checking while setting bit 1 false disables data integrity checking By default data is sent in big endian format with integrity checking i 2 Bit Weight Definition 1 Use little endian 1 2 Use data integrity checking Example STREAMOPTION 1 Send data in little endian format with no integrity checking STREAMOPTION 3 Send data in little endian format with integrity checking STREAM OFF ON i The STREAM i command turns ethernet data streaming off i 0 or on i 1 To enable streaming the command must be sent over VXI 11 from the receiving computer This determines the IP address that the data is sent to Example STREAM 1 Turn ethernet streaming on STREAM ON SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 143 System Commands TIME SEConds MiNutes HOUrs j i The TIME j i command sets the clock value for seconds j 0 minutes j 1 or hours j 2 toi Example TIME MIN 30 Set the system clock minutes to 30 TIME 1 30 TIME MIN Returns the system clock minutes DATE DAY MONth YEAr j i The DATE j i command sets the clock value for day j 0 month j 1 or year G 2 to
142. he voltage input Setup We will use the internal oscillator sine output to provide the signal Connect the Sine Out to both the A and B inputs of the lock in Use equal length cables from A and B to a BNC TEE Connect the cable from the Sine Out to the TEE This test only uses ONE Sine Out BNC Do not use any termination Procedure 1 PRESET press Save Recall then touch Recall default Touch Confirm 2 Use the front panel to make the following adjustments Touch Fint to display the internal frequency keypad Enter a frequency of 1 0 kHz Touch Ampl to display the sine out amplitude keypad Enter an amplitude of 1 0 V 3 The value of R yellow should be 1 000 V within 2 4 Use the front panel to make the following adjustments Press Couple Select DC coupling Press A B Select A B Press Sensitivity Down multiple times Set the sensitivity to 200 uV 5 Record the value of R yellow 6 This completes the CMRR measurement test The common mode rejection is 20log 1 0 R where R is in Volts Enter the results of this test in the test record at the end of this section ASRS SR865 DSP Lock in Amplifier 186 Performance Tests Appendix G Sine Output Amplitude Accuracy and Flatness This test measures the amplitude accuracy and frequency flatness of the internal oscillator Sine Out These results will also be used next for testing the input Amplitude Accuracy Setup We will use the precision ac m
143. hoose a streaming rate higher than the time constant bandwidth The lock in time constant and filter slope should be chosen to attenuate outputs at frequencies higher than 1 2 the streaming rate as much as possible STREAMCH X XY RT XYRT i The STREAMCH i command configures the data streaming to X i 0 X and Y i 1 R and 0 i 2 or X Y R and 0 i 3 Example STREAMCH 2 Capture R and 9 STREAMCH RT SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 141 STREAMCH Returns the stream configuration i STREAMRATEMAX The STREAMRATEMAX query returns the maximum allowed streaming rate at the current time constant and sync filter period in Hz This is a query only command The streaming rate sets how often data points are streamed to the ethernet interface All streamed parameters are sampled at the same rate and at the same times The maximum streaming rate is determined by the time constant of the SR865 Shorter time constants allow faster streaming rates and if sync filter is on shorter sync filter periods allow faster streaming rates up to a limit of 1 25 MHz The actual streaming rate can be set to the maximum allowed rate divided by factors of 2 Example STREAMRATEMAX Returns the maximum streaming rate in Hz STREAMRATE n The STREAMRATE n command sets the streaming rate to the maximum rate divided by 2 The value of n is limited to 0 lt n lt 20 Set n 0 for the maximum streaming rate For
144. i FFTO x FAUT FFTMAXSPAN FFTSPAN x FFTA AVG1 AVG3 AVG10 AVG30 AVG 100 i FFTL OFF ON i FCRW Line NARrow Wide i FCRX FCRY Scan Commands SCNPAR Fint REFAmp REFDc OUT1 OUT2 i SCNLOG LIN LOG i SCNEND ONce REpeat UPdown i SCNSEC x SCNAMPATTN SCNDCATTN fi SCNINRVL 4 SCNENBL OFF ON i SCNRUN SCNPAUSE SCNRST SCNSTATE SCNFREQ BEGin END j f HZ KHZ MHZ SCNAMP BEGin END j v NV UV MV V SCNDC BEGin END j v NV UV MV V SCNAUX1 BEGin END j v NV UV MV V SCNAUX2 BEGin END j v NV UV MV V Data Transfer Commands OUTR DAT1 DAT2 DAT3 DAT4 j OUTP j SNAP j k 1 Data Capture Commands CAPTURELEN n CAPTURECFG X XY RT XYRT i CAPTURERATEMAX CAPTURERATE n SR865 DSP Lock in Amplifier page description 125 SetFFT data source 125 Set vertical scale from 1 0 dB 0 to 200 dB 7 125 Set top reference level 126 FFT auto scale 126 Query maximum allowed frequency span in Hz div 126 Set frequency span in Hz div 126 Setaveraging 126 Turn FFT offfon pause live 126 Set the cursor width 127 Query the FFT cursor frequency 127 Query the FFT cursor amplitude page description 128 Setthe scan parameter 128 Setthe scan type 128 Setthe scan end mode 128 Set the scan time to x seconds 128 Setthe
145. iangle on the right edge The scale is 50 mV div so the green data is a line 2 divisions above the center The DVM should now read 2 V This is because X 100 mV is now 20 of the sensitivity 500 mV This also increases the bar graph to 20 and increases the resolution of the numeric readout SR8 amp 65 DSP Lock in Amplifier 22 Getting Started Chapter 1 Phase T deg 400 000 Hz A 4 AM 109 py 8 Press Sensitivity Down two more times to select 100 mV Phase 0 000 deg Fint 100 000 kz Harrn 1 Ampl 100 mv oc 9 Press and hold the lt CH1 Offset gt knob above the CH1 BNC to display the offset keypad Phase 0 000 deg E 100 000 kiz A 14 A 100 my 10 Touch Auto in the offset keypad screen SR865 DSP Lock in Amplifier Note that the trace of X is unchanged This is because the value of X is unchanged The Sensitivity does not affect the value of the output just the way the value is scaled to the displays and analog output The Sensitivity applies to X Y and R The DVM should now read 10 V and X is now 100 of full scale on the bar graph It is important to adjust the Sensitivity even if the analog outputs are not being used The Sensitivity determines the resolution of the numeric readouts and bar graphs Auto Scale will adjust the Sensitivity automatically X Y and R may all be offset ratioed and expanded separately Since CH1 is set to X indicated by the X LED above th
146. icated power to all sections of the instrument Multiple independent grounds are left isolated from each other within the power supply to avoid creating ground loops each circuit however is ultimately ground referenced to chassis ground Bridge rectifiers on the lower power supply board convert 50 Hz 60 Hz to dc some of these components particularly D6 run quite hot under normal conditions Unregulated dc power is routed up from the rectifier board to the upper regulator board by the 20 pin header J2 located near the rear panel of the SR865 Linear regulators on the upper power supply board establish the voltage rails for distribution across the instrument Most regulators are clamped onto the channel heat sink on the center of the board Please note that the DB 9 connections for the Reference In SR865 DSP Lock in Amplifier ASRS Appendix H Circuit Description 203 SRS module J6 and the Sine Out module J8 are not interchangeable these subassemblies have different power requirements and must be cabled correctly 24 VDC is tapped from the motherboard connector to header J9 for powering the small cooling fan mounted to the heat sink Serial SPI control signals are routed from the motherboard to each front end module Preamp Ref In and Sine Out through the regulator board signals come in via J7 and are bussed to pins 4 5 8 and 9 of the power control connectors J2 J6 and J8 While each set of related voltage rails h
147. ine NARrow Wide i SCRY DAT DAT2 DAT3 DAT4 STATus j CURDATTIM CURINTERVAL SRS 118 118 119 119 page 120 120 120 121 121 121 121 122 122 122 122 122 123 123 123 124 description Set CH1 2 to X or R Y or 0 Set CH1 2 expand Turn CH1 2 output offset off on Set CH1 2 output offset to x percent Auto Offset CH1 2 Ratio Ch1 2 with Aux In 3 or 4 description Query Aux Input j 0 3 Set Aux Output j 0 3 voltage to v description Auto Phase Auto Range Auto Scale description Turn front panel blanking off on Set Screen Layout Screen Shot to USB memory stick Assign parameter i to data channel j Turn data channel j strip graph off on Download screen capture image description Set horizontal scale from 0 5 s 0 to 2 days 16 Set channel j vertical scale to x Set channel j vertical offset Auto scale channel j Auto scale with zero center channel j Auto find for channel j Turn channel j graph off on Turn strip chart offfon pause run Move cursor to i 0 right 639 left Set cursor relative or absolute mode Set cursor horizontal mode to date time or interval Set cursor to mean maximum minimum Set the cursor width Query the cursor data values Query the cursor horizontal position as date time Query the cursor horizontal position as interval SR865 DSP Lock in Amplifier xii Commands FFT Screen Commands FFTR ADC MIXer FILTer i FFTS
148. ine out Each of these parameters can be adjusted using the knobs and buttons in the reference settings section of the front panel Touching one of these tiles brings up a numeric keypad for direct entry 0 000 deg 100 000 khz YA 100 000 clr 1 00000 kHz 1 00000 MHz F2 F4 50 0000 Hz 100 000 kHz Internal frequency entry screen Numeric entry is straightforward Close will return to the data screen The buttons F1 F2 F3 and F4 are frequency presets Touching a preset will load the preset value immediately Touch and hold a preset button to memorize the current setting Other parameters may have slightly different entry screens Strip Charts The most common way to visualize the lock in outputs is to use the strip chart display New data is plotted at the right edge and older data scrolls left The scroll rate is determined by the horizontal scale time per division For example a scale of 1s div presents the 10 most recent seconds of data and data points take 10 s to scroll completely off the left edge Horizontal scales range from 0 5 s to 2 days per division At each point along the horizontal axis the graph displays the maximum to minimum excursion of each data channel during a time interval corresponding to that point in the SR865 DSP Lock in Amplifier 6 Getting Started Chapter 1 past The time interval is determined by the horizontal scale per division and the number of pixels in the display There a
149. ir maximums The results displayed on the strip charts or available over the computer interfaces are the floating point outputs and are unaffected by output overloads Output overloads are indicated by a Scale indicator in the X Y or R displays SR865 DSP Lock in Amplifier 70 Operation Chapter 3 The Sensitivity should be chosen to conveniently and accurately display the measurement results on the output BNC the bar graph and the numerical readout The Sync filter also uses the Sensitivity as an indicator of the signal range it needs to accommodate An output overload does not necessarily mean a Sync filter overload but a Sync error indicated in the X Y R or 9 displays does signify a Sync filter overload O ranges from 180 to 180 regardless of the scale or range When CH2 outputs a voltage proportional to 0 the output scale is 18 Volt or 180 10 V The phase bar graph and numeric readout scales are also unaffected by the Sensitivity Synchronous Filters and Noise Calculations Synchronous filtering and calculation of Xyoise aNd Y noise are performed on the output scaled values of X and Y Thus their accuracy is affected by a poor choice of Sensitivity If the Sensitivity is set too large the values of X and Y in the numeric displays will lack the necessary resolution If the Sensitivity is set too low the values will be overloaded pinned and no noise will be measured In general setting the Sensitivity to dis
150. ise 3 THETa 8 output 12 PHAse Reference Phase 4 INI Aux In 13 SAMp Sine Out Amplitude 5 IN2 Aux In2 14 LEVel DC Level 6 IN3 Aux In3 15 Flint Internal Reference Frequency 7 IN4 Aux In4 16 FExt External Reference Frequency 8 OUTI Aux Outl Example CDSP DATg2 IN2 Assign Aux In2 to data channel 2 blue CDSP 3 15 Assign the Reference Frequency to data channel 4 orange CDSP DAT1 SAM Assign the Sine Out Amplitude to data channel 1 green SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 119 CDSP 1 Returns the parameter index assigned to data channel 2 blue CDSP DAT2 CGRF DAT1 DAT2 DAT3 DAT4 j OFF ON i The CGRF j i command turns the strip chart graph of data channel j off i 0 or on i 1 The value of j 0 3 corresponds to the DAT1 green DAT2 blue DAT3 yellow and DAT4 orange data channels Example CGRF DAT2 ON Turn graphing on for data channel 2 blue CGRF 1 1 CGRF 0 OFF Turn graphing off for data channel 1 green CGRF 1 Returns data channel 2 blue graphing state i CGRF DAT2 GETSCREEN The GETSCREEN query creates and returns a screen image as a binary block of data in BMP file format This query cannot be executed over the RS 232 interface The file image is the same as is written to a USB memory stick upon pressing the Screen Shot key After sending GETSCREEN the user s remote program should repeatedly perform a Status Byte query either by serial poll o
151. isualized Stored parameter values are not altered by scale changes Horizontal Scale Palette Horizontal scale changes are applied to the entire strip chart display and all data channels Zoom In and Zoom Out change the horizontal scale and scroll speed Pause stops the chart scrolling and pauses the graph When the graph is paused the cursor can be used to readout data values These readouts correspond to the min max or mean of the data in the time bin at the cursor location The time of the cursor location is displayed in the tile at the left edge of the scale bar below the graph Touch this tile to switch between elapsed time from the right edge to absolute time time and date when the point was taken Use Cursor MinMaxMean and Cursor Width to change the cursor Note that the cursor marker may not lie on the data graph for wide cursors since the marker shows the min max or mean of all the data within the cursor width m 19Nov14 12 37 57 25 A sRs SR865 DSP Lock in Amplifier 8 Getting Started Chapter 1 The cursor is only displayed when the graph is paused Zooming in and out preserves the right hand edge of the graph at the point in time when the graph was paused Data storage continues in the background while the graph is paused When live scrolling is resumed with Resume the graph is redrawn so the current point is once again at the right edge SR865 DSP Lock in Amplifier ASRS Chapter 1 Getting S
152. ively press and hold Play Pause reset to return to the Ready LED on state this will also put newly changed scan parameters into effect Aux Output SRS Press Aux Output to display the Aux Output screen SR865 DSP Lock in Amplifier 90 Operation Chapter 3 Phase 427 775 deg 250000 mz 40 AM o y 500 v 0 000 4 Out 1 Out 2 41 2 Out 3 Out 4 0 The SR865 has 4 Aux Outputs on the rear panel Use this screen to set their levels Touch one of the Out n buttons to select an aux output then use the keypad to enter a value The lt Cursor gt knob will also adjust the output value in this screen Calc system Press Calc system briefly to display the calculator screen Phase 427 775 deg 2 50000 mz gp AM o v getphs getfrq get am sin cos tan vx LES In deg Tr log rot 4 This is an RPN calculator where the operator follows all of its operands For more information ask your friends or check the internet The 2 button switches the functions of the operator keys Note that the reference phase frequency and amplitude can be copied in and out of the calculator System Menu Press and hold Calc system to display the system menu The system menu is used to set non measurement parameters such computer interface configurations file names clock setup and more Use the and buttons next to the category list to move up and down through the category list General Fil
153. k an external SR540 Optical Chopper to the SR865 s internal reference frequency This is achieved by connecting the f Reference output from the SR540 to the Ref In of the SR865 But instead of using this input as the external reference the SR865 will servo the SR540 to lock the SR540 to the SR865 internal frequency This control is via a connection from the SR865 rear panel Aux Out 4 BNC to the CONTROL VOLTAGE input of the SR540 This essentially transforms the SR540 into a frequency and phase stabilized chopper with the frequency accuracy and drift of the SR865 This is especially useful in experiments with multiple SR540 choppers Each SR540 chopper with its own SR865 can be synchronized to a common 10 MHz clock to achieve stable frequency and phase relationships with each other Lock in detection is at the measured external frequency The SR540 is driven to phase lock with this internal frequency but is subject to the mechanical limitations of its motor and slew rate The internal frequency is shown in the info bar Use the lt Frequency gt knob or the numeric keypad to adjust the internal frequency To record and view the external frequency use the Config key to assign fex to one of the four data channels By comparing f and fin the effective settling time of the SR540 to commanded frequency changes can be observed In this mode the Sine Out is at the external frequency Both the SR540 and the SR865 must be configured correctly
