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FS725 Rubidium Frequency Standard

1. NPO Capacitor Chip SMT1206 50V 5 NPO Cap Ceramic 50V SMT 1206 10 X7R Capacitor Chip SMT1206 50V 5 NPO Capacitor Chip SMT1206 50V 5 NPO Cap Ceramic 50V SMT 1206 10 X7R Capacitor Chip SMT1206 50V 5 NPO Capacitor Chip SMT1206 50V 5 NPO Cap Ceramic 50V SMT 1206 10 X7R Cap Ceramic 50V SMT 1206 10 X7R SMT Film Capacitors 50V 5 All Sizes Cap Ceramic 50V SMT 1206 10 X7R SMT Film Capacitors 50V 5 All Sizes Cap Tantalum SMT all case sizes Cap Tantalum SMT all case sizes Capacitor Chip SMT1206 50V 5 NPO Cap Tantalum SMT all case sizes Cap Tantalum SMT all case sizes Cap Tantalum SMT all case sizes Capacitor Chip SMT1206 50V 5 NPO Capacitor Chip SMT1206 50V 5 NPO Capacitor Chip SMT1206 50V 5 NPO Cap Ceramic 50V SMT 1206 10 X7R Capacitor Chip SMT1206 50V 5 NPO Cap Ceramic 50V SMT 1206 10 X7R Capacitor Chip SMT1206 50V 5 NPO Cap Ceramic 50V SMT 1206 10 X7R Cap Ceramic 50V SMT 1206 10 X7R Cap Ceramic 50V SMT 1206 10 X7R Cap Ceramic 50V SMT 1206 10 X7R Cap Ceramic 50V SMT 1206 10 X7R Cap Ceramic 50V SMT 1206 10 X7R Cap Ceramic 50V SMT 1206 10 X7R Cap Ceramic 50V SMT 1206 10 X7R Cap Ceramic 50V SMT 1206 10 X7R SMT High Voltage Porcelain Cap Cap Ceramic 50V SMT 1206 10 X7R Cap Ceramic 50V SMT 1206 10 X7R Capacitor Chip
2. 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor PRS10 Parts List 71 REF SRS PART VALUE DESCRIPTION 274 4 01191 462 5 90K Thin Film 1 50 ppm MELF Resistor R 275 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 276 4 01305 462 90 9K Thin Film 1 50 ppm MELF Resistor R 277 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 278 4 01305 462 90 9K Thin Film 1 50 ppm MELF Resistor R 279 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 280 4 01305 462 90 9K Thin Film 1 50 ppm MELF Resistor R 281 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 282 4 01305 462 90 9K Thin Film 1 50 ppm MELF Resistor R 283 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 284 4 01294 462 69 8K Thin Film 1 50 ppm MELF Resistor R 285 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 286 4 01294 462 69 8K Thin Film 1 50 ppm MELF Resistor R 287 4 01280 462 49 9K Thin Fil
3. 4 01088 462 4 0125 1 462 4 0125 1 462 4 01117 462 4 01117 462 4 01467 461 4 01467 461 4 01088 462 4 01355 462 4 01309 462 4 01479 461 4 01471 461 4 01479 461 4 01479 461 4 01447 461 4 01503 461 VALUE 10K 240 240 240 240 UNDECIDED PART 240 240 240 240 UNDECIDED PART 3 9K 100K 3 9K 100K UNDECIDED PART UNDECIDED PART 10 0K 249K 1 00M 47 220 100K 7 50K 2 7M 10 0K 30 1K 301K 100K 100K 499 499 499 24 9K 24 9K 1 00K 1 00K 330 330 499 301K 100K 1 0K 470 1 0K 1 0K 47 10K PRS10 Rubidium Frequency Standard DESCRIPTION Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Hardware Misc Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Hardware Misc Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Hardware Misc Hardware Misc Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Res
4. FB43 301 FB43 301 FB43 301 64 PIN STRIP 64 HDR PIN R A 4 7UH 5PH 4 7UH 5PH 100UH SMT 100UH SMT 6611 TYPE 43 6611 TYPE 43 FR47 FR47 FR47 FR47 027UH SMT 012UH SMT 18UH SMT 2 2U SMT 1 0U 1 5 WIRE 1 5 WIRE 50K 9MM SIDE RB MULTIPLES PRS10 Rubidium Frequency Standard DESCRIPTION SMT High Voltage Porcelain Cap SMT High Voltage Porcelain Cap SMT High Voltage Porcelain Cap SMT High Voltage Porcelain Cap Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Diode SMT Diode SMT Diode SMT Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg 2 lt wa Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Diode Step Recovery Connector D Sub Male Connector Misc SMB Connector SMB Connector SMB Connector Ferrite Beads Ferrite Beads Ferrite Beads Connector Male Connector Male Inductor Variable Inductor Variable Inductor Fixed SMT Inductor Fixed SMT Ferrite Beads Ferrite Beads Ferrite bead SMT Ferrite bead SMT Ferrite bead SMT Ferrite bead SMT Inductor Fixed SMT Inductor Fixed SMT Inductor Fixed SMT Inductor Fixed SMT Inductor Axial Hardware
5. Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Voltage Controlled Crystal Oscillator Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Voltage Reg TO 220 TAB Package Integrated Circuit Surface Mount Pkg Voltage Reg TO 220 TAB Package Voltage Reg TO 220 TAB Package Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Voltage Reg TO 220 TAB Package Voltage Reg TO 220 TAB Package Crystal Crystal Nut Mini Washer Split Washer lock Washer nylon Insulators Screw Allen Head Screw Allen Head 75 PRS10 Rubidium Frequency Standard 76 PRS10 Parts List REF SRS PART VALUE Z0 0 00607 025 4 40X1 2 SOCKET Z0 0 00608 025 6 32X1 4 BUTTON Z0 0 00609 025 6 32X5 8 SOCKET Z0 0 00629 066 FOIL CU 1 2 Z0 0 00630 034 22 INSULATING Z0 0 00641 031 4 40X3 16 M F Z0 0 00642 031 4 40X3 8 M F Z0 0 00643 020 4 40X
6. LO LO value Frequency lock loop status FC FC high low FC FC Frequency control values DS Read detected signals and 20 SF SF value Set frequency offset SS SS value SS SS Set Slope SF calibration GA GA value GA GA FLL Gain parameter PH PH value PH PH Phase angle parameter SP SP r n a SP SP Set synthesizer parameters Magnetic Tuning MS MS value Magnetic switching MO MO value MO MO Magnetic Offset MR Magnet read 1PPS Lock TT Time tag 1 input TS TS value TS TS Time slope cal 1pps input TO TO value TO 101 Time tag offset PP value Place pulse 1pps output PS PS value PS PS Pulse slope cal 1pps output PL PL value PL PL Phase lock to 1pps input PT PT value PT PT Phase lock time constant PF PF value PF PF Phase lock stability factor PI PI value Phase lock integral term PRS10 Rubidium Frequency Standard Abridged Command List Set Value ie Query Description EPROM EEPROM D A Control SD0 SD0 value SDO 00 Set DAC RF amplitude SD1 SDl value SD1 SD1 Set DAC 1005 delay SD2 SD2 value SD2 SD2 Set DAC lamp intensity SD3 SD3 value SD3 03 Set DAC lamp temperature SD4 SD4 value SD4 04 Set DAC crystal temperature SD5 SD5 value 05 05 Set DAC cell temperature SD6 SD6 value 06 06 Set DAC 10 MHz amplitude SD7
7. Misc Hardware Misc Pot Multi Turn Cermet Various sizes Printed Circuit Board 0 100 0 101 0 150 0 400 0 500 Q 501 0 502 0 503 0 504 Q 600 0 800 Q 900 100 101 102 103 104 105 106 107 108 109 110 R 11 112 R113 R114 R115 R 116 R117 118 R 119 120 121 123 150 151 153 154 200 R 201 R 202 R 203 204 R 205 206 R 207 R 208 SRS PART 3 00808 360 3 00555 360 3 00325 329 3 00895 360 3 00807 360 3 00540 360 3 00808 360 3 00809 360 3 00810 360 3 00325 329 3 00325 329 3 00665 360 4 01242 462 4 01184 462 4 01309 462 4 00954 462 4 01280 462 4 01213 462 4 01447 461 4 01088 462 4 01184 462 4 01213 462 4 01184 462 4 01067 462 4 01447 461 4 01146 462 4 01096 462 4 0125 1 462 4 0125 1 462 4 01479 461 4 01503 461 4 01503 461 4 01503 461 4 01213 462 4 00925 462 4 01407 461 4 01407 461 4 00899 43 1 4 00899 43 1 4 01280 462 4 01305 462 4 01295 462 4 01213 462 4 01557 461 4 01575 461 4 01213 462 4 01376 462 4 01230 462 VALUE MMBRS179 MMBR941L TIP 107 NE461M02 MJD47 MMBT5087 MMBRS179 MMBTHSILTI MMBTHIOLTI TIP107 TIP107 MRF134 20 0K 4 99K 100K 20 49 9K 10 0K 47 499 4 99K 10 0K 4 99K P1H104 T NTC P1H104 T NTC 49 9K 90 9K 71 5K 10 0K 1 8M 10M 10 0K 499K 15 0K PRS10 Parts List DESCRIPTION Integrated Circuit Surface Mount Pkg Integrated Circuit Surface
8. are low and the unit is not locked Example Immediately after power is applied to a unit the command ST returns 16 3 21 1 2 129 From the status byte definitions below we see that the following conditions exist 16 the lamp has not yet started 3 the RF VCXO has not yet locked 21 the lamp crystal and cells are all below their set point temperatures 1 the frequency lock has not been established 2 fewer than 256 Ipps inputs have been qualified 129 both the lamp and unit have been restarted ST1 Power supplies and Discharge Lamp STI bit Condition which sets bit Corrective Action 24 for electronic lt 22 Vde Increase supply voltage 24 for electronics gt 30 Vdc Decrease supply voltage 24 for heaters lt 22 Vdc Increase supply voltage e O PRS10 Rubidium Frequency Standard RS 232 Instruction Set 39 24 for heaters gt 30 Vdc Decrease supply voltage Lamp light level too low Wait check SD2 setting Lamp light level too high Check SD2 setting 6 Gate voltage too low Wait check SD2 setting Gate voltage too high Check SD2 setting ST2 RF Synthesizer ST2 bit Condition which sets bit Corrective Action _ 0 RF synthesizer PLL unlocked Query SP verify values _ 6 RF AGC control too high Check 00 values ST3 Temperature Controllers ST3 bit Condition which sets bit Corrective Action 0 _ Lamp temp below setpoint Wait
9. current stability factor to the EEPROM for use as the new default PF may be used to read the value of the stability factor which is stored in EEPROM PI PI value 2000 lt value lt 2000 Phase lock integrator This command is used to set the value of the integral term in the PLL s digital filter It is not necessary to set this value as it will be initialized by the PLL routine to the current frequency setting parameter when the PLL begins Users may want access to the value to alter the PLL characteristics or to investigate its operation Example PI 0 will set the integrator in the PLL s digital filter to 0 which is the center of the 2000 bit range PI will return the current value of the PLL integrator There are two terms which control the phase locking of the PRS10 to an external 1 source the integral term and the proportional term The proportional term is equal to the value returned by an SF minus the value returned by the PI Analog Control SD port SD port value 0 lt port lt 7 and 0 lt value 255 factory only SD port SD port Set DAC This command is used to set or read the settings of an octal 8bit DAC which provides analog signals to control systems parameters The command which sets values is only available to the factory The command to query values may be used by all The query command returns a single integer in the range of 0 to 255 Port Function 4 0 Controls the amp
10. measurements For a detailed description for the operation of the SR620 refer to the instrument s operation and service manual Four input connections The 10MHz reference frequency is connected to both the rear panel 10MHz input and to the A START input Place the tee on the rear panel input Connect the 10MHz from the DUT to the B STOP input Connect the IKHz TTL square wave from REF output to gate EXT input Four input setups From the front panel CONFIG menu use SET to choose the cAL menu then use SELECT to select the cLoc SourcE Use the arrow keys to set the clock source to rEAr This will allow the SR620 to use the 10MHz reference frequency which has been applied to the rear panel 10MHz input as the timebase for all measurements Set the EXT gate input LOGIC to POS TERM to 500 and LEVEL to 1 0 The TRIG LED will go on when the GATE ARM is setup properly Both START and STOP are AC coupled and terminated into 500 The SLOPE is set to and the LEVEL is turned full counter clockwise to AUTO and the UHF LED should be off The TRIG LEDs will be on when the 10MHz sources present Coarse Frequency Measurements You should verify that the DUT is very close within 0 1Hz to 10MHz To measure the frequency set MODE to FREQ set SOURCE to B set the GATE
11. 10 MHz to within 1 x 107 or 10 MHz 0 0001Hz Important details are list below 1 Be certain that the FS725 has been operated continuously for at least 24 hours 2 Ifthe FS725 has been off for more than one week it should be operated continuously for at least 72 hours before the adjustment is made Be certain that nothing is connected to the rear panel FREQ ADJ input Be certain that nothing is connected to the rear panel pps input Measure and record the dc voltage on the center pin of the FREQ ADJ BNC Power cycle the FS725 by unplugging it for at least three seconds This will make certain that the unit will track the calibration potentiometer which can be ignored if the unit was phase locked to a 1 pps input or the frequency had been previously set via the SF command The unit should lock in less than a minute as it is warm from previous operation Wait for at least one hour before proceeding with the calibration 7 The frequency standard used in the calibration must have an accuracy of better than 1 x 107 or 10 MHz 0 0001 Hz A cesium standard or a well characterized rubidium or quartz standard referenced to GPS may be used 8 The frequency counter must have sufficient resolution and accuracy to perform the measurement This may require long gate times in the frequency measurement mode of operation A 100s gate is recommended when using an SR620 Time Interval Counter in the frequency mode 9 Usually the measurement can
12. 304 PT PT Phase lock integrator time constant This command is used to set the PLL s integrator s time constant t which phase locks the PRS10 to an external 1pps input The integrator time constant is equal to seconds The default value is 8 which provides an integrator time constant of 2 9 or 65536 seconds Integrator s time constants can range from 256 to 4 194 304 seconds or from about 4 minutes to 18 days It is important to note that the natural time constant Tn is different from the integrator time constant as shown in the table below The natural time constant is the best measure of the loop response The PLL natural time constant spans between 8 minutes and 18 hours for PT values between 0 and 14 Example PT10 sets the integrator time constant to 2 9 seconds or about 72 hours Refer to Table below For PT10 the natural time constant is about 4 5 hours PT will return the current value of the time constant parameter A phase lock time constant may be stored in EEPROM as a new default with the PT command The PT command may be used to verify the value stored in EEPROM The following case will illustrate the operation of the PLL Suppose that the PRS10 has been phase locked to a stable 1 reference for a very long time several periods of tn when the Ipps reference input makes an abrupt shift of 100ns moving later in time The PRS10 s 1005 PLL algorithm will reduce the PRS10 s frequency of operati
13. 4 minutes to about 18 hours When provided with an accurate and stable 1 source the unit will automatically align its Ipps output to the 1 pps input and then adjust the frequency of rubidium reference to PRS10 Rubidium Frequency Standard 34 RS 232 Instruction Set maintain the alignment over time A typical application would lock the PRS10 to the 1005 output from a GPS receiver with a time constant of several hours Several commands and one status byte may be used to control and monitor the PLL however default values will allow units to lock to clean 1 inputs without any software interaction PL PL 0 or 1 PL PL Phase lock control This command may be used to disable the 1 PLL or to re enable and so restart the 1095 PLL The unit is shipped with the phase lock control enabled This command would be used if the 1 time tagging were being used to measure the position of Ipps inputs and phase locking is not desired Example PL 0 will disable the PLL to the Ipps inputs so that the frequency of the rubidium standard will not be affected by the 1 inputs PL will return a 1 if the PLL to the 1005 is enabled PL is used to write the current value 0 or 1 to the EEPROM for use after the next start up PL is used to query the value of the phase lock control parameter which is stored in the unit s EEPROM PT 0 lt value lt 14 t 20199 seconds 256 512 4 194
14. ARM to 15 and set the SAMPLE SIZE to 1 Hold the START button down for a few seconds to start continuous measurements Set the display to MEAN to display the frequency of the 10MHz output from the DUT Fine Frequency Measurements If the 10MHz from the DUT is within 0 1Hz of 10MHz you may use the fine frequency measurement technique to make measurements to a few parts in 10 a one second interval As explained above the frequency offset between the reference and the DUT is inferred by time interval measurements between their zero crossings To carry out these measurements Set the MODE to TIME select the SOURCE of START to A set the GATE ARM mode to TIME and EXT and set the PRS10 Rubidium Frequency Standard 64 Appendix B SAMPLE SIZE to 1000 With the external gate triggered by the SR620 s 1kHz reference output the unit will display a new MEAN every second If the DUT is adjusted so that the mean of the time interval measurements changes by less than 1005 per second then the DUT is within 1 part of 10 of the reference frequency PRS10 Rubidium Frequency Standard PRS10 Parts List 65 Parts list for Revision H Part reference numbers may be used to help locate the part per the following table Reference Designator Location fe eee oe ee ee Rubidium Oscillator PC Board Assembly Parts List REF SRS PART VALUE DESCRIPTION C 10
15. Mount Pkg Voltage Reg TO 220 TAB Package Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Voltage Reg TO 220 TAB Package Voltage Reg TO 220 TAB Package Integrated Circuit Surface Mount Pkg Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thermistor various Thermistor various Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor
16. SD7 value SD7 SD7 Set DAC RF deviation Analog Test 12 bit values ADO Spare J204 AD1 24V heater supply 10 AD2 24V electronics supply 10 AD3 Drain voltage to lamp FET 10 AD4 Gate voltage to lamp FET 10 AD5 Crystal heater control voltage AD6 Resonance cell heater control AD7 Discharge lamp heater control AD8 Amplified ac photosignal AD9 Photocell s I V converter 4 AD10 Case temperature 10 mV C AD11 Crystal thermistors AD12 Cell thermistors AD13 Lamp thermistors AD14 Frequency calibration pot AD15 Analog ground Analog Test 8bit values AD16 VCXO varactor voltage AD17 VCO varactor voltage AD18 AGC for RF AD19 RF PLL lock signal PRS10 Rubidium Frequency Standard 7 8 Theoretical Overview Theoretical Overview of Rubidium Frequency Standards Rubidium is an alkali metal like lithium sodium potassium and cesium There are two naturally occurring isotopes of rubidium Rb85 and Rb87 which have relative abundances of 72 and 28 respectively The metal has a melting point of 39 C The alkali metals behave similarly they have one electron outside an inert core Most of the chemical electronic and spectroscopic properties of these elements are determined by this outer electron The deep red glow of a low power rubidium discharge lamp is due to the resonance line transitions of the outer electron as it emits a red photon and drops back to the ground state The g
17. SMT1206 50V 5 NPO Capacitor Chip SMT1206 50V 5 NPO Cap Ceramic 50V SMT 1206 10 X7R Capacitor Chip SMT1206 50V 5 NPO Capacitor Tantalum 35V 20 Rad 67 PRS10 Rubidium Frequency Standard 68 PRS10 Parts List REF C 903 C 904 C 905 C 906 D 100 D101 D 102 D 202 D 203 D 204 D 205 D 400 D 401 D 500 D 501 D 502 D 503 D 504 D 700 J 100 J 100X J 400 J 700 J 701 J 800 J 801 J 802 JP500 JP501 L 100 L 101 L 102 L 103 L 104 L 105 L 200 L 300 L 301 L 302 L 400 L 401 L 402 L 403 L 902 LX104 LX105 P 100 PCI SRS PART 5 00487 574 5 00479 574 5 00479 574 5 00487 574 3 00803 360 3 00648 360 3 00648 360 3 00538 360 3 00854 313 3 00854 313 3 00854 313 3 00803 360 3 00803 360 3 00648 360 3 00806 360 3 00649 360 3 00544 360 3 00806 360 3 00235 308 1 00319 166 1 00320 100 1 00224 141 1 00222 141 1 00222 141 6 00017 630 6 00017 630 6 00017 630 1 00323 130 1 00324 130 6 00171 606 6 00171 606 6 00264 609 6 00264 609 6 00174 630 6 00174 630 6 00236 631 6 00236 631 6 00236 631 6 00236 631 6 00530 609 6 00513 609 6 00266 609 6 00281 609 6 00011 603 0 00772 000 0 00772 000 4 01576 459 7 00767 701 VALUE 68P 500V 18P 500V 18P 500V 68 500V MMBV609 MBRD660CT MBRD660CT MMBD352L ZMM5230B ZMMS5230B ZMM5230B MMBV609 MMBV609 MBRD660CT BAVI7OLTI BAWS6LT1 BAV70LTI BAVI7OLTI MP4025 10 PIN MALE COAX INSERT STRAIGHT PLUG REAR MT JACK REAR MT JACK
18. Standard 44 Circuit Descriptions control signals Typical phase noise is 125dBc Hz 10Hz 145dBc Hz 100Hz and 155dBc Hz gt 1kHz Circuit elements and operating points were chosen to reduce noise sources An SC cut resonator was chosen for high Q and stable motional impedances The transistors are operated at a few mA trading off base bias current noise against emitter resistance Metal film resistors are used to reduce 1 f noise Series 100UH inductors are used to reduce the Johnson noise current of bias resistors The op amp was chosen for low input current noise and it is operated with sufficient gain so that its voltage noise would not degrade the phase noise floor Finally the crystal is operated at its plateau temperature to reduce the frequency instability associated with temperature fluctuations Crystal Heater The crystal heater has the same design as the two other heaters resonance cell and lamp in the system There are two heaters in TO 220 packages an LM340 12 a 12Vdc voltage regulator and a TIP107 a pnp power Darlington The tabs of both TO 220 heaters are at ground so they are bolted directly to the block of the heater current passes through three parallel 1Q shunt resistors to sense current The block temperature is sensed by two series 100kQ thermistors which are directly beneath the TO 220 heaters in the oven block Two sensors are used because the division of power will depend on the heater voltage
19. Standard Part 001U 10 330UF 2 2U 35V 27P 01U 100P 220P 180P 390P 820P 68P 330P 39P 4 7 18 2 2 MBAV70L LED DAMIIWISD DM121073 202 BNC_RT BLOCK6 DB9F318 HEADIOF RT HEAD10M FUSE_CLIP BEAD 100UH H 56UH 1 8UH 1 5UH 100 100 MMBT3904 MMBT3906 100K 100 196 SRS P N 5 387 5 299 5 516 5 318 5 368 5 298 5 375 5 379 5 378 5 382 5 386 5 373 5 381 5 370 5 359 5 351 5 355 3 544 3 010 1 1027 1 1028 1 579 1 1029 1 1030 1 1031 1 1032 1 1033 0 1044 6 236 6 594 6 595 6 596 6 597 4 353 4 1455 3 601 3 580 4 1527 4 1021 Component Parts List 31 Item Quantity Reference Part SRS P N 41 7 R103 R104 R107 R205 R207 IK 4 1479 R213 R214 42 4 R105 R106 R108 R109 IM 4 1551 43 2 R110 R111 10K 4 1503 44 6 R112 R229 R230 R241 R242 R243 47 4 1447 45 9 R150 RI51 R152 R153 R154 330 4 1467 R155 R156 R157 R158 46 4 R200 R202 R231 R232 249 4 1059 47 1 R201 3 74K 4 1172 48 1 R203 1 74K 4 1140 49 3 R208 R212 R220 604 4 1096 50 3 R210 R218 R235 715 4 1103 29 R211 R219 R236 R237 R238 499 4 1088 52 4 R215 R216 R227 R228 1 00K 4 1117 53 2 R221 R226 1 50K 4 1134 54 1 R223 49 9 4 992 55 1 R224 301 4 1067 56 1 T100 1 1 6 0009 57 4 1200 T201 T202 1203 TOKO7P 6 195 58 1 U100 74HC540 3 748 59 1 U101 74AC00 3 1405 60 2 U102 U103 74HC4538 3 1467 61 1 0104 74 14 3 662 62 1 0105 MAX232ACSE 3 1468 63 2 U106 U107 HS 212 3 196 64 2 U200 U201 LM317 3 149 65 1 U202 MAX6250BC
20. Temp Set Madd 4 582 Crystal Heat Ctrl 2 Stability Factor sd6 Output Voltage W ad 4 550 Cell Heat Ctrl 0 Integral Term RF Modulation Wiad 3052 Lamp Heat Ctrl 1 Lock Mode SP Param ad8 2 583 AC Photosignal Last Valid Tag ad3 2 808 Photocelll adi 0 438 Temp Set Frequency Bb adii 1 138 XtalThermistors Set Slope adi2 0 943 CellThermistors Mag Offset WM adi3 1 343 Lamp Thermistors Mag Read adi4 2 501 Frequency Pot Mag Switching 915 0 003 Analog Ground Lock adi6 0 540 22MHz Varactor 7 Gain adi 2 880 3B50MHz Varactor 24 Phase adi8 1 590 Auto Gain Ctrl 1 Enable Power 919 4 780 RFLock 2126 81 10MHz DAC 230 Signal Values The screen is divided into three main sections labeled Factory Settings Analog Output Values and 1 pps Control Each section has three columns The first column contains the command name the second contains the current value of that parameter and the third contains a brief description of the parameter Colored boxes highlight various parameters that are out of the normal operating range The three sections and the meanings of the colored boxes are briefly described below For detailed descriptions of each of the commands consult the PRS10 manual Factory Settings This section includes calibration values and parameters for controlling the frequency of the PRS10 The most important parameter here is LO which indicates whether the PRS10 has locked to the Rb atomic hyperfine
21. Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor 69 PRS10 Rubidium Frequency Standard 70 PRS10 Parts List REF R 209 R 210 R211 213 214 215 216 217 218 219 220 221 222 223 224 226 227 228 229 230 231 232 233 234 235 236 237 238 240 241 248 249 250 252 256 261 262 263 264 265 266 267 268 269 270 271 272 273 PRS10 Rubidium Frequency Standard SRS PART 4 01280 462 4 01230 462 4 01278 462 4 01280 462 4 01305 462 4 01309 462 4 01213 462 4 01557 461 4 01575 461 4 01213 462 4 01376 462 4 01230 462 4 01280 462 4 01230 462 4 01278 462 4 01238 462 4 01305 462 4 01363 462 4 01213 462 4 01557 461 4 01575 461 4 01213 462 4 01309 462 4 01376 462 4 0125 1 462 4 01280 462 4 0125 1 462 4 01278 462 4 01479 461 4 01117 462 4 01439 461 4 01059 462 4 01479 461 4 01479 461 4 01503 461 4 01335 462 4 01355 462 4 01309 462 4 01309 462 4 01098 462 4 01309 462 4 01309 462 4 01405 462 4 01280 462 4 01347 462 4 01213 462 4 01280 462 4 01455 461 VALUE 49 9K 15 0K 47 5K 49 9K 90 9K 100K 10 0K 1 8M 10M 10 0K 4
22. While the integrator time constant t determines the natural time constant 1 it is the natural time constant which characterizes the loop response The PRS10 accepts stability factors ranging from 0 25 to 4 0 in powers of 2 The default value of G 1 0 corresponds to a critically damped response G lt 1 0 and G gt 1 0 correspond to under damped and over damped responses respectively PRS10 Rubidium Frequency Standard 16 10 Overview With 1 and specified the proportional gain Ap of the controller is given by the equation A 2 2 2 0 001s z With the default time constant ti of 65 536 seconds and a stability factor of 1 0 the proportional gain will be about 0 25 In this case the instantaneous frequency of the rubidium source will be adjusted by about 0 25 parts in 10 per nanosecond of time tag measured The PRS10 also provides an optional pre filter The pre filter is enabled by default but it can be disabled by sending the command LM0 which puts the PRS10 into lock mode 0 When the pre filter is enabled the PRS10 will exponentially average the time tags output by the phase detector before passing the result to the PI controller The time constant of the pre filter is hard coded to be 6 0 in order to obtain the maximum benefits of the averaging while simultaneously insuring that the PLL will be stable Use of the pre filter is recommended when locking to references that
