Agilent Microwave Component Measurements Amplifier
Contents
1. cursor search function on channel 1 the 1 dB com pression point can be found The power out can then be read off the channel 3 cursor Press CHANNEL 1 CURSOR and select MAX Activate the cursor delta function by selecting CURSOR D ON Press SEARCH then select SEARCH VALUE and enter the desired search value in this case 1 0 dB Select SEARCH RIGHT and the cursor symbol will move to the 1 dB compression point Deactivate the cursor search trace hold by pressing CURSOR The channel 3 cursor should now read the amplifier output power at the 1 dB gain compression point see Figure 8 If the message Cursor value not found appears on the analyzer then the amplifier is not reaching its 1 dB compression point in the specified sweep Increase the dB Sweep or the start power Change the frequency and repeat the measurement A convenient way to do this is to set a step size in GHz and increment the frequency using the T key on the sweeper Press SHIFT CW T to increment the frequency It is not normally necessary to adjust the power sweep parameters once they are set up The sweeper must however stay in swept CW mode Return loss SWR measurements Return loss and SWR are commonly specified for the amplifier input and output ports With the 8757A reflec tion can be displayed as return loss in dB or in standing wave ratio SWR The reflection measurement setup shown in Figure 9 could be used for simult2. such as the RF noise at the amplifier output 14 Appendik 1 cont d AC versus DC detection If there is any doubt about which detection mode is the better choice in a particular measurement try both methods and compare the results The 85025 series de tectors can operate in either mode If there is no signifi cant difference then the choice may be arbitrary If there is a difference evaluate which method is more ac curate and make all measurements in that mode 8757A Scalar Amplifier network analyzer under test jy T Sweep oscillator N Figure C Example measurement of amplifier with automatic gain control AGC 1 measurement setup and 2 measured gain in AC and DC detec tion modes DC detection is better in this application Appendix 2 8350B powersweep All 83500 series RF plug ins have a power sweep capabili ty that utilizes the internal ALC circuitry of the plug in to ramp the power out The ALC dynamic range is deter mined by the minimum settable power and the maximum leveled output power of the plug in If the plug in has the optional step attenuator installed Option 002 then the maximum leveled output power and therefore the dynamic range will decrease due to the insertion loss of the internal attenuator In addition this attenuator cannot be switched during a power sweep due to the excessive wear that would be inflicted on the attenuator switches In normal operation the A
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4. actual values of small signal or large signal gain Note again that power out or in can be displayed on channel 3 Read power at 1 dB compression CHi B A Me O ea 248 2 2 7 mi Ame hana A a Ch e i 18 98 28 9 S di 10 08 AEF 19 78 a a rd wt Large signal gain 7 Ge 7i F sra 0000mr STAT 2 0000GHz CASA 0 00000Hz STOP 14 000GHz Figure 6 Example swept frequency gain compression measurement using the Alternate Sweep mode The large signal gain trace shows amplifier gain compression Measurement sequence cont d Single frequency gain compression power sweep Using the power sweep capability of the 8350B gain and output power can be displayed as a function of input power level This measure ment performed at a single frequency is described below 1 Return to the swept small signal gain measurement B R M on channel 1 You may need to normalize the measurement again using a THRU connection as de scribed in the first section Display output power B on channel 3 Activate the adaptive normalization feature of the 8757A Press SYSTEM then select ADPT NM ON With adaptive normalization as the frequency is changed the calibration data will be adjusted 2 Set any desired CW frequency on the source within the range of the original calibration Press SHIFT CW then enter the frequency for example 1 0 GHz The left hand LED display s
5. in the thru calibration menu The default averaging factor is 8 Wait until the trace stabilizes before selecting STORE THRU 9 Insert the amplifier under test and apply the appro priate bias If averaging is on restart averaging by pressing AVG then RESTART AVG Press SCALE then AUTOSCALE to view then small signal gain Press CURSOR and use the knob for a reading of gain at any point along the trace 10 To measure gain variation or ripple in this fre quency range use the cursor delta function Press CURSOR select MAX then turn CURSOR D ON Now select MIN The active entry area now displays the total peak to peak variation in gain across the band see Figure 4 In this note the Agilent 8757A front panel keys such as PRESET appear in bold type as opposed to the softkeys labeled on the CRT which appear in regular type e g CHAN 1 OFF Gain compression There are several ways to measure amplifier gain com pression using a scalar network analysis system The methods described here show how to cause 1 dB com pression of the amplifier then how to measure Pj the amplifier output power when 1 dB compression occurs Both swept frequency and single frequency methods are described The first method is a swept frequency meas urement which uses Alternate Sweep mode to measure small signal gain and large signal gain simultaneously Swept gain compression Swept gain compression meas