154. k in Amplifier 50 Basics Chapter 2 The Input Amplifier A lock in can measure signals as small as a few nanovolts A low noise signal amplifier is required to boost the signal to a level where the A D converter can digitize the signal without degrading the signal to noise The analog gain in the SR865 ranges from roughly 1 to 100 As discussed previously higher gains do not improve signal to noise and are not necessary The gain is set by the Input Range What is the Noise Floor The input noise of the SR865 signal amplifier is about 2 5 nVrms VHz What does this noise figure mean Let s set up an experiment If the SR865 s Input Range is set to 10 mV then the input gain is sufficient that the noise floor of the measurement is determined by the input noise Suppose the PSD output is low pass filtered with a single RC filter 6 dB oct roll off with a time constant of 100 ms What will be the noise floor of the measurement Amplifier input noise and Johnson noise of resistors are Gaussian in nature That is the amount of noise is proportional to the square root of the bandwidth in which the noise is measured A single stage RC filter has an equivalent noise bandwidth ENBW of 1 4T where T is the time constant RxC This means that Gaussian noise is filtered with an effective bandwidth equal to the ENBW In this example a single 100ms low pass filter has an ENBW of 1 4x100ms or 2 5 Hz Thus the lock in noise floor will be 2 5 nVrms
155. lators External Reference Input The reference input module provides signal conditioning for sine and TTL mode external reference inputs for the digital phase locked loop of the SR865 Input impedance and signal type sine or TTL is selected by steering relays K101 and K102 and analog switch U108 The TTL path is de coupled and discriminated directly with comparator U102 with transition threshold set at 1V The polarity rising or falling edge is selected SR865 DSP Lock in Amplifier 202 Circuit Description Appendix H by U104 The sine path is ac coupled with a 1 s time constant at C117 R132 and buffered by U106 Threshold trimming from U115 is added through R192 before further amplification at U105 and discrimination at U112 The logic level reference is then converted into a fast differential transition using current mode logic from U109 and then level shifted and slew rate limited with U102 before being driven differentially through R105 R108 to the shielded twisted pair interconnect J101 leading to the motherboard Chip selects clock gating and relay control is provided by U201 through U206 dc power is re regulated from 16V to 5V with linear regulators U207 and U208 Sine Output The SR865 sine output module provides filtering attenuation and dc level generation for the differential sine output A passive 7th order differential Cauer low pass filter is implemented with components L102 L107 C101 C107 C112 C114 Th
156. lifier ASRS Appendix G Performance Tests 187 4 5 6 7 SRS b Adjust the voltmeter range step iv a iv b or iv c Wait for the ac volts reading to stabilize Record the value c Repeat 3a and 3b for each amplitude in the table Adjust the multimeter range to 100 mVAC step iv b Use the front panel to make the following adjustments Touch Ampl to display the sine out amplitude keypad Enter an amplitude of 100 mV Frequency response is checked at frequencies above 1 kHz The sine amplitude is set to 100 mV for all frequencies The test frequencies are listed below Test Frequencies 10 kHz 100 kHz 1 0 MHz a Use the front panel to make the following adjustments Touch Fint to display the internal frequency keypad Enter the frequency from the table b Wait for the multimeter readings to stabilize Record the value c Repeat steps 6a and 6b for all frequencies in the table This completes the sine output amplitude accuracy and frequency response test Enter the results of this test in the test record at the end of this section SR865 DSP Lock in Amplifier 188 Performance Tests Appendix G Amplitude Accuracy and Flatness This test measures the amplitude accuracy and frequency response Setup We will use the internal sine output to provide an accurate sine wave for testing the signal input function This test requires the preceding Sine Output Amplitude Accuracy and Flatness test results to
157. ll achieve that If the output value is changing because of parameter sweeping or a signal turning on and off then the advanced filters can be of great help In practice it is simple to try these filters at various time constants and number of poles in comparison with RC filters They often yield better results in less time Closed Loop Applications If the lock in output X is used in a feedback loop the filter should always be a single RC pole Advanced filtering may lead to oscillations in the loop SR865 DSP Lock in Amplifier ASRS Appendix A Advanced Filters 157 SRS The BlazeX output is always an RC filter 1 to 4 poles regardless of the front panel Advanced Filter setting This is because the BlazeX output is intended for feedback applications that require low latency Summary of Filter Characteristics The chart below summarizes the speed of the different filters also see Step Response graphs on the following pages Time to settle to within 1 of final value in time constants Linear Phase z i ee ee The chart below summarizes the noise bandwidth of the different filters Note that for a single stage all 3 filters are designed with the same noise bandwidth Equivalent Noise Bandwidth fraction of f 2axtime constant Linear Phase SR865 DSP Lock in Amplifier 158 Advanced Filters Appendix A Frequency Response Graphs These graphs illustrate the frequency response of the different filte
158. ll need BNC cables and a digital voltmeter DVM Specifically you will set the Aux Output voltages and measure them with the DVM These outputs will then be connected to the Aux Inputs to simulate external dc voltages which the lock in can measure 4 Do This Disconnect all cables from the lock in Turn the power on while holding down the Local key The power switch is on the power entry module on the rear panel Connect Aux Out 1 on the rear panel to the DVM Set the DVM to read dc volts Press Aux Output to show the Aux Output keypad Phase 0 000 deg t 100 000 nz 11 0 0004 clr Out 1 Out 2 1 2 Out 3 Out 4 0 Touch 1 0 Enter to set Aux 1 to 10 000 V Use the lt Cursor gt knob adjust the level to 5 000 V SR865 DSP Lock in Amplifier Explanation When the power is turned on with Local pressed the lock in returns to its standard default settings See the Standard Settings list in the Operation section for a complete listing of the settings The 4 Aux Outputs can provide programmable voltages between 10 5 and 10 5 volts The outputs can be set from the front panel or via the computer interface Aux Outputs are easily set from the front panel The DVM should display 10 00 V The lt Cursor gt knob is used to adjust values when the Aux Output keypad is shown The DVM should display 5 00 V The 4 Aux Outputs are useful for controlling other parameters in an experiment such as
159. ll the data within the cursor width m 19Nov14 12 37 57 25 Relative Chart Cursor To read chart data relative to a reference point within the chart position the cursor at the desired reference location and press the lt Cursor gt knob once A stationary vertical line is placed at the reference location and the cursor readout values will be relative to the data at this location and the Rel LED is lit Touch the cursor location tile below the cursor readouts to switch between absolute time and time relative to the cursor Note that changing the horizontal scale may move the reference location off screen to the left The reference location is preserved in this case the reference line is just not visible on the graph To clear the reference location press the lt Cursor gt knob again The cursor readout now toggles between absolute time and elapsed time from the right hand edge and the Rel LED turns off FFT Cursor The cursor is always active in the FFT screens The amplitude and frequency readout is at the upper right of the graph Use the lt Cursor gt knob to position the cursor There is no relative cursor function A sRs SR865 DSP Lock in Amplifier 84 Operation Chapter 3 Touch in the vertical scale paleete to change the width of the cursor In the FFT display the wide cursors always find the peak within the cursor region This makes reading an FFT much easier SR865 DSP Lock in Amplifier ASRS Chapter 3 Oper
160. lready floating point numbers with all of the resolution available The strip charts do reflect the offsets however What is Dynamic Reserve Really Suppose the lock in input consists of a signal at f plus noise at some other frequencies The real world definition of dynamic reserve is the ratio of the largest noise signal to the actual signal at f This ratio is usually expressed in dB For example if the fet signal is 1 uV and the noise reaches 1 mV then the dynamic reserve is 60 dB noise is 1000 times the signal Dynamic reserve is therefore a property of the input signal generated by the experiment that is the ratio of noise to signal at the BNC is determined by factors outside the lock in It is the job of the lock in to measure the signal to the best of its ability whatever the ratio of noise to signal at the input Of course it is always better to have less noise and more signal Dynamic Reserve in the SR865 Unlike most lock ins the SR865 does not have a dynamic reserve setting As mentioned above the real world dynamic reserve is the noise to signal ratio at the input The SR865 is designed to achieve the best possible measurement as easily as possible SR865 DSP Lock in Amplifier ASRS Chapter 2 Basics 49 SRS In the SR865 the only real gain is in the input amplifier which amplifies the signal before it reaches the A D converter After this point the signal is processed digitally so there s no furthe
161. ltage preamplifier or current preamplifier respectively In the de energized state these relays keep the sensitive amplifier front end components isolated from the front panel input connections The voltage input preamplifier uses a cascaded JFET input stage with transistors Q202 and Q101 with feedback provided by U202 and resistors R223 R224 and R225 R226 R230 R231 The current input preamplifier is built with transimpedance gain of 10 V A or 10 V A R305 or R302 de offset voltage trim is injected at the fully differential summing junctions for U204 which also provides CMRR trimming for the voltage preamp front end Programmable gain blocks around U402 and U404 implement the SR865 Input Range setting with U406 driving the rear panel Signal Mon after all programmable gain is added The main analog signal is driven differentially through R428 and R429 to the shielded twisted pair interconnect J401 leading to the motherboard Level translators U501A U502A U503A and U504A support overload window comparator circuits while the differentiators U501B U502B and U503B allow for slew rate overload detection U601 through U603 all perform overload detection wire or ing into buffer U611 U610 for monitoring by the motherboard Chip selects clock gating and relay control is implemented with U701 U709 Analog power for the signal amplifiers is re regulated from the main power supply 16V to 15V with U710 U711 low noise low drop out regu
162. mA should ever be applied to this input Voltage A B The voltage input can be either a single ended A or differential A B voltage The shields of A and B are connected and grounded by either 10 Q Ground or 10 kQ Float Voltage Couple This key selects the voltage input coupling The signal input can be either ac or dc coupled The ac coupling option uses a high pass filter to pass signals above 160 mHz and attenuate signals at lower frequencies ac coupling should be used at frequencies above 160 mHz whenever possible At lower frequencies dc coupling is required ac SR8 amp 65 DSP Lock in Amplifier 66 Operation Chapter 3 coupling results in gain and phase errors at low frequencies as wells as reduced CMRR performance Remember the Reference Input is ac coupled when a sine reference is used This also results in phase errors at low frequencies Voltage Ground This key chooses the shield grounding configuration The shields of the input connectors A and B are not connected directly to the lock in chassis ground In Float mode the shields are connected by 10 kQ to the chassis ground In Ground mode the shields are connected by 10 Q to ground Typically the shields should be grounded if the signal source is floating and floating if the signal source is grounded Do not exceed 1 V on the shields Voltage Input Range The voltage input range specifies the largest input signal ac or dc coupled before the vol
163. mber of points or after receiving the CAPTURESTOP command the data should be downloaded from the SR865 Retriggering will begin overwriting the buffer When operating in Continuous mode with hardware trigger the SR865 again begins capturing data upon the first falling edge of TRIG IN after the CAPTURESTART command However in this case the TRIG IN level must remain low for capture to continue In Continuous mode with hardware trigger a rising edge on TRIG IN after capture has been triggered has the same effect as the CAPTURESTOP command This allows users the flexibility for some degree of post triggering where the capture acquisition can run continuously until some externally generated signal indicates to stop Again note that captures do not stop immediately but will continue to run for a short but indeterminant time until the current 2 kbyte buffer is filled Also as with OneShot mode the data should be downloaded from the SR865 after stopping Retriggering will begin overwriting the buffer from the beginning of the buffer Aliasing Effects In any sampled data stream it is possible to sample a high frequency signal such that it will appear to be a much lower frequency This is called aliasing Aliasing occurs whenever the signal being sampled contains signals at frequencies greater than 1 2 the sample rate The effect is most noticeable when trying to sample an output frequency at or near an integer multiple of the sample rate
164. n access the same subnet Note that typical installations do not allow the unit to be seen on the internet only on the internal subnet For full internet access consult with your IT administrator The homepage is shown below Various instrument properties are displayed This is an easy way to check the ethernet settings without writing any software gt eR A e htp 172 25 98 239 SRS stanford Research Systems SR865 2 MHz Lock in Amplifier Instrument Info Interactive Control Monitor Output SR865 JOE1 Stanford Research Systems 2 MHz Lock in Description Amplifier Serial Number 00001 IP Address 172 25 98 239 GPIB Address Firmware Version alia WUE Ce WX O CEH OO 19 b3 0a 00 01 SR865 Manual Stanford Research Systems Inc 2008 2014 Click on the Interactive Control tab to display the command screen Here you can try sending commands or queries and checking the responses to SR865 commands Click on the SR865 Command List link to see the commands A SRS SR865 DSP Lock in Amplifier 170 Using the Webserver Appendix C AAA Gia e a TA Cae ot To ne 272 25 98 239 SRS stanford Research Systems SR865 2 MHz Lock in Amplifier Instrument info Interactive Control Monitor Output Send Receive Commands Command idn Send Read History gt idn lt Stanford_Research_Systems SR865 00001 v0 94 SR865 Manual Stanford Research Sy
165. n adjusting an experiment to find a maximum or minimum The lock in defaults to the internal oscillator reference set at 100 000 kHz The reference source is indicated by the Internal LED The internal oscillator should be very close to the actual generator frequency The X green and Y blue displays should read values which change slowly The lock in and the generator are not phase locked but they are at nearly the same frequency with a slowly changing 0 orange The signal magnitude R yellow is phase independent and does not change SR8 amp 65 DSP Lock in Amplifier 14 Getting Started Chapter 1 3 4 5 Use the lt Frequency gt knob to carefully adjust the frequency to 99 9998 kHz That s 0 2 Hz below 100 kHz 99 9998 kiz MM 1 Phase 0 000 deg Fint Press Screen Layout once to change the display to the full screen strip chart 1 Ampl Phase 0 000 deg t 99 9998 kiz Touch the orange scale tile 0 at the bottom to display the vertical scale palette 1 Ampl Phase 0 000 deg Fit 99 9998 kiz SR865 DSP Lock in Amplifier By setting the lock in reference 0 2 Hz away from the signal frequency the X and Y outputs are 0 2 Hz sine waves difference between fef and fsig The X and Y displays should now oscillate at about 0 2 Hz the accuracy is determined by the timebases of the generator and the lock in The most common way to visualize the lock in outputs is to use the
166. nal What does the SR865 measure The SR865 multiplies the input signal by a pure sine wave at the reference frequency All components of the input signal are multiplied by the reference simultaneously Mathematically speaking sine waves of differing frequencies are orthogonal i e the average of the product of two sine waves is zero unless the frequencies are exactly the same In the SR865 the product of this multiplication yields a dc output signal proportional to the component of the signal whose frequency is exactly locked to the reference frequency The low pass filter which follows the multiplier provides the averaging which removes the products of the reference with components at all other frequencies The SR865 because it multiplies the signal with a pure sine wave measures the single Fourier sine component of the signal at the reference frequency Let s take a look at an example Suppose the input signal is a simple square wave at frequency f The square wave is actually composed of many sine waves at multiples of f with carefully related amplitudes and phases A 2V pk pk square wave can be expressed as S t 1 273 sin t 0 4244 sin 3at 0 2546 sin Set where 2nf The SR865 locked to f will single out the first component The measured signal will be 1 273 sin wt not the 2V pk pk that you d measure on a scope In the general case the input consists of signal plus noise Noise is represented as varying signal
