Rubidium Frequency Standard
Contents
1. II Rubidium Communication COM2 loj x File Help R5232 Communications Rb Stats Terminal Type in commands here Communications Stanford Research Systems SIM900 s n000112 ver2 4 Adjust COM port here D v Connected e ie ee SIM900 response to IDN query click here to activiate 3 4 Connecting to the SIM940 Initially the RbMon program will only be communicating with the SIM900 mainframe To control a SIM940 installed in a mainframe slot we must inform the SIM900 which slot number to communicate with This is done with the CONN command The format of this command is SIM940 Rubidium Frequency Standard fs RS 3 4 Monitor and Control CONN port number esc string where port number is the slot address of the SIM940 inside the SIM900 mainframe this is the leftmost number above the module when seated in the mainframe and esc string is a quote delimited string to cause the mainframe to terminate the connection Following the CONN command issue a ID to verify that communi cations are now established with the SIM940 For example if SIM940 is installed in slot 7 of the SIM900 mainframe occupying the rightmost two slots the screen should appear as below II Rubidium Communication COM2 Oj x File Help R5232 Communications Rb Stats Terminal Communications Port com2 v Connected idn Command sent to SIM900 Stanford Research Systems SIMSO00 s n0001122. 1 00367 J104 DAM 11W1S 1 00342 J2 CONN10M156 1 01057 COAX INSERT FOR ABOVE 1 00343 J3 1 01056 J100 CONN10F156 1 01058 J200 1 01032 HARDWARE J201 J202 J203 J204 J205 J206 L1 L2 L3 L4 P1 Q100 R1 R4 R5 R6 R8 R12 R35 R38 R2 R3 R7 R10 R13 R15 R9 R14 R11 R39 R40 R41 R42 R43 R44 R16 R21 R26 R31 R17 R18 R19 R24 R29 R34 R20 R25 R30 1 00003 FIT68 1 6 00629 56UH 6 00595 100 15T 4 00353 IRF4905 3 00944 4 01478 249 4 01059 3 74K 4 01172 330 4 01467 4 01551 100K 4 01527 4 045 10 0K 4 01213 499 4 01088 401447 5 3 Schematic Diagrams RUBIDIUM STANDARD PCB SIM FRONT PANEL SIM REAR PANEL INTERNAL BRACKET SIM TOP DUAL BRACKET SIM BOT DUAL BRACKET SIM SKINS SKIN SCREWS PRS10 FRONT PANEL SCREWS PCB AND REAR PANEL DB 15 CONNECTOR SIM RUBBER FEET FRONT PANEL LEXAN FINISHED GOODS PRS 10 7 01526 REV A 7 00053 7 00063 REV B 7 00375 REV A 7 01380 REV A 7 01381 y mo CC 4 40 SLOT 0 00148 4 40 PHILIPS 0 00515 4 40 FLAT 0 00208 0 00188 7 00654 Schematics for the SIM940 follow this page For schematics of the PRS10 see the attached PRS10 Operation and Service Manual SIM940 Rubidium Frequency Standard ASRS
3. RbMon menus 3 7 1 File menu 3 7 1 1 Print 3 7 1 2 Print Setup 3 7 1 3 Exit 3 7 2 Help menu 3 7 2 1 Contents 3 7 2 2 About The RbMon application has two pull down menus located at the top of the window frame The File menu has three options to choose from 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 from 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 81 2 by 11 inch Letter size paper Printing on paper of other sizes is not supported but not prevented Print Setup enables you to configure your printer for printing You should choose Letter size paper 81 2 by 11 inch and the Portrait orientation Landscape orientation is not supported The Exit menu item will close the program The Help menu has two options to choose from Choose this menu item to display help information Choose this menu item to display the program name and version information ASRS SIM940 Rubidium Frequency Standard 4 Calibration In This Chapter This chapter describes the calibration procedure for the SIM940 41 With a frequency counter 4 2 42 Withalppsreference 2222200000 4 4 43 Calibration of the time tag offset parameter 4 6 Calibration There is a 15 turn potentiometer in the PRS
4. SIM940 uses asynchronous serial interface for communication In addition to providing power the SIM900 mainframe acts as a com ASRS SIM940 Rubidium Frequency Standard 3 3 Testing connection 3 3 3 3 Testing connection munications adaptor translating the internal data format to standard RS 232 or IEEE 488 GPIB communications The provided RbMon software is limited to communicating via RS 232 so your computer will need one unused serial port COM port for this purpose Most computers have one or more communications ports for this purpose To establish communication you will need a standard straight through 9 pin RS 232 cable connected between your com puter s communications port and the DB 9 COMPUTER connec tor on the back of the SIM900 mainframe Once the cable is in place you need to tell the software which COM port you will be using to communicate with the SIM900 This is done with the pull down list box located in the upper right portion of the main window Use the pull down list box to select the COM port For most computers this will be either COM1 or COM2 Make sure that the Connected box is checked This will activate the communications port Test the connection by sending a command to the SIM900 to see that it responds properly Type the command IDN and press ENTER TheSIM900 should respond with its identifier string A typical screen shot with all the pertinent elements pointed out is shown below