23. a 1 Hz rate The position of the 1pps pulse may be moved with the PP command Example LM Could return 1 indicating that the unit is in its default configuration so that the lock pin goes low when locked to Rb pulsing high for 10 us at a 1 Hz rate To configure the unit for no 1pps output the command string LM 2 followed by LM will change the unit s power on default for no 1pps output PRS10 Rubidium Frequency Standard 24 RS 232 Instruction Set RC1 RC Recall This command is used to return all values in EEPROM to the values which were present when the unit was first shipped from the factory except for the unit start and lamp start counters This command should be used if you have been writing values to EEPROM and have somehow corrupted the operation of the device Executing this command may require calibration of the unit as the frequency calibration values are also returned to their factory values The unit will be restarted after the values in EEPROM have been restored to their factory values Example RC 1 will return all calibration values to the values which were determined for the unit when it was manufactured and restart the unit The RC command is a factory only command which writes all of the current parameter values to the EEPROM Frequency Lock loop Parameters LO value value 0 or 1 LO Lock This command can be used to stop the frequency lock loop FLL It is essentially the same as setting the gain parameter
24. access the cLoc SourcE submenu Press the arrow keys immediately to the left of the CONFIG menu in the SCOPE AND CHART section to select which is the rear panel time base input Press the SET key in the CONFIG section once more to verify the expected time base frequency 5 MHz or 10 MHz Press the arrow keys to select the frequency of the time base which you will be using DS345 Synthesized Function Generator The DS345 automatically detects the application of an external time base and phase locks to it After applying a 5 MHz or 10 MHz sine to the rear panel time base input a front panel indicator will turn on DG535 Digital Delay Generator A rear panel switch is used to specify an external 10 MHz time base input Some revisions of the DG535 required a 10 MHz square wave input and may not operate properly with the 10 MHz sine wave output from the FS725 The user should verify proper operation of the DG535 when locked to an external time base with an SR620 Time Interval Counter 25725 Rubidium Frequency Standard 10 Operation Direct Frequency Control of the FS725 The frequency of the rubidium hyperfine transition is adjusted via a small magnetic field in the physics package The magnitude of the magnetic field is set by the microcontroller in the PRS10 The frequency offset is quadratic in the magnetic field but firmware in the PRS10 linearizes the adjustment The microcontroller reads the position of a calib
25. current flowing in the heater circuit with a range from 0 to 500mA to provide heater powers from 0 to 12W The error amplifier has a proportional gain of R205 R204 1 6 5 and an integration time constant of R204xC201 1 s for the signal at its non inverting input The inverting input is biased near 1 00Vdc so that the servo will try to maintain the temperature so that there is 1 00Vdc on the thermistors For a set point of 75 C the series thermistor pair will have a resistance of 30kQ To get 1 00Vdc at the non inverting input of U200A XTAL_ SET is set to 170 bits full scale of 4V 255 0 01568V bit so 170 bits 2 66Vdc After settling a LSB step in XTAL_SET 15 6mV will become about 5 8mV at the on inverting input and cause an immediate change of 2x5 8mV 11 6mV at the output of U200A followed by a ramp of 5 8mV per second This quickly increases the power by about 0 27W then by 0 14W S thereafter The servo will settle when the thermistors heat up decreasing their resistance so that the voltage at the non inverting input returns to 1 00Vdc The thermistor resistance decreases by about 3 C An LSB increase in XTAL_SET near the nominal 2 66Vdc will cause the current to increase by 0156 2 66 1 00 or about 0 9 So the servo will settle when the temperature of the block increases by 0 9 3 C 0 3 C There is a small temperature offset between the temperature sensor and the device whose temperature we wish to control Since the
26. have poorer short term stability than the PRS10 but better long term stability Locking to the 1095 output by GPS is a prime example of such a case Use of the pre filter dramatically reduces the digital PLL s sensitivity to the sort term jitter of 50 to 300 ns present on the GPS reference 1005 The GPS reference also has a significant amount of 1 f noise associated with it Very long time constants are therefore required to prevent the PRS10 from following this noise too closely The PRS10 provides natural time constants of up to 18 0 hours which will allow the PRS10 to follow GPS over time scales on the order of a day but retain the superior short term stability of the rubidium clock When locking to a reference that has short term stability comparable to the PRS10 disabling the pre filter is recommended because it will allow the PRS10 to better track the phase of the reference In lock mode 0 the PRS10 s digital PLL will approximate one of the following three equations depending on the value of Fy GAT 0 M RE S sin 1 t 7z AT 0 e cos 01 Ct 7 forG 1 t AT t AT 0 7 Je 0 forG 1 F 6 4 DATO c e J DATO e 212178 dmm AT 0 is the initial offset in phase of the PRS10 from the reference Fo is the initial offset in frequency of the PRS10 from the reference before the digital PLL is enabled AT t details how the PRS10 approaches the phase
27. hole in this PCB allows access to an SMB connector to sample the microwave field in the resonance cell Power on reset low voltage protection and a watch dog time out is provided by U300 a MAX705 The RESET input to the microcontroller is asserted on power up The reset will be asserted for about 1 second after power is applied to allow time for 10MHz crystal oscillator to start A non maskable interrupt XIRQ is asserted if the SPI clock 15 inactive for more than 1 6 seconds which should never occur A maskable interrupt IRQ is asserted which will also retrigger the reset cycle when the 18 supply drops below 16 0Vdc Microcontroller The system is controlled by U302 a MC68HC11E9 which is an 8 bit microcontroller with RAM ROM EEPROM A Ds UART serial interface timers and I O control bits The controller is clocked by the 10 MHz timebase which is to be disciplined to the atomic transition frequency The microcontroller communicates with external devices via a the serial peripheral interface SPI Data is clocked by SPI CLK to or from these devices on SPI DATA To reduced digital crosstalk to the most sensitive devices the SPI data and clock are gated so that these outputs are only active when necessary The microcontroller is also responsible for a variety of housekeeping tasks power on circuit checks setting and reading temperatures boost starting the discharge lamp digitally filtering the frequency lock error
28. in the RS 232 Communications tab to change the current value of PT from 8 to 14 The value for TT the last valid time tag is blank because the unit has not yet received a pulse on its 1 pps input When a pulse is received the TT command will report the time difference in nanoseconds between the input pulse and the pps output Colored Boxes The colored boxes are visual cues to let the user know that a given parameter is outside the normal operating range This most commonly occurs at start up while the unit is warming up After about 15 minutes of operation the unit should be fully warmed up and all red and blue boxes should disappear After the initial warmup the appearance of a colored box typically signals something unusual that should be looked into If the unit was being operated in an oven for example the unit may not be able to properly regulate the temperatures of the crystal cell and lamp Under these circumstances one would expect blue boxes to appear next to AD5 AD6 and as the heaters are shut off The interpretation of the boxes is as follows Red Value is higher than normal Blue Value is lower than normal Black Value is different from the value stored in EEPROM FS725 Rubidium Frequency Standard Monitoring and Control 19 File Menu Print The program can print out the current settings shown in the Rb Stats tab To do this make sure that the Rb Stats tab is visible and then select Print f
29. is switched at a 5 Hz rate The frequency lock loop averages over a full period of the switch rate to avoid injecting a spur at 5 Hz onto the 10 MHz control signal The switching of the magnetic field is enabled at power on and restart but may be turned on or off by RS 232 command see MS command The commands associated with magnetic field control MO MS and MR allow direct control of the magnetic field circuitry Most users will not want to control the magnetic field directly but will instead allow the program to read the frequency calibration pot or external control voltage and then control the magnetic field If they want software control of the unit s PRS10 Rubidium Frequency Standard 30 85 232 Instruction Set calibration they may choose to use the SF commands which disable the analog control and allow the frequency to be adjusted over a range of 2000x10 The program will linearize the magnetic field control of the frequency offset with either analog or software control MS MS 0 or 1 Magnetic switching The MS command is used to turn off or on the 5Hz switching of the frequency tuning magnetic field Magnetic switching is enabled when the unit is powered on or after a restart Since the PRS10 is calibrated with the field switching enabled turning off the field switching may alter the calibration Example MS 1 will turn on the magnetic field switching and MS 0 will turn it off MS will return a 1 if the field
30. of phase locking and the 1 pps output will stray from the 1 pps input by about 3600 x 10 or 3 6 us This is larger than the 1 us synchronization window and so the unit will indicate that it is not synchronized With the default time constant the unit will pull its 1 pps output to within the 1 us synchronization window within a few hours and will thereafter indicate that it is properly synchronized FS725 Rubidium Frequency Standard 13 The table below shows the PLL parameters for the allowed values of the PT parameter The right most column Natural Time Constant is the best measure of the PLL response characteristics The values shown are for the default stability parameter C 1 PT Parameter Integrator Time Integral Gain Proportional Natural Time Constant Gain Constant Parameter SF bits per set by PT hours hour per ns SF bits per ns Characterizes command of time tag of time tag PLL response hours 0 0 07 14 063 3 95 0 14 1 0 14 7 031 2 80 0 20 2 0 28 3 516 1 98 0 28 3 0 57 1 758 1 40 0 40 4 1 14 0 879 0 99 0 56 5 2 28 0 439 0 70 0 80 6 4 55 0 220 0 49 1 12 7 9 10 0 110 0 35 1 59 8 18 20 0 055 0 25 2 25 9 36 41 0 027 0 17 3 18 10 72 82 0 014 0 12 4 50 11 145 64 0 007 0 09 6 36 12 291 27 0 003 0 06 8 99 13 582 54 0 002 0 04 12 72 14 1 165 08 0 001 0 03 17 99 Further details of the phase locking algorithm are provided in the PRS10 Rubidi
31. of the PRS10 is quite different from other rubidium frequency standards leading to several feature and performance benefits Ovenized Oscillator The output from PRS10 comes directly from a 10 MHz oven stabilized 3rd overtone varactor tuned SC cut crystal oscillator The varactor is tuned by a 22bit digital to analog converter which provides a full scale tuning range of 2 ppm The very fine step size 71 107 maintains the low noise inherent to the SC cut resonator yet the full scale range 1 sufficient to compensate for crystal aging over the lifetime of the unit This approach provides a 10 MHz output with extremely low phase noise which is virtually free of spurs Frequency Synthesizer The 10 MHz also serves as the reference source to the frequency synthesizer which generates RF at about 359 72 MHz The RF is multiplied by a factor of 19x in a step recovery diode to provide the microwave frequency at about 6 834 GHz which is used to interrogate the physics package The apparent hyperfine transition frequency varies with each physic package due to variations in buffer gas fill pressure etc The frequency synthesizer has two important characteristics a step size of about 1 10 and very low phase noise output The small step size is required so that only small magnetic fields will be needed to tune the apparent hyperfine transition frequency between the steps of the synthesizer The low phase noise is required so as not to degrade the s
32. of the reference as a function of time With the default AT t for C gt 1 PRS10 Rubidium Frequency Standard PRS10 Overview 17 time constant t 65 5365 and stability factor 1 the PRS10 s 1003 output will exponentially approach the phase of the reference Ipps input with a time constant T 8 095 seconds or approximately 21 4 hours In lock mode 1 the equations describing AT t are qualitatively similar to those presented above but generally can only be solved numerically The locking algorithm of the PRS10 proceeds as follows e The 1 PLL is enabled when the unit is turned on or restarted if the PL parameter stored in the unit s EEPROM is 1 e The PLL will begin to control the frequency of the rubidium frequency standard when 256 consecutive good 1005 inputs 1 e 1005 inputs which are within 2048 ns of the first time tag result modulo s are received e After receiving 256 consecutive good I pps inputs the 1105 pulse delay is set to the last of the 256 time tag values For example if the last of the 256 good time tag values is 123 456 789 ns then the program will set the 1 output delay to 123 456 789 ns which moves the 1105 output by 123 456 789 ns so that new time tag values will be about zero Also the current value of the SF parameter which adjusts the frequency of the rubidium frequency standard over the range of 2000 parts in 1012 is used to initialize the
33. pps pulse train earlier by 123 456 789 ns PS PS value 100 lt value lt 255 PS PS Pulse slope calibration This command is used to calibrate the analog portion of the 1005 output time delay circuit This circuit is used to delay the 1 pulse train with 1 ns resolution and 100 ns full scale Counting logic is used for the portion of the time interval longer than 100 ns The pulse slope value corresponds to the DACS value which provides a delay closest to but not exceeding 100 ns Example PS 200 set the pulse slope to its nominal value of 200 PS value is a factory only command The PS command will return the current value of the pulse slope The PS command writes the current value of the pulse slope to the unit s EEPROM for use after the next power on or restart This command is used after the pulse output analog output is calibrated Example PS will write the current value of the pulse slope which calibrates the 100 ns analog delay portion of the 1 pulse delay circuit to the unit s EEPROM Note that PS is a factory only command 1PPS Locking Control To facilitate integration into systems which require very low aging automatic calibration or a traceable frequency standard the PRS10 may be locked to an external Ipps input A second order digital PLL is used to lock the unit s frequency both the 10 MHz and 1 outputs to an external 1pps input with time constants ranging from 256 s to 65536s about
34. shell to reduce the frequency pulling effects of external magnetic fields The apparent hyperfine transition frequency may be quadratically tuned over a range of about 2 x 10 by the magnetic field coil The frequency shift is always positive regardless of the direction of the magnetic field To further reduce the effects of external magnetic fields the current in the field coil is switched at 5 Hz An external field which adds to the coil s field will increase the apparent transition frequency and an external field which opposes the coil s field will decrease it By alternating the coil s field and averaging the effect of an external field can be reduced Control Algorithm The microcontroller is responsible for 1 generating the 70 Hz phase modulation of the RF to probe the physics package 2 synchronously detecting the amplitude and phase of the photosignals at 70 Hz and 140 Hz and 3 digitally filtering the error signal to lock the 10 MHz SC cut ovenized oscillator to the rubidium hyperfine transition The 70 Hz digitally synthesized phase modulation waveform is generated via a 12 bit DAC in 32 discrete steps A low pass filter is used to remove image frequencies from the modulation waveform The microcontroller s hardware timers are used synchronize updating of the DAC so as to eliminate sample jitter The modulation waveform has very little distortion noise or spurs and is precisely 70 Hz The photosignal is amplified a
35. switching is currently enabled MO MOf value 2300 lt value lt 3600 MO MO Magnetic offset The magnetic offset is the value determined when the unit is calibrated which calibrates the unit to 10 MHz The restricted range is necessary to allow room for user calibration via the internal frequency calibration pot or by an external voltage If the unit cannot be calibrated to 10 MHz within the allowed range of MO values then a different setting for the frequency synthesizer is required See SP command and the table in Appendix A Example MO 3000 sets the magnetic offset to 3000 which is its nominal mid linear scale value The MO command reads back the current value of the magnetic offset MO is used to store the current value of the magnetic offset parameter to EEPROM for use after the next restart MO may be used to query the value stored in EEPROM This value is used on power up or restarts MR Magnetic read This command returns the value that the 12 bit DAC is using to control the magnetic field This value is computed from the magnetic offset value see MO command and the position of the internal frequency calibration pot external calibration voltage or value sent by the SF command The value is computed from the equation DAC V SF SLOPE MO where SF is the desired frequency offset in parts per 10 from the cal pot position the SF command or the 1095 PLL and 15 in the range 2000 lt SF lt 2000 SL
36. the resonance cell The varactor tuned crystal oscillator has a tuning coefficient of about 2 ppm V The RF VCO is phase locked to this oscillator by a mixer loop filter combination with a high natural frequency about 400 000r s a high comparison frequency 22 48 and a damping factor of one Low noise components metal film resistors film capacitors and an OP27 op amp help achieve the low phase noise The varactor for the crystal is tuned by the dual modulus frequency synthesizer U400 an MC145190 which compares the divided reference 10 00M H2 to the divided RF at about 359 72MHz Since large divisors must be used to achieve the high frequency resolution the comparison frequency will be low a few kHz but the crystal oscillator provides good frequency stability in spite of the low comparison rate The gain of U400 s phase detector may be set coarsely by the CPU and it is adjusted to maintain roughly the same PLL damping factor as divisors are changed This loop has a very low natural frequency about 10 r s and a damping factor which ranges from 0 84 to 1 19 After multiplication to 6 834GHz the phase noise has been measured at 72dBc Hz This is low enough so that the S N of the dip signal is not adversely affected by the microwave phase noise RF Output Amplifier The 359 720MHz RF must be amplified to drive the SRD It is important to maintain a constant RF level optimized to provide a large frequency deviation sen
37. to the previous calibration if necessary and serves as a record of the frequency aging of the unit Connect the RS 232 serial cable between the FS725 and the PC or laptop computer and verify that you can talk to the FS725 Be certain that the FS725 has been operated continuously for at least 24 hours excluding the brief power cycling Send the command LM1 This enables the 1 pps input digital pre filter to reduce the noise of the 1 pps reference LM1 is the default value Send the command 8 This sets the natural time constant of the PLL to about 2 25 hours which reduces the noise of the 1 pps reference by averaging PT8 is the default value If the FS725 has been off for more than one week it should be operated continuously for at least 72 hours before the adjustment is made Apply the 1 pps signal to the rear panel of the FS725 The front panel 1 pps input LED should blink at 1 pps and the 1 pps sync LED should light in about 5 minutes THE FS725 SHOULD BE PHASE LOCKED FOR AT LEAST 24 HOURS BEFORE PROCEDING TO THE NEXT STEP Query the SF parameter via the RS 232 SF The response will be a number between 2000 Write the number down The SF parameter is being controlled by the phase lock loop algorithm and has a resolution of 1 10 2000 corresponds to 0 002ppm To set the FS725 to 1 10 you want to determine the correct value of SF to 10 so check back in a few hours to determine if the SF par
38. to zero It may be desirable in a particular application to stop the FLL and set the frequency control value for the 10MHz oscillator manually See the FC command Example LO 0 will stop the FLL LO will return a value of 0 if the FLL is not active or 1 if the FLL is active FC FC high low 0 lt high lt 4095 1024 lt low lt 3072 FC FC Frequency control These commands allow direct control of the 22bit value which controls the frequency of the 10 MHz ovenized oscillator Normally this value is controlled by the FLL control algorithm however the FLL may be stopped and the value adjusted manually See the LO command Two 12 bit DACS are scaled by 1000 1 and summed to provide a varactor voltage which controls the frequency of the 10 MHz oscillator The low DAC which operates over half its range to avoid FFL oscillations at the roll over to the high DAC provides a LSB frequency resolution of 1 5 10 The high DAC which has a nominal value of 2048 has a LBS resolution of 1 5 10 These DACs provide a total tuning range of about 3 ppm Example Suppose a unit s FLL has been operating for some time and has settled An FC will return the current value of the DAC pair which might be 2021 1654 Tracking the FC value over a long period of time tells you about the frequency variations of the 1OMHz PRS10 Rubidium Frequency Standard RS 232 Instruction Set 25 crystal The FC values will change to correct for variations
39. up and locking will take longer if the unit is started from a lower temperature Operation of the LOCKED LED corresponds to the state of the rear panel RB LOCK alarm relay When power is first applied the LOCKED LED will be off and the RB LOCKED relay will be in the alarm position Once the unit warms up and locks its crystal oscillator to the rubidium physics package the LOCKED LED will turn on and the RB LOCK relay will move to the non alarm state The output frequency will be about 200 ppm below nominal when power is first applied to a cold unit The frequency will gradually converge on 10 MHz as the unit warms up After about 4 minutes typically the unit will reach operating temperature and lock the crystal oscillator to the rubidium hyperfine transition Within seven minutes the unit will be locked to with 0 001ppm of 10 MHz Connection to other instruments All of the outputs on the rear of the FS725 have a source impedance of 50 Q and are intended to drive 50 Q loads Since the outputs have a source impedance of 50 Q termination into a 50 2 load is not critical in most applications Generally the amplitude will approximately double if the output is not terminated into a 50 Q load Properly terminating into a 50 load will reduce channel to channel cross talk and will reduce the probably of multiple triggering on the 1 pps output Ifa single 5 MHz or 10 MHz output is daisy chained to severa
40. would allow the PRS10 to be locked to the 1 from a GPS receiver with a long time constant to eliminate aging PRS10 Rubidium Frequency Standard Applications 19 PRS10 Applications In virtually all cases the PRS10 may be dropped into applications which use the Efratom FRS C 1A8A4C 10 MHz sine output 5 C to 65 C or the FRS N 1A8A4B 10 MHz sine output 55 C to 65 C Some customers may wish to evaluate the PRS10 on the bench To facilitate this SRS can provide a connector adapter power supply and RS 232 cable The adapter breaks out the Cannon plug on the PRS10 to a power connector 2 1 mm with 24 V to center pin three BNCs 10 MHz and 1005 output and I pps input and DB9 for RS 232 The adapter also has status indicators for power lock and RS 232 activity This kit allows the PRS10 to be operated from 110 240 Vac 50 60 Hz provides for a direct connection to a PC via a serial port typically COM2 and allows the use of standard BNC cables The PRS10 may also be operated with a customer supplied connector Cannon series DAMIIWIS with coaxial insert DM53740 5008 for RG178 cable from a bench dc power supply The power supply should be able to supply 2 2 A at 24 Vdc Interface Connector Pin Name Description gt ee a Ext freq calibration Nom 2 50 V 0 5 V for 2x10 6 2 HEAT 24 Vdesupplyfordischargelampandheaters 8 POT 15 00Vde reference output for external fre
41. 0 5 00595 569 2 2U 50V Cap Tantalum SMT all case sizes C 101 5 00299 568 JU Cap Ceramic 50V SMT 1206 10 X7R C 102 5 00370 552 39P Capacitor Chip SMT1206 50V 5 NPO C 103 5 00375 552 100P Capacitor Chip SMT1206 50V 5 NPO C 106 5 00298 568 01U Cap Ceramic 50V SMT 1206 10 X7R C 107 5 00298 568 01U Cap Ceramic 50V SMT 1206 10 X7R C 109 4 01146 462 2 00K Thin Film 196 50 ppm MELF Resistor C 110 5 00299 568 JU Cap Ceramic 50V SMT 1206 10 X7R C111 5 00387 552 1000P Capacitor Chip SMT1206 50V 5 NPO C112 5 00361 552 6 8P Capacitor Chip SMT1206 50V 5 NPO C113 5 00375 552 100P Capacitor Chip SMT1206 50V 5 NPO C114 5 00299 568 1U Cap Ceramic 50V SMT 1206 10 X7R C115 5 00586 569 4 TUF 50V Cap Tantalum SMT all case sizes C 116 5 00586 569 4 TUF 50V Cap Tantalum SMT all case sizes C 151 5 00299 568 LU Cap Ceramic 50V SMT 1206 10 X7R C 152 5 00299 568 AU Cap Ceramic 50V SMT 1206 10 X7R C 200 5 00387 552 1000P Capacitor Chip SMT1206 50V 5 NPO 201 5 00466 572 IU SMT Film Capacitors 50V 5 All Sizes C 202 5 00299 568 AU Cap Ceramic 50V SMT 1206 10 X7R C 203 5 00387 552 1000P Capacitor Chip SMT1206 50V 5 NPO C 204 5 00466 572 IU SMT Film Capacitors 50V 5 All Sizes C 205 5 00299 568 LU Cap Ceramic 50V SMT 1206 10 X7R C 206 5 00387 552 1000P Capacitor Chip SMT1206 50V 5 NPO C 207 5 00466 572 IU SMT Film Capacitors 50V 5 A