167. nce and the digital sine wave which is multiplying the signal in the PSD This is also the phase between the sine output and the digital sine wave used by the PSD in either internal or external reference mode Changing this phase shift only shifts internal sine waves The effect of this phase shift can only be seen at the lock in outputs X Y and O R is phase independent Auto Phase Touching Auto in the numeric entry screen or pressing the Auto Phase key will adjust the reference phase shift so that the measured signal phase is 0 This is done by subtracting the present measured value of O from the reference phase shift It will take several time constants for the outputs to reach their new values Auto Phase may not result in a zero phase if the measurement is noisy or changing Phase 90 and 90 9 The 90 and 90 keys add or subtract 90 000 from the reference phase shift Touching 90 and 90 in the phase numeric entry screen will do the same lt Frequency gt If the reference source is Internal Dual or Chop this knob adjusts the internal reference frequency The internal frequency is shown in the info bar at the top of the screen Touch the Fi tile to show the numeric entry screen SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 79 The internal frequency has 6 digits or 0 1 mHz resolution whichever is larger The frequency can range from 1 0 mHz to 2 50 MHz The upper limit is decreased i
168. nd can tolerate up to 1 V of applied voltage GFCI Ground Fault Circuit Interrupter GFCI protected outlets are often available in production and laboratory environments particularly in proximity to water sources GFCI s are generally regarded as an important defense against electrocution However the use of GFCI in conjunction with the SR865 SR865 DSP Lock in Amplifier ii Safety and Preparation For Use must not be regarded as a substitute for proper grounding and careful system design GFCT s must also be tested regularly to verify their functionality Always consult an electrician when in doubt Service Do not attempt to service this instrument unless another person capable of providing first aid or resuscitation is present Do not install substitute parts or perform any unauthorized modifications to this instrument Contact the factory for instructions on how to return the instrument for authorized service and adjustment Warning Regarding Use With Photomultipliers and Other Detectors The front end amplifier of this instrument is easily damaged if a photomultiplier is used improperly with the amplifier When left completely unterminated a cable connected to a PMT can charge to several hundred volts in a relatively short time If this cable is connected to the inputs of the SR865 the stored charge may damage the front end amplifier To avoid this problem always discharge the cable and connect the PMT output to the SR865
169. nd selects relative i 1 or absolute i 0 mode for the strip chart cursor The cursor is visible only when the chart is paused If the chart is paused when CURREL transitions from 0 to 1 then the reference marker is located at the present cursor position set by PCUR If the chart is live then the marker is set to the most current bin location corresponding to the right and edge of the chart the marker however will not become visible until the chart is paused Example CURREL ON Set cursor to relative mode CURREL 1 CURREL Returns the cursor relative mode state i CURDISP i The CURDISP i command sets the cursor horizontal position display mode to date and time i 0 or interval time G 1 Example CURDISP 0 Set cursor horizontal position to date and time mode CURDISP Returns the cursor horizontal position display mode i CURBUG AVG MAX MIN i The CURBUG i command selects the strip chart cursor readout mode to mean i 0 maximum i 1 or minimum i 2 This command is the same as pressing EJ in the horizontal scale palette Each pixel represents data collected over the time elapsed for one pixel of horizontal scrolling set by the GSPD command CURBUG determines whether the cursor readout shows the mean maximum or minimum value of the data from within each pixel SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 123 Example CURBUG MAX Set cursor readout to maximum value CURBUG 1 CUR
170. nerate a voltage to measure on the dc inputs Connect a 50Q termination to the A input Procedure 1 2 3 PRESET press Save Recall then touch Recall default Touch Confirm Press Screen Layout multiple times if needed Set the screen to show 8 big numerical bar graphs For the CH1 and CH2 outputs repeat steps 2a through 2e a Connect the CH1 or CH2 output to the DVM Set the DVM to a 20 V range b Use the front panel to make the following adjustments Press the CH1 or CH2 Output lt Offset gt knob Turn the offset on Make sure the OFFSET LED is on Press and hold the CH1 or CH2 Output lt Offset gt knob Display the X or Y offset keypad c For each of the offsets in the table below repeat steps 2d and 2e Offsets 100 00 50 00 0 00 50 00 100 00 d Use the front panel to make the following adjustments Turn the CH1 or CH2 Output lt Offset gt knob to set the offset to the value in the table e Record the DVM reading For each Aux Output 1 2 3 and 4 repeat steps 3a through 3e a Connect the Aux Output on the rear panel to the DVM b Use the front panel to make the following adjustments SR865 DSP Lock in Amplifier ASRS Appendix G Performance Tests 195 4 5 6 SRS Press Aux Output to display the Aux Output keypad Touch an Out button to display the correct Aux Output in the keypad c For each output voltage in the table below repeat steps 3d and
171. nge error SR865 DSP Lock in Amplifier 124 Programming Chapter 4 CURINTERVAL The CURINTERVAL query returns the strip chart cursor horizontal position as a string This string is in the format 2d 16 07 30 00 In this example the cursor position is 2 days 16 hours 7 minutes and 30 seconds before the chart was paused corresponding to PCUR 171 on the 1 day division GSPD 15 chart When the cursor is in relative mode CURINTERVAL returns the interval between the cursor position and the marker position Performing the CURINTERVAL query when the display is not paused generates a parameter range error SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 125 FFT Screen Commands The SR865 has a basic FFT function accessable from the front panel by selecting the appropriate Screen Layout display remotely by setting DLAY FFT or DLAY BARFFT A more complete discussion of FFT mode can be found in Appendix B Note that while FFT configuration commands and queries may be sent at any time the FFT data query FCRY only returns valid data while the display is showing the FFT FFTR ADC MiXer FiLTer i FFTS i FFTO x SRS The FFTR i command sets the source for the FFT to input ADC i 0 post mixer PSD i 1 or post time constant filter G 2 Example FFTR FILT Set the FFT source to post time constant filter FFTR2 FFTR Returns the FFT source i The FFTS i command sets the FFT v
172. noise as the real filter The ENBW is determined by the time constant T and slope as shown below for normal RC type filters Slope ENBW 6 dB oct 1 4T 12 dB oct 1 8T 18 dB oct 3 32T 24 dB oct 5 64T The noise is simply the standard deviation root of the mean of the squared deviations of the measured X or Y values averaged over a period of time This involves computing the average Of Xvaiue KaD where Xvyajue 1s the current X output and X mean 1s the mean X output The noise result is the square root of the average The averaging time is roughly 200 time constants for example if the time constant is 100 mS the noise measurement would take about 20 seconds to settle If the Sync filter is enabled that also affects the noise averaging time The averaging time due to the Sync filter is 100 Sync filter periods The final noise averaging time is the longer of these two averaging times or 0 5 seconds Shorter averaging times yield a very poor estimate of the noise the mean varies rapidly and the deviations are not averaged well Longer averaging times while yielding better results take a long time to settle to a steady answer Noise and Sensitivity Beginning with firmware version 1 20 noise is calculated using floating point math and it is independent of the Sensitivity setting However earlier firmware versions used data from the scaled output values of X and Y In order to accurately compute the noise on instr
173. nput shield grounding Set voltage input range Set current input gain Query the signal strength low to overload 0 4 Set sensitivity from 1V 0 to 1 nV 27 Set time constant from 1 us 0 to 30 ks 21 Set filter slope from 6 dB 0 to 24 dB 3 Turn synchronous filter off on Turn advanced filtering off on Query the equivalent noise bandwidth Commands xi CH1 CH2 Output Commands COUT OCH1 OCH2 j XY RTHeta i CEXP X Y R j OFF X10 X100 i COFA X Y R j OFF ON i COFP X Y R jl x OAUT X Y R Ij CRAT X Y R j OFF ON i Aux Input and Output Commands OAUX j AUXV j 4 v NV UV MV V Auto Function Commands APHS ARNG ASCL Display Commands DBLK OFF ON i page 114 114 114 114 114 115 page 116 116 page 117 117 117 page 118 DLAY TREnd HISTory BARHist FFT BARFft BAREight i 118 DCAP CDSP DAT1 DAT2 DAT3 DAT4 j parameter i CGRF DAT1 DAT2 DAT3 DAT4 j OFF ON i GETSCREEN Strip Chart Commands GSPD i GSCL DAT1 DAT2 DAT3 DAT4 j x GOFF DAT1 DAT2 DAT3 DAT4 j x GAUT DAT1 DAT2 DAT3 DAT4 j GACT DAT1 DAT2 DAT3 DAT4 j GAUF DAT1 DAT2 DAT3 DAT4 j CGRF DAT1 DAT2 DAT3 DAT4 j OFF ON i GLIV OFF ON i PCUR i CURREL OFF ON i CURDISP i CURBUG AVG MAX MIN i FCRW L
174. nt After checking the setup repeat the test from the beginning to make sure that the test was performed correctly If the test continues to fail contact Stanford Research Systems for further instructions Make sure that you have the unit s serial number and firmware revision code handy Have the test record on hand as well A sRs SR865 DSP Lock in Amplifier 184 Performance Tests Appendix G dc Offset This test measures the dc offset of the signal input Setup Connect a 50Q terminator to the A input This shorts the input so the lock in s own dc offset will be measured Procedure 1 PRESET press Save Recall then touch Recall default Touch Confirm 2 Use the front panel to make the following adjustments Touch Fint to display the internal frequency keypad Enter a frequency of 1 00 Hz Press Input Range Down multiple times Set the input range to 10 mV Press Sensitivity Down multiple times Set the sensitivity to 10 mV 3 Wait at least 10 seconds then record the reading of R yellow 4 Use the front panel to make the following adjustments Press Couple Select DC coupling 5 Wait 10 seconds then record the reading of R yellow 6 This completes the dc offset test Enter the results of this test in the test record at the end of this section SR865 DSP Lock in Amplifier ASRS Appendix G Performance Tests 185 Common Mode Rejection This test measures the common mode rejection of t
175. o shunt the signal current and generate a voltage signal The lock in then measures the voltage across the shunt resistor Select the resistor value to keep the shunt voltage small so it does not affect the source current while providing enough signal for the lock in to measure Which current range should you use The current range determines the input current noise of the lock in as well as its measurement bandwidth Signals far above the input bandwidth are attenuated by 6 dB oct The noise and bandwidth are listed below Range Input Noise Bandwidth lpA 130 fA VHz 400 kHz 10nA 13fA VHz 2kHz The current to voltage preamplifier is always dc coupled SR865 DSP Lock in Amplifier 54 Basics Chapter 2 Intrinsic Random Noise Sources Random noise finds its way into experiments in a variety of ways Good experimental design can reduce these noise sources and improve the measurement stability and accuracy There are a variety of intrinsic noise sources which are present in all electronic signals These sources are physical in origin Johnson Noise Every resistor generates a noise voltage across its terminals due to thermal fluctuations in the electron density within the resistor itself These fluctuations give rise to an open circuit noise voltage V oise MS V4kKTRAL where k Boltzmann s constant 1 38x10 J K T is the temperature in kelvins typically 300 K R is the resistance in ohms and Af is the bandwidth in he
176. og outputs proportional to the measurement The effect of offsets and expands on the displayed values and the analog outputs will be explored Do This Explanation Disconnect all cables from the lock in Turn the power on while holding down the Local key The power switch is on the power entry module on the rear panel Connect the Sine Out on the front panel to the When the power is turned on with Local pressed the lock in returns to its standard default settings See the Standard Settings list in the Operation section for a complete listing of the settings The lock in defaults to the internal oscillator reference set at 100 000 kHz The default data screen is the Trend Graph The 4 displayed parameters default to X Y R and 9 Each parameter has a numeric display a bar graph and a trend graph The trend graph is a continuously autoscaling graph of the recent history of each parameter This data screen has no adjustments available A input using a BNC cable The default sine amplitude is 0 Vrms Thus the data displays will read 0 for X Y and R 0 will be just noise The lock in parameters shown in the info bar at the top of the screen may be entered using a numeric keypad simply by touching them 3 Touch Ampl in the info bar along the top of the screen Then 1 HOHOH mV Phase 0 000 cei 100 000 kelm 1414 00 mv o v The Sine Out amplitude is specified for differential output Sine Sine In thi
177. ohnson noise However there is excess noise in addition to Johnson noise which arises from fluctuations in resistance due to the current flowing through the resistor For carbon composition resistors this is typically 0 1 u V 3 uV of rms noise per Volt of applied across the resistor Metal film and wire wound resistors have about 10 times less noise This noise has a 1 f spectrum and makes measurements at low frequencies more difficult Other sources of 1 f noise include noise found in vacuum tubes and semiconductors Shot Noise Electric current has noise due to the finite nature of the charge carriers There is always some non uniformity in the electron flow which generates noise in the current This noise is called shot noise This can appear as voltage noise when current is passed through a resistor or as noise in a current measurement The shot noise or current noise is given by Loise LMS y 2GIAF where q is the electron charge 1 6x10 coulomb I is the RMS ac current or de current depending upon the circuit and Af is the bandwidth When the current input of a lock in is used to measure an ac signal current the bandwidth is typically so small that shot noise is not important Total Noise All of these noise sources are incoherent The total random noise is the square root of the sum of the squares of all the incoherent noise sources External Noise Sources In addition to the intrinsic noise sources discussed previously th
178. omputer s responsibility to keep up there is no retry on dropped packets Select the output Channels X X and Y R and 0 or X Y R and 0 and the desired Format 32 bit Float or 16 bit Integer Integers are scaled to the sensitivity The number of bits per sample point can range from 16 X integer to 128 XYR9 float This has an impact on the network bandwidth necessary to maintain the stream The Maximum Sample Rate can be set to Native depends on time constant or set to a fixed rate 1 25 MHz down to 1 2 Hz The total bandwidth required is the sample rate times the number of bits per point Choose the Packet Size to work with your host software The host Status is displayed Update Firmware Update Firmware Update Telnet Streaming Update oF SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 95 The SR865 s firmware can be updated via the USB memory stick Follow the directions that accompany the update to prepare the memory stick Insert the memory stick into the front panel and press Update to start the process A sRs SR865 DSP Lock in Amplifier 96 Operation Chapter 3 Rear Panel 5 Par es 5 x SMON ASRS STANFORD RESEARCH SYSTEMS e DESIGNED amp ASSEMBLED IN U S A 4 1 2 3 4 Power Entry Module The power entry module is used to fuse the ac line voltage input select the line voltage and block high frequency noise from entering or exiting the instrument Re
179. onfirm 2 Use the front panel to make the following adjustments Touch Fint to display the internal frequency keypad Enter a frequency of 1 0 kHz Press Input Range Down multiple times Set the input range to 10 mV Press Sensitivity Down multiple times Set the sensitivity to 100 nV Press Filter Slope adv Select 12 dB oct Press Couple Select DC coupling Press Ground Select shield Ground Press Config to display the data configuration screen Touch Datal Display to highlight the green data source Touch Xnoise to choose Xnoise for Datal Touch Close 3 Wait at least 1 minute for the reading of Xnoise green to stabilize Record the value of Xnoise 4 This completes the noise test Enter the results of this test in the test record at the end of this section SR865 DSP Lock in Amplifier ASRS Performance Test Record 197 SR865 Performance Test Record Serial Number Tested By Firmware Revision Date Equipment Used dc Offset Input Coupling Reading Upper Limit AC 0 500 mV DC 0 500 mV Common Mode Rejection Frequency Reading Upper Limit 1 kHz 30 uV Sine Output Amplitude and Flatness Sine Output Ampl Lower Limit Reading Upper Limit 1 00 V 0 490V_ A 0 5100 V 300 mV 147 0mV B 153 0 mV 100 mV 49 0mV C 51 0 mV 30 mV 14 70mV D 15 30 mV 10 0 mV 4 98mV E 5 10 mV 1 00 mV 0 490 mV F 0 510 mV Sine Ampl Frequency Lower Limit Reading Upper Limit 100 mV 10 kHz
180. ons in the SR865 are output functions they do not affect the calculation of R or 0 R has its own output offset expand and ratio Phase ranges from 180 to 180 regardless of the sensitivity When CH2 outputs a voltage proportional to 0 the output scale is 18 Volt or 180 10 V The phase bar graph and numeric readout scales are also unaffected by the Sensitivity Phase has no offset ratio or expand capability To offset phase simply use Auto Phase To expand phase expand the value of Y in quadrature A sRs SR865 DSP Lock in Amplifier 72 Operation Chapter 3 Offset Output offsets are reflected in the displays For example when the X output is offset to zero the displayed value will drop to zero also This means that the bar graph and numeric readout both drop to zero In addition if X is being charted on the graph its graph will drop to zero Any display which is showing a quantity which is affected by a non zero offset will display a highlighted Offset indicator within its display Ratio The X Y and R may be normalized to an Aux Input voltage This is called a ratio measurement An output ratio is reflected in the displays The numeric values bar graphs and strip charts all reflect the normalized output Any display which is showing a quantity which is affected by a ratio will display a highlighted Ratio indicator within its display Expand Output expands do not increase the displayed numeric values of X Y or R E