5. 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 4 1 Calibration with a frequency counter The procedure is to adjust the calibration potentiometer so that the SIM940 produces 10 MHz to within 41x10 or 10 MHz 0 0001 Hz Important details are list below 1 Be certain that the SIM940 has been operated continuously for at least 24 hours 2 If the SIM940 has been off for more than one week it should be operated continuously for at least 72 hours before the adjust ment is made 3 Be certain that nothing is connected to the rear panel FREQ CTL input ASRS SIM940 Rubidium Frequency Standard 4 1 With a frequency counter 4 3 10 Be certain that nothing is connected to the rear panel 1pps input Measure and record the dc voltage on the center pin of the FREQ CTL BNC Power cycle the SIM940 by switching off the SIM900 mainframe 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 opera tion Wait for at least one hour before proceeding with the calibration The freque
6. input or the 1 pps output from another instrument such as another SIM940 if the signal is terminated into 50 O A classical 24 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 Fre quency 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 standard to the 1 pps input The 1 pps from another SIM940 will work fine providing it is not terminated into 50 ohm SIM940 Rubidium Frequency Standard fs RS Operation The locking algorithm of the PRS10 proceeds as follows e ThelppsPLLisenabled 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 standard when 256 consecutive good 1 pps inputs i e 1 pps inputs which are within 2048 ns of the first time tag result module 1 s are received e 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 1 PPS Sync LED will turn on as th
7. is added to the transformer sec ondary so that the 5 Vpp output is centered in the common mode input range of the op amps which are operated from 20 VDC The op amps are operated with a nominal gain of 2 73 The 10 MHz input amplitude can be adjusted with P1 to calibrate the amplitude of the rear panel 10 MHz outputs The op amps drive 50 O 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 RC filters on each power sup ply improve channel to channel isolation The output filter reduces the harmonic distortion by an additional 15 dB ASRS SIM940 Rubidium Frequency Standard 5 2 Component parts list 3 5 5 2 Component parts list Reference Value SRS p n Reference Value SRS p n C1 C45 330UF 5 00516 R22 R27 R32 604 4 01096 C2 C3 C4 C5 C14 C46 2 2U 35V 5 00318 R23 R28 R33 866 4 01111 C6 C7 C8 C9 C11 C16 C17 C26 C35 C44 5 00299 R36 R37 4 01021 C10 C12 C13 001U 5 00387 T1 T2 T3 T4 TOKO7P 6 00195 C15 C21 C31 C39 5 00368 T5 6 00009 C18 C27 C36 5 00351 U1 LM317 3 00149 C19 C28 C37 2 2P 5 00355 U2 LM340 5 3 00112 C20 C23 C30 C33 C38 C42 5 00375 U3 3 00662 C22 C24 C32 C34 C41 C43 220P 5 00379 U4 74HC00 3 00902 C25 C29 C40 5 00298 U5 U6 3 01467 D1 RED LED 3 00425 U7 U8 U9 LM7171 3 00819 D2 D3 D4 GREEN LED 3 00424 U10 74HC540 3 00748 J1 DB15 HM
8. outputs have a source impedance of 50 O termination into a 50 O load is not critical in most applications Generally the amplitude will approximately double if the output is not terminated into a 50O load Properly terminating into a 50 O load will reduce channel to channel cross talk and will reduce the probability of multiple triggering on the 1 pps output If a single 10 MHz output is daisy chained to several instruments it will be important that all of the instruments have a high impedance gt 1kQ and that a 50 O terminator be placed at the far end of the line ASRS SIM940 Rubidium Frequency Standard 2 4 Connection to SRS Instruments 2 3 Generally it is better practice to use a separate output to each instru ment 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 SIM940 Sometimes a rear panel switch or front panel configuration menu is required to tell the instrument to use the 10 MHz input as a reference instead of using the instrument s own time base Improved accuracy can be achieved by using the SIM940 as an external time base to all of the instruments in a particular laboratory In addition all errors associated with the cali