42. 0 ppm MELF Resistor R 333 4 01405 462 1 00M Thin Film 1 50 ppm MELF Resistor R 334 4 01251 462 24 9K Thin Film 1 50 ppm MELF Resistor R 335 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 336 4 01575 461 10M Thick Film 5 200 ppm Chip Resistor R 337 4 01455 461 100 Thick Film 5 200 ppm Chip Resistor R 338 4 01503 461 10K Thick Film 5 200 ppm Chip Resistor R 339 4 01479 461 1 0K Thick Film 5 200 ppm Chip Resistor R 340 4 01479 461 1 0K Thick Film 5 200 ppm Chip Resistor R 341 4 01503 461 10K Thick Film 5 200 ppm Chip Resistor R 342 4 01479 461 1 0K Thick Film 5 200 ppm Chip Resistor PRS10 Rubidium Frequency Standard 72 PRS10 Parts List REF R 343 R 344 R 345 R 346 R 347 R 348 R 349 R 350 R351 R 352 353 354 355 356 357 358 359 360 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 R419 R 420 R 421 R 422 R 423 R 424 R 425 R 426 R 427 R 428 R 429 SRS PART 4 01503 461 4 01464 461 4 01464 461 4 01464 461 4 01464 461 0 00000 000 4 01464 461 4 01464 461 4 01464 461 4 01464 461 0 00000 000 4 01493 461 4 01527 461 4 01493 461 4 01527 461 0 00000 000 0 00000 000 4 01213 462 4 01347 462 4 01405 462 4 01447 461 4 01463 461 4 01527 461 4 01201 462 4 01561 461 4 01213 462 4 01259 462 4 01355 462 4 01309 462 4 01527 461 4 01088 462 4 01088 462
43. 08505 6293 3537 4 0 953926 0 027455 69 4684 2632 45 0 963357 10 009431 Appendix A 61 uv TS oo N vsa e N en N en 6 oo t 6043 7223 0 032616 PRS10 Rubidium Frequency Standard 62 Appendix B Appendix Precision Frequency Measurement One goal for the calibration of the PRS10 is to set the frequency to within 1 part in 10 of 10MHz which is 10MHz 0 0001Hz 10MHz 100uUHz Two things are required to make this measurement 1 a very good 10MHz frequency reference and 2 a very good time interval counter The frequency reference should be stable and accurate to a few parts in 10 Another PRS10 locked to the 1pps from a GPS receiver or a cesium beam standard such as HP 5071 are two possibilities The time interval counter needs to measure time intervals with a resolution of better than 50ps and should be able to do fast averaging of the time interval measurements Suitable instruments include the SR620 or an HP5370B The time interval counter may be used to directly measure the frequency of the device under test DUT In this case the frequency reference is used as the timebase for the time interval counter Unfortunately the time interval counter will require about 100 seconds to measure the frequency to a resolution of 1 part in 10 when used in the frequency measurement mode A faster way to make the comparison
44. 1 1 0K Thick Film 5 200 ppm Chip Resistor R 513 4 01059 462 249 Thin Film 1 50 ppm MELF Resistor 514 4 01167 462 3 32K Thin Film 1 50 ppm MELF Resistor 515 4 01487 461 2 2 Thick Film 5 200 ppm Chip Resistor R 516 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R517 4 01405 462 1 00M Thin Film 1 50 ppm MELF Resistor R518 4 01242 462 20 0K Thin Film 1 50 ppm MELF Resistor R519 4 01146 462 2 00K Thin Film 1 50 ppm MELF Resistor R 520 4 01088 462 499 Thin Film 1 50 ppm MELF Resistor R 521 4 01230 462 15 0K Thin Film 1 50 ppm MELF Resistor R 522 4 01117 462 1 00K Thin Film 1 50 ppm MELF Resistor R 523 4 01251 462 24 9K Thin Film 1 50 ppm MELF Resistor R 524 4 01447 461 47 Thick Film 5 200 ppm Chip Resistor R 525 4 01479 461 1 0K Thick Film 5 200 ppm Chip Resistor R 526 4 01242 462 20 0K Thin Film 1 50 ppm MELF Resistor PRS10 Rubidium Frequency Standard 74 PRS10 Parts List REF R527 R 528 529 530 531 532 533 534 536 537 538 539 540 541 542 543 544 545 546 547 548 550 551 552 553 600 601 602 603 604 605 800 801 803 804 900 100 100 101 102 150 200 201 202 205 206 207 208 c SRS PART 4 01230 462 4 01213 462 4 01213 462 4 01213 462 4 01213 462 4 01213 462 4 01493 461 4 01213 462 4 01213 462 4 01213 462
45. 3 16PF UND Z0 0 00644 020 4 40X1 4PF UNDR Z0 0 00645 055 34AWG MAGNET Z0 0 00902 034 S S SEAMLESS Z0 0 00908 030 PROTO MATERIAL Z0 0 00915 034 CU TUBING 1 16 Z0 1 00323 130 64 PIN STRIP Z0 1 00324 130 64 HDR PIN R A Z0 3 00668 312 PHOTODIODE Z0 7 00557 717 RB 10 Z0 7 00560 72 1 Z0 7 00636 720 4 Z0 7 00638 72 6 Z0 7 00639 721 7 Z0 7 00641 72 9 Z0 7 00862 720 SPACER Z0 9 00571 924 SPECIALTY 56 Miscellaneous Parts List REF SRS PART VALUE R 901 4 01620 409 536 2W U 302 3 00646 360 68HC711E20CFN Z0 0 00096 041 4 SPLIT Z0 0 00602 060 4 40X3 32 SET Z0 0 00606 025 4 40X1 4 BUTTON Z0 0 00628 065 4 40X10 32X1 4 Z0 0 00644 020 4 40X1 4PF UNDR Z0 0 00659 044 LAMP WINDOW Z0 0 00668 025 4 40X3 16 HEX Z0 0 00669 025 4 40X5 16 HEX Z0 0 00670 025 4 40X3 8 BUTTON Z0 1 00355 150 LAMP Z0 6 00505 600 RB CLOCK Z0 7 00556 717 RB 12 Z0 7 00637 720 RB 5 Z0 7 00640 720 RB 8 Z0 7 00764 720 RB 15 Z0 9 00805 917 RUBIDIUM SERIAL PRS10 Rubidium Frequency Standard DESCRIPTION Screw Allen Head Screw Allen Head Screw Allen Head Copper Foil Tape Self Adhesive Tubing Standoff Standoff Screw Flathead Phillips Screw Flathead Phillips Wire Other Tubing Spacer Tubing Connector Male Connector Male Photodiode Deep Drawn or Stamping Machined Part Fabricated Part Machined Part Machined Part Machined Part Fabricated Part Tape All types DESCRIPTION Resistor Wire Wound In
46. 3 to the output of U512B The selected 1PPS output turns Q503 s current down and turns Q504 s current up discharging C513 As C513 passes through 9 0Vdc the comparator output U514 is forced low C513 continues to discharge down to 8 V44 2 where it stays until the 1PPS PRS10 Rubidium Frequency Standard Circuit Descriptions 57 pulse goes low When the 1PPS pulse goes low the process is reversed and Q504 s current is reduced while Q503 s current is increased charging C513 back towards 10 Vaac 2 This time as C513 passes through 9 0V the comparator s output is set high In this way both the leading and trailing edges off the 1PPS output are delayed the same amount Baseplate Temperature Sensor 0505 LM45 centigrade temperature sensor has an output of 10mV C This sensor is in thermal contact with one of the baseplate standoffs that hold the thermal shield which encloses the lamp The sensor s output may be read by the CPU via the 12 bit DAC so that the baseplate temperature may be read with 0 125 C resolution The output of the temperature sensor is also used to tweak the setpoints of the temperature control servos which will reduce the affect of ambient temperature changes on the temperatures of the lamp resonance cell and crystal ovens Schematic RB_F6 Sheet 6 of 6 Resonance Cell and Lamp Heaters The heater and control circuits for the lamp and resonance cell are identical to the circuit described for the
47. 37 38 2 Table of Contents FS725 Rubidium Frequency Standard Specifications Standard outputs Optional outputs 1 pps output Sine outputs Phase noise 5 MHz amp 10 MHz Spurious Harmonics Return loss Accuracy Aging Allan variance Holdover Frequency retrace Settability Trim range Warm up time Specifications 3 Two 10 MHz sine waves one 5 MHz sine wave one pps with 10 us width Up to 3 distribution modules each with four 10 MHz one 5 MHz and one 1 pps 2 5 V into 50 5 V into high impedance 10 us pulse width 0 5 Vims 10 1 41 Vpp or 7 dBm lt 130 dBc Hz 10 Hz lt 140 dBc Hz 100 Hz lt 150 dBc Hz 1 kHz lt 155 dBc Hz 10 kHz lt 100 dBc within 100 kHz lt 60 dBc gt 25 dB 10 MHz 5 x 107 lt 5 x 10 monthly after 1 month lt 5 x 107 yearly after first year lt 2 x 10 1s lt x 10 10 s lt 2 x 10 100 s 72 hour Stratum 1 1x10 5 x10 72 hrs off then 72 hrs on lt 5 x 10 2 x 10 pot or by 0 5 VDC input 0 5 ppm via RS 232 lt 6 minutes time to lock lt 7 minutes time to 1 x 10 FS725 Rubidium Frequency Standard 4 Specifications Front Panel Indicators Power Locked 1 PPS input 1 PPS sync Receive Send On when AC power is applied On when frequency is locked to rubidium hyperfine transition Blinks with each 1 pps reference input applied to rear panel On when 1 pps output is synchroni
48. 4 01213 462 4 01489 461 4 01184 462 4 01184 462 4 01184 462 4 01146 462 4 01218 462 4 01117 462 4 01146 462 4 01447 461 4 01469 461 4 01405 462 4 01479 461 4 00925 462 4 00925 462 4 01407 461 4 01407 461 4 01407 461 4 00899 431 4 00899 431 4 01407 461 4 01407 461 4 01407 461 4 00899 431 4 00899 431 4 01597 405 6 00195 610 3 00542 360 3 00819 360 3 00773 360 3 00346 329 3 00774 360 3 00774 360 3 00774 360 3 00653 360 3 00659 360 3 00774 360 3 00661 360 VALUE 15 0K 10 0K 10 0K 10 0K 10 0K 10 0K 3 9K 10 0K 10 0K 10 0K 10 0K 2 7K 4 99K 4 99K 4 99K 2 00K 11 3K 1 00K 2 00K P1H104 T NTC P1H104 T NTC 1 1 1 P1H104 T NTC P1H104 T NTC 10K 10 7 MHZ AD587JR LM7171AIM ADS8561AR OP284FS LMC662C 74HC4051 PRS10 Rubidium Frequency Standard DESCRIPTION Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resistor Thick Film 596 200 ppm Chip Resistor Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resistor Thick Film 596 200 ppm Chip Resistor Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resisto
49. 52 360 3 00652 360 3 00652 360 3 00658 360 3 00946 360 3 00581 360 3 00654 360 3 00660 360 6 00193 625 3 00650 360 3 00773 360 3 00655 360 3 00751 360 3 00149 329 3 00581 360 3 00149 329 3 00112 329 3 00775 360 3 00742 360 3 00742 360 3 00742 360 3 00813 360 3 00812 360 3 00581 360 3 00581 360 3 00534 360 3 00346 329 3 00561 329 6 00132 620 6 00194 620 0 00045 013 0 00096 041 0 00098 042 0 00231 043 0 00243 003 0 00605 025 0 00606 025 VALUE 74HC4051 AD822 MAX705CSA 74HC14 74HC08 74HC14 DG211BDY LM358 OP284FS AD822 LTC1452CS8 LTC1452CS8 LTC1452CS8 LTC1452CS8 AD7896AR MC145193F AD822 SA602D OP27GS 380 MHZ MC12026AD LM358 TLC5628 74HC574 LM317T AD822 LM317T 7805 LM45CIM3 74HC74 74HC74 74HC74 LM311M 74HC153 AD822 AD822 AD790JR 7812 7808 10 MHZ SC CUT 22 4825 MHZ 4 40 MINI 4 SPLIT 6 LOCK 1 32 4 SHOULD TO 220 4 40X1 4 SOCKET 4 40X1 4 BUTTON PRS10 Parts List DESCRIPTION Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg
50. 99K 15 0K 49 9K 15 0K 47 5 18 2K 90 9K 365K 10 0K 1 8 10M 10 0K 100K 499K 24 9K 49 9K 24 9K 47 5 1 0K 1 00K 22 249 1 0K 1 0K 10K 187K 301K 100K 100K 634 100K 100K 1 00M 49 9K 249K 10 0K 49 9K 100 DESCRIPTION Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5
51. C setting 0 DAC lt 4095 and 5x10 x DAC The DAC setting is changed from the nominal calibration value see MO command in various ways including calibration pot position external calibration voltage direct setting see SF command and external 1pps PLL control When the unit is first turned on or restarted the internal frequency calibration pot position will be used to set the DAC relative to the calibration value stored in EEPROM If a voltage is applied to pin 2 of J100 POT W then this voltage will override the pot position An SF command may be sent or a 1pps input may be applied to control the frequency offset directly If either the SF command or the 1 input control the frequency offset then the pot position or external control voltage will not be used again until the power is cycled or the unit is restarted All the various ways to adjust the frequency of the 10 MHz output are linearized and they have a span of 2000 x 107 or 0 020 Hz PRS10 Rubidium Frequency Standard 22 RS 232 Instruction Set RS 232 Instruction Set Syntax Commands consist of a two letter mnemonic and one or more parameters Commands which end with a question mark will return a value Commands which end with an exclamation point write the current parameter value to EEPROM for use after the next restart Commands which end in an exclamation point and a question mark return the value stored in EEPROM All da
52. CD DSR and CTS are all pulled high via 10kQ resistors An XON XOFF protocol is used to pause communications as needed The LOCK IPPS function may be configured via RS 232 The factory default is a low level to indicate lock with 10us pulse to 5V at IPPS with the leading edge being defined as the IPPS timing reference This BNC output is a CMOS logic output via a 1kQ resistor LEDs are used to indicate 24 power electronics and heaters lock status and RS 232 data received and RS 232 data transmitted PRS10 Rubidium Frequency Standard 60 Appendix A Appendix A Frequency Synthesizer Table This table provides a list of frequency synthesizer parameters and the frequency offset relative to the settings for a nominal cell Also listed is the frequency step between adjacent settings This information is needed to calibrate units which have aged by more than 2x10 or in the case that an application may require operating the unit at a frequency up to 0 6Hz away from 10MHz Number R N ffo Hz df hz 6900 5878 15 0000000 6 685 3476 23 0 032570 0 007103 5899 3315 39 0 047809 0 007800 5156 3235 15 0 055996 0 008187 27 6193 3818 6 0290617 0 011800 35 5649 3175 6 0406527 0 016993 PRS10 Rubidium Frequency Standard Number R f fo Hz 902 46 4505 2532 6 0 581306 0 009036 60 151 4019 20 0 805666 0 007200 64 8361 1889 6 0 875065 0 016177 66 6579 3697 52 0 900209 0 0
53. DC supply Front Panel LEDs A small PCB mounted on the front panel can hold up to 9 LEDs and their current limiting resistors only six are used in this product An LED 15 when the corresponding line is pulled low via a 10 conductor ribbon cable FS725 Rubidium Frequency Standard Circuit Description 27 Rubidium Power Supplies and RF Amplifiers Sheet 2 of 2 Power Supplies A universal input 24VDC 55W output switching supply powers the FS725 After warm up the FS725 draws only about 1 3 of the rated power from the supply which improves reliability and reduces ripple The output of the power supply is further filtered by passive LC networks L1 L2 C200 L200 and C201 before being applied to the PRSIO rubidium frequency standard Three terminal regulators U200 U201 and U203 are used to generate systems voltages of 20VDC 10VDC and 5VDC A very quiet 5 00VDC reference is provided by U202 A transistor Q200 is used as a current boost for the 5 00VDC reference 10 MHz Outputs The PRS10 provides a 10 MHz sine wave output on an RF connector The output has a source impedance of 50 Q and amplitude of 1 41V with very low phase noise The FS725 s 10 MHz output drivers were designed to maintain the low phase noise of the PRS10 s outputs and to reduce the harmonic distortion with high output to output isolation The output amplifiers LM7171 s U204 amp U206 have high bandwidth low distortion low current noise but r
54. F Resistor R 439 4 01309 462 100K Thin Film 1 50 ppm MELF Resistor R 440 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 441 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 442 4 01184 462 4 99K Thin Film 1 50 ppm MELF Resistor R 443 4 01439 461 22 Thick Film 5 200 ppm Chip Resistor R 444 4 00992 462 49 9 Thin Film 1 50 ppm MELF Resistor R 445 4 01511 461 22K Thick Film 5 200 ppm Chip Resistor R 446 4 01503 461 10K Thick Film 5 200 ppm Chip Resistor R 447 4 01455 461 100 Thick Film 5 200 ppm Chip Resistor R 448 4 01447 461 47 Thick Film 5 200 ppm Chip Resistor R 449 4 01551 461 1 0M Thick Film 5 200 ppm Chip Resistor R 450 4 01439 461 22 Thick Film 5 200 ppm Chip Resistor R 500 4 01059 462 249 Thin Film 1 50 ppm MELF Resistor 501 4 01021 462 100 Thin Film 1 50 ppm MELF Resistor R 502 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 503 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 504 4 00925 462 10 Thin Film 1 50 ppm MELF Resistor R 505 4 00925 462 10 Thin Film 1 50 ppm MELF Resistor R 506 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 507 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 508 4 01309 462 100K Thin Film 1 50 ppm MELF Resistor R 509 4 01309 462 100K Thin Film 1 50 ppm MELF Resistor R 510 4 01347 462 249K Thin Film 1 50 ppm MELF Resistor R511 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor 512 4 01479 46
55. FS725 Rubidium Frequency Standard Operation and Service Manual S RS Stanford Research Systems 1290 D Reamwood Avenue Sunnyvale California 94089 Phone 408 744 9040 Fax 408 744 9049 email info thinkSRS com www thinkSRS com Copyright O 2002 by SRS Inc Rights Reserved Version 1 1 11 2005 25725 Rubidium Frequency Standard Table of Contents Specifications Outputs Front Panel Rear Panel General Overview Operation Power Warm up Connection SRS instruments Frequency Control 1 pps input Monitor and Control Setting up RS232 Getting Connected Testing connection Terminal program Rubidium Stats Factory Settings tab Analog Values tab 1 pps control tab Colored boxes File Menu Help Menu Terminal program 2 WwW V 14 14 14 15 15 16 17 18 18 18 19 19 19 Table of Contents Calibration With a frequency counter With a 1 pps reference Time tag offset parameter Circuit description PRS10 connector 1 pps input Lock and pps output RS 232 interface Front panel LEDs Power supplies 10 MHz output 5 MHz output Signals to option cards Option cards Component parts list FS725 main board FS725 option board FS725 hardware Circuit Schematics Main board 1 2 Main board 2 2 Option board Addendum PRS10 Manual 25725 Rubidium Frequency Standard 20 20 22 24 25 25 26 26 26 27 27 28 28 28 30 31 32 35
56. Film Capacitors 50V 5 All Sizes C 420 5 00456 572 015U SMT Film Capacitors 50V 5 All Sizes C 421 5 00299 568 JU Cap Ceramic 50V SMT 1206 10 X7R PRS10 Rubidium Frequency Standard 422 423 424 425 426 427 428 429 C 430 431 432 433 434 436 437 438 439 C 440 C 500 C 502 C 504 C 506 C 507 C 508 C 509 C 510 511 512 513 514 515 516 517 518 519 520 521 C 600 601 602 700 C 800 801 802 803 804 805 900 SRS PART 5 00298 568 5 00299 568 5 00387 552 5 00387 552 5 00299 568 5 00375 552 5 00387 552 5 00298 568 5 00359 552 5 00356 552 5 00299 568 5 00364 552 5 00366 552 5 00299 568 5 00299 568 5 00466 572 5 00298 568 5 00466 572 5 00595 569 5 00595 569 5 00387 552 5 00595 569 5 00595 569 5 00595 569 5 00387 552 5 00356 552 5 00387 552 5 00299 568 5 00376 552 5 00299 568 5 00387 552 5 00299 568 5 00299 568 5 00299 568 5 00299 568 5 00299 568 5 00299 568 5 00299 568 5 00299 568 5 00299 568 5 00480 574 5 00299 568 5 00299 568 5 00387 552 5 00387 552 5 00299 568 5 00387 552 5 00100 517 PRS10 Parts List DESCRIPTION Cap Ceramic 50V SMT 1206 10 X7R Cap Ceramic 50V SMT 1206 10 X7R Capacitor Chip SMT1206 50V 5 NPO Capacitor Chip SMT1206 50V 5 NPO Cap Ceramic 50V SMT 1206 10 X7R Capacitor Chip SMT1206 50V 5
57. HEAD PHILLIPS X 1 4 0 149 3 4 6 32 1 4 FLAT HEAD CHASSIS SCREWS 0 181 3 2 DB 9 CHASSIS HARDWARE 0 79 929 3 4 40 KEP NUTS 0 43 36 4 4 40 ROUND THREADED STANDOFF 0 293 37 2 4 40 FLAT HEAD BLACK SCREWS FOR 0 240 0 401 38 1 55 1024 24VDC POWER SUPPLY 6 182 39 1 LINE CORD 1 053 40 1 RS 232 SERIAL CABLE 1 277 41 1 12 10 CONDUCTOR CABLE WITH 0 1 PITCH 42 2 4 40 NYLON SHOULDER WASHER 0 231 43 2 TO 220 INSULATOR PADS 0 243 44 2 HOLE COVERING BRACKET NEW FAB 7 1345 45 1 FRONT PANEL LEXAN 7 1366 46 1 PRS10 FROM SRS FINISHED GOODS PRS10 25725 Rubidium Frequency Standard 34 Component Parts List 25725 Rubidium Frequency Standard Model PRS10 Rubidium Frequency Standard Operation and Service Manual S RS Stanford Research Systems 1290 D Reamwood Avenue Sunnyvale California 94089 Phone 408 744 9040 Fax 408 744 9049 email info thinkSRS com www thinkSRS com Copyright 2002 by Stanford Research Systems Inc All Rights Reserved Version 1 3 November 2005 PRS10 Rubidium Frequency Standard Table of Contents 1 Introduction 3 Crystal Oscillator 42 Crystal Heater 44 Specifications 4 Schematic RB_F2 Sheet 2 of 6 44 Temperature Control Servos 44 Abridged Command List 5 Conversion to 10MHz TTL 45 Photocell Amplifier 46 Theoretical Overview 8 Signal Filters for Oscillator Control 47 Rubidium Frequency Standards 8 Analog Multiplexers 47 Schematic RB F3 Sheet 3 of 6 48 PRS10 Overview 11 M
58. OPE is the SF calibration factor with a nominal value of 1450 see SS command and MO is the magnetic offset value The returned value should be in the range of 1000 to 4095 PRS10 Rubidium Frequency Standard RS 232 Instruction Set 31 Example MR would return a value of 3450 if the magnetic offset is at 3000 the SF command requested an offset of 2000 x 10 and the SS CAL factor has the nominal value of 1450 Frequency Control The frequency of the 10 MHz output may be adjusted in a number of ways the internal calibration potentiometer may be set accessible via a hole in the bottom plate an external voltage 0 to 5 00 Vdc applied to the interface connector pin 2 can override the internal pot or these analog channels may be overridden with a software command which sets the frequency directly When the unit turns on or after a restart command the control program will default to reading the analog channel for frequency calibration This is important to maintain compatibility with existing sockets The calibration pot and the external voltage control provide a full scale tuning range of 2000 x 10 with a worst case resolution of 5 x 107 All of the channels for calibrating the unit are linearized so that the frequency characteristic will be linear with applied voltage pot setting or SF value even though the transition frequency changes quadratically with field strength One pulse per second 1pps control To f
59. SA 3 1469 66 1 U203 LM340 5 3 112 67 3 U204 U206 U208 LM7171 3 819 68 1 0205 27 3 660 69 1 0207 AD8037 3 897 70 1 0209 74HC74 3 742 71 1 PCB REV B 7 1337 FS725 OPTION BOARD PARTS LIST REV B Item Quantity Reference Part SRS P N 1 1 C300 2 2U 35V 5 318 2 8 C301 C302 C304 C312 C320 AU 5 299 C328 C336 C346 3 9 C303 C305 C308 C313 C315 01U 5 298 C321 C323 C329 C332 4 8 C306 C310 C314 C318 C322 100P 5 375 C326 C330 C334 5 7 C307 C316 C324 C331 C339 27P 5 368 C340 C342 6 8 C309 C311 C317 C319 C325 220P 5 379 C327 C333 C335 7 1 C337 180P 5 378 8 1 C338 390P 5 382 9 1 C341 820P 5 386 10 1 C343 68P 5 373 25725 Rubidium Frequency Standard 32 Item 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 FS725 REV B HARDWARE PART LIST Item e e e vex 06 CO Component Parts List Quantity Reference C344 C345 C347 C348 C349 C350 C352 C354 C356 C358 C351 C353 C355 C357 C359 J300 301 302 1303 304 1305 306 307 1308 L305 L306 L307 L308 L309 L310 1311 0 R300 R301 R305 R307 R310 R312 R315 R317 R320 R322 8 R303 R304 R308 R309 R313 R314 R318 R319 R306 R311 R316 R321 R323 R325 R326 R327 R328 R329 R330 R331 R332 R333 R334 301 7302 7303 7304 7305 T306 U300 U301 U302 U303 U304 0305 0306 eS eee LN De RR 9 e Un Un Qnt Descri
60. X7R C 306 5 00299 568 AU Cap Ceramic 50V SMT 1206 10 X7R C 308 5 00298 568 01U Cap Ceramic 50V SMT 1206 10 X7R C 309 5 00375 552 1008 Capacitor Chip SMT1206 50V 5 310 5 00375 552 100 Capacitor Chip SMT1206 50V 5 C311 5 00586 569 4 TUF 50V Cap Tantalum SMT all case sizes C 400 5 00299 568 IU Cap Ceramic 50V SMT 1206 10 X7R C 401 5 00299 568 AU Cap Ceramic 50V SMT 1206 10 X7R C 402 5 00387 552 1000P Capacitor Chip SMT1206 50V 5 NPO C 403 5 00387 552 1000P Capacitor Chip SMT1206 50V 5 NPO 404 5 00299 568 IU Cap Ceramic 50V SMT 1206 10 X7R C 405 5 00299 568 AU Cap Ceramic 50V SMT 1206 10 X7R C 406 5 00387 552 1000P Capacitor Chip SMT1206 50V 5 NPO C 407 5 00466 572 AU SMT Film Capacitors 50V 5 All Sizes C 408 5 00298 568 01U Cap Ceramic 50V SMT 1206 10 X7R C 409 5 00466 572 1U SMT Film Capacitors 50V 5 All Sizes 410 5 00462 572 047U SMT Film Capacitors 50V 5 All Sizes 411 5 00299 568 LU Cap Ceramic 50V SMT 1206 10 X7R 413 5 00299 568 IU Cap Ceramic 50V SMT 1206 10 X7R C 414 5 00373 552 68P Capacitor Chip SMT1206 50V 5 NPO C 415 5 00375 552 100P Capacitor Chip SMT1206 50V 596 NPO C 416 5 00387 552 1000P Capacitor Chip SMT1206 50V 5 NPO C 417 5 00387 552 1000P Capacitor Chip SMT1206 50V 596 NPO C 418 5 00375 552 100P Capacitor Chip SMT1206 50V 5 NPO C 419 5 00456 572 0150 SMT
61. a 1PPS input signal on pin 5 of the main connector can be time tagged with Ins resolution The time may reported via RS 232 or used to servo the unit to another frequency standard such as GPS PRS10 Rubidium Frequency Standard 52 Circuit Descriptions Hardware on the bottom board provides two signals TIME LATCH and INTERPOLATE These signals latch the value of a free running counter clocked by the E_CLK which is part of the microcontroller TIME LATCH is just the IPPS input re synchronized to the CPU s E CLK which allows the processor to time tag the input to 400ns resolution INTERPOLATE will go low for a time equal to about 2000 times the interval between the I PPS input and the next E CLK Measuring the duration of INTERPOLATE allows the position of the 1PPS input to be measured to about 400ns 2000 0 2ns Schematic RB F4 Sheet 4 of 6 High Resolution Low Phase Noise RF Synthesizer The pressure tuned Rb hyperfine transition lies at about 6 834 685 850Hz This will vary depending on the fill pressure and gas composition of the Rb resonance cell In order to lock the crystal oscillator to this transition we need to synthesize and sweep frequencies in this region In order to minimize the amount of magnetic field tuning needed the frequency synthesizer should be capable of being set with high resolution about 1 10 In order to detect the transition with good signal to noise the synthesizer will need to have very low phase nois
62. a demonstrated MTBF several hundred thousand hours and so a malfunction of the PRS10 is very unlikely In the event of a failure the alarm relay will activate but the 10 MHz output will be provided by the unit s ovenized SC cut 10 MHz crystal oscillator with typical aging of 0 05 ppm year The FS725 will find application as an autonomous frequency standard in virtually any calibration and R amp D laboratories where precision time or frequency measurements are made or in any other facility requiring an accurate frequency standard The low cost of the unit FS725 Rubidium Frequency Standard Overview 7 together with its extremely low aging will allow customers to deploy precision frequency references throughout a facility without the need to install cables or antennas 25725 Rubidium Frequency Standard 8 Operation Operational Description Power The unit is powered from the AC mains The universal input power supply accommodates any voltage in the ranges 90 to 132VAC or 175 to 264VAC with a frequency in the range of 47 to 63Hz There is no power switch as the unit should be left all the time The rubidium frequency standard is operating and outputs are available whenever AC power is applied Warm up The front panel POWER LED should be on whenever power is applied to the unit The front panel LOCKED LED should come on within six minutes after power is applied if the unit is started from room temperature Warm