181. onverter s range as indicated by the signal strength LEDs The sensitivity determines the scale factor for the analog outputs CH1 and CH2 as well as the numeric readouts and bar graphs The sensitivity does not affect the measurement values it simply determines how much signal corresponds to a full scale 10V output from CH1 and CH2 outputs and a 100 bar graph It also sets the scale for the 5 digit numeric displays The sensitivity should be viewed as an output function only Reference The reference settings configure the lock in reference frequency and source In addition to internal and external reference the SR865 includes dual reference detect at Ifin fextl and chop lock an SR540 chopper TO the SR865 fin modes The SR865 can be synchronized to an external 10 MHz frequency reference from another SR865 or other source This allows multiple SR865 s to run in phase sync with each other in internal reference mode SR865 DSP Lock in Amplifier ASRS Chapter 1 Getting Started 3 The sine output from the SR865 is differential This provides improved performance at low amplitudes A variable dc offset is provided in both differential and common mode Use either sine out for single ended excitation Outputs The CH1 output can be proportional to either X or R while the CH2 output can be proportional to Y or 0 Output functions include offset up to 999 of the sensitivity expand up to x100 and ratio These functions a
182. orizontal scale palette repeatedly to zoom out Phase 0 000 deg E 999998 kiz 1 repeatedly to zoom back in 14 Press Screen Layout once to change the display to the half screen strip chart SRS O because these values are affected by signal overload Conditions such as overload or reference unlock are displayed in violet along the bottom This provides visual feedback about the validity of the data in those regions The overload condition goes away Zoom out on the horizontal time scale to show more and more history When the region where the signal was overloaded is shown the overload is indicated by the violet points along the bottom edge The half screen strip chart behaves the same as the full screen version The only difference is that the numeric and bar graphs are full sized SR8 amp 65 DSP Lock in Amplifier 18 Getting Started Chapter 1 99 9998 khz HM 4 Amel Phase 0 000 deg Fint 15 Press Screen Layout again to change the display to the full screen FFT Phase 0 000 deg 99 9998 kiz HM Ar 7 018 dB 97 656 kHz 4 9902 MHZ Src Rawanc dB div 40 4 H2 Div 6259 Avgs 1 16 Press Screen Layout again to change the display to the half screen FFT 17 Press Screen Layout again to change the display to the full numeric display Phase 0 000 ogl 999998 uz HM 1 Out 1 0 000 VTOt2 ATE I SR865 DSP Lock in Amplifier 0 000 V Cut4 0 000 V i i
183. ow pass filter removes the 2 sig component leaving a dc result proportional to A ig In some experiments there is an additional modulation of the signal This results from the mixing of 2 frequencies in the experiment Often one of the frequencies is much lower than the other Let s call these frequencies Ocar and moa for carrier and modulation and ASSUME Omod lt lt Ocar If the carrier signal is Acar SIM Oat then the experiment modulates this by multiplying by the modulation signal resulting in Aas Amod l Asar SINO oat X Anoa SINO moat cos a nod y COSO a O nod ir car The experimental signal consists of equal amplitude components at the sum and difference frequencies Two Lock in Detection This signal can be measured using 2 lock in amplifiers The signal is input to the first lock in which is set to detect at car The output of this lock in will contain sum and difference components from both signal components One of these will be SR865 DSP Lock in Amplifier 176 Dual Reference Detection Appendix E foe 2 SINGO O mod a Dear 1 Fat Sin Cg The time constant is adjusted to remove the high frequency sum components at 20 and the analog output of this first lock in is the signal input to the second lock in The second lock in is set to detect at mog The output of this second lock in is mod A A SIn poat X SiM 4f c08 20 540 mod The second lock in time constant is
184. perates from a 100V 120V 220V or 240V nominal ac power source having a line frequency of 50 or 60 Hz Before connecting the power cord verify that the LINE VOLTAGE SELECTOR card located in the rear panel fuse holder is set so that the correct ac input voltage value is indicated by the white dot Conversion to other ac input voltages requires a change in the voltage selector card position and fuse value See Appendix F page 177 for detailed instructions Line Fuse Verify that the correct line fuse is installed before connecting the line cord For 100V 120V use a 1 Amp fuse and for 220V 240V use a 1 2 Amp fuse See Appendix F page 177 for detailed fuse installation instructions Line Cord The SR865 has a detachable three wire power cord for connection to the power source and to a protective ground The exposed metal parts of the instrument are connected to the outlet ground to protect against electrical shock Always use an outlet which has a properly connected protective ground Power Cord Grounding A chassis grounding lug is available on the back panel of the SR865 Connect a heavy duty ground wire 12AWG or larger from the CHASSIS GROUND lug directly to a facility earth ground to provide additional protection against electrical shock Grounded BNC shields are connected to the chassis ground Do not apply any voltage to the grounded shields The A and B signal input shields are connected to chassis ground through resistors a
185. plastic retaining hook but rather shifted towards the right hand side see image above left If the fuse is not propertly aligned the fuse holder will not seat properly and line voltage will not be provided to the instrument 6 Replace fuse holder into housing fuse first Be sure to align the circular alignment opening on the fuse holder with the orienting pin within the power entry module 7 Swing and push to snap the door back in place SR865 DSP Lock in Amplifier ASRS Performance Tests 181 Appendix G Performance Tests Introduction The performance tests described in this section are designed to verify with a high degree of confidence that the unit is performing correctly The results of each test may be recorded on the test sheet at the end of this section Serial Number and Firmware Revision If you need to contact Stanford Research Systems please have the serial number of your unit available The 6 digit serial number is printed on a label affixed to the rear panel Press and hold Local and then Auto Range at the same time to display the unit serial number and version numbers The Screen Layout key exits this screen Preset Throughout this section it will be necessary to preset the lock in into a known default state To do this press Save Recall then touch Recall default Touch Confirm to reset the SR865 to its default settings Each test generally starts with a preset This procedure will
186. play a reasonable amount of bar graph is sufficient SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 71 CH1 and CH2 Outputs Offset Ratio and Expand CH 1 OUTPUT Overload Overload aX x10 sY x10 aR x100 Las x100 ka s OFFSET a RATIO Push to O Hold for more 500 lt 20 mA 500 lt 20 mA Analog Outputs Analog voltages proportional to X Y R and 6 can be output from the SR865 The X and Y rear panel outputs always output X and Y The CH1 output voltage is proportional to either X or R The CH2 output voltage is proportional to either Y or 8 The output voltages are determined by X Y orR Sensitivity Ratio In 1 000 V The output is normally 10 V for an input signal equal to the sensitivity The offset subtracts a percentage of full scale up to 999 from the output Expand multiplies the difference by a factor of 1 10 or 100 This result may be divided by a ratio input Aux Input voltage An Aux Input voltage of 1 000 V corresponds to unity Offset Output x Expand x 10V Outputs which would exceed 10 V generate an Output Overload and the red output Overload LED will light Any corresponding displayed numeric value bar graph will indicate Scale in the display The actual measurement is unaffected since it is done in floating point and has no overload Data displayed in the strip chart will still be accurate The X and Y offset expand and ratio functi
187. psa2 i Vig sin8 Now we have two outputs one proportional to cos and the other proportional to sin0 If we call the first output X and the second Y X Vig cosO Y Vig sin these two quantities represent the signal as a vector relative to the lock in reference oscillator X is called the in phase component and Y the quadrature component This is because when 0 0 X measures the signal while Y is zero By computing the magnitude R of the signal vector the phase dependency is removed RaQ Y Vig SR865 DSP Lock in Amplifier 40 Basics Chapter 2 R measures the signal amplitude and does not depend upon the phase between the signal and lock in reference A dual phase lock in such as the SR865 has two PSD s with reference oscillators 90 apart and can measure X Y and R directly In addition the phase O between the signal and lock in reference can be measured according to 0 tan Y X What Does a Lock in Measure So what exactly does the SR865 measure Fourier s theorem basically states that any input signal can be represented as the sum of many sine waves of differing amplitudes frequencies and phases This is generally considered as representing the signal in the frequency domain Normal oscilloscopes display the signal in the time domain Except in the case of clean sine waves the time domain representation does not convey very much information about the various frequencies that make up the sig
188. r edge In this mode the SINE OUT provides a sine wave phase locked to the external reference Phase locking is accomplished digitally by the SR865 The internal oscillator may be used without an external reference In the Internal Reference mode the frequency is set in the lock in and the SINE OUT provides the excitation for the experiment The phase locked loop is not used in this mode The BlazeX output on the rear panel can be configured to provide the sync output The internal oscillator s rising zero crossings are detected and the output is a square wave Reference Oscillators and Phase The internal oscillator sine wave is not the reference signal to the phase sensitive detectors The SR865 computes a second sine wave phase shifted by 0 from the internal oscillator and thus from an external reference as the reference input to the X phase sensitive detector This waveform is sin t Oet The reference phase shift is adjustable in 0 001 increments SR865 DSP Lock in Amplifier ASRS Chapter 2 Basics 43 The input to the Y PSD is a third sine wave computed by the SR865 shifted by 90 from the second sine wave This waveform is sin t Oef 90 The phase shifts Oef and the 90 shift are exact numbers and accurate to better than 0 000001 Neither waveform is actually output in analog form since the phase sensitive detectors are actually digital multipliers inside the SR865 Phase Jitter When an external r
189. r Limit Reading Upper Limit CH1 100 00 9 90 V 10 10 V 50 00 4 95 V 5 05 V 0 00 0 010 V 0 010 V 50 00 5 05 V 4 95 V 100 00 10 10 V 9 90 V Test Record sheet 2 of 4 SR865 DSP Lock in Amplifier Appendix G Performance Test Record 199 Output Offset Lower Limit Reading Upper Limit CH2 100 00 9 90 V 10 10 V 50 00 4 95 V 5 05 V 0 00 0 010 V 0 010 V 50 00 5 05 V 4 95 V 100 00 10 10 V 9 90 V Output Voltage Lower Limit Reading Upper Limit AUX OUT 1 10 000 9 90 V 10 10 V 5 000 4 95 V 5 05 V 0 000 0 010 V 0 010 V 5 000 5 05 V 4 95 V 10 000 10 10 V 9 90 V Output Voltage Lower Limit Reading Upper Limit AUX OUT 2 10 000 9 90 V 10 10 V 5 000 4 95 V 5 05 V 0 000 0 010 V 0 010 V 5 000 5 05 V 4 95 V 10 000 10 10 V 9 90 V Output Voltage Lower Limit Reading Upper Limit AUX OUT 3 10 000 9 90 V 10 10 V 5 000 4 95 V 5 05 V 0 000 0 010 V 0 010 V 5 000 5 05 V 4 95 V 10 000 10 10 V 9 90 V Output Voltage Lower Limit Reading Upper Limit AUX OUT 4 10 000 9 90 V 10 10 V 5 000 4 95 V 5 05 V 0 000 0 010 V 0 010 V 5 000 5 05 V 4 95 V 10 000 10 10 V 9 90 V Input Voltage Lower Limit Reading Upper Limit AUX IN 1 10 000 10 10 V 9 90 V 5 000 5 05 V 4 95 V 0 000 0 010 V 0 010 V 5 000 4 95 V 5 05 V 10 000 9 90 V 10 10 V A sRs SR865 DSP Lock in Amplifier 200 Performance Test Record Appendix G Input Voltage Lower Limit Reading Upper Limit AUX IN 2 10 000 10 10 V
190. r a TTL logic signal The first case is called External Sine The input is ac coupled above 1 Hz and the input impedance is 1 MQ A sine wave input greater than 200 mV pk is required Positive zero crossings are detected and considered to be the zero for the reference phase shift TTL reference signals can be used at all frequencies up to 2 5 MHz For frequencies below 1 Hz a TTL reference signal is required Many function generators provide a TTL SYNC output which can be used as the reference This is convenient since the generator s sine output might be smaller than 200 mV or be varied in amplitude The SYNC signal will provide a stable reference regardless of the sine amplitude When using a TTL reference the reference input trigger can be set to Pos TTL detect rising edges or Neg TTL detect falling edges In each case the internal oscillator is locked at zero phase to the detected edge Internal Oscillator The internal oscillator in the SR865 is basically a 2 5 MHz function generator with sine and sync outputs The oscillator generates a digitally synthesized sine wave The internal oscillator sine wave is output differentially at the SINE OUT BNC s on the front panel An attenuator sets the amplitude of the output to a value between 1 nV and 2 V rms When an external reference is used this internal oscillator sine wave is phase locked to the reference The rising zero crossing is locked to the detected reference zero crossing o
191. r common mode 3 digits or 0 1 mV whichever is greater 6 V sum of dc offset and peak amplitude Logic level sync on rear panel via BlazeX output 4 data channels are displayed and graphed green blue yellow orange Each data channel can be assigned any of these data sources X Y R 0 Aux In 1 4 Aux Out 1 2 Xnoise Y noise Sine Out Amplitude Sine Out DC Level reference phase fint OF fext All data sources are continuously stored at all chart display time scales The complete stored history of any data source can be displayed at any time X Y and R may be offset up to 999 of the sensitivity X and Y may be ratioed by Aux In 3 R may be ratioed by Aux In 4 X Y and R may be expanded by x10 or x100 1 Mpoints internal data storage Store X X and Y R and 8 or X Y R and 6 at sample rates up to 1 25 MHz This is in addition to the data histories for the chart display Realtime streaming of data either X X and Y R and 8 or X Y R and 6 at Sample rates up to 1 25 MHz over Ethernet interface One of the following parameters may be scanned fin Sine Out Amplitude Sine Out DC Level Aux Out 1 or 2 Input ADC demodulator output or filter output 1024 bins exponential rms Proportional to X or R 10 V full scale thru 50 Q Proportional to Y or 6 10 V full scale thru 50 Q Proportional to X and Y 10 V full scale thru 50 Q rear panel Low latency output of X 2 0 V full scale or logic level referenc
192. r gain Therefore the only question is how to set the gain of the input amplifier In principle the gain only needs to be high enough such that the input noise of the signal and the amplifier is greater than the input noise of the A D converter Increasing the analog gain beyond this point will not reduce the output noise Input Range The Input Range sets the analog gain in the SR865 The settings reflect the largest signal at the input before the amplifier overloads Setting the Input Range to a smaller value increases the analog gain The signal strength LEDs on the front panel indicate how much of the A D range is being used The general rule is to decrease the Input Range as much as possible without overloading the amplifier This increases the A D range used and optimizes the output noise performance of the lock in At an Input Range of 10 mV no additional gain is available since noise performance will not improve with more gain Remember we have defined dynamic reserve as a property of the input signal generated by the experiment The ratio of noise to signal at the BNC is determined by factors outside the lock in The Input Range must be set to accommodate the largest signal present at the input whether it is at f or is just noise Once the Input Range is set use the Time Constant filters to achieve the best results Then set the Sensitivity to optimize the bar graphs and numeric displays as well as the X Y and R outputs SR865 DSP Loc
193. r through the VISA Read STB function but not with the STB query until the MAV bit becomes set Note that it may take several seconds before MAV is set Once the MAV bit is set in the Status Byte the user may then read the binary block data The binary block is in the following format NCCCCCXXXXXXX Where is the hash character character code 35 n is a single digit specifying the number of digits to follow in the block length count ccecce is the n digit integer size of the binary block to follow and xxxxxxx is the ccccc byte binary image file itself This binary image should be saved as a BMP format file Example GETSCREEN Request a screen capture file The SR865 responds with 53521 9XXXXXXX corresponding to a 35219 byte BMP file image the xxxxxxx data SR865 DSP Lock in Amplifier ASRS 120 Programming Chapter 4 Strip Chart Commands GSPD i The GSPD i command sets the horizontal time div according to the table below i time div i time div 0 0 5 s 9 10 min 1 1s 10 30min 2 2s 11 1 hour 3 5s 12 2 hour 4 10s 13 6 hour 5 30 s 14 12 hour 6 1 min 15 1day 7 2 min 16 2day 8 5 min Example GSPD 6 Set the strip chart to 1 min div horizontal scale GSPD Returns the horizontal scale i GSCL DAT1 DAT2 DAT3 DAT4 j x The GSCL j x command sets the vertical scale of data channel j to x div The scale will be set to the nearest allowed scale in a 1 2 5 sequence