9. panel 1 pps input Results of the time tag are returned over the serial interface in re sponse to the TT command in integer nanoseconds with respect to the 1 pps output If the 1 pps input arrives 5 ns after the 1 pps output the TT should return a value of 5 If the 1 pps input arrives 5ns 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 is a calibration parameter TO which is added to the measured time tag before the result is returned over the serial interface 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 conse quence Substantial 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 5 VDC with a rise time of less than 10 ns To calibrate the TO parameter 1 Connect the RS 232 serial cable between the SIM900 mainframe and the PC or laptop computer Verify that you can talk to the SIM940 with RbMon by typing the ID command in the RS232 Communications tab see procedure in section 3 4 2 Send the PLO to disable phase locking to the 1 pps input 3 Verify that the PLO command was received by typing the PL command which should return a 0 indicating tha
10. the SIM940 circuit design A complete parts list and circuit schematics are included In This Chapter 5 1 Circuit Discussion 5 2 5 11 Rubidium Logic Interface 5 2 Del Pps input os 6 reas Fa dem utat rg 5 2 5 13 Lockandlppsoutput or 5 3 5 1 4 Serial Interface 2222222222220 5 3 515 BowerSupplies zu acce so ar ai Buche 5 3 5 1 6 10MHz Outputs ee rd 5 4 5 2 Componentpartslit 5 5 5 3 Schematic Diagrams 222200000 5 5 5 2 Circuitry 5 1 5 1 1 Circuit Discussion The SIM940 provides an operating environment for the PRS10 Rubid ium 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 J104 is the connector to the PRS10 rubidium frequency standard The pin out for J104 is detailed in the table below 5 1 2 1pps input Pin No Name Function 1 LOCK 1 pps OUT TTL HIGH UNLOCK PULSES HIGH AT 1 pps 2 POT WIPER 0 5 VDC 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 24 V HEATER 24 VDC SUPPLY FOR HEATERS 7 RXD RS 232 RECEIVER INPUT CMOS LEVELS 8 POT 5 00 VDC ANALOG REFERENCE 9 24 ELECTRONICS 24 VDC SUPPLY FOR ELECTRONICS 10 GROUND POWER SUPPLY CURRENT R
11. users may wish to directly interface the module to their own systems without the use of additional hardware The mating connector needed is astandard DB 15 receptacle such as Amp part 747909 2 or equivalent Clean well regulated supply voltages of 24 VDC must be provided following the pin out spec ified in Table 1 1 Ground must be provided on pins 1 and 8 with SIM940 Rubidium Frequency Standard ASRS Overview chassis ground on pin 9 The STATUS signal may be monitored on pin 2 for a low going TTL compatible output indicating a status message The SIM940 has no internal protection against reverse polarity missing supply or overvoltage on the power supply pins 1 8 2 1 Direct interface cabling 1 3 2 2 Serial settings If the user intends to directly wire the SIM940 independent of the SIM900 Mainframe communication is usually possible by directly connecting the appropriate interface lines from the SIM940 DB 15 plug to the RS 232 serial port of a personal computer Connect RXD from the SIM940 directly to RD on the PC TXD directly to TDS In other words a null modem style cable is not needed To interface directly to the DB 9 male DTE RS 232 port typically found on contemporary personal computers a cable must be made with a female DB 15 socket to mate with the SIM940 and a female DB 9 socket to mate with the PC s serial port Separate leads from the DB 15 need to go to the power supply making what is someti
12. 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 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 1 pps output The colored boxes are visual cues to let the user know that a given parameter is outside the normal operating range This most com monly 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 tem peratures of the crystal cell and lamp Under these circumstances one would expect blue boxes to appear next to AD5 AD6 and AD7 as the heaters are shut off The interpretation of the boxes is as follows lll Red Value is higher than normal u Blue Value is lower than normal Black Value is different from the value stored in EEPROM SIM940 Rubidium Frequency Standard fs RS 3 8 Monitor and Control 3 7