63. acilitate system integration the PRS10 provides a 1 output which may be set over an interval from 0 to 999 999 999 ns with 1ns resolution The unit also has the ability to measure the arrival time of 1005 input over the same interval and with the same resolution The ability to time tag a 1 input allows the PRS10 to be phase locked to other clock sources such as the 1005 output from a GPS receiver with very long time constants This is a very useful feature for network synchronization and allows the configuration of a reliable Stratum I source at a very low cost TT Time tag This command returns the value of the most recent time tag result in units of nanoseconds If a new time tag value is not available then 1 the only case for which the returned value is negative will be returned Example TT would return the value 123456789 to indicate that the most recent 1 input arrived 123 456 789ns after the 1 output Returned values range from 0 to 999999999 PRS10 Rubidium Frequency Standard 32 RS 232 Instruction Set TS TS value 7000 lt value 25000 TS TS Time slope This command is used to calibrate the analog portion of the time tagging circuit The analog portion is used to digitize the time of arrival with 1 ns resolution and 400 ns full scale Counters are used for the portion of a time interval longer than 400 ns The analog circuit stretches the time interval between the 1pps input and the n
64. al 95 non condensing General AC Power 90 to 132 VAC or 175 to 264 VAC 47 to 63 Hz 50 VA Dimensions 8 5 x 3 5 x 13 WHL Weight 9 Ibs Warranty One year parts and labor on materials and workmanship 25725 Rubidium Frequency Standard 6 Overview Overview The FS725 is a stand alone 10 MHz frequency standard with an estimated 20 year aging of less than 0 005 ppm The unit integrates an SRS PRS10 Rubidium Frequency Standard a universal AC input power supply and distribution amplifiers in a half width 2U chassis 3 5 height The unit s low aging eliminates the need for an external frequency reference in many applications however units can phase lock to an external 1 pps GPS signal for ageless performance or periodic calibration as required The FS725 provides exceptionally low phase noise 5 MHz amp 10 MHz sine outputs as well as a low jitter 1 pps output The 10 MHz outputs have a phase noise of lt 130 dBc Hz at 10Hz offset from carrier dropping to 155 dBc Hz at 10 kHz offset The Allan Variance is less than 2x10 at one second and 2 x 107 at 100 seconds The 1 PPS TTL output has less than Ins of jitter and may be set with Ins resolution The standard FS725 has two 10 MHz outputs one 5 MHz output and one PPS output Up to three distribution modules can be added to the unit Each module provides six additional outputs four 10 MHz outputs one 5 MHz output and one 1 PPS output With three modules insta
65. ameter has settled Repeat as necessary When you have a satisfactory SF value unplug the 1 pps input and power cycle the FS725 to return control of the SF parameter to the potentiometer While monitoring the SF parameter via the RS 232 adjust the calibration potentiometer to set the SF value to the value seen when the unit was phase locked to the 1 pps reference Now each time the unit turns it will be operating at the same frequency as when it did when it was phase locked to the 1 pps reference 25725 Rubidium Frequency Standard 24 Calibration Calibration of the time tag offset parameter The FS725 can time tag the leading edge of a rear panel 1 pps input Results of the time tag are returned over the RS 232 in response to the TT command in integer nanoseconds with respect to the 1 pps output Ifthe 1 pps input arrives 5ns after the 1 pps output the TT should return a value of 5 If the 1 pps input arrives Sns before the 1 pps output the TT should return a value of 999999995 If no input was applied and so no time tag result is available the TT will return a value of 1 There 15 a calibration parameter which is added to the measured time tag before the result is returned over the RS 232 Offsets in time tag results can be eliminated by adjusting the TO parameter Few users will need to perform this calibration as fixed offsets of a few nanoseconds in the reported time tags will have little consequence S
66. and C102 has a capacitive reactance equivalent to an 87pF capacitor At the fundamental 3 3MHz and at the B mode frequency 10 8MHz this network is inductive and so there will be no gain provided by Q100 In addition to this network C103 C104 the varactor D100 and C106 which connects to the ac ground at the emitter of Q101 are all in series with the crystal C104 15 selected when the unit is calibrated so that the crystal will operate at 10 0MHz with the nominal EFC voltage applied to the varactor The crystal frequency tunes linearly with the net series reactance with a tuning coefficient of 1Hz 20Q Series capacitors tune the crystal to higher frequencies series inductors tune the crystal to lower frequencies Only NPO capacitors are to be used and inductors should be either air or iron powder core no ferrites in order to preserve the relative insensitivity to ambient temperature variations To move the oscillator to higher frequencies C pF 808 Af Hz To move the oscillator to lower frequencies L uH 0 29Af Hz PRS10 Rubidium Frequency Standard Circuit Descriptions 43 The MMBV609 varactor provides an approximate linear tuning characteristic over 2 ppm This will allow the unit to correct for aging of the crystal for a nominal 27 year life given a daily aging of 2 parts in 107 The crystal is operated at its temperature plateau of about 80 C The plateau temperature is determined at calibration for each unit The frequ
67. applied to the unit At the operating temperature of 75 C each thermistor will have a resistance of about 15kQ The control circuit will allow operation up to 90 C For the lamp the nominal operating temperature is 105 C for which each thermistor will have a resistance of about 5 5kQ The maximum setpoint for the lamp is 122 C The control circuits for all of the heaters are on the top analog PCB The control circuit can vary the heater current from 0 to 0 7A to maintain the set point In the case of a control failure the LM340 12 will turn off the current if the junction temperature reaches 125 C Schematic RB F2 Sheet 2 of 6 The components contained on this schematic are all located on the top analog PCB This board contains most of the analog circuitry for the system including temperature servos photodiode amplifier and filter analog signal multiplexers and noise filters for the crystal s EFC and amplitude control signals Temperature Control Servos There are three temperature control servos for the crystal oven the Rb discharge lamp and the Rb resonance cell The three servos are identical except for the maximum set point 122 C for the lamp and 90 C for the others The circuit description will refer to the crystal temperature controller PRS10 Rubidium Frequency Standard Circuit Descriptions 45 The controller is a proportional integral controller The output of the error amplifier U200A is used to control the
68. ather high voltage noise 14nV VHz A tuned transformer 200 is used to increase the voltage and source impedance of the PRS10 s 10 MHz output to avoid degrading the output noise The 10 7MHz transformer is tuned down to 10 MHz by an additional capacitor on the primary side of the transformer The transformers have a 47pF capacitor internally The 3 5 1 transformer increases the 10 MHz amplitude to about 5Vpp the source impedance to about 6000 and reduces harmonic distortion A 10VDC bias from U205 is added to the transformer secondary so that the 5Vpp output is centered in the common mode input range of the op amps which are operated from 20VDC The op amps U204 and U206 are operated with a nominal gain of 2 4 The gain can be adjusted with P200 or P201 to calibrate the amplitude of the rear panel 10 MHz outputs The op amps drive 50 Q loads via a tuned output transformer and a 3 degree Cauer low pass filter The transformer impedance matches the user load to the amplifier reduces voltage noise and insures that all of the output current will return to the isolated ground Passive LC filters on each power supply also improves channel to channel isolation The output filter reduces the harmonic distortion by an additional 15 dB Another amplifier U208 provides a buffered 10 MHz sine wave to the optional output modules This amplifier is operated with nominal gain of 2 4 which is adjusted to provide 5 Vpp to the optional output mod
69. be done more quickly and accurately by comparing the phase of a 10 MHz reference to the phase of the FS725 10 MHz output A time interval counter may be used for this purpose In this case the 10 MHz reference is used as a timebase for the time interval counter The inputs to the time interval counter will be two 10 MHz signals one from the 10 MHz reference and one from the FS725 The time ON Ud FS725 Rubidium Frequency Standard Calibration 21 interval counter should be set up to report the average of 1000 measurements The reported time interval measurements will be between 0 and 100 ns the period of a 10 MHz signal Using the SR620 the resolution and jitter of the average of 1000 time interval measurements will be less than 1 ps rms and values will be reported about once a second Observe how the average time interval measurement changes with time If the reported time intervals are changing by Ins per second then the two 10 MHz source are off by 1 x 10 or 0 001 ppm Adjust the calibration potentiometer until the drift between time interval measurements slows to less than 1 10 or 10 ps per second 10 Measure and record the dc voltage on the center pin of the FREQ ADJ BNC 25725 Rubidium Frequency Standard 22 Calibration Calibration using a 1 pps signal such as from a GPS receiver The FS725 may also be calibrated by phase locking the unit to the 1 pps signal from a GPS receiver The calibration potentiometer will be a
70. between the reference frequency and the DUT is to use the time interval measurement mode of the counters In this case the time intervals between the 10MHz zero crossings of the reference frequency and the DUT are measured and averaged If this time interval changes by less than 1005 per second then the DUT is within 1 part in 10 of the frequency reference This technique is very similar to the technique of offsetting the reference frequency from the DUT mixing the two sources amplifying and filtering and measuring the frequency of the beatnote Often referred to as a heterodyne measurement However the time interval measurement technique does not require mixers or amplifiers or offsetting the reference from the DUT The resolution of the time interval technique is remarkable Each time interval measurement has an rms jitter of about 25ps in the case of the SR620 As the jitter is randomly distributed the jitter of the mean is reduced by the square root of the number of samples For a 1000 sample measurement which takes less than one second to complete the rms jitter of the mean will be less than 1ps and the difference between two time interval measurements will have a jitter of less than 2ps This provides a relative frequency measurement to 2 parts in 10 in 2 seconds PRS10 Rubidium Frequency Standard Appendix 63 Set up for an SR620 Described here is the set up for an SR620 Time Interval Counter to make precision frequency
71. cillator affects the operating conditions of the lamp temperature Rb vapor pressure and discharge intensity which will affect the apparent hyperfine transition frequency The drain voltage and current are controlled by the lamp regulator The gate voltage to the MOSFET is controlled so that the drain current is about 60mA 10mA V V arain The gate voltage is supplied by U502B which measures the drain current through the shunt resistors R504 R505 R552 and R553 The offset and slope of the drain current vs drain voltage is set by R510 and R511 The drain voltage is controlled by an 8bit DAC whose output is multiplied by 6 and buffered by U502 and Q500 An adjustable regulator U501 is bootstrapped at 1 75Vdc above the drain voltage This regulator will provide the drain current for drain voltages above 6 25V dc When the drain voltage is set below 6 25Vdc the drain current is sourced from a 8V regulator which is part of the lamp heater circuit This is done to reduce the power required by the unit by redirecting the heat of the regulator to the lamp block which needs to operate at a high temperature To start the discharge the drain voltage to the MOSFET is set to about 20Vdc which is regulated from the 24 HEAT supply The drain voltage is reduced to about 5V after the lamp starts 1PPS Input Time Tag The rising edge of a IPPS input signal on pin 5 of the main connector can be time tagged with 1ns resolution The time may repor
72. crystal oscillator See Sheet 1 of 1 The resonance cell heaters U600 and 0600 are located on the back of the resonance cell The lamp heaters 0800 and Q800 are located on the bottom of the lamp block The other passive components are located on the small vertical PBCs attached to the back of the resonance cell and lamp blocks The control circuits of the heaters are located on the top PCB Resonance Cell Components shown inside the resonance cell include L700 a 50 turn magnetic field coil D700 the SRD with its input matching network mounted on an SMB connector and D701 the photodiode Another SMB connector J701 is used to pick up some of the microwave field to allow diagnostic tests with an RF spectrum analyzer Discharge Lamp A plasma discharge is maintained inside a small bulb filled with a few Torr of an inert gas and some Rb metal by an RF oscillator The oscillator operates at about 150MHZ with a peak to peak voltage of about 10 times the dc voltage applied to the FET s drain Q900 an MRF134 medium power n channel FET is used as the active element in the oscillator circuit This part is characterized for operation at 28Vdc and 150MHz and is rated for a dissipation of 9 5W derated for our 105 C operation Our most severe operation is during lamp ignition with an total input power of about 3 2W The total input power PRS10 Rubidium Frequency Standard 58 Circuit Descriptions during normal operation 15 0 5W The p
73. d when 256 consecutive good 1 pps inputs i e 1 pps inputs which are within 2048 ns of the first time tag result modulo 1 s are received After receiving 256 consecutive good 1 pps inputs the 1 pps output pulse is moved to coincide with the last of the 256 time tag values The front panel 1 pps synchronized LED will turn on as the 1 pps input and the 1 pps output are now within lys of each other The unit will lock the frequency ofthe PRS10 to the good 1 pps input pulses Bad 1 pps inputs 1 pps inputs with time tags greater than 1 024 ns from the last good 1 pps input will be rejected A digital filter is used to implement the classical 2 order FS725 Rubidium Frequency Standard 12 Operation proportional integral phase lock loop The output of the digital filter controls the frequency of operation via the SF set frequency command which is updated once a second The PLL will be aborted and restarted if there are 256 consecutive bad 1 pps inputs This could happen if the 1 pps input is moved suddenly by more than 1 024 ns The time constant of the PLL is set with the PT command The default value is 8 providing a natural time constant of about 2 25 hours The PT value should be set to a smaller number if the 1 pps reference is very clean for example if the 1 pps reference is provided by another FS725 Using a smaller PT value will reduce the peak to peak variations swim
74. dc Supply current A Protection Ve RF protection mA Ext calibration Cal reference out 5 00 0 05 RS 232 9600 8 bits no parity 1 stop bit 0V 5V levels with x baud on x off protocol lpps measurement 10 accuracy 1 resolution 1 output set 10 accuracy 1 resolution Miscellaneous pU ns ns Temperature 20 to 65 baseplate Storage Size inches Weight Ibs Warranty ears Baseplate threads Connector lt E Extcalibration Cal reference out Va Miscellaneous Temperature Storage Wamany year PRS10 Rubidium Frequency Standard Abridged Command List 5 Abridged Command List Commands consist of two letter ASCII mnemonics A command may be followed one or more numeric values and punctuation Command sequences end with a carriage return ASCII 1310 All commands are case insensitive Spaces ASCII 3210 and linefeeds ASCII 1010 are ignored A command followed by a value is used to set a parameter to the value A command followed by an exclamation point or ASCII 3310 indicates that the current value should be saved to EEPROM to be used as the initial value after the next reset A command followed by a question mark or ASCII 6310 1s used to request that the current value be returned A command followed by an exclamation point and a question mark is used to return the EEPROM value For example the gain parameter determines the time constant us
75. djusted so that the frequency control parameter SF matches the value determined by the phase locking algorithm When a precise 1 pps signal is applied to the 1 pps input on the rear panel of the FS725 the rubidium frequency standard will phase lock to the signal The 1 pps signal usually comes from a GPS receiver which can have a long term accuracy of a few parts in 10 Because the stability of the 1 pps reference in the short term can be much worse than that of the PRS10 the PRS10 phase locks to the 1 pps with a very long time constant of several hours or more A classical 2 order phase lock control loop is implemented in firmware The phase comparator is a time tagging circuit which measures the timing of the 1 pps input with nanosecond resolution A proportional integral phase lock algorithm controls the frequency of the rubidium frequency standard with the SF parameter Set Frequency Normally the SF parameter is set by the firmware reading the calibration potentiometer However when a 1 pps signal is seen the phase lock algorithm hijacks the SF parameter to phase lock the rubidium frequency standard to the 1 pps input To calibrate with a precise 1 pps input the FS725 will be phase locked to the 1 pps input and the SF parameter will be read via the RS 232 interface Then the 1 pps input will be removed the unit will be power cycled so that the SF 15 again set by the calibration potentiometer and the potentiometer will be ad
76. e on the order of 70dBc Hz at 6 8GHz Since we want to stabilize a 10 000MHz crystal to an essentially arbitrary frequency with low phase noise we will need a dual loop synthesizer a fast loop to stabilize an RF VCO to a crystal for good phase noise and a slow loop to stabilize the crystal to the 10 000MHz reference Typical numbers A typical microwave frequency is 6 834 685 853Hz Which is the 19th harmonic of the RF frequency 359 720 308Hz Which is 16 times the crystal frequency 22 482 519Hz In this case dividing the RF frequency by 1386 64 39 4053 5Hz Which equals the reference frequency 10 000 000Hz divided by 2467 The dual modulus frequency synthesizer will be programmed with R 2467 N 1386 and 39 The microwave frequency is generated by frequency multiplication of RF frequency in a step recovery diode SRD The RF frequency was chosen to give good conversion efficiency with favorable numerology so that the gaps between available frequency steps from the dual modulus synthesizer are small A reasonable crystal frequency is the RF frequency divided by 16 dividers and oscillator mixers are available in this frequency range A third overtone crystal resonator with a nominal frequency of 22 48252MHz is used to provide a low noise flywheel We PRS10 Rubidium Frequency Standard Circuit Descriptions 53 only need to tune over a range of 0 1ppm to accommodate for fill pressure variations of
77. e about 6Q Q101 also helps to isolate the crystal circuit from variations from the external 10MHz load as does U101 so that the frequency of operation of the circuit will not be pulled by the load The op amp operates as a transconductance amplifier with a transconductance gain of about 20000 at 1OMHz The dc output of the op amp is midway between the supplies at about 8 25Vdc which is controlled by the current drawn by Q101 and the value of R111 There is a 10Vpp sine at 1OMHz at the output which is ac coupled reverse terminated and matched to 500 load by C111 R114 and 1100 The primary of T100 is tuned to 1OMHz so that spurs and harmonics are attenuated The 7 2 turns ratio transforms the 50Q into a 612Q load at 10MHz The output amplitude into 500 is 0 50Vrms 1 414Vpp 7dBm Extremely low phase noise is an important specification for this oscillator The phase noise close to carrier 10Hz offset and below is dominated by 1 f components including crystal parameters temperature stabilization amplitude limiting and gain mixing Far away from carrier gt 1kHz the noise floor is determined by ratio of broadband noise sources to the signal current at 1OMHz Examples of broadband sources include the shot noise current on base currents the Johnson noise current from bias resistors and the op amp s input current and voltage noise It is also important to maintain very low noise on the EFC and amplitude PRS10 Rubidium Frequency
78. e discharge by a process called optical pumping Suppose that the filter can completely remove one of the two discharge lines The remaining light can be absorbed by Rb87 atoms in the resonance cell which are in the lower ground state moving them to the upper state When they decay from the upper state they fall with equal probability into either ground state As this continues population will be moved from the lower ground state to the upper ground state circulating through the upper state As the population in the lower ground state is decreased the amount of light which reaches the photodetector will increase as the number of atoms which can absorb the radiation 1s reduced PRS10 Rubidium Frequency Standard Theoretical Overview 9 Rb Rb Isotopic Discharge Filter Resonance lamp Cell Photocell gt gt gt Qt 6 834GHz Photons 6 834 682 612 8Hz Rb RbU Emission Scattering Optical Pumpin Figure 1 Hypothetical Rubidium Physics Package Now if we apply a microwave field at the frequency corresponding to the hyperfine transition frequency 6 834 682 612 8 GHz the populations in the ground state will mix and the amount of light reaching the photodetector will decrease The PRS10 uses the integrated filter topology rather than a separate filter cell the resonance cell contains a mixture of the two rubidium isotopes along with a buffer gas The lamp al
79. e frequency of the inner loop s crystal keeping it locked to the 10 MHz reference PRS10 Rubidium Frequency Standard PRS10 Overview 13 The frequency synthesizer is set to the nearest frequency above the apparent hyperfine transition for the unit s physics package A magnetic field is used to tune the physics package s apparent hyperfine transition frequency up to the synthesizer frequency A 70 Hz digitally synthesized sine wave is used to phase modulate the inner loop The outer loop bandwidth is too small to suppress this modulation This generates an RF output which when multiplied to 6 834 GHz sweeps by about 300 Hz around the apparent hyperfine transition frequency By sweeping through the transition at 70 Hz the output from the photocell will have an ac component at 140 Hz when centered on the transition There will be an ac component at 70 Hz if we are off to one side of the transition the phase of the 70 Hz component is used to determine if the RF is above or below the transition Physics Package The physics package consists of a discharge lamp enriched with Rb87 and an integrated filter and resonance cell The discharge lamp operates at about 150 MHz The lamp oscillator can provide up to 300 to start the lamp which drops to about 100 V during normal operation The lamp oscillator voltage and current are carefully regulated to provide a consistent intensity and low noise The resonance cell is inside a mu metal
80. ed Details are available in the PRS10 operation and service manual 4 By software calibration via the MO Magnetic Offset parameter The MO parameter is summed in quadrature with the SF parameter to control the frequency of operation At the factory the MO parameter is calibrated so that the unit will operate at 10 MHz with an SF value of 0 The SF parameter is set by either the potentiometer or via RS 232 commands and will be zero when the potentiometer is centered The MO parameter may be adjusted and retained in the PRS10 EEPROM allowing frequency calibration via the RS 232 Details are available in the PRS10 operation and service manual 25725 Rubidium Frequency Standard Operation 11 Using an External 1 pps Reference The FS725 has sufficient accuracy 0 005 ppm estimated over 20 years that most users will not need to use an external timing reference However for critical applications requiring a frequency accuracy which is better than 110 or to provide continuous traceablity the FS725 may be locked to an external 1 pps reference When a precise pps signal is applied to the 1 pps input on the rear panel of the FS725 the rubidium frequency standard will phase lock to the signal The 1 pps signal usually comes from GPS receiver which can have a long term accuracy of a few parts in 10 Because the stability of the 1 pps reference in the short term can be much worse than that of the PRS10 the PRS10 phase locks to the 1 pps
81. ed to lock the 10MHz oscillator to the rubidium hyperfine transition Examples of the four forms of the gain parameter command are GA returns the current value of the frequency lock loop gain parameter 047 sets the frequency lock loop gain parameter to 7 GA writes the value of the gain parameter to EEPROM for use after reset GA returns the value of the gain parameter which is stored in EEPROM All strings returned by the unit are terminated with a carriage return ASCII 1310 In the verbose mode strings are preceded with a linefeed ASCII 1010 and terminated with a carriage return and a linefeed If more than one value is returned by a command the values will be separated by a comma ASCII 4410 When a unit is first turned it will send the string PRS_10 without the quotes followed by a carriage return Only commands in bold type are available to the end user The other commands are factory only commands which disabled at the factory PRS10 Rubidium Frequency Standard 6 Abridged Command List x Value or an Query Description ctivate EPROM EEPROM Initialize RS RS 1 Restart VB VB value Verbose mode ID Read ID string SN SN value SN SN Read unit serial number ST Read six status values LM LM value LM LM Lock pin mode RC 1 RC Recall factory calibration Freq Lock