194. re 640 pixels across 10 divisions of the graph Thus there are 64 pixels in each division At a scale of 0 5 s div each pixel represents about 8 ms of data At a scale of 1 min div each pixel represents about 1s of data This binning is fundamental to the SR865 strip chart display All time scales are stored all of the time This allows the horizontal scale to change without re acquiring any data The caveat is that all graphs are drawn with the most recent point at the right hand edge Zooming in and out changing the horizontal scale always displays the most recent point at the right edge There is no zooming in about a point in the distant past All parameters which may be assigned to a data channel are continuously recorded even when they are not displayed This means that historical data can be viewed for all parameters simply by assigning them to a data channel and viewing the strip chart Strip charts may be paused When the graph is paused the cursor can be used to readout data values Data storage continues in the background while the graph is paused When live scrolling is resumed the graph is redrawn so the most recent point is once again at the right edge Graph Scale Bar Strip Chart displays have a scale bar at the bottom of the screen This bar shows tiles indicating the vertical scale per division for the 4 data channels green blue yellow and orange and the horizontal time scale per division white Touch a data ch
195. re generally only used when the CH1 or CH2 outputs are being used to drive other parts of an experiment A sRs SR865 DSP Lock in Amplifier 4 Getting Started Chapter 1 SR865 Touchscreen The SR865 screen displays the lock in outputs both numerically and graphically Touch buttons and icons are used to adjust the data displays as well as enter certain lock in parameters Screen Layout Press Screen Layout to cycle through the different screen layouts 90 40 43 43 20 70 dB 595 703 kHz Half Screen Strip Chart Phase 595 700 ite Horm 81 80 59 50 101 15 40 22 91 93 100 34 20 16 dB 595 703 KHz c d dv x 4 He div 6 Half Screen FFT Big Numbers The SR865 displays up to 4 channels at a time in green blue yellow and orange Each channel is assigned a parameter using the Config key Parameters are chosen from X SR865 DSP Lock in Amplifier ASRS Chapter 1 Getting Started 5 SRS Y R 0 detected fins fex phase setting Sine Amplitude DC Level any Aux Input Aux Output 1 or 2 Xnoise or Ynoise Displayed parameters can be re assigned at any time Data is being stored for all possible parameters all of the time Info Bar and Numeric Entry Each of the data screens always displays a lock in info bar across the top 0 000 deg 400 000 wiz aT JA This bar always shows tiles displaying the phase frequency detect harmonic sine out amplitude and dc offset of the s
196. re is no differential noise pickup between the cables The signal coupling and input impedance is now determined by the preamp The SR865 does NOT compensate for the gain of the preamp The preamps will default to their maximum gain Measurements made by the SR865 with a preamp need to be divided by the gain of the preamp A sRs SR865 DSP Lock in Amplifier Programming 101 Chapter 4 Programming Introduction The SR865 DSP Lock in Amplifier may be remotely programmed via the RS 232 GPIB IEEE 488 USB or ethernet interfaces Any computer supporting one of these interfaces may be used to program the SR865 All interfaces can receive commands at all times and the SR865 will send responses to the querying interface only A few specialized commands such as interface status and data streaming are interface specific Most instrument commands are common to all interfaces Communicating With GPIB The SR865 supports the IEEE 488 2 2004 interface standard The SR865 s device address must be set in the system menu hold Calc system Communicating With RS 232 The RS 232 interface baud rate and parity must be set in the system menu hold Calc system The RS 232 word length is always 8 bits The transmit terminator character s may also be specified Communicating With USB The SR865 is a Test and Measurement Class instrument and easily interfaces with LabVIEW NI VISA and Matlab Communicating With Ethernet The SR865 supports
197. re phase in degrees Block diagram A simplified block diagram of the SR865 s lock in circuit is shown below and explained in the following sections Timebase input Timebase Timebase in out 10 MHz crystal Clock generator Digital signal processor Reference input Phase e Sine TTL locked Internal CPU L loop oscillator Attenuator Ref in 1O Ch 2 out 1MQ Sine I Sine 500Q 1MQ amp L Sync DAC Out 500 Bref Low pass filters gt F p RC Blazex S DAC gt Blazex Signal input Filter i T out AC DC couple Gain m RC FIR fs Differential x Offset A amplifier Expand A input Binput Input IDAC gt X out Gnd p a ay amp JDAC gt Ch 1 out Float R 8 x 1022 210kQ gt 4 lt ADC amp Calc g ine R x0 1 Y PSD EE ange x0 3 IIR Sync 1009 x100 Filter Filter DAc Y out linput i 1kOQ i Virtual r j Signal a ground gt onito Sensitivity A sRs SR865 DSP Lock in Amplifier 42 Basics Chapter 2 The Reference Oscillator A lock in amplifier requires a reference oscillator phase locked to the signal frequency In general this is accomplished by phase locking an internal oscillator to an externally provided reference signal This reference signal usually comes from the signal source that is providing the excitation to the experiment Reference Input The SR865 reference input can trigger on an analog signal like a sine wave o
198. re should not be a large loop area enclosed by the two cables Large loop areas are susceptible to magnetic pickup Common Mode Signals Common mode signals are those signals which appear equally on both center and shield A or on the center of both A and B A B With either connection scheme it is important to minimize both the common mode noise and the common mode signal Notice that the signal source is held near ground potential in both illustrations above If the signal source floats at a nonzero potential the signal which appears on both the A and B inputs will not be perfectly cancelled The common mode rejection ratio CMRR specifies the degree of cancellation For low frequencies the CMRR of 100 dB indicates that the common mode signal is canceled to 1 part in 10 Even with a CMRR of 100 dB a 100 mV common mode signal behaves like a 1 uV differential signal This is especially bad if the common mode signal is at the reference frequency this often happens due to SR865 DSP Lock in Amplifier ASRS Chapter 2 Basics 53 SRS ground loops The CMRR decreases by about 6 dB octave 20 dB decade starting at around 1 kHz ac vs dc Coupling The signal input can be either ac or dc coupled The ac coupling passes signals above 160 mHz 0 16 Hz and attenuates signals at lower frequencies ac coupling also degrades the common mode rejection of differential inputs At low signal frequencies dc coupling is required A dc signal
199. rence Ext Sine Reference Single ended A or differential A B 1 nV to 1 V voltage input 1 fA to 1 uA current input 10 MQ 25 pF ac gt 1 Hz or dc coupled 10 mV to 1 V peak max input before overload 1 below 200 kHz and 2 to 2 MHz signal amplitude lt 30 of input range 2 5 nV NHz at 1 kHz 10 mV input range typical Greater than 90 dB at 1 kHz dc Coupled 80 dB below 100 kHz 60 dB above 100 kHz Greater than 120 dB 1 uA or 10 nA 1 mHz to 2 MHz specified operates to 2 5 MHz 10 MHz In Out phase locks the internal frequency to other SR865 units Minimum 2 V logic level rising or falling edge 400 mV pk pk minimum signal ac coupled gt 1 Hz Ext Reference Input Impedance 1 MQ or 50 Q Acquisition Time Phase Setting Resolution Phase Noise Phase Drift Harmonic Detect Dual F Reference Chopper Reference Demodulator dc Stability Time Constants Low Pass Filters Filter Slope Synchronous Filter Harmonic Rejection Low Latency Output Internal Oscillator Frequency Frequency Accuracy External Timebase Frequency Resolution 2 cycles 5 ms or 40 ms whichever is greater 360 222 deg Ext TTL reference lt 0 001 rms at 1 kHz 100 ms 12 dB oct typical Internal reference lt 0 0001 rms at 1 kHz 100 ms 12 dB oct Sine Out to Signal In 200 mVrms lt 0 002 C below 20 kHz dc coupled input lt 0 02 C below 200 kHz lt 0 2 C below 2 MHz Detect at N xfrer where N l
200. ress Input Range Down to select 30 mV Press Input Range Up to select 100 mV 11 Press Sensitivity Down multiple times to select 50 mV 12 Press Time Constant Down multiple times to SRS converter without overload In this case the Input Range should change to 10 mV The Input Range is the peak allowable voltage at the input whether noise or signal In this case the signal is 5 mVrms or 7 mVpk so 10 mV is the best allowed setting The signal strength increases from the minimum yellow to something in the middle The peak signal exceeds the input range so the Input Range Overload LED lights Ovld indicators also appear on the screen when a displayed value is invalidated by an input overload During Input Range Overload the Auto Range function selects the 1 V range From the 1 V range the Auto Range function changes the Input Range to maximize the signal at the A D converter without overload The Input Range should change to 100 mV Settings which have many options such as Input Range Time Constant and Sensitivity are changed with up and down keys The setting is indicated by LEDs The peak signal exceeds the input range so the Input Range Overload LED lights Ovld indicators also appear on the screen when a displayed value is invalidated by an input overload The Sensitivity is indicated with 3 LEDs In this case the 5 x10 and mV should be lit The Sensitivity sets full scale for the bar graphs
201. ring would be required This is increasingly useful the lower the reference frequency Imagine what time constant would be needed at 0 001 Hz SR865 DSP Lock in Amplifier 46 Basics Chapter 2 In the SR865 synchronous filters are available at detection frequencies up to 4 8 kHz At higher frequencies synchronous filters are not required because 2f is easily removed with the other lowpass filters The synchronous filter is applied after the other lowpass filters have removed the noise components from the PSD output leaving just the synchronous signals for the synchronous filter to remove This combination of filters removes all multiples of the reference frequency and provides overall noise attenuation as well Synchronous Filters and Sensitivity It is important to note that the synchronous filter requires the Sensitivity to be set appropriately This is because the synchronous filter is an integer filter and requires the floating point output to be converted to integer values This conversion is based upon the Sensitivity The output of the synchronous filter is then converted back to floating point Choose a Sensitivity as if you will be using the analog X and Y outputs This will typically result in the optimum scale If the Sync filters are overloaded the Sync error indicators on the display will light Outputs and Scales The SR865 has X and Y outputs on the rear panel and Channel 1 and 2 CH1 and CH2 outputs on the front panel X
202. rise time and overshoot and different frequency domain characteristics such as noise bandwidth and attenuation slope These characteristics are inextricably linked It is impossible to arbitrarily specify both the frequency response of the filter and its time domain response That is why when we impose frequency domain restrictions such as brick wall type attenuation characteristics they come back to haunt us in the form of poor time domain behavior such as large overshoot Lock in outputs are fundamentally time domain outputs and this limits how aggressively we can specify the frequency domain characteristics of our time constant filters Nevertheless there are filters other than simple RC filters which offer quantifiable benefits to lock in users What are these benefits For two filters with the same noise bandwidth i e whose outputs would be equally noisy if the input was white noise a lock in user would always prefer a filter with a faster transient response and minimal overshoot And a lock in user would always prefer a filter with higher stop band attenuation to ensure greater suppression of out of band spurs These are the characteristics we will concentrate on To use the advanced filters in place of the RC filters press and hold the Slope adv key until the Advanced LED turns on Brief presses of Slope adv cycles the number of poles from 1 to 4 6 to 24 dB Another press and hold of the Slope adv key reverts the filters ba
203. rs at 12 and 24 dB oct Each stage of the Gaussian filter has the same ENBW as a single stage of the RC filter Each stage of the Linear Phase filter has the same roll off as a single stage of the RC filter 0 T T T T T T is ap T T T T T 1 joie Saal Advanced filters aan more bandwidth Gaussian FIR x Linear Phase 20 j i i bakai s Sx p S N ao a E N 2 40 duradusddindied 3 hous i a o a D E 60 i Peel i i A OE ON T EE EA f en ifii R Li I a Ea ETIA 23dB better S fy I a Tejection PTR LLL A PiE in i l r 80 f pyr i il al vei LEME i poe ees 0 1 1 10 100 Frequency f 0 H rm T T i 1 F ij T T T T T i E T T T T T T TTT ie a Oe a a ee mn TPS Advanced filters 24dB oct Tahar more bandwidth RC i Gaussian FIR al 320 Linear Phase N 40 H 4 60 4 oO z 2 c 80 H o re k3 Me faa 3 100 bs E A 120 i eu H es N i Er l A Gaussian i 140 H l fit Jf 46dB better N I lf rejection WIN MVM i 160 H L i basje i it it SVESTAN E peepee py Me 0 1 1 10 100 Frequency f SR865 DSP Lock in Amplifier Appendix A Advanced Filters 159 ASRS Step Response Graphs These graphs illustrate the step response of the different filters at 12 and 24
204. rtz Af is the equivalent noise bandwidth of the measurement Since the input signal amplifier in the SR865 has a bandwidth of approximately 3 MHz the effective noise at the amplifier input is V noise 220VR nVrms or 1 1VR uV pk pk This noise is broadband and if the source impedance of the signal is large can determine the required Input Range of the lock in The amount of noise measured by the lock in is determined by the measurement bandwidth Remember the lock in does not narrow its detection bandwidth until after the phase sensitive detectors In a lock in the equivalent noise bandwidth ENBW of the low pass filter time constant sets the detection bandwidth In this case the measured noise of a resistor at the lock in input typically the source impedance of the signal is simply Voice rms 0 13VR VENBWnV where R is in ohms and ENBW is in hertz The ENBW is determined by the time constant T and slope as shown below for normal RC type filters Slope ENBW 6 dB oct 1 4T 12 dB oct 1 8T 18 dB oct 3 32T 24 dB oct 5 64T SR865 DSP Lock in Amplifier ASRS Chapter 2 Basics 55 The signal amplifier bandwidth determines the amount of broadband noise that will be amplified This affects the Input Range analog signal gain The time constant sets the amount of post demodulation noise which will be measured at the reference frequency 1 f Noise Every 10 Q resistor no matter what it is made of has the same J
205. s case each BNC has an amplitude of 50 mV rms with a 50Q output The lock in input is high impedance so the output of each BNC is doubled and the lock in measures 100 mV Since the phase shift of the sine output is very close to zero X green should read about A ee 1 0 1000 and Y blue should read close to 0 0000 V A2 A4 1 00 pv 1 23 mV 0 SR865 DSP Lock in Amplifier Chapter 1 Getting Started 21 4 Connect the CH1 Output on the front panel to the DVM Set the DVM to read de Volts 5 Press Screen Layout twice to show the half screen strip chart 6 Touch the green scale tile X to display its scale palette LEN EA to auto scale the green X trace 0 000 deg 100 000 kz 4 Touch the green highlighted scale tile again to dismiss the scale palette 7 Press Sensitivity Down to select 500 mV SRS The CH1 output defaults to X The output voltage with ratio disabled is given by X CH1 _ Sensitivity a ones x Expand x 10V In this case X 0 1 V Sensitivity 1 V the offset is zero percent and the expand is 1 The output should thus be 1 V or 10 of full scale Note that the bar graph for X and R is at 10 The Sensitivity 1 V sets the full scale for the bar graphs of X Y and R Now let s look at how the Sensitivity affects the different displays EA auto scales the trace keeping zero at the center The zero location is indicated by the small green tr
206. s are less noisy with 2 poles of filtering With 4 poles of low pass filtering even this short time constant attenuates the 2f component reasonably well and provides steady readings Let s leave the filtering short and the outputs noisy for now This turns on synchronous filtering whenever the detection frequency is below 4 8 kHz Synchronous filtering effectively removes output components at multiples of the detection frequency At low frequencies this filter is a very effective way to remove 2f without using extremely long time constants The outputs are now quiet and steady even though the time constant is very short The response time of the synchronous filter is equal to the period of the detection frequency 1 ms in this case This concludes this measurement example You should have a feeling for the basic operation of the front panel Basic lock in parameters have been introduced and you should be able to perform simple measurements ASRS Chapter 1 Getting Started 13 Using Displays This measurement is designed to use the internal oscillator and an external signal source to explore some of the display types You will need a synthesized function generator capable of providing a 500 mVrms sine wave at 100 000 kHz BNC cables and a terminator appropriate for the generator function output Specifically you will display the lock in outputs when measuring a signal close to but not equal to the internal referenc