13. 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 10 or 36ns The SIM940 indicates that it is synchronized to the 1 pps input by a front panel LED The unit will indicate that 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 ASRS SIM940 Rubidium Frequency Standard 2 6 Using an External 1 pps Reference 2 7 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 lost synchronization depending on the initial freqneyc offset between the SIM940 and the 1 pps reference Suppose that a SIM940 is out of cali bration by 1x107 anda 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 SIM940 s frequency of opera tion during the first hour of phase locking and the 1 pps output will stray from the 1 pps input by about 3600 x 109 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 hou
14. 10 rubidium frequency standard which adjusts the operating frequency The potentiometer is accessible by removing the right hand side cover from the SIM940 the side closest to the FREQ CTL IN BNC connector 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 To operate the SIM940 with the potentiometer accessable install the module in slot 1 of the SIM900 mainframe the left most slot with the remaining 6 slots empty When installing the module with the right hand side cover removed be careful to properly align the mod ule in slot 1 so that the rear panel DB 15 connecter mates cleanly Alternatively the SIM940 may be operated outside the SIM900 main frame by providing 24 VDC and ground on the DB 15 connector as described in section 1 3 2 1 The position of the potentiometer may be determined by measuring the DC voltage on the center pin of the rear panel FREO CTL IN BNC The voltage will read 0 VDC at the counter clockwise extreme 5 VDC at the clockwise extreme and 2 50 VDC when the poten tiometer is centered This voltage should be recorded before any calibration is attempted This will allow you to return to the previ ous calibration if necessary and serves as a record of the frequency aging of the unit When the SIM940 is shipped from the factory the potentiometer is set to its center position
15. ETURN Table 5 1 Connector Pin Assignments J104 In addition there is an RF connector which is part of J104 which is used for the 10 MHz sine output from the unit The connector shell of J104 is also grounded The rear panel 1 pps input is connected directly to the 1 pps 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 1 pps input by adjusting its frequency of operation The rear panel 1 pps input also triggers a 1 us one shot U5A to detect the synchronization of the 1 pps input to the 1 pps output to within a 1 us window The 1 pps output also triggers a 1 us one shot U6A If the two 1 pps pulses have rising edges within 1 us of each other the NAND gate U4D will re trigger the 1 6 s one shot U6B which holds the 1 PPS Sync LED on The 1 us one shot triggered by the 1 pps input also triggers a 70 ms one shot U5B which is used to blink the front panel 1 PPS Input ASRS SIM940 Rubidium Frequency Standard 5 1 Circuit Discussion 5 3 LED The 70 ms one shot also holds off additional triggers of the 1us one shot until it times out reducing the probability of false sync detection in the case that the user s 1pps input is running much faster than 1 pps 5 1 3 Lock and 1 pps output 5 1 4 Serial Interface 5 1 5 Power Supplies The PRS10 outputs lock status and 1 pps on P
16. OM2 m v Connected idn Command sent to SIM900 Stanford Research Systems SIMS00 s n000112 ver2 4 Response from SIM 900 conn 7 x Command sent to SIM900 to connect to Slot 7 SIM940 PRS 10 power on message from SIM940 PRS10 id Command sent to SIM940 PRSIO PRS10 3 24 SN 15830 Response from SIM940 PRSIO pt Command sent 8 Response pt14 Command sent pt Command sent Response New commands can be entered on the last line only 3 6 Rubidium 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 that the SIM900 mainframe has been CONNected to the correct slot number see section 3 4 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 A sample screen shot taken while the PRS10 was warming up is shown below SIM940 Rubidium Frequency Standard JE RS 3 6 Monitor and Control WE Rubidium Communication COM2 File Help R5232 Communications Fb Stats PRS10 3 24 SN 16830 Communications Port COM2 r Iv Ce Factory Settings Analog Output Yalues 1pps Control sd 58 Step Rec Diode ad 0 003 Spare TO 1814 Time Offse