6. refer to the section AC versus DC Detection in Amplifier Measurements If using AC mode skip step 6 and go to step 7 6 Zero the DC detectors To do this press CAL then select DC DET ZERO Press AUTOZRO This detector zeroing should be repeated once every 5 10 minutes when using DC mode To do this automatically turn on REPT AZ in the DC DET ZERO menu and set the REPT AZ TIMER to the desired time value 7 Set the desired START and STOP frequencies on the source Also set the source power level keeping in mind that you will have about 6 dB loss through the power s plitter To measure power at the test port press CHANNEL 1 then MEAS then select B Press SCALE then select AUTOSCALE to view the data Press CURSOR and use the knob for a reading of power and frequency across the trace Adjust the power level of the source to the desired level Press CHANNEL 1 then press CHAN 1 OFF to turn channel 1 off again 8 Calibrate with the THRU connection Press CAL and select THRU Be sure the THRU connection is made then press STORE THRU to place the calibra tion data into memory Press DISPLAY then select MEAS MEM to view the normalized trace B R M Sometimes averaging may be required during calibration to remove the effects of noise This becomes particularly important when the detectors are measuring low level signals lt 40 dBm To activate averaging during cali bration select AVG ON
7. soft key Erase any limits that may have already been entered by selecting DELETE ALL LNS 2 Select FLAT LIMIT and the label FLAT FREQ 1 appears on the display Enter the frequency of the start of the limit line and terminate the entry using the ap propriate soft key NG A To display SWR the 8757A requires firmware revision 2 0 or higher If your 8757A has revision less than 2 0 order the 11614A Firmware Enhancement to upgrade 10 3 The analyzer will prompt you for the upper and lower limit values In this case we only use a lower limit since we are testing against a minimum gain specification Just press the ENT key when prompted for the upper limit When prompted for the lower limit enter the minimum gain specification and terminate the entry with the dB dBm key 4 Repeat steps 1 through 3 above as necessary until all limits are entered 5 Turn on limit lines by selecting DONE then LIM LNS ON Figure 12 shows an example limit entry with 3 flat segments Saving the measurement Once a measurement is configured it can be stored for future use in one of the nine SAVE RECALL registers of the 8757A The first four of these registers will also save limit lines and calibration data for channels 1 and 2 To store a measurement press SAVE then a number 1 through 9 No terminator is required This saves your measurement settings of both the analyzer and the source in non volatile memory Reca
8. will then display input A R 3 Press SYSTEM and select the desired detection mode AC or DC Either detection can be used with any of the 85027 series directional bridges If using AC detection mode skip step 4 and go to step 5 4 Zero the DC accessories To do this press CAL then select DC DET ZERO Press AUTO ZERO This detector zeroing should be repeated once every 5 10 minutes To do this automatically turn on REPT AZ in the DC DET ZERO menu and set the REPT AZ TIMER to the desired time value 5 Set the desired START and STOP frequencies on the source Also set the source power level Remember you will have approximately 6 8 dB loss through the direc tional bridge 6 Press CAL and select SHORT OPEN This initiates the short open calibration procedure As prompted connect the short circuit and press STORE SHORT Again as prompted connect the shielded open circuit and press STORE OPEN This procedure places the short open average into the channel 1 memory Press DISPLAY then select MEAS MEM to normalize the display trace 7 Connect the device under test Connect in the forward direction to measure input return loss or in the reverse direction for output return loss 8 Press SCALE and select AUTOSCALE Press CURSOR and use the knob to read the return loss in dB at any point along the trace Figure 10 shows an example XY Figure 11 Ex
9. A Deviation dB XY I Figure 15 Worst case deviation from ideal sguare law operation due to second harmonics when using a scalar network analyzer Notice that the un certainty is greater for high harmonic levels and at the higher power levels when the detector is in its linear operating region 12 If the power level at the detector must remain high fil tering can be used to reduce the effect of harmonics on amplifier measurements This can be done using a lowpass filter for measurements covering less than one octave An example is shown in Figure 16 a For more broadband measurements a tracking filter can be con nected as shown in Figure 16 b In either case the effects of the filter mismatch frequency response etc must be included in any uncertainty analysis Sweep oscillator a Low ass Amplifier aa Detector Sweep b oscillator LL Tracking filter Figure 16 The effects of harmonics can be reduced using a a low pass filter when measuring less than one octave or b using a tracking filter for broadband measurements If filtering is not practical operate the detectors in their square law region whenever possible to minimize the effects of harmonics Return loss The uncertainty of return loss measurements is de scribed by the following equation Ap A Bp Cp where Ap is the uncertainty in the reflection coefficient p is the reflec
10. K Agilent gn e Pi Microwave Component Measurements Amplifier Measurements Using the 6 e Scalar Network Analyzer O oe Application Note 345 1 6 6 e J e e one Agilent Technologies Introduction A scalar network analyzer provides fast economical measurements of many amplifier parameters This note describes gain gain compression isolation and return loss SWR measurements using the Agilent 8757A Scalar Network Analyzer and the Agilent 8350B Sweep Oscillator to illustrate the techniques Definitions and specific step by step instructions are included along with a description of accuracy considerations The system features such as alternate sweep power sweep trace cursor and pass fail limit lines are described All measurements described in this note are possible without the use of a computer However it is possible to automate these measurements using the same measure ment sequences For more information on the Agilent 8757A and amplifier measurements refer to the refer ences listed on page 15 This note describes how scalar network analysis can be used to measure several important amplifier parameters as a function of both frequency and input power It is important to keep in mind that many other factors can affect amplifier performance such as bias level temper ature and time In amplifier measurements all these variables must be taken into account for complete device characterizatio