207. s at all frequencies The ideal lock in only responds to noise at the reference frequency Noise at other frequencies is removed by the low pass filter following the multiplier This bandwidth narrowing is the primary advantage that a lock in amplifier provides Only inputs at the reference frequency result in an output RMS or Peak Lock in amplifiers as a general rule display the input signal in Volts RMS When the SR865 displays a magnitude of 1V rms the component of the input signal at the reference frequency is a sine wave with an amplitude of 1 Vrms or 2 8 V pk pk SR865 DSP Lock in Amplifier ASRS Chapter 2 Basics 41 Thus in the previous example with a 2 V pk pk square wave input the SR865 would detect the first sine component 1 273 sin wt The measured and displayed magnitude would be 0 90 V rms e g 1 273 V2 Degrees or Radians In this discussion frequencies have been referred to as f Hz and 2zf radians sec This is because people measure frequencies in cycles per second and math works best in radians For purposes of measurement frequencies as measured in a lock in amplifier are in Hz The equations used to explain the actual calculations are sometimes written using to simplify the expressions Phase is always reported in degrees Once again this is by custom Equations written as sin t 0 are written as if O is in radians mostly for simplicity Lock in amplifiers always manipulate and measu
208. s the frequency components of the input signal Now here s the interesting part The original digital time record comes from discrete samples taken at the sampling rate The corresponding FFT yields a spectrum with discrete frequency samples In fact the spectrum has half as many frequency points as there are time points Remember Nyquist s theorem Suppose that you take 1024 samples at 256 kHz It takes 4 ms to take this time record The FFT of this record yields 512 frequency points but over what frequency range The highest frequency will be determined by the period of 2 time samples or 128 kHz The lowest frequency is just the period of the entire record or 1 4 ms or 250 Hz Everything below 250 Hz is considered SR865 DSP Lock in Amplifier 162 The FFT Display Appendix B to be dc The output spectrum thus represents the frequency range from dc to 128 kHz with points every 250 Hz Advantages and limitations The advantage of this technique is its speed The entire spectrum takes only 4 ms to measure The limitation of this measurement is its resolution Because the time record is only 4 ms long the frequency resolution is only 250 Hz Suppose the signal has a frequency component at 260 Hz The FFT spectrum will detect this signal but place part of it in the 250 Hz point and part in the 500 Hz point In fact the way to measure the signal accurately is to lengthen the time record and change the resolution of the spectrum Of course ev
209. s the operating mode for the output attenuators when scanning dc level to automatic i 0 or fixed G 1 Normally the sine out dc level hardware will automatically switch one or more balanced attenuators into the dc level signal path to optimize performace for the commanded level During scans however it can be helpful to force the attenuators to remain in a fixed configuration inhibiting the relay switching that would otherwise introduce transients in the sine output Example SCNDCATTN 1 Set the scan dc level attenuator mode to fixed SCNDCATTN Returns the mode for dc level output attenuators SCNINRVL i The SCNINRVL i command sets the parameter update interval according to the table below 1 Update Interval i Update interval 0 8 ms 9 9 375 s 1 16 ms 10 28 12s 2 31 ms 11 56 25s 3 78 ms 12 1125s 4 155 ms 13 5m37s 5 469 ms 14 lilmI5s 6 938 ms 15 22m30s 7 1 875 s 16 45m00s 8 4 688 s The parameter update interval is the approximate time spent at each scan step along the way during a scan Example SCNINRVL 8 Set parameter update interval to 4 688 s SCNINRVL Returns parameter update interval index i SCNENBL OFF ON i The SCNENBL i command turns scanning off i 0 or on i 1 This sets the scan parameter to its begin value but does not start a scan Example SCNENBL ON Turn scanning on SR865 DSP Lock in Amplifier SRS 130 Programming Chapter 4 SCNRUN SCNPAUSE SCNRST SCNSTATE S
210. sed scan 21 Press and hold Play Pause reset to reset the This resets the scan parameter back to the begin scan back to the begin value value The Done LED turns off 22 Press Play Pause reset briefly to start the scan This starts the scan again The Run LED turns again SR865 DSP Lock in Amplifier on and the frequency ramps upward Chapter 1 Getting Started 35 Phase 0 000 deg Fint While the scan is in progress press Scan setup Turn the scan off before the end and the internal briefly to turn scanning off frequency returns to its original value 100 000 kHz as shown in white in the info bar A sRs SR865 DSP Lock in Amplifier 36 Getting Started Chapter 1 SR865 DSP Lock in Amplifier Basics 37 Chapter 2 Lock in Amplifier Basics What is a Lock in Amplifier Lock in amplifiers are used to detect and measure very small ac signals all the way down to a few nanovolts Accurate measurements may be made even when the small signal is obscured by noise sources many thousands of times larger Lock in amplifiers use a technique known as phase sensitive detection to single out the component of the signal at a specific reference frequency and phase Noise signals at frequencies other than the reference frequency are rejected and do not affect the measurement Why use a lock in Let s consider an example Suppose the signal is a 10 nV sine wave at 10 kHz Clearly some amplification is required A
211. sensitive detectors for X and Y This is the filter that removes signals at frequencies other than fet In general longer time constants provide more noise filtering and quieter measurements but longer response times The time constant also determines the equivalent noise bandwidth ENBW of the low pass filter This is the measurement bandwidth for X and Y noise and depends upon the time constant and filter slope See the Noise discussion in the Basics section In some experiments output latency delay from signal input to analog output at short time constants is important Use the rear panel BlazeX output for the lowest latency analog X output Otherwise the front panel outputs CH1 and CH2 as well as the rear panel X and Y outputs have sufficient bandwidth for all time constants Filter Slope adv This key selects the number of poles in the low pass filter Choosing 6 12 18 or 24 dB oct selects 1 2 3 or 4 poles Using more poles can decrease the required time constant and make a measurement faster The normal low pass filter is an RC filter This is equivalent to the traditional filter found in analog lock ins To use advanced filters in place of the RC filters press and hold the Slope adv key until the Advanced LED turns on Brief presses of Slope adv cycles the number of poles A sRs SR865 DSP Lock in Amplifier 68 Operation Chapter 3 from 1 to 4 6 to 24 dB Another press and hold of the Slope adv key reverts th
212. st the data display as well as change certain lock in parameters Warm Up The lock in should be turned on and allowed to warm up for at least an hour before any tests are performed It is necessary to turn the unit off and on to preset it As long as the unit is powered on immediately this will not affect the test results Necessary Equipment The following equipment is necessary to complete the performance tests The suggested equipment or its equivalent should be used 1 Frequency Synthesizer Freq Range 1 Hz to 2 MHz Freq Accuracy better than 5 ppm Harmonic Distortion lt 65 dBc Spurious lt 55 dBc TTL SYNC available 2 AC Voltmeter Freq Range 10 Hz to 2 MHz Input Ranges 10 mV 100 mV 1V Accuracy 0 5 below 200 kHz 2 below 2 MHz Suggested instrument Keysight model 3458A 3 DC Voltmeter Range 19 999 V 4 1 2 digits Accuracy 0 005 4 Feedthrough Terminations Impedance 50 Q precision to 0 1 Test Record Make a copy of the SR865 Performance Test Record at the end of this section Fill in the results of the tests on this record This record will allow you to determine whether the tests pass or fail and also to preserve a record of the tests SR865 DSP Lock in Amplifier ASRS Appendix G Performance Tests 183 If A Test Fails If a test fails you should check the settings and connections of any external equipment and if possible verify its operation using a DVM scope or some other piece of test equipme
213. stems Inc 2008 2014 SRS stanford Research Systems SR865 2 MHz Lock in Amplifier Instrument Info Interactive Control Monitor Output SR865 Manual Stanford Research Systems Inc 2008 2014 SR865 DSP Lock in Amplifier ASRS Data Streaming and Capture 171 Appendix D Data Streaming and Capture Ethernet streaming of lock in data X Y R and 9 is implemented using UDP packets The STREAM ON command must be sent via VXI 11 from the computer that will receive the streamed data This is because ethernet streaming sends data to the IP address that sent the STREAM ON command UDP packets must be received on a port which is a virtual numerical address on the receiving computer Common ports like 80 for web servers or 22 for ssh should be avoided By default port 1865 is used but that can be changed with the STREAMPORT command All data in the UDP packet are binary so the endianness of the data matters In big endian format a 32 bit 4 byte word is stored as B3 B2 B1 BO where B3 is the most significant byte of the word and BO is the least significant byte Little endian format reverses the byte order so that the same 32 bit 4 byte word is stored as BO B1 B2 B3 Similarly a 16 bit 2 byte integer is stored as B1 BO in big endian format while it is BO B1 in little endian format If the streamed data is not in the same endianness as the receiving computer then the byte order of the data must be reversed Note th
214. strip chart display New data is plotted at the right edge and older data scrolls left The scroll rate is determined by the horizontal scale time per division The fastest rate is 0 5 s div and the shows 5 s of history The info bar is at the top of the screen Touch a tile to change a parameter using a keypad The numeric and bar graph displays shrink to fit above the chart The scale bar is shown below the strip chart This bar shows tiles indicating the vertical scale per division for the 4 data channels green blue yellow and orange and the horizontal time scale per division white Use the palette functions to adjust the vertical scale of the selected data channel s trace Touch the scale tile again to dismiss the palette The trace may be moved up and down auto scaled zoomed in and out and dismissed entirely Chapter 1 Getting Started 15 Touch BA to auto scale the orange 0 graph 99 9998 khz HM 4 Amel Phase 0 000 deg Fint Touch the green scale tile X to display its scale palette Touch BE to auto scale the green X trace Touch the blue scale tile Y to display its scale palette PF Touch BE to auto scale the blue Y trace Touch the yellow scale tile R to display its scale palette Touch to move the trace so the newest points are vertically centered j repeatedly to zoom in about the center Keep zooming in until the yellow trace shows some noise The scale will probably
215. t This is because a single RC filter requires about 5 time constants to settle to its final value The time constant reflects how slowly the output responds and thus the degree of output smoothing The time constant also determines the equivalent noise bandwidth ENBW for noise measurements The ENBW is not the filter 3 dB pole it is the effective bandwidth for Gaussian noise More about this later The digital signal processing in the SR865 handles all of the low pass filtering Each PSD can be followed by up to four filter stages for up to 24 dB oct of roll off Why are multiple filter stages desirable Consider an example where the reference is at 1 kHz and a large noise signal is at 1 05 kHz The PSD noise outputs are at 50 Hz difference and 2 05 kHz sum Clearly the 50 Hz component is the more difficult to low pass filter If the noise signal is 80 dB above the full scale signal and we would like to measure the signal to 1 40 dB then the 50 Hz component needs to be reduced by 120 dB To do this in two stages would require a time constant of at least 3 seconds To accomplish the same attenuation in four stages only requires 100 ms of time constant In the second case the output will respond 30 times faster and the experiment will take less time SR865 DSP Lock in Amplifier ASRS Chapter 2 Basics 45 SRS Advanced Filters The SR865 also provides advanced filtering in place of the traditional 6dB octave RC filters
216. t 99 and N Xfrer lt 2 MHz Detect at fuai fint fext All frequencies less than 2 MHz for specified performance operates to 2 5 MHz SR865 drives SR540 Chopper via Aux Out 4 to lock the chopper to fint Digital output values have no offset drift 1 us to 30 ks Typical RC type filters or Advanced Gaussian Linear P hase filters 6 12 18 24 dB oct rolloffs Available below 4 kHz 80 dB Rear panel BlazeX output with lt 2 us delay plus low pass filter rise fall times 1 mHz to 2 MHz specified operates to 2 5 MHz 25 ppm 30 u Hz with internal timebase 10 MHz timebase input output on rear panel 6 digits or 0 1 mHz whichever is greater SR865 DSP Lock in Amplifier ASRS viii Specifications Sine Output Outputs Output Impedance Amplitude Amplitude Resolution dc Offset Offset Resolution Output Limit Sync Data Data Channels Data Sources Data History Offset Ratio Expand Capture Buffer Data Streaming Scanning FFT Source Record length Averaging Inputs and Outputs CH 1 Output CH 2 Output X and Y Outputs Blazex Aux Outputs Aux Inputs Trigger Input Monitor Output HDMI Timebase Input Output Differential or Single ended 50 Q source 1 nVrms to 2 Vrms Specified amplitude is differential into 50 Q loads Output amplitude is halved when used single ended Output amplitude is doubled into a high impedance load 3 digits or 1 nV whichever is greater 5 V differential o
217. t count that is one greater than the previous packet modulo 256 Packet content indicates the data contained in the packet Packet length the number of bytes of data that follow Sample rate is the sample rate of the data in the packet A value of N corresponds to a sampling rate of 1 25 MHz 2 where N is a value between 0 and 31 inclusive This sample rate already takes into account the subsampling requested in the command STREAMRATE Status bit 24 is high when an input overload sync filter overload if sync filter is on or output overload if integer data is contained in the packet condition existed at the beginning of the packet Bit 25 is high when a PLL unlock or sync filter out of range condition existed at the beginning of the packet Bit 28 is high if data is stored in little endian format and low if it is in big endian format Bit 29 indicates that integrity checking UDP packet checksumming is enabled After reading the header information it is possible to correctly interpret the data in the rest of the packet If you need to do endian conversion floating point data must be converted 32 bits 4 bytes at a time whereas integer data must be converted 16 bits 2 bytes at a time XYR data is recorded in that order followed by the next sample of XYR As an example let s say the packet contains floating point XYR data Data bytes Byte O 1 2 3 4 5 6 7 8
218. t to 100 ms OFLT Returns the time constant i The OFSL i command sets the filter slope to 6 dB oct i 0 12 dB oct i 1 18 dB oct i 2 or 24 dB oct i 3 Example OFSL 1 Set the filter slope to 12 dB oct OFSL Returns the filter slope i SYNC OFF ON i The SYNC i command turns the synchronous filter off G 0 or on i 1 Example SYNC ON Set the synchronous filter to on SYNC 1 SYNC Returns the state of the synchronous filter i ADVFILT OFF ON i ENBW SRS The ADVFILT i command turns the advanced filter off i 0 or on G 1 Example ADVFILT ON Set the advanced filter to on ADVFILT 1 ADVEFILT Returns the state of the advanced filter i The ENBW query returns the equivalent noise bandwidth of the output filter in hertz Note that the effect of the SYNC filter is neglected by ENBW Example ENBW Returns the equivalent noise bandwidth in hertz SR865 DSP Lock in Amplifier 114 Programming Chapter 4 CH1 CH2 Output Commands COUT OCH1 OCH2 j XY RTHeta i The COUT j i command sets CH1 j 0 or CH2 j 1 to either XY 0 or RO G 1 Example COUT OCH2 RTHeta Set CH2 to 0 COUT 1 1 COUT 0 RTH Set CH1 to R COUT 0 XY Set CH1 to X COUT 1 Returns the CH2 output i COUT OCH2 CEXP X Y R j OFF X10 X100 i The CEXP j i command sets the output expand for X j 0 Y G 1 or R G 2 to off G 0 X10 G 1 or X100 i 2 Setting an expand for phase is not
219. tage input is overloaded The smaller the input range value the higher the amplifier gain leading to the A D converter The signal strength LEDs indicate how much of the A D range is being used When the lowest yellow LED is on try decreasing the input range increasing the gain In general use the smallest input range possible without overload Remember the largest signal whether it s at f or just noise will be the first to overload This setting has no effect when the current input is selected The Signal Monitor on the rear panel is the amplifier output Current Range The current input uses the I connector The input burden resistance is 1 kQ 10 nA range or 100 Q 1 uA range to a virtual ground The shield is chassis ground The largest allowable current ac plus dc before overload is approximately 4 uA or 40 nA No current larger than 10 mA should ever be applied to this input The current range determines the input current noise as well as the input bandwidth The 10 nA range has 10 times lower noise but 200 times lower bandwidth Be sure the signal frequency is below the input bandwidth limit The noise and bandwidth are listed below Range Noise Bandwidth 1 pA 130 fA VHz 400 kHz 10nA 13fA VHz 2 kHz The impedance of the current source should be greater than 1 MQ when using the 1 pA range and greater than 100 MQ when using the 10 nA range The signal strength LEDs indicate how much of the A D range is being