82. ency is a maximum at the plateau and so the oscillator will typically be a few hundred Hertz low when the unit is turned on at room temperature Near the plateau top the frequency deviation verses temperature is about Af Hz 0 061x AT C Note that if the crystal oven were to lose regulation by 12 8 C perhaps the baseplate is too hot that this would cause a 1 frequency error which could be corrected by the Rb frequency lock loop Power to overcome losses to sustain oscillation is provided by Q100 The dominate loss is the series resistance of the crystal about 800 0100 provides power by injecting a current at the top of L100 which is in phase with the 10MHz voltage at this node The amount of current injected depends on the size of C103 and R103 the current injected is equal to the ac voltage across C103 divided by the resistance of R103 assuming emitter following action of Q100 The magnitude of the oscillation will grow until the peak voltage at the base exceeds the collector voltage causing Q100 to saturate The circuit is designed to allow about ImA rms to circulate through the crystal The ac current is high enough to provide low phase noise but low enough to minimize aging This ac current is cascoded to the inverting input of the high speed op amp U101 Q101 Q101 provides a good ac ground for the crystal circuit to maintain high in circuit Q With an emitter current of 4mA the emitter resistance of Q101 will b
83. ency step of about 1 10 We would like for noise the EFC to be less than one LSB To arrange this the DAC22 signal is filtered with a time constant of 1s and buffered by a FET input op amp U210B an AD822 The FET op amp has 1 f noise of about 2uVpp in the two decade band from 0 1Hz to 10Hz Both the op amp and the 10 0kQ feed back resistor will have noise of about 30nV VHz at 10Hz which is well under the target of 1 6V VHz required to meet the specification of 125dBc Hz at 10Hz offset The oscillator s amplitude control is filtered is a similar fashion using U210A Noise on this signal would be detrimental to the phase noise spectrum but would not affect zero crossings of the sine output Analog Multiplexers There are 16 analog signals which may be multiplexed to the 12 bit A D converter One of these signals PHOTO_AMP is be digitized 32 times during each cycle of the 70Hz modulation 2240 Hz in order to lock the crystal to the Rb hyperfine transition The other 15 signals are monitored intermittently and in response to RS 232 requests AO Amplified and filtered photocell signal 2 Case temperature 10mV C 3 Crystal thermistor voltage 24 CLEAN 10 7 Discharge lamp FET s drain voltage PRS10 Rubidium Frequency Standard 48 Circuit Descriptions Schematic RB F3 Sheet 3 of 6 of the components shown on this schematic reside on the vertical PCB on the left side of the unit The large
84. ented The transmit drive level is 0 and 5V not the 12V normally associated with RS 232 These levels are compatible with RS 232 line receivers but does not require their use a TTL inverter may be used instead hence simplifies the interface when used inside an instrument at the sacrifice of degraded noise immunity over long lines 12 Bit A D Conversion A serially interfaced 12 bit A D converter is used to measure the ac and de components of the photocell signal The analog input to the ADC is buffered by U309A a FET input op amp configured as a unit follower The quantization noise of this converter will not degrade the S N of the ac signal even in the case when the ac signal occupies a relatively small portion of the converter s full scale range The A D converter can also measure the position of a 10 turn user cal pot which has a software defined range of 2E 9 The 12 bits of resolution will provide a frequency trim of 1E 12 12 Bit Digital to Analog Converters There are four 12 bit DACs Two of the DACs are scaled summed and offset to provide level with 22 bits of resolution to control the crystal frequency One of the DACs is used to control the magnitude of the magnetic field in the resonance cell The forth DAC is used to digitally synthesize the 70Hz phase modulation of the 6 834GHz microwave field Two of the DACs the upper DAC of the 22 bit pair and the DAC which controls the magnetic field are rarely changed and
85. ext edge of a internal 2 5 MHz clock by a factor of about 2000 and measures the duration of the stretched pulse by counting a 2 5 MHz clock The analog portion of the time tag result is calculated from the equation AT ns counts TS 2 5 where TS is the time slope value which has a nominal value of 13 107 Example TS might return 14 158 which is a time slope parameter value a bit above the nominal value which would be required if the analog portion of the time tagging circuit stretched the pulse by a bit less than a factor of 2000 TS will return the current value of the time slope The TS command is used to write the current value of the time slope parameter into the unit s EEPROM The TS value and TS are factory only commands Example TS will write the current value of the time slope which may be queried with the TS command to the unit s EEPROM TS will return the time slope calibration factor which is in the unit s EEPROM TO 32767 lt value lt 32768 TO TO Time offset This calibration value in ns is added to the measured time tag value to reference the result to the 1005 output To calibrate the 1103 output is connected to the 1 input and the time tag is read with the TT command The returned value is subtracted from the current TO value and sent with the TO command to calibrate the offset Example Suppose the 1 output is connected to the 1 pps input A time tag value read w
86. flops drives the rear panel 5 MHz output via R231 amp R232 a tuned transformer T203 and a 5 order Cauer low pass filter C241 248 amp L209 210 A standard 10 7 MHz transformer is tuned down to 5 MHz by additional capacitance on the primary of the transformer The tuned transformer and filter attenuate harmonics of the 5 MHz square wave by more than 60dB producing a low distortion 5 MHz sine wave at the rear panel output The other 5 MHz square wave is used as a clock reference for the optional output modules Signals to the Optional Output Modules A 10 line bus connects signals from the main board to each of optional output modules Signals and ground planes on this bus are arranged to reduce cross talk Three power supplies 20V 5V and the 5 00V reference three grounds all in common and four signals 10 MHz sine 5 MHz and its compliment and the gated 1 pps signal are passed to all the option modules Optional Output Modules Up to three optional output modules can be installed in the FS725 Each module provides four 10 MHz sine outputs one 5 MHz sine output and one 1 pps TTL output All of the outputs are intended to drive 50 2 loads and all have the same drive levels phase noise and distortion as the standard outputs Signals passed from the main board are buffered on each option module The 10 MHz sine wave is buffered by U300 which operates with a gain of 2 to overcome the attenuation of the input divider used on eac
87. for warm up 6 Casetemperaturetoolow Wait for warm up ST4 Frequency Lock Loop Control ST4 bit Condition which sets bit Corrective Action 0 _ Frequency lock control is off 2 3 Frequency lock is disabled Enable w LO1 command 2 10MHz EFC is too high SD4 SP 10MHz cal Tamb 3 10 MHz EFC 15 too low SP 10 MHz cal PRS10 Rubidium Frequency Standard 40 RS 232 Instruction Set Analog cal voltage gt 4 9 V Int cal pot ext cal volt Analog cal voltage lt 0 1 Int cal pot ext cal volt CN 1 ST5 Frequency Lock to External 1pps ST5 bit Condition which sets bit Corrective Action ee a ee 5 O PLLdisabed Send PL 1 to enable PLL active ee 6 feontrol saturated Wait check Ipps inputs ST6 System Level Events ST6 bit Condition which sets bit 0 Lamp restart Watchdog time out and reset _ 6 Bad command parameter PRS10 Rubidium Frequency Standard PRS10 Calibration Procedures 41 Calibration Procedures Many applications for the PRS10 only require that the frequency of the 10 MHz output be calibrated This may be done by adjusting a potentiometer which is accessible through a hole in the bottom of the unit The unit should be operating for at least 24 hours before it is calibrated The 15 turn pot has a range of 0 020 Hz The frequency increases if the pot is turned clockwise by about 0 001 Hz for 3 8 s of a turn Note the
88. g the gate pulse driving the output of the comparator U509 low C509 is then recharged by Q501 5 4uA constant current source When C509 reaches 11 0V the output of the comparator goes high The ratio of the collector currents of Q501 and Q502 sets the stretch multiplier The circuit is temperature compensated against variations in the transistors base emitter voltages as both the charge and discharge currents are equally affected by their junction temperature leaving the ratio unchanged 1PPS Output Pulse Delay A port bit on the microcontroller PA7 may be used to output a 105 pulse at a rate of 1Hz This port bit is controlled by the microcontroller s timer which has a resolution of one E CLK cycle 400ns Hardware on this circuit board provides delays in 100ns steps under control of the port bits IPPS SELO and IPPS SEL and in steps of about 0 5ns via an analog signal from 8 bit DAC The combination of these three delays allows the 1PPS output pulse to be placed with an accuracy resolution and differential non linearity of about Ins The 1PPS port bit from the CPU is synchronized to E_0 by U506B then synchronized and delayed by U500 The multiplexer U510 selects one of the four phases of the 1PPS output delayed in steps of 100ns by the 10MHz clock The selected 1PPS pulse may be delayed by an analog control signal C513 is charged to a level of 10 V4 2 by Q503 s collector current which turns on D503 connecting C51
89. h channel To improve distortion biases are arranged so that none of the 10 MHz amplifiers has any dc current at their output FS725 Rubidium Frequency Standard Circuit Description 29 The 5 MHz square waves are buffered by U305 The 1 pps signal is buffered by U306 To reduce channel to channel cross talk all channels are placed on separate ground planes and operate from locally bypassed power supplies 5725 Rubidium Frequency Standard 30 Component Parts List FS725 Component Parts List FS725 REV B MAIN BOARD PARTS LIST Item Quantity Reference 1 2 UJ 5 30 NO RR RR PLR W De BWR lt NK BR DR KF AN W C100 C103 C104 C256 C257 C101 C106 C107 C108 C109 110 111 113 114 115 C116 C117 C211 C212 C213 C214 C221 C222 C229 C231 C232 C234 C235 C236 C237 C238 C239 C250 C253 C254 C102 C200 C201 C105 C112 C202 C203 C204 C205 C206 C207 C208 C249 C210 C217 C225 C242 C243 C245 C215 C223 C230 C216 C219 C224 C227 C218 C220 C226 C228 C240 C241 C244 C246 C247 C248 C255 C251 C252 C258 C260 C262 C259 C261 C263 D100 D101 D150 D151 D152 D153 D154 D155 J100 COAX PART OF J100 7101 7102 7108 202 7203 1204 J103 EXTERNAL PART OF J103 J104 J205 250 1251 1252 253 J254 J255 J256 J257 L102 L206 L208 L211 L200 L202 L204 L209 L210 P200 P201 P202 R209 R217 R234 R240 0100 0101 0200 R100 R101 R102 R204 R206 R222 R225 25725 Rubidium Frequency
90. h the oscillation which stops the discharge which allows the oscillation to start again This cycle can occur at several kHz which makes frequency locking impossible PRS10 Rubidium Frequency Standard Circuit Descriptions 59 Schematic RB_F7 Sheet 1 of 1 Connector Interface Board Not part of standard product This board is not part of the standard product and is available from the factory at and additional charge It is intended to facilitate customer evaluation of the product by adapting the standard product s interface connector to connector types which are more readily available in the laboratory such as BNCs and DB9 for RS 232 This board connects to the outside of the unit Three BNCs are used to source 10MHz and the 1PPS outputs and to receive the IPPS input A DB9 female allows direct connection to a computer usually via COM2 2 1mm power connector allows the unit to be connected to a standard 24V 2 5A power supply center conductor must be positive The 10MHz output should be terminated into 50 2 load The output will be about 0 5Vrms about 1 41 Vpp The RS 232 interface uses CMOS logic levels OV and 5 which will work with standard RS 232 line drivers and receivers The 12V of the standard RS 232 line driver will not harm the logic input and the 0 5V RS 232 output from the rubidium standard will work with virtually all computers provided the cable is less than 25 feet long The RS 232 control lines
91. icrocontroller 48 Block Diagram 11 RS 232 50 Ovenized Oscillator 11 12 Bit A D Conversion 50 Frequency Synthesizer 11 12 Bit Digital to Analog Converters 50 Physics Package 13 Magnetic Field Control 50 Control Algorithm 13 Phase Modulation 51 Initial Locking 14 1PPS Output 51 Locking to External 1 14 1PPS Input Time Tag 51 CPU Tasks 18 Schematic RB F4 Sheet 4 of 6 52 High Resolution Low Phase Noise Applications 19 RF Synthesizer 52 Interface Connector 19 RF Output Amplifier 53 Configuration Notes 19 Step Recovery Diode Matching 53 Hardware Notes 20 Analog Control 54 Operating Temperature 21 Schematic RB_F5 Sheet 5 of 6 54 Frequency Adjustment 21 Power Supply Lamp Control and 1PPS Timing PCB 54 RS 232 Instruction Set 22 Linear Power Supplies 54 Syntax 22 Lamp Regulator 55 Initialization 22 1PPS Input Time Tag 55 Frequency Lock loop Parameters 24 1PPS Output Pulse Delay 56 Frequency Synthesizer Control 28 Baseplate Temperature Sensor 57 Magnetic field Control 29 Schematic RB_F6 Sheet 6 of 6 57 Frequency Control 31 Resonance Cell and Lamp Heaters 57 One pulse per second control 31 Resonance Cell 57 1PPS Locking Control 33 Discharge Lamp 57 Analog Control 36 Schematic RB F7 Sheet 1 of 1 59 Analog Test Voltages 97 Connector Interface Board 59 Status Bytes 38 Appendix A Frequency Synthesizer Calibration Procedures 41 Table 60 Circuit Description 42 Appendix B Precision Frequency Schematic RB sheet 0660 42 Measurement 62 I
92. ignal from the physics package which would lead to a noisy frequency lock and degraded output stability These two characteristics require a dual loop design for the frequency synthesizer The inner loop consists of the 359 72 MHz VCO which is directly phase locked to a 3rd overtone 22 48252 MHz crystal oscillator This loop has a large natural frequency of about 400 000r s The VCO s phase noise at 359 72 MHz is very close to the phase noise of the crystal plus 24 dB for the multiplication factor of 16 PRS10 Rubidium Frequency Standard 12 10 Overview 22 bit DAC 10MHz Low Noise Matching 10 bit Ovenized Oscillator Transformer C 500 EE CPU 4 3 gt gt 10MHz Synthesizer Reference Dual Modulus 22 4825MHz VCXO Synthesizer 359 72MHz Af 1E 9 step 10r s 359 72MHz Gain Leveling Amp E Temperature Controlled 150MHz Lamp Oscillator LT Step Recovery Diode FEED Rb87 Enriched Transimpedance 12 bit Discharge Lamp Resonance Cell Amplifier ADC Figure 2 Rubidium Frequency Standard Block Diagram The outer loop compares the RF frequency to the 10 MHz This loop provides high resolution by dividing the RF and 10 MHz by large numbers and consequently operates at a low comparison rate typically 4 kHz This loop has a low natural frequency about 10 1 5 so the phase noise of the RF more than a few Hz from carrier will be determined by the inner loop The outer loop slowly disciplines th
93. ilter is a digital filter consisting of an optional pre filter and a standard proportional integral controller PRS10 Rubidium Frequency Standard PRS10 Overview 15 PI controller with programmable proportional and integral gains The VCO is the rubidium frequency standard whose frequency f is tuned by the magnetic field via the SF command parameter with a sensitivity for its 1 output of Kyco 0 001 ns bit s or 1 part in 10 bit The response function for each of the elements of the digital PLL is also indicated in the figure in terms of the standard Laplace variable s External 1 input Proportional and Integral Pre filter a Time tag circuit Figure 3 External Ipps Phase Locking Block Diagram The PI controller is programmed by choosing an appropriate integrator time constant and a stability factor t determines the natural time constant Tn of the PLL for following a step in phase of the reference while G determines the relative rise time and ringing of the PLL in response to the step The value of G also represents the tradeoff in the equivalent noise bandwidth verses peaking in the passband near the natural frequency of the response function The PRS10 accepts integrator time constants T1 ranging from 2 to 2 seconds in powers of 2 The natural time constant is given by 7 7 J 1000s 7 Thus the PRS10 provides natural time constants ranging from 506 seconds to 18 0 hours
94. in the crystal frequency due to aging and ambient conditions Both DACs may be set to any value in the range specified above Example FC 2048 2048 will set the 1OMHz oscillator back to the middle of its tuning range However it is possible to set the frequency of the 10 MHz oscillator so far from the correct frequency that the FLL signal disappears making the lock impossible If this happens the last saved FC value may be read from EEPROM with the FC command and restored with the FC high low command The FC command is used to save the current FC values in the unit s EEPROM The FC Command may be used to read the value which is stored in the EEPROM The value stored in EEPROM is used to set the 10 MHz at startup before the FLL can be established Occasionally while the unit is operating at about 20 minutes after power on and once a day there after the program will write a new value to EEPROM to correct the value for crystal aging Example will return four values separated by commas the number of power cycles the unit has undergone the number of times the FC pair has been written to EEPROM and the value of the FC pair high low which is used at turn on and restart DS Detected signals This command returns two numbers corresponding to the synchronously detected signals at the modulation frequency Moa and at twice the modulation frequency 20moa The first number the amplitude of the signal at moa is the error sig
95. integrator Int 0 The current value of the SF parameter may be from the internal calibration pot position an external calibration voltage the value from a previously received SF command or the value left over from a previous PLL lock If the pre filter is enabled the exponential filter for the time tags is zeroed e The unit will lock the frequency of the PRS10 to the good I pps input pulses Bad 1 inputs 1 inputs with time tags greater than 1 024 ns from the last good 1105 input will be rejected The frequency parameter f to the SF command will be updated with each good time tag result AT n as follows The pre filter LMO AT n 1 AT n The pre filter LM1 AT n 1 1 0 At t3 AT n At AT n The integral term Int n 1 Int n n1 tj KasAt The proportional term Pro n 1 A AT n 1 K get The frequency setting f n 1 1 Int n 1 In the above equations At is the time between phase comparisons which is one second for the PRS10 The frequency control value f ranges over 2000 bits If the new f value exceeds 2000 it is set to 2000 If the new f value is less than 2000 it is set to 2000 If the new integral term exceeds 2000 it is set to 2000 If the new integral term is less than 2000 it is set to 2000 This will prevent integrator wind up in the case that the f value is pinned for a long time to slew the 1 out
96. istor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thin Film 1 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor PRS10 Parts List 73 REF SRS PART VALUE DESCRIPTION R 430 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 431 4 01117 462 1 00K Thin Film 1 50 ppm MELF Resistor R 432 4 01447 461 47 Thick Film 5 200 ppm Chip Resistor R 433 4 01447 461 47 Thick Film 5 200 ppm Chip Resistor R 434 4 01447 461 47 Thick Film 5 200 ppm Chip Resistor R 435 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 436 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 437 4 01503 461 10K Thick Film 5 200 ppm Chip Resistor R 438 4 01355 462 301K Thin Film 1 50 ppm MEL
97. ith the TT query returns a value of 25ns The TO parameter read via the TO query returns a value of 1750ns The command TO 1775 is sent to correct for the offset After waiting about one second to allow another time tag value to be acquired the next TT query returns a value of 2ns indicating a measurement of 2ns after the 1pps output Waiting another second the next TT query returns the value 999 999 999ns indicating 1 ns before the 1005 output These values are consistent with a well calibrated time tag offset Following calibration of the TO parameter the TO command is used to write the current value of the time offset to the unit s EEPROM Example TO will write the current time tag PRS10 Rubidium Frequency Standard RS 232 Instruction Set 33 offset value to the unit s EEPROM for use after the next power up cycle or restart command TO will return the value which is burned in the unit s EEPROM Note Firmware revisions prior to Rev 3 23 do not allow user TO commands Check the firmware revision with the ID command PP value 0 lt value lt 999999999 Place pulse This command is used to move the I pps output from its current position The Ipps output can be moved earlier in time by 1 ns to 999999999 ns Since the Ipps input time tag is referenced to the 1 output changing the Ipps output placement will change the report time tag values as well See the TT and TO commands Example PP 123456789 will move the I
98. justed to provide the same SF value as was seen when the FS725 was phase locked to the 1 pps To use this procedure you will need a source of a precise 1 pps signal such as a GPS timing receiver You will also need a way to communicate with the FS725 via the RS 232 such as a PC or laptop computer with a serial port operating at 9600 baud Any Windows computer together with the FS725 application program RbMon will work well FS725 Rubidium Frequency Standard Calibration 23 Procedure for frequency calibration using a precise 1 pps input 1 3 10 11 12 13 Be certain that nothing is connected to the rear panel FREQ ADJ input Be certain that nothing is connected to the rear panel 1 pps input Power cycle the FS725 by unplugging it for at least three seconds This will make certain that the unit will track the calibration potentiometer which can be ignored if the unit was phase locked to a 1 pps input or the frequency had been previously set via the SF command The unit should lock in less than a minute as it is warm from previous operation The position of the calibration potentiometer may be determined by measuring the dc voltage on the center pin of the rear panel FREQ ADJ BNC The voltage will read OV at the counter clockwise extreme or 5 VDC at the clockwise extreme and will read 2 50VDC when the potentiometer is centered This voltage should be recorded before any calibration is attempted This will allow you to return
99. l instruments it will be important that all of the instruments have a high impedance input gt 1 kQ and that a 50 2 terminator be placed at the far end of the line Generally it is better practice to use a separate output to each instrument to avoid cross talk and amplitude variations due to uncertain loading and standing waves along the line Many instruments which generate or measure precise frequencies or time intervals have a 10 MHz input usually located on the rear of the instrument Most of these instruments accept a sine input with the amplitude provided by the FS725 Sometimes a rear panel switch or front panel configuration menu is required to tell the instrument to use the 10 MHz input as a 25725 Rubidium Frequency Standard Operation 9 reference instead of using the instrument s own time base Improved accuracy can be achieved by using the FS725 as an external time base to all of the instruments in a particular laboratory In addition all errors associated with the calibration of an individual instrument s time base are eliminated Connection to SRS Instruments SR620 Time Interval Counter This instrument is told to use the external 10 MHz input via the front panel CONFIG menu To configure the SR620 to use an external time base press the SEL key in the CONFIG section once to display CONFIG menus then press SEL one more time to select the cAL menu which will flash when selected Press the SET key twice to
100. les VBO disables the verbose mode this is the power on default mode VB1 will enable the verbose mode ID Identify This command returns an identification string which includes the serial number and firmware version of the PRS10 Example ID will return the identification string PRSIO 3 15 SN 12345 model firmware version serial number PRS10 Rubidium Frequency Standard RS 232 Instruction Set 23 SN SN value SN SN Serial number This command returns the unit s serial number Example SN will return 21567 for a unit with serial number 21567 The command to write and burn a serial number are for factory use only ST Status This command will return a six number string corresponding to the values of the six status bytes Each number will range between 0 and 255 and will be separated by commas For definitions of the status bytes refer to the end of the detailed command descriptions LM value value 0 1 2 or 3 LM LM LM Lock mode pin configuration This command is used to configure the LOCK IPPS output pin 1 on the main connector J100 The LOCK 1PPS pin may be configured per the following table LM Description of LOCK Ipps Output Output goes low when locked to Rb pulses high for 10 us at 1 Hz Ipps locking pre filter disabled Ipps locking pre filter enabled default The default value is 1 so that pin 1 will go low when the unit is locked to rubidium and will pulse high for 10 us at