11. LC and at tenuator are coupled and the values are automatically set when the power level is entered However the two can be decoupled and controlled independently by pressing SHIFT SLOPE to set the attenuator value and SHIFT POWER LEVEL to set the ALC power level Decoupling the ALC from the attenuator potentially allows you to use the full range of the ALC circuitry Changing just the attenuator value internal and or exter nal will not change the ALC level Changing just the ALC power level will change both the output power level and the ALC range of operation Let s look at an example using the 8350B with the 89592A RF plug in The ALC dynamic range of this plug in is 5 dBm to 10 dBm allowing 15 dB of power sweep range Assume that we want to sweep power from 21 dBm to 6 dBm On the 83592A plug in press POWER LEVEL and enter 21 dBm 20 dB of attenuation is automatically switched in and the low end of the ALC range is set to 1 dBm Press POWER SWEEP and enter 15 dB sweep The power is now being swept from 21 dBm to 6 dBm and the ALC is operating from 1 dBm to 14 dBm This is not within the allowable 5 dBm to 10 dBm ALC range of the plug in To remedy this decouple the ALC from the attenuator and add a 6 dB external attenuator Press SHIFT SLOPE and enter 10 dB of attenuation This plus the 6 dB of external attenuation provides 16 dB of attenuation for the system Press SHIFT POWER SWEEP a
12. Measurement seguence Gain compression measurements he following procedure describes how to measure ampli fier gain compression This measurement assumes that you have already measured the small signal gain as de scribed in the previous section The setup and calibra tion data remain the same Swept gain compression alternate sweep 1 Be sure channel 2 is measuring normalized gain B R M Note the power level indicated by the sweeper 2 Compression can be seen easily by simply increasing the power level from the source When the amplifier saturates the gain trace will fall Return the power level to the small signal input level 3 Display small signal gain B R M on both channels 1 and 2 Normalize both channels using the CAL THRU sequence described in step 8 of the section on gain measurements 4 Connect the amplifier under test Set the scale and reference level to identical levels on both channels so that the traces overlap Both traces should show the amplifier s small signal gain 5 Enable ALTERNATE SWEEP On the source press SAVE 1 then press ALT n 1 The system is now alternating between two identical states and the traces should still overlap 6 Press CHANNEL 1 then increase the source power level on the present state by pressing POWER LEVEL and turning the knob As the amplifier satu rates the channel 1 trace will fall Figure 6 shows an example Channel 1 shows the large signal ga
13. Ratio B R Li T External leveling a Sweep oscillator Crystal detector Power splitter To DUT Sweep oscillator b Power meter Power meter Power sensor Figure 17 Comparison of leveling and ratioing techniques Frequency re sponse at the test port using a internal source leveling b the ratio B R and c external source leveling When ratioing the power variation at the test port occurs because of mismatch between the detector and the source during calibration and between the amplifier and the source during measurement In either case the R detector tracks the variations and they are removed from the ratio B R While ratioing removes the effect of the source power variations external source leveling actually reduces the variations directly As shown in Figure 18 a an external detector provides feedback to an automatic leveling circuit ALC which then modulates the source output power to compensate for mismatch at the test port The result is that the source output power is flatter as a function of frequency see Figure 17 For further infor mation on external source leveling see Product Note 8350 9 The source power can also be leveled using the recorder output of a power meter as shown in Figure 18 b The major advantage of external or power meter source leveling is that the power at the test port is controlled by a feedback loop and is
14. ample input SWR plot in dB Measurement Sequence cont d Measuring SWR The reflection data can also be displayed as standing wave ratio SWR 9 To view SWR simply change the trace format Press DISPLAY then select TRC FMT SWR The trace is then displayed in SWR Press CURSOR and use the knob to view the SWR at any point along the trace see Figure 11 The trace format can be changed at any time without affecting the calibration in trace memory Even if the calibration is performed while in SWR trace mode the calibration stored in dB is still valid Figure 10 Example input return loss plot in dB Using limit lines and SAVE RECALL Using the limit line feature of the 8757A specification limits can be entered on the screen for comparison to the measured data Up to 12 limit entries can be entered as point limits sloped lines or flat lines The minimum gain specification for example can be entered as a lower limit If the gain falls below this lower limit then a FAIL condition exists Upper and lower limits could also be entered for testing of gain ripple or flatness For re flection measurements a SWR upper limit could be entered and displayed as the pass fail criterion The following procedure describes how to enter a series of flat limit line entries for testing the minimum gain on channel 2 1 Press CHANNEL 2 to activate channel 2 Press SPCL then press the ENTER LMT LNS