17. Operation and Service Manual Rubidium Frequency Standard SIM940 SRS Stanford Research Systems Revision 1 3 e May 8 2007 Certification Stanford Research Systems certifies that this product met its published specifications at the time of shipment Warranty This Stanford Research Systems product is warranted against defects in materials and workman ship for a period of one 1 year from the date of shipment Service For warranty service or repair this product must be returned to a Stanford Research Systems authorized service facility Contact Stanford Research Systems or an authorized representative before returning this product for repair Information in this document is subject to change without notice Copyright Stanford Research Systems Inc 2003 2007 Allrights reserved Stanford Research Systems Inc 1290 D Reamwood Avenue Sunnyvale CA 94089 USA Phone 408 744 9040 e Fax 408 744 9049 www thinkSRS com e e mail info thinkSRS com Printed in U S A Document number 9 01568 903 SRS SIM940 Rubidium Frequency Standard Contents General Information iii Service aues MUS eate SEU UR Rh Pus ds iii SyMmMDOlS ah e PE NE iv Specifications euros Er re Me he vi Overview 1 1 11 Introduction to the Instrument 1 2 1 2 Quick Start Guide 2 2 cc 1 3 13 SIMInterface 2 2222 222mm 1 5 Operation 2 1 231 Power 4 Atle ea e RE ee 2 2 2 2 WN ATID aae ata erg
18. a are daten 2 2 23 Connection to other instruments 2 2 24 Connection to SRS Instruments 2 3 25 Direct Frequency Control of the SIM940 2 4 2 6 Using an External 1 pps Reference 2 5 Monitor and Control 3 1 3 1 Setting up communications 3 2 3 2 Getting connected 2 2 32 parks o Ne Feet 3 2 3 3 Testing connection e ac ee 3 3 3 4 Connecting to the SIM940 3 3 3 5 Terminal procera ser a 3 4 36 Rubidium Stats 2 2222 22mm 3 5 3 7 RbMonmenus ees 3 8 ii Contents 4 Calibration 4 1 4 1 With a frequency counter 2 e see 64044 4 2 4 2 With a 1pps reference x s kon ma a Ra 4 4 43 Calibration of the time tag offset parameter 4 6 Circuitry 5 1 5 1 Circuit Discussion cc 5 2 5 2 Component parts list 2 2 2 2 222er 5 5 5 3 Schematic Diagrams a we xe der d sare e 5 5 Addendum PRS10 Manual ASRS SIM940 Rubidium Frequency Standard General Information Service The SIM940 Rubidium Frequency Standard part of Stanford Re search Systems Small Instrumentation Modules family is a 10 MHz frequency standard with an estimated 20 year aging of less than 0 005 ppm Do not install substitute parts or perform any unauthorized modifi cations to this instrument The SIM940 is a double wide module designed to be used inside the SIM900 Mainframe Do not turn on the power until the module is completely inserted into the mainframe
19. and locked in place iii iv General Information Symbols you may Find on SRS Products Alternating current Caution risk of electric shock Frame or chassis terminal Caution refer to accompanying documents Earth ground terminal SRS SIM940 Rubidium Frequency Standard General Information v Notation The following notation will be used throughout this manual e Front panel indicators are set as Overload e Remote command names are set as ID e Literal text other than command names is set as OFF SIM940 Rubidium Frequency Standard fs RS vi General Information Specifications Performance Characteristics Output Noise Accuracy Control Output frequency 10 MHz sine 10 us wide 1 pps pulse Amplitude 10 0 5 Vrms 7 dBm into 50 Q 1 pps pulse amplitude 2 5 V into 50 O 5 V into high Z Phase noise SSB lt 130 dBc Hz 10 Hz lt 140 dBc Hz 100 Hz lt 150 dBc Hz 1 kHz lt 155 dBc Hz 10 kHz Spurious lt 100 dBc 100 kHz BW Harmonics 60 dBc Short term stability 2x 1071 1s Allan variance 1x10 10s lt 2 x 10 12 100 s Accuracy at shipment 5 x 10711 Aging after 30 days lt 5x 1071 monthly lt 5x107 yearly 5 x 107 20 years typ Holdover 72h Stratum 1 level 1 x 10 11 Frequency retrace 5 x 1011 72h off then 72h on Settability lt 5x 10 12 Tri