101. ling a 10 resistor for another R353 on the microcontroller PCB Pin 5 IPPS IN PHOTO The default configuration uses this pin as a 1pps input to allow time tagging or phase locking to an external 1 source The function of this pin may be changed to allow monitoring of the amplified photo signal When configured as a IPPS IN R241 will be omitted on the top PCB and a 1 kQ resistor will be installed for R242 When configured for PHOTO output R242 will be omitted on the top PCB and a 1 KQ resistor will be installed for R241 10 MHz coax shield The default configuration floats the shield of the 10 MHz coaxial connector with respect to ground The 10 MHz output is transformer coupled and the shield may be ground referenced by installing the jumper between J101 and J102 located near the connector on the 10 MHz oscillator PCB Hardware Notes of the pins on the interface connector are protected against continuous connection to any potential up to 24 Vac The power supply pins are protected against polarity reversal and may be operated up to 30 Vac In most applications both 24 Vac supplies heater and electronic supplies will be connected together and operated from 24 V supply Logic outputs LOCK 1PPS and TXD PHOTO have a 1 kQ output resistance driven by a CMOS logic device operating between 5 Vac and ground Logic inputs RXD EFC and IPPS IN have 100 to ground and 3 9 kQ CMOS gate inputs which have inpu
102. litude of RF to multiplier in resonance cell Controls the analog portion 0 to 99 ns of the delay for the 1003 output PRS10 Rubidium Frequency Standard RS 232 Instruction Set 37 Controls the drain voltage for the discharge lamp s FET oscillator Controls the temperature of the discharge lamp Controls the temperature of the 10 MHz SC cut crystal Controls the temperature of the resonance cell 6 Controls the amplitude of the 10 MHz oscillator Controls the peak deviation for the RF phase modulation Example SD2 could return the value 255 indicating that the unit has set the discharge lamp s FET drain voltage to the maximum which it does while it is trying to start the lamp The SD port is a factory only command which writes the data from the corresponding SD port to the unit s EEPROM for use on subsequent restarts Example SD3 will return the start up value for SD3 lamp temperature control value which is stored in the unit s EEPROM Analog Test Voltages AD port port 0 1 2 15 Analog to digital This command reads the voltage at the corresponding 12 bit ADC port and returns the voltage as a floating point number Values can range from 0 000 to 4 998 The voltages correspond to various test points in the system per the following table Note that this command can only query Examples AD10 could return the value 0 710 indicating that the case temperature sensor 15 at 71 C this sensor indicates a tempe
103. ll Sizes PRS10 Rubidium Frequency Standard 66 PRS10 Parts List REF SRS PART VALUE DESCRIPTION C 208 5 00299 568 1U Cap Ceramic 50V SMT 1206 10 X7R 210 5 00387 552 1000 Capacitor Chip SMT1206 50V 5 C 212 5 00299 568 AU Cap Ceramic 50V SMT 1206 10 X7R C 216 5 00387 552 1000P Capacitor Chip SMT1206 50V 5 NPO C217 5 00466 572 1U SMT Film Capacitors 50V 5 All Sizes C 218 5 00299 568 AU Cap Ceramic 50V SMT 1206 10 X7R 219 5 00375 552 100 Capacitor Chip SMT1206 50V 5 NPO C 220 5 00466 572 SMT Film Capacitors 50V 5 All Sizes 221 5 00454 572 01U SMT Film Capacitors 50V 5 All Sizes C 222 5 00454 572 01U SMT Film Capacitors 50V 5 All Sizes C 223 5 00299 568 AU Cap Ceramic 50V SMT 1206 10 X7R C 224 5 00299 568 LU Cap Ceramic 50V SMT 1206 10 X7R C 226 5 00466 572 AU SMT Film Capacitors 50V 5 All Sizes C 227 5 00466 572 IU SMT Film Capacitors 50V 5 All Sizes C 228 5 00299 568 LU Cap Ceramic 50V SMT 1206 10 X7R C 229 5 00299 568 AU Cap Ceramic 50V SMT 1206 10 X7R C 230 5 00299 568 LU Cap Ceramic 50V SMT 1206 10 X7R C 231 5 00375 552 1008 Capacitor Chip SMT1206 50V 5 C 232 5 00355 552 2 2P Capacitor Chip SMT1206 50V 5 NPO 301 5 00299 568 AU Cap Ceramic 50V SMT 1206 10 X7R C 302 5 00299 568 LU Cap Ceramic 50V SMT 1206 10 X7R C 304 5 00299 568 AU Cap Ceramic 50V SMT 1206 10
104. lled the FS725 will provide fourteen 10 MHz outputs four 5 MHz outputs and four PPS outputs each with the same low phase noise harmonic distortion and jitter An RS 232 interface allows for direct communication with the PRS10 Rubidium frequency standard This interface is used for frequency calibration 1 PPS timing status determination and to assist in the evaluation of the PRS10 rubidium frequency standard as a component for OEM applications A Windows application program is provided FS725 will time tag the rising edge of a user supplied 1 pps input which usually comes from a GPS receiver The unit can phase lock to this input to cancel long term aging The time tag results may be read back via the RS 232 interface Timing is referenced to the leading edge of the 1 pps output pulse The 1 pps output pulse will be moved into coincidence with the user supplied 1 pps input after 256 input pulses Thereafter the PRS10 will adjust its frequency to maintain the phase lock between the 1 pps input and the 1 pps output The phase lock algorithm uses an adjustable natural time constant from a 8 minutes to 18 hours which may be adjusted via the RS 232 Two alarm relays are provided to signal the status of the PRS10 rubidium frequency standard and its synchronization to an external 1 pps input Both relays are SPDT providing both normally open and normally closed operation Several thousand PRS10s have been operating in various OEM applications with
105. lution These values may be reported back via RS 232 and or used to phase lock the unit to an external reference such as GPS with a time constant of several hours This feature can provide Stratum 1 performance at a very low cost In addition to reading time tag results the RS 232 interface allows the user to set the frequency adjust the phase of the 1005 output read the value of virtually every parameter lamp drive level rf level temperature set point of the crystal lamp and resonance cell and 10 MHz output level and measure many test points lamp light level heater currents power supply voltages and case temperature The PRS10 establishes a new level of features and performance in atomic frequency standards Its design provides for the lowest phase noise and easiest path to system integration of any rubidium frequency standard available PRS10 Rubidium Frequency Standard 4 Specifications Units Output Frequency 10 Sine wave into 500 MHz Amplitude V Accuracy afi Allan variance afi SSB phase noise dBc Hz Spurious dBc Harmonics dBc Aging after 30 days lt 5 10 monthly Af f lt 5x10 yearly Af Return loss gt 25 at IOMHZ Temperature X1x10 over 20 C to 65 baseplate Af f Voltage afi Magnetic field lt 2x10 for 1 Gauss field reversal At f Retrce afi aft Trim Range afi Time to lock minutes Time to 1x10 minute lt Other Electrical ee a ea O Power supply V
106. m 1 50 ppm MELF Resistor R 288 4 01294 462 69 8K Thin Film 1 50 ppm MELF Resistor R 289 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 290 4 01479 461 1 0K Thick Film 5 200 ppm Chip Resistor 20 4 01575 461 10M Thick Film 5 200 ppm Chip Resistor R 292 4 01479 461 1 0K Thick Film 5 200 ppm Chip Resistor R 293 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 294 4 01251 462 24 9K Thin Film 1 50 ppm MELF Resistor R 295 4 01575 461 10M Thick Film 5 200 ppm Chip Resistor R 296 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 297 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 298 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 299 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 300 4 01405 462 1 00M Thin Film 1 50 ppm MELF Resistor R 301 4 01302 462 84 5K Thin Film 1 50 ppm MELF Resistor R 303 4 01479 461 1 0K Thick Film 5 200 ppm Chip Resistor R 304 4 01479 461 1 0K Thick Film 5 200 ppm Chip Resistor R 305 4 01527 461 100K Thick Film 5 200 ppm Chip Resistor R 324 4 01249 462 23 7K Thin Film 1 50 ppm MELF Resistor R 325 4 01230 462 15 0K Thin Film 1 50 ppm MELF Resistor R 326 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 327 4 01455 461 100 Thick Film 5 200 ppm Chip Resistor R 329 4 01455 461 100 Thick Film 5 200 ppm Chip Resistor R 331 4 01073 462 348 Thin Film 1 50 ppm MELF Resistor R 332 4 01117 462 1 00K Thin Film 1 5
107. ming between the 1 pps input and the unit s 1 pps output With a clean reference and a short time constant the 1 pps output typically tracks the 1 pps input within a 1 ns window The default time constant of 2 25 hours is appropriate for locking to the typical GPS receiver However if there is a rapid change in the ambient temperature which causes the PRS10 to change frequency by 1x10 the change will not be appreciably effected by the PLL during the first hour and one would expect to see a drift between the 1 pps input and the 1 pps output on the order of 3600 x 107 or 36 ns The FS725 indicates that it is synchronized to the 1 pps input by a front panel LED and a rear panel alarm relay The unit will indicate that it is synchronized when the 1 pps input is present and if it occurs within 1 us of the 1 pps output A single dropped pulse on the 1 pps input will cause the unit to indicate that it is not synchronized but there is no detrimental effect from missing a small number of 1 pps inputs It is quite common for a unit to indicate that it has acquired and then lost synchronization depending on the initial frequency offset between the FS725 and the 1 pps reference Suppose that a FS725 is out of calibration by 1x10 and a 1 pps input is applied while using the default PLL time constant of PT8 or 2 25 hours With this time constant the PLL algorithm has little effect on the FS725 s frequency of operation during the first hour
108. nal in the rubidium frequency lock loop The value is proportional to the instantaneous frequency error of the 10 MHz oscillator as detected by the physics package The value may be large when the unit is first locking and will bobble around zero in steady state Each LSB corresponds to about 15 u Vrms of signal at mod The second number is the amplitude in millivolts rms of the synchronously detected signal at twice the modulation frequency 205 4 The amplitude of this signal is proportional to the strength of the rubidium hyperfine transition signal The returned value is a spot measurement taken over just one cycle of the modulation frequency Since the signals have several Hz of equivalent noise bandwidth they will be rather noisy Example DS could return 55 800 indicating a small error signal and a strong resonance signal PRS10 Rubidium Frequency Standard 26 RS 232 Instruction Set SF value 2000 lt value lt 2000 SF Set frequency This command is used to override the internal calibration pot or external calibration voltage to set the frequency directly relative to the calibration values in EEPROM see the SP and MO commands The command sets the frequency offset in units of parts in 107 corresponding to a frequency resolution of 10 Hz at 10 MHz The SF command will return the currently set relative frequency value with a range of 2000 whether the value comes from the internal calibration pot position an exte
109. nd PLL Table for all PT values assuming a stability factor C 1 PT Parameter Integrator Time Integral Gain Proportional Natural Time Constant Gain Constant Parameter SF bits per set by PT hours hour per ns SF bits per ns Characterizes command of time tag of time tag PLL response hours 0 0 07 14 063 3 95 0 14 1 0 14 7 031 2 80 0 20 2 0 28 3 516 1 98 0 28 3 0 57 1 758 1 40 0 40 4 1 14 0 879 0 99 0 56 5 2 28 0 439 0 70 0 80 6 4 55 0 220 0 49 1 12 7 9 10 0 110 0 35 1 59 8 18 20 0 055 0 25 2 25 9 36 41 0 027 0 17 3 18 10 72 82 0 014 0 12 4 50 11 145 64 0 007 0 09 6 36 12 291 27 0 003 0 06 8 99 13 582 54 0 002 0 04 12 72 14 1 165 08 0 001 0 03 17 99 PRS10 Rubidium Frequency Standard 36 A RS 232 Instruction Set PF value 0 lt value lt 4 0 1 4 1 1 2 2 1 3 2 or 4 4 PF PF Phase lock stability factor This command is used to set the stability factor of the 1 PLL The stability factor is equal to 20 The default value is 2 which provides a stability factor of 20 2 1 Stability factors can range from 0 25 to 4 0 Example PF 1 sets the stability factor to 0 5 which will reduce the equivalent noise bandwidth of the PLL at the cost of increasing the ringing near the natural frequency relative to the default settings PF will return the current value of the stability factor parameter PF may be used to write the
110. nd bandpass filtered before being converted by a 12 bit ADC The microcontroller multiplies the ADC samples by table data corresponding to sines and cosines at 70 Hz and 140 Hz The products are summed over a frame of 14 modulation cycles PRS10 Rubidium Frequency Standard 14 PRS10 Overview which completely eliminates signal components at 5 Hz and at any integer multiple of 5 Hz including 50 Hz 60 Hz 70 Hz and 140 Hz from the error signal so that there will be no spurs at the modulation frequency in the 10 MHz output The summed product corresponding to the detected signal at 70 Hz and 0 is used to frequency lock the 10 MHz oscillator to the Rb hyperfine transition frequency This value is filtered in a simple first order IIR digital filter The filter coefficient determines the frequency lock loop time constant Time constants from 1 s to 128 s are available to optimize the output stability of the 10 MHz Initial Locking When power 1 first applied to the unit the EFC the electronic frequency control or the voltage applied to the varactor in the 10 MHz SC cut oscillator is set to the last value for which the unit was locked As the 10 MHz oscillator heats to its operating temperature the output frequency will increase smoothly to converge on 10 MHz In most cases the output frequency will be within 0 1 Hz of 10 MHz even before the lock to rubidium is achieved After the lamp starts and the physics package settles to its ope
111. ndix A details the values for R N and A for the range of frequencies needed SP SP R N A 1500 lt lt 8191 800 lt lt 4095 0 lt lt 63 SP SP Set Parameters This command is used to set or query the frequency synthesizer s parameters which will coarsely adjust the unit s output frequency These parameters may need to be adjusted if the unit cannot be calibrated by magnetic field adjustment PRS10 Rubidium Frequency Standard RS 232 Instruction Set 29 Example During calibration a unit s 10 MHz output frequency is found to be low by 0 010 Hz and the magnetic field offset adjustment is already at its maximum See the MO command Sending the SP command returns the current values of R N and A which are 2610 1466 63 in this example This corresponds to line 38 in the table in Appendix A To increase the frequency of the 10 MHz output we select the next higher setting line 37 which will increase the frequency by 0 01986 Hz To do this we send the command SP 5363 3014 22 which are the parameters from line 37 Waiting for the frequency to settle we now measure the output to be about 0 0098 Hz high Now the magnetic field is adjusted down to calibrate the unit to exactly 10 MHz The SP command is used to save these new values in EEPROM for the next power on or restart Also see the MO command for adjusting the magnetic field The SP command is used to write the current frequency synthesizer parameters to
112. nput Power 42 Set up for an SR620 63 Voltage Reference 42 Four input connections 63 PRSIO Rubidium Frequency Standard 2 Table of Contents Four input setups 63 Coarse Frequency Measurements 63 Fine Frequency Measurements 63 Parts List for Revision H 65 PRS10 Rubidium Frequency Standard Introduction 3 PRS10 Rubidium Frequency Standard Introduction The PRS10 is a ultra low noise 10 MHz frequency standard which disciplines an SC cut ovenized oscillator to a hyperfine transition in the ground state of rubidium The PRS10 was designed to fill a variety of communication synchronization and instrumentation requirements The phase noise of the 10 MHz output is low enough to be used as the reference source for cellular synthesizers The unit s short term stability and low environmental coefficients make it an ideal component in network synchronization systems Also the low aging rate makes it an excellent choice as a timebase for precision frequency measurements The unit is compatible in fit form and function to the Efratom FRS frequency standards with improvements in features and performance The PRS10 allows closed case diagnostics and calibration via an RS 232 interface its digital synchronous detection and filtering eliminate spurs on the 10 MHz output and the PRS10 has 1000x less phase noise than the Efratom unit 130 dBc vs 90 dBc at 10 Hz The PRS10 can time tag an external 1 input with very high reso
113. oltage regulator is used to supply 18Vdc to the system The 18V is used on the analog PBC on the frequency synthesizer for the crystal oscillator and for the analog switches and ADC buffer on the CPU PCB PRS10 Rubidium Frequency Standard Circuit Descriptions 55 0504 LM340 5 three terminal regulator is used to provide 5 0Vdc This supply is used for all logic circuits and for analog circuits which interface to analog devices which must not be driven above their logic supplies Lamp Regulator A discharge is ignited and maintained by a MOSFET powered oscillator operating at about 150MHz inside the lamp enclosure It is very important that the voltage provided to the lamp circuit be well regulated as the lamp intensity is nearly proportional to this voltage Since the synchronously detected light signal at 70Hz is used to lock to the hyperfine transition noise at 70Hz will add noise to the frequency lock loop Also noise at other frequencies may be heterodyned by the 2 signal 140Hz which is really a modulation of the attenuation of light through the resonance cell For example if the power supply has noise at 210Hz the lamp will have an intensity fluctuation at 210Hz which when mixed by the 140Hz attenuation modulation will create a component at 70Hz which will interfere with the frequency lock loop Long term stability thermal and aging of the lamp voltage regulator is also important The voltage provided to the lamp os
114. on by adjusting its SF PRS10 Rubidium Frequency Standard RS 232 Instruction Set 35 parameter to eliminate the 100ns phase shift between the 1 reference input and the I pps output After the phase shift is eliminated the PRS10 will settle to the correct operating frequency The PLL algorithm computes integral and proportional terms from time tag measurements adjusting the SF parameter to phase lock the 1 output to the 1105 input The table below shows the integral and proportional gain terms For the nominal PT value of 8 the integral term is 0 055 SF bits per hour per ns of time tag and the proportional gain is 0 25 SF bits per ns of time tag Per the table below for PT8 if the input reference shifts by 100ns the proportional term will adjust the SF by 0 25bits ns 100ns 25 bits Each SF bit corresponds to 1 10 of the operating frequency and so the PRS10 frequency will be shifted by about 25 x 10 The integral term will begin ramping by 0 055bits hour ns 100ns or by 5 5 bits per hour The phase shift between the I pps input and 1 output will be gradually eliminated Phase jumps of 100ns are quite common on I pps outputs from GPS receivers which are a likely 1 reference to the PRS10 The corresponding frequency jumps of 25 x 10 may be excessive in some applications and so a digital pre filter is used to smooth the time tag values before they are used by the PLL algorithm See LM comma
115. on the vertical PCB which holds the main connector to the outside This board has a 10MHz SC cut ovenized oscillator which is frequency locked by the microprocessor to the hyperfine transition in rubidium via a high resolution DAC This will overcome two important shortcomings of the oscillator circuit frequency aging of a few parts in 10 day and a sensitivity of a few parts in 10 over the ambient temperature range of 0 C to 65 C Input Power D101 and D102 MBRD660CT Schottky diodes in DPAKs protect the unit from input power supply polarity reversals 24 CLEAN and 24 HEAT The supplies are filtered L104 and L105 ferrite beads with about 34H and 15 at 100kHz and C115 and C116 These filters are designed to reduce EMI emission and susceptibility but they have a low Q resonance at about 40kHz Voltage Reference U100 provides a 10 00V low noise reference for the entire unit and various biases for the crystal oscillator The reference voltage is divided by two and buffered by U102B to provide 5 00V reference for the internal calibration pot and for the POT output Crystal Oscillator The crystal oscillator uses a Colpitts configuration The 3rd overtone SC cut crystal is specified to operate at 10 0MHz with a series load of 20pF Hence the crystal will operate slightly above its series resonance to contribute an inductive reactance equal in magnitude to the series capacitive reactance At IOMHZ the network L100 L101
116. ower dissipated in the MRF 134 is probably about 1 2 the total input power The oscillator current circulates through the series LC network consisting of C903 906 and L903 The coil L903 is in contact with the bulb The high voltage end of the coil connects to C905 When oscillating the drain of the FET swings between ground and twice the dc drain voltage C903 is in parallel with the FET s drain source capacitance about 10pF for a total capacitance of 78pF or a reactance of about j13 6Q at 150MHz With a drain voltage of 20Vdc the drain will have about 40V peak to peak so there will be a circulating current of 2 9A peak to peak The series capacitance of C904 C905 is 9pF a reactance of about j118Q so they will have about 340Vpp across the pair due to the circulating current which is in phase with the 40Vpp drain voltage for a total of 380Vpp at the top of the coil It is very important that C903 906 be very low loss and high stability capacitors Porcelain capacitors are used in this circuit they have Qs of about 500 for ESRs of about 0 03Q for the 56pF part at 150MHz Low loss is important to reduce self heating which can destroy other types of capacitors and high stability is important to maintain a constant discharge intensity The operation of the oscillator depends somewhat on the conditions of the discharge Over certain temperature ranges which are carefully avoided the losses caused by the discharge can quenc
117. played The program does not provide any facility for customizing the COM port settings for communication with other devices FS725 Rubidium Frequency Standard 20 Calibration Calibration There is a 15 turn potentiometer in the PRS10 rubidium frequency standard which adjusts the operating frequency The potentiometer is accessible beneath a calibration sticker on the left side of the instrument The total adjustment range using the potentiometer is 0 002 ppm or 0 02 Hz at 10 MHz Turning the potentiometer clockwise will increase the 10 MHz output by about 0 0025 Hz per turn The position of the potentiometer may be determined by measuring the dc voltage on the center pin of the rear panel FREQ ADJ BNC The voltage will read 0 VDC at the counter clockwise extreme 5 VDC at the clockwise extreme and 2 50 VDC when the potentiometer is centered This voltage should be recorded before any calibration is attempted This will allow you to return to the previous calibration if necessary and serves as a record of the frequency aging of the unit When the FS725 is shipped from the factory the potentiometer is set to its center position and the frequency is calibrated via the RS 232 The user can return the potentiometer to the center position by adjusting the potentiometer until the dc voltage on the center pin of the rear panel FREQ ADJ BNC measures 2 50VDC The procedure is to adjust the calibration potentiometer so that the FS725 produces
118. potentiometer position will not affect the frequency of operation if 1 it is turned to either extreme 2 an external control voltage is applied to pin 2 of the main connector J100 3 an SF set frequency command has been sent via the RS 232 interface or 4 the unit is locked to an external 1pps input The time constant for pot adjustments depend on the setting of the frequency lock loop gain see command the default is about 2 seconds In the case that the unit cannot be calibrated because the internal pot has reached an extreme position it will be necessary to modify a calibration values which are stored in the unit s EEPROM To verify that the pot has been turned to a limit of its motion measure the voltage on pin 2 POT W of J100 the main connector with respect to the chassis Zero volts on pin 2 indicates that the pot has been adjusted for the lowest frequency and 5 0 Vdc indicates that the pot has been adjusted for the highest frequency To modify EEPROM calibration values it will be necessary to establish RS 232 communications with the PRS10 This can be done with a three wire connection between the port and the PRS10 s main connector A communication program see Windows Accessories or other will be needed as well See MO and SP commands PRS10 Rubidium Frequency Standard 42 Circuit Descriptions Circuit Descriptions Schematic RB F1 sheet 1 of 6 Components shown on this schematic are located
119. ps input to the PRS10 The PRS10 can time tag the rising edge of this input and report the time tag values via the RS 232 with Ins resolution The PRS10 can also phase lock to the external pps input by adjusting its frequency of operation The rear panel 1 pps input also triggers a 1us one shot U102B to detect the synchronization of the 1 pps input to the 1 pps output to within a lus window The 1 pps output also triggers a lus one shot U102A If the two 1 pps pulses have rising edges within lus of each other the NAND gate U101B will re trigger the 1 6s one shot U103A which holds the 1 pps SYNC relay U107 The lus one shot triggered by the 1 pps input also triggers a 70ms one shot U103B which is used to blink the front panel 1 pps input LED The 70ms one shot also holds off additional triggers of the lus one shot until it times out reducing the probability of false sync detection in the case that the user s 1 pps input is running much faster than 1 pps 5725 Rubidium Frequency Standard 26 Circuit Description Lock and 1 pps output The PRS10 outputs lock status and 1 pps on Pin 1 This output will have a CMOS high level 5V until the unit locks its crystal oscillator to the rubidium physics package When the unit locks Pin 1 will go to a CMOS low level OV and pulse high for 10 at 1 pps An RC filter R100 amp C100 together with the Schmitt trigger U104A recovers the lock status and gates the 1 pps outp