15. an uncertainty which could add in phase or out of phase with the original signal With detector match of 16 dB and effective source match of 20 dB the worst case mismatch error in calibration is approximately 0 14 dB from mismatch calculator Mismatch during measurement is caused by two sepa rate mismatches the mismatch between the amplifier input and the source and the mismatch between the am plifier output and the detector For example with an ef fective source match of 20 dB and an amplifier input match of 10 dB the first mismatch error is approximately 0 3 dB With an amplifier output match of 10 dB and detector match of 16 dB the second mismatch error is approximately 0 4 dB So the total mismatch error during measurement is 0 7 dB In the worst case calibration and measurement mis match uncertainties combine to form a larger window of uncertainty as shown in Figure 14 It is important to minimize the mismatches to minimize the total measure ment uncertainty 4 N Measurement F L pitt ttt uncertainty Actual gain pe Aeneas Calibration ial uncertainty bia J Figure 14 Total mismatch uncertainty includes both calibration and meas urement uncertainty as shown The uncertainty due to mismatches is a function of three things e effective source match e detector match e match of the amplifier under test The effective source match can be improved using three techniques ratioing external s
16. aneous reflection and transmis sion measurements The 8757A can be easily configured to display return loss on channel and gain on channel 2 If the reflection measurement is made with a separate setup ratioing may not be required since the measure ment is often only made at one power level If reflection is tested at several power levels then ratioing is recom mended Scalar network analyzer Sweep oscillator 116678 Power splitter Directional bridge Amplifier under test Mas A Figure 9 Test setup for simultaneous measurement of amplifier gain and input return loss Reflection parameters can be measured with a direction al bridge or with a directional coupler and another de tector The setup of Figure 9 describes the measurement with an 85027B Directional Bridge but also applies to a directional coupler such as the 11692D Calibration for reflection measurements is performed by normalizing to the average response of a short circuit and an open circuit This removes errors that occur during calibration due to directivity and source match This is discussed further in Accuracy Considerations Measurement Sequence The following procedure describes basic reflection measurements with the 8757A 1 Connect the equipment as shown in Figure 9 Ratioing with the power splitter may not be required 2 Activate channel 1 and turn all other channels off Press MEAS and select A R Channel 1
17. d from the amplifier is actually at a lower level than the amplifier s broadband noise floor AC detection rejects the noise and detects the return signal With DC detection the noise masks the return signal Some amplifiers can be affected by the 27 778 kHz modu lation Some examples are the following e Amplifiers with Automatic Gain Control AGC e Amplifiers with high gain at very low frequencies lt 1 MHz e Amplifiers with slow responding self bias The measurement of an amplifier with AGC is shown in Figure C Note that the function of an AGC is to provide a desired output power over a whole range of input power levels Figure C shows the measurement made in both AC and DC detection modes In DC mode the am plifier input and output are unmodulated The detector downconverts to a DC level then chops the signal to form a 27 778 kHz square wave for the receiver The same measurement in AC mode RF modulated shows that the leveling circuit is adversely affected by the mod ulation The AGC tries to adjust its gain to track the modulation but cannot The resulting square wave is distorted and the scalar analyzer response is degraded Note that DC detection is the better choice in this appli cation Input from 4 sweep oscillator Directional bridge _ Termination Amplifier J l f under test SUL Signal to analyzer Figure B AC detection rejects unwanted signals that are unmodulated
18. ef descriptions of the amplifier parameters that can be measured using a scalar network analyzer Gain Amplifier gain is defined as the ratio in milliwatts of the amplifier output power delivered to a Zo load to the input power delivered from a Zo source where Zo is the characteristic impedance in which the amplifier is used 50Q in this note In logarithmic terms the gain is the difference in dB between the output and input power levels expressed in dBm Amplifier gain is most commonly specified as a minimum gain value in the linear operating range This would guar antee a given output power for a given input power Since variations in frequency response can cause distortion gain flatness is often specified over the frequency range of the amplifier Gain compression Figure 1 a shows an example plot of amplifier output power versus input power at a single frequency Gain at any power level is the slope of this curve Figure 1 b Notice that the amplifier has a region of constant gain where gain is independent of input power level Reverse isolation Reverse isolation is the measure of transmission from output to input The measurement of isolation is similar to the measurement of gain except that the amplifier is reversed Reverse isolation is typically 1 5 to 2 times the forward gain This is commonly referred to as small signal gain As the input power is increased to a level that causes the amplifier t
19. es frequency response and improves the ef fective source match However normalization is also required even when ratioing to remove tracking differ ences between the two arms of the power splitter and between the two detectors B and R This normalization is also described in the small signal gain measurement procedure 8350B 83592A Sweep oscillator Attenuator optional 11667B Power splitter XS Thru q Attenuator Amplifier under test 8757A Scalar network analyzer 00000000 recommended 85025B Detector Figure 3 Test setup for measuring amplifier gain and gain compression characteristics Measurement seguence Small signal gain measurement The following procedure describes the measurement of small signal gain versus frequency 1 Connect the instruments as shown in Figure 3 Connect the B detector directly to the test port Do not connect the device under test yet 2 Press PRESET on the 8757A This brings the ana lyzer and sweeper to a knon state 3 Press the CHAN 1 OFF soft key on the 8757A This turns channel off and leaves channel 2 being displayed as the active channel 4 Press MEAS then select B R This displays the ratio B R on channel 2 5 Press SYSTEM then select the desired detection mode AC or DC DC mode can be only used with the 85025 and 85026 series detectors For a description of these detection modes