20. anual 3 6 1 Factory Settings This section includes calibration values and parameters for control ling the frequency of the PRS10 The most important parameter here is LO which indicates whether the PRS10 has locked to the Rb atomic hyperfine transition In the screen shot the value of LO is zero indicating that the unit has not yet locked to the atomic tran sition It is highlighted with a blue box to indicate that this value is lower than normal SRS SIM940 Rubidium Frequency Standard 3 6 Rubidium Stats 3 6 2 Analog Output Values 3 6 3 1pps control tab 3 6 4 Colored boxes 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 AD5 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 thermis tors and are highlighted with red boxes to indicate that these volt ages are above normal Highlighted boxes are a normal occurrence during the warm up of the PRS10 This section contains calibration parameters that control the PRS10 s behavior when locking to an external 1 pps reference In the example screen shot PT is highlighted with a black box This is meant to indicate that its current
21. bration of an individual instrument s time base are eliminated 2 4 Connection to SRS Instruments 2 4 1 SIM900 Mainframe The SIM900 will automatically sense the presence of a 10 MHz sine wave or square wave signal at its rear panel TIMEBASE IN BNC connector and phase lock to it For convenience it is best to install the SIM940 in the slot 7 position of the SIM900 filling slots 7 amp 8 so that a short length of RG58 coax cable can tie one of the 10 MHz outputs of the SIM940 to the SIM900 TIMEBASE IN The TIMEBASE status LED on the fron the the SIM900 should move from Internal 10 MHz to External Lock 2 4 2 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 C AL menu which will flash when selected Press the SET key twice to access the c Loc Sourc E submenu Press the arrow keys immediately to the left of the CONFIG menu in the SCOPE AND CHART section to select r ERr 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 10 MHz 2 4 3 DS345 Synthesized Function Generator The DS345 automatically detects the application of an external time base and phase locks
22. calibration section 2 By applying a voltage in the range of 0 VDC to 5 VDC to the rear panel FREQ CTLIN 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 overrid den by a user input The tuning rate at 10 MHz is 0 008 Hz V The input source impedance is between 10 kO and 35 kQ de pending on the potentiometer position 3 By software command via the serial interface The SF Set Frequency parameter allows the user to set the frequency with a resolution of 1 10 7 The parameter has a range of 2000 which allows frequency control over the range of 10 002 ppm The position of the frequency adjustment potentiometer is used to set the SF parameter until a SF command is received via the serial interface or until phase locking to an external source is activated by the application of 1 pps inputs to the rear panel BNC Control is returned to the potentiometer when the power is cycled Details are available in the PRS operation and service manual ASRS SIM940 Rubidium Frequency Standard 2 6 Using an External 1 pps Reference 2 5 4 By software calibration via the MO Magnetic Offset parame ter 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 10MHz with an SF value of 0 The SF parameter is set by eith
23. ding 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 SIM940 will be phase locked to the 1 pps input and the SF parameter will be read via the serial interface Then the 1 pps input will be removed the unit will be power cycled so that the SF is again set by the calibration potentiometer and the potentiometer will be adjusted to provide the same SF value as was seen when the SIM940 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 commu nicate with the SIM940 via the serial interface such as a PC or laptop computer with a serial port operating at 9600 baud Any Windows computer together with the SIM940 application program RbMon will work well The procedure for frequency calibration using a precise 1 pps input is 1 Be certain that nothing is connected to the rear panel FREQ CTL input 2 Be certain that nothing is connected to the rear panel 1 pps input 3 Power cycle the SIM940 by switching off the SIM900 mainframe ASRS SIM940 Rubidium Frequency Standard 4 2 With a 1 pps reference 4 5 10 11 12 for at least three seconds This will make certain that the unit will track the calibration potentiometer wh