220. tarted 9 The Basic Lock in This measurement is designed to use the internal oscillator to explore some of the basic lock in functions Specifically you will measure the amplitude of the Sine Out at various frequencies amplitudes time constants and phase shifts Do This 1 Disconnect all cables from the lock in Turn the power on while holding down the Local key The power switch is on the power entry module on the rear panel 2 Connect the Sine Out on the front panel to the A input using a BNC cable 3 Touch Ampl in the info bar along the top of the screen Then 5 0 O mV 4 Press the Auto Phase key SRS Explanation When the power is turned on with Local pressed the lock in returns to its standard default settings See the Standard Settings list in the Operation section for a complete listing of the settings The lock in defaults to the internal oscillator reference set at 100 000 kHz The reference source is indicated by the Internal LED In this mode the lock in generates a synchronous sine output at the internal reference frequency The default data screen is the Trend Graph The 4 displayed parameters default to X Y R and 9 Each parameter has a numeric display a bar graph and a trend graph The trend graph is a continuously autoscaling graph of the recent history of each parameter This data screen has no adjustments available The default sine amplitude is O Vrms Thus the data
221. tay in phase indefinitely The Ext 10 MHz LED is on when the unit is locked to an external timebase The SR865 also outputs its own 10 MHz timebase to another unit Note that external function generators even a direct digital synthesizer locked to the same 10 MHz timebase will not stay in phase with the SR865 This is because the resolution of the frequency tuning word in the external direct digital synthesizer differs from the SR865 HDMI Output Use the HDMI output to display the SR865 screen on a large computer monitor or television The output has a resolution of 640x480 and a frame rate of 60 Hz Aux In 1 4 These are auxiliary analog inputs which can be digitized by the SR865 The range is 10 5 V the resolution is 1 mV and the inputs are 1 MQ These inputs can be displayed on the screen read over the computer interfaces and graphed on the strip charts Aux Out 1 4 These are auxiliary analog outputs The range is 10 5 V and the resolution is 1 mV These outputs may be programmed from the front panel via Aux Output scanned via Scan setup or controlled via the computer interfaces Aux Out 4 is used to control an SR540 chopper when the reference mode is Chop X and Y Outputs The X and Y lock in outputs are always available at these connectors The output sample rate is 1 25 MHz An input signal equal to the sensitivity will generate 10 V at these outputs These outputs are identical to the front panel CH1 and
222. te correctly or a parameter is out of range An illegal command is received Set by any user front panel action Set by power on usage CH1 output overload CH2 output overload unused External reference or Chop unlock detected Input range overload detected Sync filter frequency out of range Sync filter overload Set when data storage is triggered Data Channel 1 output overload Data Channel 2 output overload Data Channel 3 output overload Data Channel 4 output overload Display capture to USB stick completed Scan started Scan completed SR865 DSP Lock in Amplifier xvi Commands Error Status Byte bit name usage 0 CLK External 10 MHz clock input error 1 BACKUP Battery backup failed 2 unused 3 unused 4 VXI VXI 11 error 5 GPIB GPIB fast data transfer mode aborted 6 USBDEV USB device error interface error 7 USBHOST USB host error memory stick error SR865 DSP Lock in Amplifier Getting Started 1 Chapter 1 Getting Started Introduction The sample measurements described in this section are designed to acquaint the first time user with the SR865 DSP Lock In Amplifier Do not be concerned that your measurements do not exactly agree with these exercises The focus of these measurement exercises is to learn how to use the instrument It is highly recommended that the first time user step through some or all of these exercises before attempting to perform an actual experiment Keys Knobs and Touch Buttons
223. te sign dc level on each BNC or in Common mode same sign dc level on both BNC s SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 81 Display Blank Se Screen Screen Layout Shot Screen Layout Press Screen Layout to cycle through the different screen layouts Trend Graph Half Screen FFT Big Numbers Screen Shot Pressing Screen Shot saves a screen shot to the USB memory stick as a BMP file Set the Print Mode in the system menu Screen is an exact screen shot Print replaces the A sRs SR865 DSP Lock in Amplifier 82 Operation Chapter 3 black background with white and Monochrome is black and white Screen shots also contain the time date and summary of input parameters The Busy LED indicates that the USB stick is busy and should not be removed Blank Press the Blank key to operate with the front panel display and LEDs off The SR865 is still operating the outputs are active data collection continues and the unit responds to interface commands To change a setting press Blank to return to normal operation change the desired parameter then press Blank again The Blank LED is lit while the rest of the display is off Config The SR865 displays up to 4 channels at a time in green blue yellow and orange Each channel is assigned a parameter using the Config key Parameters are chosen from X Y R 0 detected Fin Fe
224. ted or a TTL pulse or square wave rising or falling edge The input impedance is either 1 MQ or 50 Q The Sine Trig is ac coupled gt 1 Hz For low frequencies lt 1 Hz it is necessary to use a TTL reference signal The TTL input provides the best overall performance and should be used whenever possible Sine Out The internal oscillator outputs are differential with 50 Q output impedance each The output amplitude is specified as Vrms differential into 50 Q loads amplitude sine sine If the output is terminated in a high impedance the amplitude will be double If only a single output is used the amplitude will be half Thus a single output into high impedance will actually have the specified amplitude The amplitude can be set from 1 nVrms to 2 Vrms The dc level of the sine output can also be specified The dc can be applied differentially outputs move apart or in common outputs move together The dc level can be set from 0 1 mV to 5 V The maximum output is about 6 V These outputs are active even when an external reference is used In this case the sine wave is phase locked to the reference and its amplitude is programmable A 2 5 V sync logic output is provided on the rear panel via the configurable BlazeX output This output is useful for triggering scopes and other equipment at the reference frequency The sync output is a square wave derived from the zero crossings of the sine output SR865 DSP Lock in Ampli
225. teger variables must be expressed in integer format no decimal point or exponent SR865 DSP Lock in Amplifier 104 Programming Chapter 4 Real number variables may be expressed in integer floating point or exponential formats i e the number five can be either 5 5 0 or SE1 Strings are a sequence of ASCII characters within quotes Units For numerical values of frequency phase or volts a units string may be appended to a set command If the unit string is omitted then the value is assumed to be the base unit either Hz degrees or Volts The accepted unit strings are NV nanoVolts HZ Hertz UDEG microDegrees UV microVolts KHZ kiloHertz MDEG milliDegrees MV milliVolts MHZ megaHertz DEG Degrees y Volts For example the following commands to set the internal frequency are all equivalent FREQ 1234 56 FREQ 1234 56 HZ FREQ 1 23456 KHZ Queries of f p or v always return the value in the base units of Hz degrees or Volts Enumerated Strings Many commands have an enumerated list of allowed settings It is possible to specify the setting with a string instead of the integer index Using the enumeration strings improves the readability of your source code Enumerated strings can either be the substring shown in CAPS or the full string For example the enumeration string ABCDefgh can be sent as abcd or abcdefgh The string can be sent
226. therwise be done by moving the position of the chopper or beam The lt Cursor gt knob will also adjust the phase value in this screen External Input This key sets the termination of the Ref Input BNC to either 50 Q or 1 MQ Be careful that the termination does excessively attenuate the signal or cause ringing on its edges External Trig This key selects the external reference input trigger mode TTL When either Pos TTL or Neg TTL is selected the SR865 locks to the selected edge of a TTL square wave or pulse train For reliable operation the TTL signal should exceed 1 5 V when high and be less than 0 5 V when low This trigger mode is dc coupled This input mode should be used whenever possible since it is less noise prone than the sine wave discriminator For very low frequencies lt 1 Hz a TTL reference MUST be used Sine Sine input trigger locks the SR865 to the rising zero crossings of an analog signal at the Ref In BNC This signal should be a clean sine wave at least 200 mVpp in amplitude In this input mode the Ref In is ac coupled above 1 Hz Sine reference mode cannot be used at frequencies far below 1 Hz At very low frequencies the TTL input modes must be used Unlock SRS In External or Dual reference source the Unlock indicator turns on if the SR865 cannot lock to the external reference This can be because the external reference amplitude is too low or the frequency is out of range In Chop r
227. ts in the Serial Poll Enable register A service request is only generated when an enabled Serial Poll Status bit becomes set changes from 0 to 1 An enabled status bit which becomes set and remains SR865 DSP Lock in Amplifier 152 Programming Chapter 4 set will generate a single SRQ If another service request from the same status bit is desired the requesting status bit must first be cleared In the case of the ERR LIA and ESB bits this means clearing the enabled bits in the ERR LIA and ESB status bytes by reading them Multiple enabled bits in these status bytes will generate a single SRQ Another SRQ from ERR LIA or ESB can only be generated after clearing the ERR LIA or ESB bits in the Serial Poll status byte To clear these bits ALL enabled bits in the ERR LIA or ESB status bytes must be cleared The controller should respond to the SRQ by performing a serial poll to read the Serial Poll status byte to determine the requesting status bit Bit 6 SRQ will be reset by the serial poll For example to generate a service request when a reference UNLK occurs bit 3 in the LIA Status Enable register needs to be set LIAE 3 1 command and bit 3 in the Serial Poll Enable register must be set SRE 3 1 command When a reference unlock occurs bit 3 in the LIA Status byte is set Since bit 3 in the LIA Status byte AND Enable register is set this ALSO sets bit 3 LIA in the Serial Poll Status byte Since bit 3 in the Serial Poll
228. ts or 0 1 mV whichever is greater The levels may be programmed from 5 00 V to 5 00 V Example PSTL 0 1 23 V Set dc level preset L1 to 1 23 V PSTL 0 1230 MV PSTL 0 Returns the dc level preset L1 in Volts SR865 DSP Lock in Amplifier SRS Chapter 4 Programming 111 Signal Commands IVMD VOLTage CURRent i The IVMD i command sets the signal input to voltage i 0 or current i 1 Example IVMD VOLT Set the signal input to voltage IVMD VOLTAGE IVMD 0 IVMD Returns the signal input i ISRC A A B i The ISRC i command sets the voltage input mode to A i 0 or A B i 1 Example ISRC A B Set the voltage input to A B ISRC 1 ISRC Returns the voltage input mode i ICPL AC DC i The ICPL i command sets the voltage input coupling to ac i 0 or de i 1 Example ICPL DC Set the voltage input coupling to dc ICPL 1 ICPL Returns the voltage input coupling mode i IGND FLOat GROund i The IGND i command sets the voltage input shields to float G 0 or ground i 1 Example IGND FLO Set the voltage input shields to float IGND FLOAT IGND 0 IGND Returns the voltage input grounding mode i IRNG 1Volt 300Mvolt 100Mvolt 30Mvolt 10Mvolt i The IRNG i command sets the voltage input range to 1 V i 0 300 mV i 1 100 mV i 2 30 mV i 3 or 10 mV i 4 Example IRNG 1V Set the voltage input range to 1 V IRNG 1VOLT IRNG 0 IRNG Returns the voltage inp
229. tte selected turns on a status display across the top Now touch anywhere within the graph area al me a the looi sianal settings Phase 0000 deg FT 999998 kz P 1 This status is useful when the HDMI port on OR the rear panel is used to drive an external monitor or TV Users who are looking at the monitor can see the lock in front panel settings Touch anywhere within the graph area to dismiss The status display is dismissed when the graph the status display area is touched or a scale palette is displayed Simply turn it back on with a touch if desired 11 Touch the white scale tile Time to display the Horizontal scale changes are applied to the horizontal scale palette entire strip chart display and all data channels S repeatedly to zoom out Zooming changes the horizontal scale and scroll speed The chart always displays the most Phase 01000 deg F 999998 kz MA 1 recent point at the right edge In this case zooming out displays more history and more cycles of X Y and 0 appear 12 Increase the amplitude of the function generator The signal now exceeds the input range of 1 V to 1 5 Vrms SR865 DSP Lock in Amplifier peak so the Input Range Overload LED is on Ovid indicators are displayed for X Y R and ASRS Chapter 1 Getting Started 17 Phase 0 000 deg E 99 9998 kiz HAM 1 Decrease the amplitude of the function generator back to 500 mVrms 13 Touch the white scale tile Time to display the h
230. ugh the reference sources Press and hold to display the SR540 Chopper setup screen see Chop below Internal When the source is Internal the SR865 s synthesized internal oscillator is used as the reference The Ref In BNC is ignored in this case The internal frequency is shown in the info bar Use the lt Frequency gt knob or the numeric keypad to adjust the frequency In this mode the Sine Out is at the internal frequency External When the source is External the SR865 will phase lock to the external reference provided at the Ref In BNC The SR865 will lock to frequencies between 0 001 Hz and 2 5 MHz The external frequency is shown in the info bar The lt Frequency gt knob has no effect on the external frequency In this mode the Sine Out is at the external frequency Dual When the source is Dual the SR865 will detect signals at factect lfint fextl where fext 1S the external reference frequency and fj is the internal frequency The internal frequency is shown in the info bar Use the lt Frequency gt knob or the numeric keypad to adjust the internal frequency In this mode the Sine Out is at the internal frequency To record and view the external frequency use the Config key to assign fex to one of the four data channels You can find more about dual reference mode in Appendix E page 175 SR8 amp 65 DSP Lock in Amplifier 76 Operation Chapter 3 Chop When the source is Chop the SR865 will phase loc
231. ultimeter to measure the Sine Out Connect the Sine Out to the ac Volts input of the voltmeter using a 1 meter BNC cable with the precision 50Q terminator at the voltmeter connection This test only uses ONE Sine Out BNC This means that each measurement with the ac voltmeter will nominally be 50 of the programmed sine amplitude Set the multimeter for ac volts If using the model 3458A it should be configured as 1 Reset press blue shift key then Reset ii SETACV RNDM press blue S then U3 times then gt then U2 times then Enter iii ACV press ACV iv a for RANGE 1 press blue R then 1 then Enter iv b for RANGE 0 1 press blue R then 0 1 then Enter iv c for RANGE 0 01 press blue R then 0 01 then Enter Procedure 1 PRESET press Save Recall then touch Recall default Touch Confirm 2 Amplitude accuracy is verified at 1 kHz Use the front panel to make the following adjustments Touch Fint to display the internal frequency keypad Enter a frequency of 1 0 kHz 3 For each sine amplitude use the voltmeter range setting in the table below perform steps 3a through 3b Sine Output Amplitude ac voltmeter range 1 00 V 1 300 mV 1 100 mV 0 1 30 mV 0 1 10 0 mV 0 01 1 00 mV 0 01 a Use the SR865 front panel to make the following adjustments Touch Ampl to display the sine out amplitude keypad Enter the amplitude from the table SR865 DSP Lock in Amp
232. uments running earlier firmware the Sensitivity must be set appropriately so that the displayed values of X or Y have enough resolution If the displayed values of X or Y are unchanging then the computed noise will be almost zero Noise Display To display a noise measurement choose X or Y noise Xn or Yn as one of the displayed parameters in the Config screen The SR865 is calculating the noise all of the time whether or not X or Y noise are being displayed Thus as soon as noise is displayed the SR865 DSP Lock in Amplifier 60 Basics Chapter 2 value shown is up to date and no settling time is required If the time constant is changed then the noise measurement will need to settle to the new value X and Y noise are displayed in units of V rms The ENBW of the time constant is NOT factored into the calculation To convert to spectral noise density divide the reading by VENBW SR865 DSP Lock in Amplifier ASRS Operation 61 Chapter 3 Operation n Display functions Auto functions re x i Ns LCD touchscreen data display XorR output Y or 8 output Introduction Power The power switch is on the rear panel The SR865 is turned on by pushing the switch up Keys The keys are grouped and labeled according to function This manual will refer to a key with square brackets such as Key A complete description of the keys follows in this section Knobs Knobs are used to adjust the internal r