20. hould display zeroes This indicates swept CW mode SHIFT CW in which the source SWEEP OUT drives the horizontal axis of the 8757A display to make this axis power instead of fre quency 3 Activate power sweep mode on the source Press POWER SWEEP and the power sweep LED should be lit On the sweeper enter the sweep range required to saturate the amplifier e g 10 dB per sweep Most 8350B RF plug ins can sweep up to 15 dB from the start power See Appendix 2 for more information on using the 4 The 8757A display should now show the gain as a function of input power level at a single frequency Figure 8 shows an example Notice that the gain de creases as the amplifier enters saturation If the gain does not decrease by the desired amount usually 1 dB then increase the dB sweep value or the start power 5 Press CHANNEL 1 then select CHANNEL 3 Press SCALE then select AUTOSCALE This trace shows the amplifier output power Notice that this power in creases linearly until saturation occurs kan CHa ogame 18 32 9 5 dh REF 16 11 di CHS B 11 03 dBm 2 0 08 REF 6 00 dBm jl a a KANA KA STAT BA CRSA 10 000GHz STOP 10 0000Hz I Figure 8 Single freguency gain compression measurement using the Power Sweep feature Gain and power are displayed as a function of input power 6 Find the power out at 1 dB compression Use the
21. in and channel 2 shows the small signal gain The system alternates between the two input power levels Gain compression at any frequency is simply the differ ence between the two traces 7 In this configuration power can be measured on channel 3 Press CHANNEL 1 then select CHANNEL 3 Press MEAS and select B to display the amplifier output power in compression or R to display the input power to cause compression Since channels 1 and 3 are alternated with channels 2 and 4 channel 3 in this configuration will display the large signal output power see Figure 6 8 Press ALT n to deactivate alternate sweep Return the source power level to the small signal input Turn off channel 2 by pressing CHANNEL 2 then CHAN 2 OFF r 4 Figure 7 Example normalized swept frequency gain compression measure ment Gain compression is measured relative to small signal gain Normalized swept gain compression 1 With channel 1 still measuring normalized small signal gain B RM press CHANNEL 1 DISPLAY then select MEAS gt MEM The channel 1 display is now normalized to the ampli fier s small signal gain Press SCALE and select AUTOSCALE The trace should be a flat line near 0 dB 2 Increase the source power level until the trace falls by 1 dB at some frequency An example is shown in Figure 7 This display shows compression from a flat trace but does not show the
22. lected signals and permits gain measurements to be made at different RF power levels without recali brating Since source power level variations are moni tored in both the reference and measurement channels their effect is removed from the ratio This same source match improvement can also be ac complished using source leveling as described in Ratioing or Leveling in the section Accuracy Considerations Also note that ratioing or source level ing could be accomplished using a directional coupler such as the 11692D instead of a power splitter The coupler causes less power loss but typically is not as broadband as the power splitter Attenuation of the amplifier output is recommended as required to keep the power level to the detectors in their square law region of operation below approxi mately 15 dBm especially when gain compression will be measured Note that the attenuators will be included in the transmission thru normalization so their fre quency response will be removed from the measurement is described more in the section Accuracy Considerations CH2 B A Me 0 3 17 98 1 0 o8 REF 19 75 ab N Figure 4 Example 8757A plot of amplifier gain vs frequency Small signal gain Small signal gain is the gain in the amplifier s linear region of operation Figure 4 shows an example swept frequency gain measurement Ratioing in this measure ment remov
23. ll that measurement at any time by pressing RECALL and entering the same number ra N d TERE yE A M AE RE R b a PASS 24 00 o8 J Figure 12 a Example limit line entries for minimum gain specification and b plot showing limit lines gain trace and PASS FAIL status Accuracy considerations The accuracy of amplifier measurements with a scalar analyzer is determined by many factors This section summarizes the key accuracy considerations for gain gain compression and return loss measurements and discusses possible ways of reducing these errors Some applications may require better accuracy than the scalar analyzer can provide A vector network analyzer such as the 8510A not only provides phase data but also provides vector accuracy enhancement and immuni ty to harmonics and other spurious signals The result is significantly better measurement accuracy Gain The major sources of error in measuring amplifier gain with a scalar analyzer are the following e mismatch during calibration e mismatch during measurement e system dynamic accuracy Mismatched errors are caused by re reflection signals within the measurement system Mismatch during calibration results because the detec tor reflects a portion of the signal back toward the ef fective source actually the power splitter This reflected signal is then re reflected from the source causing