120. ption STANDOFF FOR P S TOP SKIN BOTTOM SKIN 6 32 FLAT HEAD FOR P S 6 32 PAN HEAD FOR P S RIGHT FRONT FOOT LEFT FRONT FOOT REAR FEET BAIL WIRE BIG SKIN SCREWS LITTLE SKIN SCREWS REAR FOOT SCREWS FRONT FOOT SCREWS BEZEL SCREWS BEZEL BNC WASHERS BNC NUTS AANA KP KN NN 25725 Rubidium Frequency Standard WASHER FOR P S MOUNTING Part 330P 39P 330UF 001U 18 2 2 HEADIO BNC RT STANDOFFS 56UH BEAD 1 8UH 1 5UH 499 2 00K 604 249 100 196 100 47 TOKO7P T1 1 LM7171 74HC04 74HC540 REV B SRS P N 5 381 5 370 5 516 5 387 5 313 5 355 1 1032 1 579 0 293 6 595 6 236 6 596 6 597 4 1088 4 1146 4 1096 4 1059 4 1021 4 1455 4 1447 6 195 6 009 3 819 3 741 3 748 7 1338 SRS P N 0 550 7 260 7 259 0 195 0 185 0 97 0 99 0 179 0 180 0 204 7 122 0 248 0 167 0 185 0 150 7 217 Component Parts List 33 Item Qnt Description SRS P N 19 1 INSIDE FRONT CHASSIS REV B 7 1320 20 1 INSIDE REAR CHASSIS REV B 7 1321 21 1 CORCOM 3EEA1 DIGIKEY CCM1622 ND 0 1045 22 1 3 BLACK 0 548 23 1 3 WHITE 0 549 24 2 3 GREEN YELLOW 0 521 25 1 17 BLACK THREE TURNS THROUGH FERRITE 0 225 26 1 17 RED THREE TURNS THROUGH FERRITE 0 227 27 2 BIG FERRITE BEADS FOR POWER LEADS 6 598 28 1 ADHESIVE CLIP 0 418 29 1 CHASSIS BINDING POST 0 517 30 1 LUG FOR BINDING POST 31 8 4 40 PAN X 1 4 PCBs AND ROUND STANDOFFS 0 187 32 7 4 40 FLAT
121. put in line with the 1005 input PRS10 Rubidium Frequency Standard 18 10 Overview The output of the digital filter f is used as the frequency control parameter for the SF set frequency command which is updated once a second e The PLL will be aborted and restarted if there are 256 consecutive bad I pps inputs This could happen if the 1005 input is moved suddenly by more than 1 024 ns The PLL will also be aborted and restarted if the measured time tag value for a good 1 input exceeds 4 ns s T1 For T s default value of 65 536 seconds the PLL will restart if the absolute value of a good time tag is greater than 262 144 ns This could happen if the 1pps input is more than a few parts in 10 off the correct frequency for a long time CPU Tasks In addition to the frequency lock loop control the microprocessor is responsible for a variety of other tasks The CPU sets D A values which control the microwave amplitude the lamp intensity the 10 MHz output amplitude and set the temperature of the crystal lamp and resonance cell The CPU will also controls peripheral electronics to output a 1 pulse with Ins placement and measure the time for a 1pps input pulse with 1 ns resolution There is an RS 232 interface which allows closed case calibration of the PRS10 This capability may also be used to servo the 10 MHz or Ipps outputs to another frequency or time source in a system For example this
122. q cal pot 9 2CLEAN 124 Vde supply for electronics not heaters or lamp Configuration Notes The functions of three pins 4 5 and 7 on the interface connector may be modified by internal hardware jumpers The function of the LOCK output may be modified via RS 232 Pin 1 LOCK IPPS output The default configuration is 5 V indicates that the unit is not locked to rubidium as during warm up 0 V indicates a successful lock of the 10 MHz oscillator to rubidium pulsing high for 10 us at a 1pps rate The 1pps output may be moved earlier by any interval from 1ns to 999 999 999 ns via RS 232 command The unit may be configured to omit the 1005 output via the LM command via RS 232 PRS10 Rubidium Frequency Standard 20 Applications Pin 4 TXD PHOTO The default configuration uses this pin as an output for RS 232 data Many system parameters including the lamp intensity may be monitored via the RS 232 interface The function of this pin may be changed to an analog monitor for the lamp intensity by removing one resistor R347 and installing a 10 resistor for another R348 on the microcontroller PCB Pin 7 RXD EFC The default configuration uses this pin as an input for RS 232 data Many system parameters including the EFC electronic frequency control may be monitored via the RS 232 interface The function of this pin may be changed to an analog monitor for the EFC by removing one resistor R354 and instal
123. r Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resistor Thick Film 596 200 ppm Chip Resistor Thick Film 596 200 ppm Chip Resistor Thin Film 196 50 ppm MELF Resistor Thick Film 596 200 ppm Chip Resistor Thin Film 196 50 ppm MELF Resistor Thin Film 196 50 ppm MELF Resistor Thick Film 596 200 ppm Chip Resistor Thick Film 596 200 ppm Chip Resistor Thick Film 596 200 ppm Chip Resistor Thermistor various Thermistor various Thick Film 596 200 ppm Chip Resistor Thick Film 596 200 ppm Chip Resistor Thick Film 596 200 ppm Chip Resistor Thermistor various Thermistor various Resistor Carbon Film 1 8W 596 Transformer Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Voltage Reg TO 220 TAB Package Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg Integrated Circuit Surface Mount Pkg 209 210 300 301 303 304 306 307 308 309 310 311 312 313 314 400 401 402 403 404 405 406 407 511 cuc e Cb EE C UTC G GCC Gre CG ce cerco SRS PART 3 00661 360 3 00581 360 3 00563 360 3 00662 360 3 00663 360 3 00662 360 3 00643 360 3 00773 360 3 00659 360 3 00581 360 3 00652 360 3 006
124. r of 19 is from frequency multiplication in the step recovery diode and the other terms come from the operation of the dual modulus frequency synthesizer integrated circuit The apparent transition frequency is different for each physics package due mostly to variations in the fill pressure of the resonance cell The frequency synthesizer parameters R N and A are used to adjust the frequency synthesizer s output frequency to the closest frequency just above the apparent transition frequency then the magnetic field is set to move the transition frequency up to the synthesizer frequency During frequency locking the frequency of the 10 MHz OCXO is adjusted to maintain the output of the frequency synthesizer on the rubidium hyperfine transition frequency Initial calibration of the unit will involve finding the synthesizer parameters and magnetic field value which will lock the 10 MHz OCXO at exactly 10 MHz During the lifetime of the unit there will be some aging of the physics package which will cause the apparent transition frequency to change This is usually corrected by minor calibration adjustments of the magnetic field strength which provides a setting resolution of a few parts in 10 7 See the MO command However if the magnetic field strength reaches its lower or upper limit it is necessary to change the frequency synthesizer parameters which can change the output frequency in steps of about one part in 10 The table in Appe
125. rating temperature a resonance signal will be detected by the processor and used to lock the crystal oscillator to rubidium In the case that no signal is detected or if the signal is lost during normal operation the processor will suspend the frequency lock loop and maintain the varactor voltage to the 10 MHz ovenized oscillator at a fixed level Any of the following conditions would cause the CPU to suspend lock 1 The detected signal at 140Hz 1 very low 2 The discharge lamp light level is outside an acceptable range 3 The RF synthesizer is unlocked 4 The RF AGC level is pinned high or low 5 The VXCO varactor voltage is outside the acceptable range Suspending lock will prevent a radical change in output frequency in the case of a physics package failure So in the case of most failures which cause loss of the lock to rubidium the 10 MHz will maintain a stable output with an aging of a few parts in 10 per day Locking to External 1pps The PRS10 may be locked to an external 1 source from a GPS or LORAN receiver for example by applying a l1 pps pulse to the Ipps input pin 5 on the main connector A second order digital phase lock loop PLL is used to adjust the frequency of the PRS10 to match the frequency of the 1pps source over long time intervals The block diagram of this PLL is shown in Figure 3 The phase detector is the time tagging circuit and firmware which has a gain of Kaet Ibit ns The loop f
126. ration potentiometer accessible on the side panel of the instrument or responds to software commands via the RS 232 to set the magnetic field which controls the operating frequency The operating frequency may be directly controlled over the range of 0 002ppm in four ways 1 the adjustment of a 15 turn potentiometer accessible on the side of the instrument Turning the potentiometer clockwise will increase the frequency by about 0 0025Hz per turn The adjustment procedure is detailed in the calibration section 2 By applying a voltage in the range of 0 to 5 VDC to the rear panel FREQ ADJ BNC The nominal voltage is 2 50 VDC The voltage on this BNC corresponds to the position of the 15 turn potentiometer described above The voltage may be overridden by a user input The tuning rate at 10 MHz is 0 008 Hz volt The input source impedance is between 10 kQ and 35 kQ depending on the potentiometer position 3 By software command via the RS 232 The SF Set Frequency parameter allows the user to set the frequency with a resolution of 1 10 The parameter has a range of 2000 which allows frequency control over the range of 0 002 ppm The position of the frequency adjustment potentiometer is used to set the SF parameter until a SF command is received via the RS 232 or until phase locking to an external source is activated by the application of pps inputs to the rear panel BNC Control is returned to the potentiometer when the power is cycl
127. rature which is about midway between the baseplate temperature and the lamp temperature Command _ Returned voltage Spare J204 24V heater supply divided by 10 PRS10 Rubidium Frequency Standard 38 RS 232 Instruction Set AD port 16 port 19 A D via CPU s E port This command returns a value corresponding to the voltage present at the input to the microcontroller s octal 8bit ADC port E on MC68HC11 Only the first four ports are in use The voltage corresponds to various test point in the system per the following table Example AD17 could return a value of 4 81 indicating that the 360 MHz RF synthesizer has acquired lock Command Returned voltage AD 16 Varactor voltage for 22 48 MHz VCXO inside RF synthesizer 4 AD 17 Varactor voltage for 360 MHz VCO output of RF synthesizer 4 Gain control voltage for amplifier which drives frequency multiplier 4 AD 19 RF synthesizer s lock indicator voltage nominally 4 8 V when locked Status Bytes ST Status query This command returns the six system status bytes which are used to indicate the health and status of the unit The values ranges from 0 to 255 The six status bytes are detailed in the tables below A status bit will remained set until it is read even though the condition which caused the error has been removed Some status bits are not errors for example during warmup the status bytes may indicate that the lamp is not lit temperatures
128. rd 46 Circuit Descriptions sees a signal with 0 91 Vdc offset and an amplitude of 0 91 Vpp while the inverting input is biased at 0 91Vdc Photocell Amplifier The output from the photocell is a sink current which is proportional to the light intensity of the discharge lamp as attenuated by the resonance cell The light transmission through the resonance cell decreases slightly by about 1 part in 1000 when the microwave synthesizer sweeps through the hyperfine transition frequency The microwave frequency is modulated at 70Hz so the light output will dip at 140Hz when centered on the hyperfine transition The S N of the photocell is limited by shot noise the shot noise current on a dc current of I amps is given by V 2qI amps VHz where q 1 6x10 On a 50 dc current the best we can do is 4pA Hz of noise A good design requires that the shot noise be the dominate noise term U206A is a low noise bipolar input op amp whose input range includes ground A 150kQ metal film resistor shunted by a 1nF film capacitor is used in feedback providing a transconductance bandwidth of 1kHz The input current noise of the op amp 0 4pA VHz and the Johnson noise current of the feedback resistor 0 33pA VHz are not important noise terms Also the voltage noise of the op amp 3nv VHz times the noise gain which is about 10 for a photocell whose shunt resistance is 1 at 25 C but drops to 15k at the operating temperature of 80 C is not impor
129. rnal frequency control voltage SF command from the external 1pps phase lock loop control algorithm However SF set command is ignored if the unit is phase locked to an external signal To re establish direct control via the SF command the PLL must be disabled See PL 0 command Example SF 100 will set the frequency 100 x 10 or 0 001 Hz above the stored calibration value and the SF command will return 100 Data from the SF command cannot be saved when the power 15 turned off To do this type of calibration see the SP and MO commands Once executed the SF command will disable the analog channels internal calibration pot and external calibration voltage until the power is cycled or the unit is restarted SS SS value 1000 lt value lt 1900 SS SS Set slope This command is used to read the slope calibration parameter for the SF command This parameter compensates for a variety of factors which affect the magnitude of the coefficient between magnetic coil current and transition frequency Example SS might return 1450 the nominal parameter value This calibration parameter may not be altered by the end user The factory only SS command is used to store the current value of the SS parameter to the unit s EEPROM The SS will return the value of the SS parameter which is used on power up or restart GA GA value Oz value lt 10 GA GA Gain This command sets the gain parameter in
130. rom the File menu The current values displayed in Rb Stats will be sent to the printer The formatting of the printed output is intended for standard 8 by 11 inch Letter size paper Printing on paper of other sizes is not supported but not prevented Print Setup Print Setup enables you to configure your printer for printing You should choose Letter size paper 8 by 11 inch and the Portrait orientation Landscape orientation is not supported Exit The Exit menu item will close the program Help Menu Contents Choose this menu item to display help infromation About Choose this menu item to display the program name and version information Communicating with Other Devices RbMon was designed to communicate with the PRS10 however many people have used the RS 232 Communications tab as a general purpose dumb terminal for communicating with other instruments and devices If you wish to do this keep in mind the following technical limitations e The communications port configuration is fixed at 9600 baud 8 bits per byte no parity and 1 stop bit e A carriage return CR is appended to each command sent e Commands are sent on a line by line basis Nothing is sent until the user hits the lt gt key after which the entire line is sent with a CR appended to the end e Responses to commands from the device are expected to be terminated by a carriage return CR e Unprintable characters will not be dis
131. round state of Rb87 is split by a very small energy due to the relative orientation of the magnetic spins of the electron and the nucleus The split corresponds to the energy of a photon with a microwave frequency of 6 834 682 612 8 GHz It is this hyperfine transition frequency which will be used to stabilize the 10 MHz output of the PRS10 To see how this is might be done Figure 1 shows a typical physics package which uses discharge lamp an isotopic filter and a resonance cell We will see that the amount of light which passes through the resonance cell to the photodetector can be reduced when the resonance cell is exposed to microwaves at the hyperfine transition frequency To simplify the discussion we will assume that the light from the Rb87 discharge lamp consists of just two lines corresponding to transitions from a single excited state to the split ground state The filter cell contains Rb85 vapor which also has a split ground state and an isotopic shift relative to Rb87 as well An important coincidence exists one of the lines from the Rb87 discharge corresponds one of the transitions in Rb85 This will allow us to reduce the intensity of this line by passing the Rb87 discharge light through the Rb85 vapor Normally atoms in the ground state will be equally distributed between the split states as the splitting is much less than the thermal energy of the atoms in the vapor This distribution is modified by the filtered light from th
132. s common practice and so there is no need to tune the length of the cavity or worry about the affect of coming off resonance Sufficient field strength at 6 834GHz is available without resonant enhancement due to the high RF drive frequency and efficient coupling into the SRD The SRD loop is oriented inside the can in such a way as to minimize the drive level required for a good hyperfine optical signal Analog Control Various analog voltages are provided by an octal 8 bit DAC to control temperatures intensities and for system tests 0407 a TLC5628 is connected to the microcontroller via the gated serial interface Each of the eight analog outputs may be set from 0 to 4 00V with 10mV resolution Except for the PHASE DEV output which has a full scale of 2 00V and a step size of 5mV The outputs are dedicated as follows OUTPUT NAME DESCRIPTION RF LEVEL Controls the RF power level to the SRD B IPPS DELAY Analog control of 1 PPS output delay 0 5ns bit LAMP DRAIN Drain voltage to lamp FET D LAMP _ Controls the lamp temperature Tmax 122 C Schematic RB F5 Sheet 5 of 6 Power Supply Lamp Control and 1PPS Timing PCB Components shown on this schematic are located on the bottom PCB Three TO 220 power regulators are mounted to the back wall of the device Linear Power Supplies of the power supplies operate from the 24 CLEAN input pin 9 of the main connector U503 an LM317 adjustable v
133. sensor is located very near the heat source the sensor will be warmer and the temperature offset will increase as more heat is required To compensate for this effect a small portion of a voltage proportional to the baseplate temperature 10mV C is summed to the voltage at the inverting input of the error amplifier This is the electronic equivalent of a double oven as the errors due to changes in ambient temperature are greatly reduced U200B controls the current in the heaters in proportion to the signal from error amplifier U200A When the output of the error amplifier goes up the output of U200B goes down increasing the current in the heaters causing the signal XTAL SHUNT to go down The gain from error amplifier to shunt voltage is set by R207 and R208 Offsets are arranged so that the heaters will be off when the output of the error amplifier is less than 5Vdc Conversion to 10MHz TTL U205 converts a 1OMHz offset sinewave from the crystal oscillator into complimentary 10MHz TTL level signals The 10MHZ signal is used as a reference for the microwave frequency synthesizer and the 10MHZ signal is used as a clock for the microprocessor Separate signals are used to improve the isolation between the CPU and the synthesizer The 10MHz sine has an offset of 8 2Vdc and an amplitude of 10Vpp and is sourced via a 2 0kQ resistor After attenuation by R249 R250 and C210 the non inverting input to U205 PRS10 Rubidium Frequency Standa
134. signal passing the filtered error signal to the 22 bit D A converter and responding to commands and queries via the RS 232 interface A description of I O from the controller follows Name Function a a eee iets certs a PORTA Mixed inputs and outputs Time tag input to measure 1 PPS input to 400ns Interpolation input to measure 1PPS input to 0 2ns PRS10 Rubidium Frequency Standard Circuit Descriptions 49 bes oe ee 5522 PORT B_ Eight TTL outputs pu gt PORT Chip select outputs PCT 5 Ee PI and RS 232 555 RS 232 IN D PDI PD2 PD3 RENT PORTE Octal 8 bit A D converter with 5 12V full scale RF XVCO Should be between 0 2 and 3 5Vdc RF VCO Should be between 3 0 and 4 0Vdc Should be 1 0 and 3 75Vdc PRS10 Rubidium Frequency Standard S0 Circuit Descriptions LOCK Should be gt 4 0 4 Spare analog input 1302 with 100kQ to ground Spare analog input J303 with 100kQ to ground Spare analog input J304 with 100kQ to ground RS 232 The system may be controlled by commands sent via the RS 232 Two pins on the system connector J100 are used for transmit and receive Data is sent to the host on pin 4 received from the host on pin 7 The baud rate is fixed at 9600 baud 8 bits no parity with 1 start and 2 stop bits No DTR or CTS controls have been used rather the XON XOFF protocol has been implem
135. sitivity and immunity to RF level variations The variable gain output amplifier is designed to provide a conjugate match of Q400 an MFR5812 medium power RF transistor to the 500 source U404 the VCO and to the 50Q load the SRD which has its own matching network The gain of Q400 is adjusted by changing its dc collector current U406A compares the DAC signal RF LEVEL to the rectified RF current in the SRD which is the dc current sourced by R444 If the detected RF is low the output of U406A will ramp up increasing the output of U406B which increases the base current to Q400 increasing the available power from Q400 The output of U406A linearly controls the collector current of Q400 from 0 to about 35mA U406A s output settles when the detected RF signal on R444 is exactly 1 10th ofthe RF LEVEL DAC signal Step Recovery Diode Matching The output of the RF amplifier is connected to the SRD via an SMB connector The SRD can be modeled at RF frequencies and at our drive level as a resistor 20 40Q depending on PRS10 Rubidium Frequency Standard S4 Circuit Descriptions drive level with a shunt capacitor about 1pF and a series inductor a few nH Matching for best return loss is achieved by adding a shunt capacitor 5pF across the SMB and with a series inductor 10nH to the SRD The SRD is inside the mu metal can which encloses the resonance cell and photodetector The can is not resonant at the microwave frequency as i
136. so contains a mixture of isotopes The isotopic mixtures buffer gases and operating conditions are chosen so as to minimize temperature coefficients and intensity shifts of the apparent hyperfine transition frequency PRS10 Rubidium Frequency Standard 10 Theoretical Overview The apparent transition frequency will be shifted by about 3 kHz from the natural transition frequency by the buffer gas and discharge lamp spectral profile The transition frequency differs slightly for each unit depending on the fill pressure etc The transition frequency is also tuned over a few Hertz by a magnetic field which may be varied In the PRS10 the rubidium physics package acts as a very stable frequency detector for a frequency around 6 834 GHz By using a microwave frequency synthesizer which is referenced to the 10 MHz OCXO the 10 MHz may be stabilized to the rubidium hyperfine transition frequency PRS10 Rubidium Frequency Standard 10 Overview 11 PRS10 Overview All compact rubidium frequency standards discipline a crystal oscillator to the hyperfine transition frequency in the ground state of rubidium Several different topologies have been developed A major difference in these designs is the method chosen to lock a standard output frequency usually 10 MHz to the essentially arbitrary hyperfine transition frequency at about 6 834 GHz Block Diagram Figure 2 shows a block diagram for the PRS10 Rubidium Frequency Standard The design
137. such as lock indicators signal levels heater control status 1 pps time tags status flags and calibration parameters This provides the systems integrator with the ability to monitor and control the operation of the PRS10 Communicating with the PRS10 is not required in most applications The PRSIO comes from the factory calibrated to 5x10 and the FS725 provides front panel LEDs to indicate power lock and 1 pps synchronization You may want to communicate with the PRS10 to access advanced features or for evaluation of the PRSIO in an OEM application For instance when locking the PRS10 to an external 1 pps reference you may want to optimize the time constant and lock mode of the digital PLL to match the stability characteristics of your reference Getting Connected The PRS10 uses the RS 232 protocol for communication Most computers have one or more communications ports for this purpose To establish communication with the PRS10 you will need a standard straight through 9 pin RS 232 cable connected between your computer s communications port and the DB9 connector on the back ofthe FS725 Once the cable is in place you need to tell the software which COM port you will be using to communicate with the FS725 This is done with the pull down list box located in the upper FS725 Rubidium Frequency Standard Monitoring and Control 15 right portion of the main window Use the pull down list box to select the COM port For most comp
138. t protection diodes to 5 V and ground RS 232 data 15 sent to the host on pin 4 received from the host on pin 7 The baud rate is fixed at 9600 baud 8 bits no parity with 1 start and 1 stop bit No DTR or CTS controls are used rather the XON XOFF protocol has been implemented The transmit drive level is 0 and 5 V not the 12 V normally associated with RS 232 These levels are compatible with most RS 232 line receivers but does not require their use a TTL inverter may be used instead hence simplifies the interface when used inside an instrument at the sacrifice of degraded noise immunity over long lines PRS10 Rubidium Frequency Standard Applications 21 The PRS10 may be connected directly to PC s COM2 port with three wires TXD RXD and ground As PRS10 sources only 5 0 V for the RS 232 via 1 the connecting cable should be kept short PC s COM DB9 Connector PC s COM DB25 Connector Pin4 TXD Pin2 RXD Pin 2 RXD Pin 10 GND Pin 5 GND Pin 7 GND Operating Temperature The unit should be operated so that the baseplate temperature stays below 65 This requirement is usually met by units operating on the bench at room temperature when powered by 24 Vac Frequency Adjustment A magnetic field coil inside the resonance cell is used to tune the hyperfine transition frequency The magnetic field is controlled by a 12 bit DAC The output frequency at 10MHz tunes quadratically with the DA