233. unction output Specifically you will scan the lock in internal reference frequency through the signal frequency Do This 1 Disconnect all cables from the lock in Turn the power on while holding down the Local key The power switch is on the power entry module on the rear panel 2 Turn on the function generator set the frequency to 100 000 kHz exactly and the amplitude to 500 mVrms Connect the function output sine wave from the synthesized function generator to the A input using a BNC cable and appropriate terminator 3 Press and hold Scan setup to display the scan menu Parameter Update Interval 0 008 S Scan Parameter Scan Duration 01 40 Begin Value 100 000 t i Scan Type End Mode End Value 200 000 re 8 4 5 F3 1 00000 kHz 1 00000 MHz 1 2 0 F2 F4 50 0000 Hz 1 0 mHz SR865 DSP Lock in Amplifier Explanation When the power is turned on with Local pressed the lock in returns to its standard default settings See the Standard Settings list in the Operation section for a complete listing of the settings The input impedance of the lock in is 10 MQ The generator may require a terminator Many generators have either a 50Q or 600Q output impedance Use the appropriate feedthrough or T termination if necessary In general not using a terminator means that the function output amplitude will not agree with the generator setting The internal oscillator should be very
234. used The Signal Monitor on the rear panel is the amplifier output The current range setting has no effect when the voltage input is selected SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 67 Input Overload The Overload LED in this section indicates an input overload This occurs for voltage inputs greater than the voltage input range unless removed by ac coupling or current inputs exceeding 4 uA 1 uA range or 40 nA 10 nA range An input overload compromises the measurement of X Y R and 0 Any display of these quantities will be accompanied by an Ovld indication Either increase the input range or reduce the input signals to remove the overload Slew Rate Overloads can also be caused by high slew rate voltage signals These are large signals with fast rise or fall times This can be a square wave at any frequency with fast transitions or simply a large amplitude high frequency sine wave In these cases the amplifier may become slew rate limited and unable to accurately amplify other components of the signal When this occurs the Overload LED may light even though the signal strength indicator is not at its maximum Once again increase the input range or reduce the input signal Time Constant The time constant may be set from 1 us to 30 ks The time constant is indicated by 1 or 3 times 1 10 or 100 with the appropriate units This time constant sets the bandwidth of the low pass filter after the phase
235. ut range i ASRS SR865 DSP Lock in Amplifier 112 Programming Chapter 4 ICUR 1MEG 100MEG i The ICUR i command sets the current input gain to 1 MQ 1 pA i 0 or 100 MQ 10 nA G 1 Example ICUR 100MEG Set the current input gain to 100 MQ 10 nA ICUR 1 ICUR Returns the current input gain i ILVL The ILVL query returns the signal strength indicator from lowest 0 to overload 4 Example ILVL Returns the signal strength indication SCAL i The SCAL i command sets the sensitivity according to the table below 1 sensitivity 1 sensitivity 0 1 V pA 15 10 uV pA 1 500 mV nA 16 5uV pA 2 200 mV nA 17 2uYV pA 3 100 mV nA 18 1 pV pA 4 50 mV nA 19 500 nV fA 5 20 mV nA 20 200 nV fA 6 10 mV nA 21 100 nV fA 7 5 mV nA 22 50nvV fA 8 2 mV nA 23 20n fA 9 1 mV nA 24 10nV fA 10 500 uV pA 25 SnvV fA 11 200 uV pA 26 2nV fA 12 100 uV pA 27 I nV fA 13 50 uV pA 14 20 nV pA Example SCAL 6 Set the sensitivity to 10 mV nA SCAL Returns the sensitivity i SR865 DSP Lock in Amplifier Chapter 4 Programming 113 OFLT i OFSL i The OFLT i command sets the time constant according to the table below i time constant i time constant i time constant O lus 8 10ms 16 100s 1 34s 9 30ms 17 300s 2 10us 10 100 ms 18 1ks 3 30 us 11 300 ms 19 3ks 4 100 us 12 Is 20 10ks 5 300 us 13 3s 21 30ks 6 lms 14 10s 7 3ms 15 30s Example OFLT 10 Set the time constan
236. value 0 or 1 of bit j 0 7 Reading the entire byte will clear it while reading bit j will clear just bit j Example ERRS Returns decimal value of the error status byte ERRS 4 Returns bit 4 of the error status byte LIAE j i SRS The LIAE i command sets the lock in LIA status enable register to the decimal value i 0O 4095 The LIAE j i command sets bit j 0 11 toi 0 or 1 The LIAE command queries the value of the LIA status enable register The LIAE j command queries the value 0 or 1 of bit j Example LIAE 257 Set bits 1 and 8 in the lock in status enable register LIAE 8 1 Set bit 8 in the lock in status enable register LIAE Returns decimal value of the lock in status enable register LIAE 8 Returns bit 8 of the lock in status enable register SR865 DSP Lock in Amplifier 150 Programming Chapter 4 LIAS j The LIAS command queries the value of the lock in LIA status word The value is returned as a decimal number from 0 to 4095 The LIAS j command queries the value 0 or 1 of bit j 0 11 Reading the entire word will clear it while reading bit j will clear just bit j Example LIAS Returns decimal value of the lock in status word LIAS 8 Returns bit 8 of the lock in status byte CUROVLDSTAT The CUROVLDSTAT command queries the present overload states of the lock in These overloads reflect the state of the lock in at the time the command is processed They are NOT set until read The value
237. verify specified performance Connect the Sine Out to the Voltage A input of the SR865 using a 1 meter BNC cable with the precision 50Q terminator at the A input BNC This test only uses ONE Sine Out BNC This means that each measurement will nominally be 50 of the programmed sine amplitude Preliminary calculations i Refer to the Reading values recorded for the Sine Output Amplitude and Flatness test ii Results labeled A through J are used to set performance Lower Limits and Upper Limits for Amplitude Accuracy and Flatness iii On the Performance Test Record for Lower and Upper limits perform the multiplication indicated with the referenced value of Sine Output Amplitude and Flatness For example Assume result A is recorded as 0 4966 V The Lower Limit at 1V will be 0 99x A 0 99x0 4966 V 0 4916 V The Upper Limit at 1V will be 1 01x A 1 01x0 4966 V 0 5016 V Procedure 1 PRESET press Save Recall then touch Recall default Touch Confirm 2 Use the front panel to make the following adjustments Touch Fint to display the internal frequency keypad Enter a frequency of 1 0 kHz Press Filter Slope adv multiple times Select 24 dB oct 3 Amplitude accuracy is verified at 1 kHz and various sensitivities For each input range setting in the table below perform steps 3a through 3c Sine Out Amplitude Input Range Sensitivity 1 0000 Vrms 1V 1V 300 00 mVrms 300 mV 200 mV 100 000 mVrms 100 mV 100 mV 3
238. with any capitalization For example the IVMD i command sets the input to either voltage i 0 or current i 1 This command is specified as IVMD VOLTage CURRent i The following commands are all equivalent ways to set the input to voltage IVMD 0 IVMD VOLT IVMD VOLTAGE These commands are all equivalent ways to set the input to current IVMD 1 IVMD CURR IVMD CURRENT The IVMD query always returns the setting index i 0 or 1 in this case SR865 DSP Lock in Amplifier ASRS Chapter 4 Programming 105 Example Commands RSRC 1 Set reference source to external RSRC EXT RSRC Query the reference source FREQ 10E3 Set the internal reference frequency to 10000 Hz FREQ 10 KHZ FREQ 10000 0 APHS Execute Auto Phase A sRs SR865 DSP Lock in Amplifier 106 Programming Chapter 4 Reference Commands TBMODE AUTO INternal i TBSTAT The TBMODE i command sets the external 10 MHz timebase mode to auto i 0 or internal i 1 Example TBMODE AUTO Set the timebase mode to auto TBMODE 0 TBMODE Returns the timebase mode i The TBSTAT query returns the current 10 MHz timebase source either external 0 or internal 1 Example TBSTAT Returns the current 10 MHz timebase PHAS p UDEG MDEG DEG URAD MRAD RAD APHS The PHAS p command sets the reference phase shift to p degrees The value of p is set with a resolution of 0 0000001 The phase may be programmed from
239. x Reference Phase Sine Amplitude DC Level any Aux Input or Output Xnoise or Ynoise Highlight one of the channel s Display tile then select a parameter from below to assign it to the channel Displayed parameters can be re assigned at any time Data is being stored for all parameters all of the time X noise Y noise Out 1 Out 2 SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 83 Cursor Rel Push for re 3 lt Cursor gt The screen cursor readout is only available for strip chart and FFT displays Trend and Big Number screens do not have a cursor function Strip Chart Cursor The strip chart cursor is only active when the strip chart is paused and the graph is stationary Use Pause in the horizontal scale palette to stop the chart scrolling and pause the graph Use the lt Cursor gt knob to position the display cursor and readout the data values These readouts correspond to the min max or mean of the data in the time bin at the cursor location The time of the cursor location is displayed in the tile at the left edge of the scale bar below the graph Touch this tile to switch between elapsed time from the right edge to absolute time time and date when the point was taken Use Cursor MinMaxMean and Cursor Width on the horizontal scale palate to change the cursor Note that the cursor marker may not lie on the data graph for wide cursors since the marker shows the min max or mean of a
240. xpand increases the resolution of the displayed X Y or R numeric value This is because the expand function increases the resolution of the output not the size of the input signal The displayed value will show an increased resolution but will continue to display the original value minus the offset Any display which is showing a quantity which is affected by a non unity expand will display a highlighted Expand indicator within its display Output expands affect the bar graphs The bar graphs are simply a reflection of the BNC outputs and as such are expanded to provide more visual resolution Output expands do not affect the strip charts The values being charted are already floating point numbers with all of the resolution available The strip charts do reflect the offsets however See the SR865 Basics section for more information Output Select This key selects the source for the CH1 CH2 output BNC CH1 can select either X or R CH2 can select either Y or 0 This also determines which parameter s offset and expand are adjusted with the Expand key and the lt Offset gt knob Output Expand Pressing this key selects the X or R Expand CH1 or the Y Expand CH2 Use the Select key to select either X or R CH1 and Y or 0 CH2 0 cannot be expanded To expand phase expand the value of Y in quadrature The expand can be none x1 or no expand x10 or x100 If the expand is x10 or x100 the corresponding LED will light Th
241. xternal harmonic detect to 2 HARMDUAL Returns the dual external harmonic number i BLADESLOTS SLT6 SLT30 i The BLADESLOTS i command configures the lock in for operation with an external SR540 chopper Before proper operation in RSRC EXT mode the SR865 must be configured for either 6 slot i 0 or 30 slot i 1 operation based on the hardware installed on the SR540 chopper You can read more about operation in chop mode on page 76 Example BLADESLOTS SLT30 Configure for 30 slot chopper blade BLADESLOTS 1 BLADESLOTS Returns chopper blade configuration BLADEPHASE p UDEG MDEG DEG URAD MRAD RAD The BLADEPHASE p command sets the phase of the SR540 chopper blade When operating a single chopper this has little effect since the SR865 will follow the chopper If multiple SR865 SR540 systems are being used in a single experiment BLAADEPHASE can be used to modify the relative phase of the choppers Blade phase may be specified in degrees default or millidegrees microdegrees radians milliradians or microradians You can read more about chopper blade phase on page 77 Example BLADEPHASE 43 21 DEG Set the blade phase to 43 21 deg BLADESLOTS 4 321E1 BLADEPHASE Returns chopper blade phase SLVL v NV UV MV V The SLVL v command sets the sine out amplitude to v The value of v will be rounded to 3 digits or 1 nV whichever is greater The amplitude may be programmed from 1 nV to 2 0 V Example
242. y corresponding displayed numeric value bar graph will indicate Scale in the display The actual measurement is unaffected since it is done in floating point and has no overload Data displayed in the strip chart will still be accurate A sRs SR865 DSP Lock in Amplifier 74 Operation Chapter 3 Reference REFERENCE FREQUENCY O Hold for fundamental Hold to zero Difference Common EXTERNAL Unlock SOURCE a Internal Sine External m Pos TTL 500 Dual Neg TTL 1MQ Chop CO Oe Ref In 0 000 deg 100 000 ir 2 fre This bar always shows tiles displaying the reference phase frequency detected harmonic sine out amplitude and dc level Each of these parameters can be adjusted using the knobs and buttons in this section of the front panel Touching one of these tiles brings up a numeric keypad for direct entry SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 75 Phase 0 000 deg 15127012 4A tot mv oy 1 51270 clear al F3 1 00000 kHz 1 00000 MHz close F2 F4 50 0000 Hz 1 0 mHz Numeric entry is straightforward Close will return to the data screen The buttons F1 F2 F3 and F4 are frequency presets Touching a preset will load the preset value immediately Touch and hold a preset button to memorize the current setting Other parameters may have slightly different entry screens Source Ref chop SRS Press this key briefly to cycle thro
243. y since the temperature of the detector and experiment generally changes slowly This effect is large on the scale of many detector outputs and can be a problem for low frequency measurements especially in the mHz range Some ways to minimize thermocouple effects are 1 Hold the temperature of the experiment or detector constant 2 Use a compensation junction i e a second junction in reverse polarity which generates an emf to cancel the thermal potential of the first junction This second junction should be held at the same temperature as the first junction Noise Measurements Lock in amplifiers can be used to measure noise Noise measurements are generally used to characterize components and detectors SR865 DSP Lock in Amplifier ASRS Chapter 2 Basics 59 SRS The SR865 measures input signal noise AT the reference frequency Many noise sources have a frequency dependence which the lock in can measure How Is Noise Measured Remember that the lock in detects signals close to the reference frequency How close Input signals within the detection bandwidth set by the low pass filter time constant and roll off appear at the output at a frequency f fsig fret Input noise near f ef appears as noise at the output with a bandwidth of dc to the detection bandwidth For Gaussian noise the equivalent noise bandwidth ENBW of a low pass filter is the bandwidth of the perfect rectangular filter which passes the same amount of
244. ynchronous filter on or measurements of Xnoise or Ynoise may take many time constants to return to their steady state values SR865 DSP Lock in Amplifier 118 Programming Chapter 4 Display Commands DBLK OFF ON i The DBLK i command turns front panel blanking off i 0 or on i 1 Example DBLK OFF Turn blanking off displays on DBLK 1 Turn blanking on displays off DBLK Returns the blanking state i DLAY TREnd HISTory BARHist FFT BARFft BAREight i The DLAY i command sets the screen layout to trend i 0 full strip chart history i 1 half strip chart history i 2 full FFT i 3 half FFT G 4 or big numerical i 5 Example DLAY HIST Set the screen layout to full screen strip chart history DLAY 1 DLAY Returns the screen layout i DCAP The DCAP command saves a screenshot to a USB memory stick This command is the same as pressing the Screen Shot key A USB memory stick must be present in the front panel USB port CDSP DAT1 DAT2 DAT3 DAT4 j parameter i The CDSP j param command assigns a parameter to data channel j This is the same parameter assignment as pressing the Config key The value of j 0 3 corresponds to the DAT1 green DAT2 blue DAT3 yellow and DAT4 orange data channels The parameter list is i enumeration parameter i enumeration parameter 0 xX X output 9 OUT2 Aux Out2 1 Y Youtput 10 XNOise Xnoise 2 R R output 11 YNOise Yno
245. ynchronous filtering removes outputs at harmonics of the reference frequency most commonly 2xf This is very effective at low reference frequencies since 2xf outputs would require very long time constants to remove The synchronous filter does not attenuate broadband noise very well The low pass filters should be used to remove outputs due to noise and other non harmonic interfering signals The synchronous filter computes moving averages of the X and Y outputs over a complete reference period 1 f In this way outputs with periods 1 nxf e average to zero and are removed Thus frequency components in the output at nxf are all removed The synchronous filter follows the normal low pass filters time constant and is applied at the outputs Sensitivity In order for the synchronous filter to perform accurately the sensitivity must be set appropriately This is because the synchronous filter acts on the output scaled values of X and Y If the sensitivity is set too high the values of X and Y in the numeric displays will lack the necessary resolution If the sensitivity is set too low the values will overload In general setting the sensitivity to display a reasonable amount of bar graph is sufficient for accurate synchronous filtering If the synchronous filter overloads because of the sensitivity the Sync warning is displayed with displays of X Y R or 0 SR865 DSP Lock in Amplifier ASRS Chapter 3 Operation 69 Intern

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