24. n Equipment required The following equipment is used in the measurements described in this note 8757A Scalar Network Analyzer 8350B 83592A Sweep Oscillator 85027B Directional Bridge 85025B Detectors 11667B Power Splitter 85023B Verification Kit 85022A system cable kit The Agilent 8757A is a powerful easy to use scalar ana lyzer It provides three detector inputs a fourth input is optional and four independent display channels With the 11664A E Detectors the 8757A offers 60 dBm sensi tivity at sweep speeds as fast as 50 ms With the Agilent 85025A B Detectors and the Agilent 85027A B C Directional Bridges the 8757A offers the choice between two detection modes In AC mode the detectors detect the envelope of signals modulated by a 27 778 kHz square wave This modulation is provided internally by the Agilent 8350B sweep oscillator Spurious unmodulated signals and broadband noise are undetected In DC detec tion mode modulation is not required and the detector responds to all signals in its frequency range The 8350B Sweep Oscillator is used in all of the following measurement setups An 8340B or 8341B Synthesized Sweeper could also be used for ap plications requiring higher frequency accuracy and stability When higher source power is required to deliver more power to the device under test the 8349B amplifier can be used to provide up to 18 dBm from 2 20 GHz Amplifier definitions This section contains bri
25. nd enter 5 dBm The plug in now operates over the full 5 dBm to 10 dBm ALC dynamic range The same method can be applied to the 8340B or 8341B The internal step attenuator is standard on both instru ments SHIFT POWER SWEEP decouples the ALC from the attenuator ATTN XX dB ALC X XX dBm appears in the ENTRY DISPLAY The ALC power level is set with the keypad or knob and the attenuator is set with the step keys The performance of the synthesized sweeper is opti mized when the ALC operates within the range of 20 dBm to 20 dBm As an example assume that the 8340B or 8341B has a maximum leveled output power of 10 dBm This means that the ALC can operate from 20 dBm to 10 dBm as opposed to 5 dBm to 10 dBm for the 8350B The synthesized sweeper has 15 dB more dynamic range available as compared to the 8350B A minimum power sweep range of 20 dB can be achieved in any part of the dynamic range without using any external attenua tion List 0f references Application Note 183 High Frequency Swept Measurements literature no 5952 9200 Application Note 329 Performance Characteristics of Microwave Signal Sources literature no 5953 8883 Application Note 326 Principals of Microwave Connector Care literature no 5954 1566 Barretter and Diode Comparison For Insertion Loss Measurements in the Presence of Harmonics by Fritz K Weinert Dr Bruno 0 Weinschel and Donald D Woodruff Mic
26. nmodulated RF then chops the detected signal The receiver sees the same type square wave signal in either mode In DC mode the detectors respond to all signals present and 27 778 kHz source modulation is not required As shown in Figure A 2 when in DC mode the 85025 series Detectors chop the signal after detection to provide a 27 778 kHz square wave to the receiver The receiver cir cuitry used is identical in both modes The choice between detection modes depends on the par ticular application In many applications the choice is arbitrary because the results in either mode would be i dentical But some applications are more suited to one mode or the other Absolute measurements of power dBm are usually more accurate in DC detection mode because the meas urement is not subject to variations in source modula tion DC mode is usually a better choice for measuring power out at 1 dB gain compression for example In AC mode power measurements are subject to changes in and square wave duty cycle In addition DC mode is more easily referenced to a power meter In AC mode since the source is square wave modulated the power meter reading would be nominally 3 dB lower than the scalar analyzer reading This is not the case in DC mode AC mode is the better choice whenever the low levels signals must be detected in the presence of higher level broadband noise Figure B shows an output return loss measurement where the signal returne
27. not subject to variations due to mismatch at the test port However ratioing is often more convenient and easy to use than source leveling 13 Splitter X Figure 18 Source leveling techniques a using an external crystal detector and b using a power meter Appendix 1 AC versus DC detection The 8757A offers a choice of detection modes AC detec tion which uses modulation for immunity to broadband noise and thermal drift and DC detection which offers fast accurate power measurements without modulation This section describes the capabilities and advantages of each mode when measuring amplifiers In AC mode the RF source is modulated by a 27 778 kHz square wave The detector then processes only the modu lated signal see Figure A 1 The 8350B sweep oscilla tor provides this modulation internally Other signals such as DC or thermal drift broadband noise and spuri ous signals from other sources are unmodulated and therefore go undetected AC detection is ideal for most relative measurements such as gain and return loss par ticularly in the presence of undesired unmodulated signals AC detection also requires no detector zeroing 2 85025B Detector Chopping circuit converts DC level to squarewave DUT HaHa ee Loe ad DC Voltage Squarewave aaa SUL AW RF RF Ov Figure A Comparison of detection modes 1 AC detection uses RF modula tion 2 DC detection detects u