139. t line is available for typing in new commands FS725 Rubidium Frequency Standard 16 Monitoring and Control Here is a sample screen shot pointing out the pertinent items of interest WE Rubidium Communication COM2 id Command sent PRS10 3 23 SM 20489 PRS10 response Command sent Comunand sent PRS10 response Command sent Comunand sent PRS10 response New commands can be entered on last line only Rb Stats The tab labeled Rb Stats is used to monitor virtually all of the PRS10 s settings To view the current PRS10 settings click on the Rb Stats tab make sure that the selected communications port is correct and click the Connected check box to enable the connection The screen should fill in with the current PRS10 settings The settings will be continuously updated until you disconnect from the communications port by clicking on the Connected check box a second time to uncheck it FS725 Rubidium Frequency Standard Monitoring and Control 17 A sample screen shot taken while the PRS10 was warming up is shown below i Rubidium Communication COM2 Factory Settings Analog Output Values 1005 Control 90 Step Diode ad 0003 Spare TO 1805 Time Offset 91 Delay Value 2 276 24 Volt Heat TS 14081 Time Slope 92 Fet Voltage Set ad2 2 307 24 Volt Elec 193 Pulse Slope sd3 Lamp Temp Set 0 495 Lamp Drain 1 Phase Lock 94 Crystal Temp Set ad4 0 327 Lamp Gate 14 Time Constant 95 Cell
140. ta is communicated in ASCII codes Commands are case insensitive and spaces ASCII 3210 are ignored Commands are processed when a carriage return is received Returned values are delimited with commas in the case of multiple returned values or a carriage return in the case of a single or the last returned value Commands available to the end user are in bold some commands are for factory use only and a special code must be transmitted to enable these commands Parameter lists are enclosed in curly brackets the brackets are not part of the command On reset the unit will transmit the characters PRS_10 with a carriage return Initialization RS 1 Restart This command will restart the PRS10 s microcontroller just like power on It is not necessary to send a RS command on power up All values will return to the values stored in EEPROM verbose mode disabled 10 MHz set to last stored value etc The frequency lock loop will be disabled until the microcontroller verifies that the unit is warmed up and that a useful signal level is present Example RS 1 will cause the unit to restart 0 or 1 VB Set verbose mode The verbose mode is useful when a human is communicating with the PRS10 using a terminal program the PRS10 will provide an command prompts etc The verbose mode should be disabled when a computer program is communicating with the PRS10 where format characters would interfere Examp
141. tant as the expected shot noise current times the transconductance gain is about 600nV VHz The transconductance amplifier is followed by a high gain amplifier x288 for ac signals This amplifier has a pass band from 16Hz to 1 6kHz The non inverting input to this amplifier is biased to place the output of the following bandpass filter at midscale A two pole Butterworth low pass filter 300Hz bandwidth is used to reduce noise at the A D input while preserving gain between 70Hz and 140Hz The filter has a gain of 1 59 for signals in the pass band The input voltage noise specifications for the high gain and filter amplifiers are not particularly important as there is about 600nV VHz of noise on the output of the transconductance amplifier With an noise equivalent bandwidth of about 400Hz we expect a total noise from the shot noise of the photocell s de current of about 3 4mVrms or about 17mVpp This is much larger than the LSB 1 25mV of the A D converter so the quantization noise of the A D will not be important PRS10 Rubidium Frequency Standard Circuit Descriptions 47 Signal Filters for Oscillator Control The amplitude and frequency of the crystal oscillator are controlled by signals from D A converters In order to preserve low phase noise these signals must have very little voltage noise The EFC signal has a full scale of 17Vdc and a resolution of 22 bits A LSB represents a step of about 4uV which is a fractional frequ
142. te and adjust the frequency of the 10 MHz timebase so as to null the component at the sweep rate which keeps the optical dip centered in the middle of the sweep The CPU digitally synthesizes the 70Hz sinewave which phase modulates the RF frequency synthesizer U313 an 12 bit DAC outputs 32 samples during each cycle of the 70Hz sinewave The amplitude of the sinewave is controlled by the signal PHASE DEV which comes from an 8 bit DAC on the frequency synthesizer PCB The amplitude of the sinewave controls the magnitude of the frequency deviation which is adjusted to optimize the deviation sensitivity of the resonance cell The frequency deviation is about 300Hz at 6 834GHz 1PPS Output A port bit on the microcontroller PA7 may be used to output a 10us pulse at a rate of 1Hz This pulse is combined with the LOCK output signal on the main connector pin 1 of J100 The function of the LOCK 1PPS output may be configured via RS 232 This port bit is controlled by the microcontroller s timer which has a resolution of one CLK cycle 400ns Hardware on the bottom circuit board provides delays in 100ns steps under control of the port bits 1 SELO 1PPS_SEL1 and in steps of about 0 5ns via an analog signal from an 8 bit DAC The combination of these three delays allows the 1PPS output pulse to be placed with an accuracy resolution and differential non linearity of about Ins 1PPS Input Time Tag The rising edge of
143. te 6 to the unit s EEPROM which will be used to initialize the gain parameter after the next power on or restart Then GA will return a 6 PH PH value Os value lt 31 PH PH Phase This command is used to set the phase of the synchronous detection algorithm The frequency lock loop FLL uses the in phase component of the photo signal at the modulation frequency 70 Hz as the error signal for the FLL The phase between modulation source and the error signal is affected by phase shifts in the modulation and signal filters and by optical pumping time constants This parameter corrects for the accumulation of all of these phase shifts Each modulation cycle consists of 32 phase slots so each phase increment corresponds to 11 25 Example PH would typically return a value of 24 PRS10 Rubidium Frequency Standard 28 RS 232 Instruction Set The PH command is used to write the current phase parameter into the unit s EEPROM This is a factory only command The value which is burned in EEPROM is used on power on and restart and may be queried by the PH command Example could return typical value of 24 Frequency Synthesizer Control A frequency synthesizer which uses the 10 MHz OCXO as a frequency reference is used to generate the RF which sweeps the rubidium hyperfine transition The frequency synthesizer multiplies the 10 MHz by a factor M 19 64 N A to generate a frequency near 6 834 GHz The facto
144. ted via RS 232 or used to servo the unit to another frequency standard such as GPS PRS10 Rubidium Frequency Standard S6 Circuit Descriptions Hardware on this board provides two signals TIME LATCH and INTERPOLATE These signals latch the value of a free running counter clocked by the E CLK which is part of the microcontroller TIME LATCH is just the 1PPS input re synchronized to the CPU s E CLK which allows the processor to time tag the input to 400ns resolution INTERPOLATE will go low for a time equal to about 2000 times the interval between the 1PPS input and the next E CLK Measuring the duration of INTERPOLATE allows the position of the IPPS input to be measured to about 400ns 2000 0 2ns The E CLK is synchronized to the 10MHz clock and four phases are generated by U500 an octal latch E 0 is used to synchronize EN CLR U506A and E 90 is used to arm the time tagging circuit U507A A gate pulse the output of U507B will start with the first 1PPS input after U5074A is set and end synchronously with the first E 180 rising edge after the first E 90 rising edge after the 1PPS input This will generate a gate pulse of 100ns to 500ns duration that is a measure of the position of I PPS input relative to the E CLK The width of the gate pulse is multiplied by a factor of about 2000 by the pulse stretcher circuit Initially C509 is charged to 11 4Vdc C509 is rapidly discharged by Q502 s collector current about 10 8mA durin
145. tegrated Circuit Surface Mount Pkg Washer Split Screw Misc Screw Allen Head Inserts Threaded Screw Flathead Phillips Window Screw Allen Head Screw Allen Head Screw Allen Head Socket THRU HOLE Misc Components Deep Drawn or Stamping Fabricated Part Fabricated Part Fabricated Part Product Labels PRS10 Parts List 77 a 4 00 3 60 4 2 00 Y I 0 20 TYP 1637 amp gt 3 00 2 60 BOTTOM VIEW 0 144 a END VIEW BASEPLATE 5 Y U v gt 4 m Y ERE 0 30 FREQUENCY ADJUSTMENT MOUNTING HOLES 1 03 gt 4 40 UNC 2B x 0 25 DEEP 4 PLACES CONNECTOR MATES WITH 1 POSITRONIC CBM11W1F2 NOTE ALL DIMENSIONS IN INCHES WITH COAX INSERT MS4104D 2 CANNON DAM11W1S WITH COAX INSERT DM53740 5000 Figure 4 Mechanical Dimensions PRS10 Rubidium Frequency Standard
146. the frequency lock loop algorithm Higher gain values have shorter time constants the time constant is the time it takes for the frequency lock loop to remove 67 of the frequency error but have larger equivalent noise PRS10 Rubidium Frequency Standard RS 232 Instruction Set 27 bandwidths which will reduce the short term stability of the 10 MHz output A gain of 0 will stop the frequency lock loop so that the frequency of the output is determined by the 10MHz ovenized oscillator alone The gain setting approximate time constants and approximate equivalent noise bandwidths are detailed in the following table The gain parameter is set automatically by the program however the user may want control over the parameter in special circumstances Example GA7 will set the gain parameter to 7 which has a time constant of about 2 s which is a typical value for normal operation GA could return a value of 8 just after restart which has a short time constant of about s to assist the initial frequency locking Setting the gain parameter during the first 6 minutes after turn on or restart will abort the automatic gain sequencing Command Time Constant seconds Noise Bandwidth Hz GAO 0 GAS 8 0 The GA command stores the current value of the frequency lock loop gain parameter into the unit s EEPROM Example If the current value of the gain is 6 the command will wri
147. the unit s EEPROM for use after the nest restart or power on cycle This command is used after the SP command is used during the calibration of the unit Example SP will write the frequency synthesizer parameters R N and A which are currently in use to the unit s EEPROM SP will return the values for R N and A which are currently in the unit s EEPROM The SP command may be used to verify that the SP write command executed correctly Magnetic field Control A magnetic field coil inside the resonance cell is used to tune the apparent hyperfine transition frequency The magnetic field is controlled by a 12 bit DAC Increasing the magnetic field will increase the hyperfine transition frequency which will increase the frequency of the 10 MHz output The transition frequency may be tuned over about 3 x 10 by the magnetic field which corresponds to 40 030 Hz at 10 MHz The output frequency at 10 MHz tunes quadratically with field strength and 0 08 DAC 4096 2s A minimum magnetic field should always be present to avoid locking to the wrong Zeeman component of the hyperfine transition so the 12 bit DAC may be set from 1000 to 4095 with 3000 being the nominal midscale value DAC value of 1000 corresponds to about 6 of the full scale frequency tuning range 3000 corresponds to about 53 while 4095 15 100 of the full scale range To help cancel frequency shifts due to external magnetic fields the current in the coil
148. transition In the screen shot the value of LO 15 zero indicating that the unit has not yet locked to the atomic transition It is highlighted with a blue box to indicate that this value is lower than normal FS725 Rubidium Frequency Standard 18 Monitoring and Control Analog Output Values This section contains the current values of 20 internal voltage test points within the PRS10 These values allow you to monitor the heaters temperatures power supplies phase lock loops and lamp characteristics of the PRS10 In the example screen shot the values ADS AD6 and AD7 are highlighted with red boxes to indicate that the heaters are at maximum power Values AD11 AD12 and AD13 indicate the current voltages on the temperature sensitive thermistors Values AD11 and AD13 are highlighted with red boxes to indicate that these voltages are above normal In the above screen shot AD12 is not highlighted because the cell has almost reached the correct operating temperature Highlighted boxes are a normal occurrence during the warm up of the PRS10 1 pps Control This section contains calibration parameters that control the PRS10 s behavior when locking to an external pps reference In the example screen shot PT is highlighted with a black box This is meant to indicate that its current value 14 is different from the value stored in EEPROM which is the value it will have at start up In this particular case the command PT14 was entered manually
149. ubstantial offsets are usually caused by low or slow pulses provided to the 1 pps input The 1 pps input should make a clean transition from 0 to SVDC with a rise time of less than 10ns To calibrate the TO parameter Connect to the FS725 s serial port using the RbMon program Select the RS 232 Communications tab Verify the connection by typing the ID command Send the PLO to disable phase locking to the 1 pps input Verify that the PLO command was received by typing the PL command which should return a 0 indicating that 1 pps phase locking is disabled 6 Connect the 1 pps output directly to the 1 pps input with a 3 RG58 coax cable Do not use any 50 terminators 7 Query the time tag with the TT command If the result is 5ns which is the time delay of 3 of RG58 coax cable then you are done 8 Ifthe result ofthe TT is not 5ns then query the TO parameter with a TO A typical value ofthe TO parameter is 1700ns This value is added to the time tag measurement before is it returned via RS 232 9 Ifthe result of the TT was greater than 5ns then increase the magnitude of the TO parameter making it more negative For example if TT returned 15ns and TO returned 1700 then send TO 1710 10 Ifthe result ofthe TT less greater than 5ns then decrease the magnitude of the TO parameter making it less negative For example if TT returned 1ns and TO returned 1700 then send TO 1696 11 Iterate as necessar
150. ules The source impedance of the line is 250 and it has an offset of 10 VDC 5725 Rubidium Frequency Standard 28 Circuit Description 5 MHz Output 5 MHz output is created by converting the 10 MHz sine into a square wave clocking a D type flip flop to divide the frequency by two and filtering the 5 MHz square wave to produce a low distortion 5 MHz sine wave output It is necessary to increase the slew rate of the 10 MHz clock signal to the flip flop so that the noise at the clock input does not produce jitter and so degrade the phase noise A low noise high bandwidth linear clamping amplifier is used to condition the 10 MHz clock instead of a comparator The clamp amplifier is operated with a gain of 7x so that the slew rate of its 10 MHz output will be about 1V ns The clamping operation restricts the output to an amplitude of 4 5 Vpp with a DC offset of 5 00VDC This signal is AC coupled into the clock of the D type flip flop with a bias of Vcc 2 The clock input of the flip flop has a nominal threshold of Vcc 2 The flip flop is powered from the quiet 5 00V reference for two reasons to reduce the noise of the clock threshold and to reduce amplitude modulation of the output The outputs of the flip flop swing between 0 and Vcc and so any noise on Vcc translates directly into amplitude noise thereby degrading the phase noise of the output Two flip flops are clocked with the output of the clamping amplifier One of the flip
151. um Frequency Standard manual 25725 Rubidium Frequency Standard 14 Monitoring and Control Monitoring and Control via the RS 232 A Windows based program RbMon is provided on a 37 floppy disc The program may also be downloaded from the SRS web site at www thinkSRS com The program facilitates monitoring control and calibration of the FS725 Rubidium Frequency Standard via the computer s COM port over a serial RS 232 interface Running the Install program will place the RbMon icon on the Desktop and in the Start Programs list and place the program components in the folder RbMon The provided Uninstall utility should be used to remove the program and its icons from your computer System requirements include 2150MHz Pentium or comparable Windows 95 or later and an available COM port The default COM port is COMI If your computer uses COMI as a mouse port you will need to change the COM port to COM2 The FS725 uses the PRS10 rubidium frequency standard also manufactured by Stanford Research Systems All commands sent to the FS725 are passed directly to the PRS10 commands received from the FS725 come directly from the PRS10 No buffering or filtering of commands is performed by the FS725 The PRS10 operation and service manual which is an addendum to this document serves as a programming reference for the FS725 Setting up Communications The PRS10 frequency standard can communicate operating and calibration settings
152. ut off when the unit is unlocked The Schmitt trigger output holds the LOCK relay on when the unit is locked The 1 pps output from the PRS10 is gated by U101A and buffered by the octal driver U100 The gated 1 pps is also passed to the optional output modules The 8 outputs of U100 are ganged together through two 1000 resistors R101 amp R102 to drive the rear panel 1 pps OUT via balun T100 The 1 pps output 15 intended to drive 50 loads to 2 5VDC requiring 50mA of current during the 10 pulse Care was taken to confine this large current pulse to the immediate area around the 1 pps driver there is an isolated ground plane which is locally bypassed by a 330uF capacitor C102 and a 1uF capacitor C101 the system s 5VDC supply is isolated by a 470 resistor R112 and the balun T100 requires that all of the output current returns to the local ground RS 232 Interface The 10 can receive serial data on Pin 7 and transmit serial data on Pin 4 The CMOS levels from the PRS10 are level shifted by an RS 232 interface U105 which has a charge pump to generate the 10VDC required by the RS 232 Activity on the RS 232 lines is detected by diodes D100 and D101 and RC networks R108 C107 amp R109 and C108 which drive the front panel LEDs RS 232 Receive and Send via Schmitt trigger U104D and U104E The charge pump RS 232 interface has a separate ground plane and is locally bypassed and isolated from the system s 5V
153. uters this will be either COMI or COM2 Make sure that the Connected box is checked This will activate the communications port Testing the Connection Test the connection by sending a command to the PRS10 to see that it responds properly Type the command ID and press lt ENTER gt The PRS10 should respond with its identifier string A typical screen shot with all the pertinent elements pointed out is shown below A Rubidium Communication COM2 QjPRS10 3 23 SN 20495 PRS10 response to id command Check here to activate RS 232 Communications Use the tab labeled RS 232 Communications to send commands to the PRS10 manually This tab is meant to provide the user with an easy method for manually typing commands to the PRS10 To use it make sure that the proper communications port is selected and that the Connected box is checked Once that is complete sending a command to the PRS10 is as simple as typing it in and pressing lt ENTER gt For ease of interpretation commands that you type will be formatted in bold Responses from the PRS10 will have normal formatting Keep in mind that the PRS10 will not respond to all commands It only responds to query commands ones ending with a question mark One important point to keep in mind is that commands can only be entered on the last line Lines above the last line show the previous commands sent and responses received These lines can not be overwritten Only the las
154. with an adjustable time constant of from 8 minutes to 18 hours which reduces the noise of the reference 1 pps by averaging The rear panel 1 pps input to the FS725 is a high impedance input that is discriminated by a CMOS Schmitt trigger with a threshold of about 2 5 VDC It is important to provide a 1 pps input with CMOS levels 0 5VDC The 1 pps input may not work reliably with a TTL input or the 1 pps output from another instrument such as another FS725 if the signal is terminated into 50 Q The 1 pps from another FS725 will work fine providing it is not terminated into 50 Q A classical 2 order phase lock control loop is implemented in firmware The phase comparator is a time tagging circuit which measures the timing of the 1 pps input with nanosecond resolution A proportional integral phase lock algorithm controls the frequency of the rubidium frequency standard via the parameter SF Set Frequency Normally the SF parameter is set by the firmware reading the calibration potentiometer However when a 1 pps signal is seen the phase lock algorithm hijacks the SF parameter to phase lock the rubidium frequency standard to the 1 pps input The locking algorithm of the PRS10 proceeds as follows e The 1 pps PLL is enabled when the unit is turned on or restarted if the PL parameter stored in the unit s EEPROM is 1 This is the default value e The PLL will begin to control the frequency of the rubidium frequency standar
155. would be very sensitive to digital crosstalk and so are communicated with via the gated SPI interface Magnetic Field Control R331 a 348Q shunt resistor is used to measure the current through the magnetic field coil which is in the resonance cell U307B an LM358 op amp maintains a current through the field coil so that the voltage across the shunt resistor matches the output from the 12 bit DAC U310 an LTC1452 The coil current can be programmed from 0 to 8mA but a minimum level 3mA is always maintained to spread out the non 0 0 Zeeman transitions The PRS10 Rubidium Frequency Standard Circuit Descriptions 51 frequency offset is quadratic in the field strength with a fractional frequency resolution of about 1x10 at 3mA and of 2 5x 10 at 8mA To reduce the susceptibility of the transition frequency to external magnetic fields the polarity of the magnetic field is chopped at 5Hz by the CPU control signal MAG SIGN and U306 a DG211 quad analog switch The apparent transition frequency is synchronously filtered by the CPU over the field reversal period so as to notch out any 5Hz noise from the EFC signal Phase Modulation The main task for the microcontroller is to modulate the microwave carrier to sweep through the Rb hyperfine transition frequency The microcontroller will A D the optical signal via a 12 bit A D converter synchronously detect the components of the optical signal at the sweep rate and at twice the sweep ra
156. y adjusting TO until the TT returns 5ns 12 Save the TO parameter to EEPROM by sending the command TO 13 Verify that the TO was saved to EEPROM by reading it back from EEPROM with the TO command pe FS725 Rubidium Frequency Standard Circuit Description 25 FS725 Circuit Description The FS725 provides an operating environment for the PRS10 Rubidium Frequency Standard The PRS10 operation and service manual details operation circuits and software interface for the rubidium frequency standard as an addendum to this manual Rubidium Logic Interface Sheet 1 of 2 J100 is the connector to the PRS10 rubidium frequency standard The pin out for J100 is detailed in the table below Pin No Name Function 1 LOCK 1 pps OUT TTL HIGH UNLOCK PULSES HIGH AT 1 pps 2 POT WIPER 0 SVDC ANALOG FREQUENCY ADJUST 3 POT ANALOG GROUND VOLTAGE REFERENCE 4 TXD RS 232 TRANSMIT OUTPUT CMOS LEVELS 5 1 pps IN 1 pps TIME TAG INPUT FOR PHASE LOCKING 6 24V HEATER 24VDC SUPPLY FOR HEATERS J RXD RS 232 RECEIVER INPUT CMOS LEVELS 8 POT 5 00VDC ANALOG REFERENCE 9 24 ELECTRONICS 24VDC SUPPLY FOR ELECTRONICS 10 GROUND POWER SUPPLY CURRENT RETURN In addition there is an RF connector which is part of J100 which is used for the 10 MHz sine output from the unit The connector shell of J100 is also grounded 1 pps Input The rear panel 1 pps input is connected directly to the 1 p
157. zed to within 115 of 1 pps input Blinks when RS 232 characters are received by FS725 Blinks when RS 232 characters are sent by FS725 Rear Panel Connections FREQ ADJ 1 PPS Input 10 MHz Out 5 MHz Out 1 PPS Out Optional Relays Rb Lock 1 PPS RS 232 0 5 VDC to adjust frequency by 0 002 ppm normally unconnected One 100 kQ input Requires CMOS level pulses 0 5 VDC If an external 1 pps input is applied lock is maintained between the 1 pps input and 1 pps output with computer adjustable natural time constant from 8 minutes to 18 hours Default is 2 25 hours Two isolated 50 Q 10 MHz sine outputs One 50 Q 5 MHz sine output One 50 Q pulse output Each option board provides four 10 MHz one 5 MHz and one 1 pps outputs Up to 3 boards may be installed at the time of order Max current 3A SPDT normally open or normally closed May be wired in parallel with other relays to wire or a single alarm Relay status matches front panel Locked LED Relay status matches front panel 1 pps sync LED 9 pin connector configured as DCE 9600 baud A Windows Control program is provided 25725 Rubidium Frequency Standard Environmental Operating temperature Storage temperature Temperature stability Magnetic field Relative humidity Specifications 5 10 C to 40 C 55 C to 85 C Aff 1 x 107 10 C to 40 C Af f lt 2 10 for 1 Gauss field revers

FS725 Rubidium Frequency Standard

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