28. o saturate gain decreases causing the large signal response showing the limitation in the amplifi er s output power In this note gain compression is measured by measuring the output power when gain is decreased by 1 dB see Figure 1 b This 1 dB gain compression point Pp is a common measure of an amplifier s output capability Both single frequency and swept gain compression tests are described in this note Another common measure of amplifier output is Pu the maximum power an amplifier can deliver Pu is the output power level for which further increase in input power yields no further increase in output power P at measurements are not described in this note b Power dBm Gain dB i Input power dBm l Input power dBm Figure 1 Typical amplifier s characteristics a Output power versus input power and b gain versus input power Return loss SWR Another amplifier parameter commonly specified is the impedance match at the input and output ports The most common scalar parameters are defined by the fol lowing equations p V AN reflected incident Return loss 20 log p SWR 1 p 1 p Gain compression and isolation measurements Gain gain compression and power can all be measured using the setup shown in Figure 3 Note that ratioing is performed using the power splitter This ratioing im proves the effective source match to remove the effects of re ref
29. ource leveling and isola tion Ratioing was used in the above example For a comparison of ratioing and leveling see Ratioing or Leveling Isolation can be accomplished most easily with an attenuator on the output of the source This attenua tion decreases the magnitude of the re reflected signal Effective detector match can be improved by inserting an attenuator before the detector This improvement would be significant only if the attenuator has far better match than the detector As the attenuation is in creased the effective detector match approaches the match of the attenuator Amplifier match is normally a function of the amplifier design and cannot be further im proved However it is important to realize that measurements of a well matched amplifier will typically contain less uncertainty than those of a poorly matched amplifier Dynamic accuracy also influences gain measurement un certainty Gain is a relative measurement that is the output power relative to the input power measured with the thru normalization This normalization accounts for the frequency response of the detectors However the system s response is also a function of power level Specifically if the power level seen by the detector is d ifferent between calibration and measurement then the detector response causes an additional uncertainty in the measurement of gain The uncertainty due to the detector is a function of how much the power change
30. ovides a systematic method of measuring the power at 1 dB compression for a given frequency This can be accomplished using the power sweep feature of the 8350B At a single CW frequency a power ramp is input to the amplifier under test and gain is measured directly as a function of input power see Figure 8 Again output power P1dB can be displayed on a sepa rate channel When using power sweep with the 8350B with RF plug in and the optional step attenuator it is important to under stand the interaction between the Automatic Leveling Circuit ALC and the attenuator For details on how to sweep over a given range of power see Appendix 2 Reverse isolation Transmission from output to input is defined as the reverse isolation of an amplifier It can be measured using the setup shown in Figure 8 Note that Figure 5 is very similar to Figure 3 used to measure small signal gain The differences are that the amplifier is reversed and that the test port power level should be significantly higher as close as possible to the amplifier s typical output power level To measure isolation with the 8757A follow the instruc tions for measuring small signal gain making adjust ments as needed 8757A Scalar network analyzer 8350B 83529A Sweep oscillator R 85025B Detector n Amplifier under test 85025B 11667B Detector Power splitter Figure 5 Test setup for measuring amplifier isolation
31. performance and functionality When you are choos ing new equipment we will help you with product information including realistic performance specifications and practical recommendations from experienced test engineers When you use Agilent equipment we can verify that it works properly help with product operation and provide basic measurement assistance for the use of specified capabilities at no extra cost upon request Many self help tools are available Your Advantage Your Advantage means that Agilent offers a wide range of additional expert test and measurement services which you can purchase accord ing to your unique technical and business needs Solve problems effi ciently and gain a competitive edge by contracting with us for calibration extra cost upgrades out of warranty repairs and on site education and training as well as design system integration project management and other professional engineering services Experienced Agilent engineers and technicians worldwide can help you maximize your productivity optimize the return on investment of your Agilent instruments and systems and obtain dependable measure ment accuracy for the life of those products By internet phone or fax get assistance with all your test and meas urement needs Online assistance www agilent com find assist Phone or Fax United States tel 1 800 452 4844 Latin America tel 305 269 7500 fax 305 269 7599 Canada tel 1
32. rowave journal March 1975 pp 39 43 Product Note 8350A 6 Reduced Harmonic Distortion Using the Integra TMF 1800H Tracking Filter with the 8350 Sweep Oscillator literature no 5952 9345 Integra Microwave Santa Clara California Application Note FT2 Ferretrac Hands Off Tracking Filter Reduces Spurious and Harmonic Output Signals from RF Sources Ferretec Inc San Jose California 8757A Scalar Network Analyzer Operating Manual part no 08757 90034 8350B Sweep Oscillator Operating and Service Manual part no 08350 90034 Product Note 8350 9 Improving the output power flatness of the 8350B Sweep Oscillator literature no 5954 8344 Agilent Technologies Test and Measurement Support Services and Assistance Agilent Technologies aims to maximize the value you receive while mini mizing your risk and problems We strive to ensure that you get the test and measurement capabilities you paid for and obtain the support you need Our extensive support resources and services can help you choose the right Agilent products for your applications and apply them successfully Every instrument and system we sell has a global warran ty Support is available for at least five years beyond the production life of the product Two concepts underlie Agilent s overall support policy Our Promise and Your Advantage Our Promise Our Promise means your Agilent test and measurement equipment will meet its advertised
33. s between calibration and meas urement This is specified on systems as dynamic accu racy Detector error over a 30 dB range is typically 0 1 dB It is possible to reduce the effects of dynamic accuracy by inserting attenuation during the measurement after cali bration to keep the calibration power level as close as possible to the measurement power level For high gain measurements gt 30 dB this post attenuation or RF sub stitution technique is recommended However the fre quency response of the external attenuator must be removed from the measurement since it is not included during the calibration process The 8757A detector offset function can remove a nominal attenuation value from the measured data Gain compression The measurement of gain compression is subject to the same errors as the measurement of gain in addition to the following e detector power measurement accuracy e harmonics at the detector Power measurement accuracy A gain compression measurement really consists of two measurements first of gain then of power when gain compresses by 1 dB Pn Normally the uncertainty in the gain meas urement will not translate directly to the uncertainty of the power measurement When measuring compres sion large signal gain is normally compared to small signal gain at a single frequency Only the power level is changed So the uncertainty in the measurement of 1 dB compression is the uncertainty in
34. the measure ment of the difference between small signal gain and large signal gain This will be determined by the dynamic accuracy of the detectors and by how much the amplifier s match varies as it saturates The ability to measure power dBm also determines the accuracy of the measurement of P Power accuracy is specified for the 85025 series detectors in DC mode typ ically 0 15 dB and is enhanced by referencing the detector to a power meter sensor as described in the 85025A B Operating and Service Manual Harmonics Another important factor in measuring am plifier compression is the presence of harmonics most commonly second and third harmonics of the test signal Harmonics are also present in the measurement of gain but their magnitude is typically low enough that their effect is negligible When in compression the amplifier under test tends to generate higher harmonic levels for example 20 dBc and this affects the accuracy of the scalar analyzer Figure 15 shows the worst case uncertainty of a scalar power measurement in the presence of second harmon ics 15 20 25 and 30 dB below the fundamental Notice that the errors are insignificant at the lower power levels where the detector measures total rms power square law region For this reason attenuation is rec ommended to keep the power level at the B detector as low as possible below 15 dBm a N T Measured using 85025B Detector and 8757
35. tion coefficient of the device under test A is directivity B is calibration uncertainty and C is the effective source match The A term can be kept to a minimum using a high direc tivity directional bridge and high quality adapters The B term can be removed through open short averaging if a coaxial system is used In waveguide calibration can be accomplished with a short only B A C The C term can be reduced by improving source match using the tech niques already discussed for gain measurements Ratioing or leveling In a swept frequency measurement effective source match can be improved either by ratioing or by leveling the source externally Both methods provide similar source match improvement However there are impor tant distinctions between the two techniques This section describes the differences Ratioing improves effective source match by reducing the effect of source power variations versus frequency Because the power variations appear in both detectors B and R for example they are not seen in the ratio B R Figure 17 shows a typical plot of B R and the ratio B R Notice that while B R is relatively flat B can vary by approximately 0 5 dB Some amplifier measure ments can be adversely affected by this type of ripple particularly with fast sweeps STR CH2 eA ctis Bos mer 7 50 obe it 969 ner ao 08 Ons 8 1 0 OB REF 7 50 68m TJ T ternal source leveling n adl ee aT Sa a
36. urements can be made using the Alternate Sweep feature of the 8757 and the 8350 to alternate between two instrument states at dif ferent power levels see Figure 6 Both small signal gain and large signal gain can be viewed in real time The dif ference between traces is due to compression The source power can be adjusted so that 1 dB compression occurs at any desired frequency and then the power out Pap can be measured with a separate channel Notice in this measurement that both traces are active so it is possible to see how device tuning affects both small and large signal gain This makes device tuning more effective Normalized swept gain compression In the alternate sweep measurement it is difficult to see exactly at which frequency 1 dB gain compression first occurs This can be more easily seen using normalization to the small signal gain Small signal gain is placed into memory then normalized As the power level is increased compression can be observed as the drop from a flat ref erence line The worst case frequency can be easily deter mined see Figure 7 Notice that in this measurement the actual values of small and large signal gain are not displayed Single frequency gain compression The normalized compression measurement is a very useful test for finding the worst case compression point Again it is a swept frequency measurement of gain com pression Notice however that neither of the above methods pr
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