24. e 1 pps input and the 1 pps output are now within 1 us of each other e The unit will lock the frequency of the PRS10 to the good 1 pps input pulses Bad 1 pps inputs 1 pps inputs with time tags greater than 1024ns from the last good 1 pps input will be rejected A digital filter is used to implement the classical 2nd order 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 e The PLL will be aborted and restarted if there are 256 consecu tive bad 1pps inputs This could happen if the 1 pps input is moved suddenly by more than 1024 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 SIM940 Using a smaller PT value will reduce the peak to peak variations swimming between the 1 pps input and the unit s lpps output With a clean reference and a short time constant the 1pps 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 PRS to change frequency by 1 x 10 11 the change
25. er the potentiometer or via serial interface commands and will be zero when the potentiometer is centered The MO pa rameter by be adjusted and retained in the PRS10 EEPROM allowing frequency calibration via the serial interface Details are available in the PRS10 operation and service manual 2 6 Using an External 1 pps Reference The SIM940 has sufficient accuracy 0 005 ppm estimated over 20 years that most users will not need to use an external timing refer ence However for critical applications requiring a frequency accu racy which is better than 1x10 or to provice continuous traceability the SIM940 may be locked to an external 1 pps reference When a precise 1 pps signal is applied to the 1 pps input on the rear panel of the SIM940 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 12 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 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 SIM940 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 5 VDC The 1 pps input may not work reliably with a TIL
26. f the internal mainframe slots or the remote cable interface It is also possible to operate the SIM940 directly without using the SIM900 Mainframe This section provides details on the interface SIM interface connector The DB 15 SIM interface connector carries all the power and commu nications lines to the instrument The connector signals are specified in Table 1 1 Direction Pin Signal Src 2 Dest Description 1 SIGNAL GND MF SIM Ground reference for signal 2 STATUS SIM MEF Status service request GNDcasserted 5V idle 3 RIS MF gt SIM HW Handshake unused in SIM940 4 CTS SIM MF HW Handshake unused in SIM940 5 REF 10MHZ MF gt SIM 10 MHz reference unused in SIM940 6 5V MF SIM Power supply No connection in SIM940 7 15V MF gt SIM Power supply No connection in SIM940 8 PS RTN MF gt SIM Power supply return 9 CHASSIS GND Chassis ground 10 TXD MF gt SIM Async data start bit 0 5 V 1 ZGND 11 RXD SIM gt MF Async data start bit 0 5 V 1 GND 12 REF_10MHz MF gt SIM 10 MHz reference unused in SIM940 13 5V MF gt SIM Power supply No connection in SIM940 14 15V MF gt SIM Power supply No connection in SIM940 15 24V MF gt SIM Power supply 1 3 2 Direct interfacing Table 1 1 SIM Interface Connector Pin Assignments DB 15 The SIM940 is intended for operation in the SIM900 Mainframe but
27. ht 5 lbs Dimensions 1 5 W x 3 6 H x 7 0 D SIM940 Rubidium Frequency Standard ASRS viii General Information SRS SIM940 Rubidium Frequency Standard 1 Overview This chapter gives the necessary information to get started quickly with the SIM940 Rubidium Frequency Standard In This Chapter 11 Introduction to the Instrument 1 2 12 Quick Start Guide 1 3 13 SIM Interface lees 1 5 1 31 SIM interface connector 1 5 1 3 2 Direct interfacing ois st De osx qa 1 5 1 2 Overview 1 1 Introduction to the Instrument The SIM940 is a 10 MHz frequency standard with an estimated 20 year aging of less than 0 005 ppm The module integrates an SRS PRS10 Rubidium Frequency Standard in a standard double width SIM module The module s low aging eliminates the need for an ex ternal frequency reference in many applications however moduless can phase lock to an external 1 pps signal such as GPS for ageless performance or periodic calibration as required The SIM940 provides exceptionally low phase noise 10 MHz sine outputs as well asa low jitter 1 pps output The three 10 MHz outputs have a phase noise of lt 130dBc Hz at 10 Hz offset from carrier dropping to 155 dBc Hz at 10 KHz offset The Allan variance is less than 2 x 1071 at one second and 2 x 10 12 at 100 seconds The 1 PPS OUT outputhas less than 1 ns of jitter and may be set with Ins resolut
28. ich 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 inless than a minute as itis warm from previous operation The position of the calibration potentiometer may be deter mined by measuring the dc voltage on the center pin of the rear panel FREQ CTL IN BNC The voltage will read 0 V at the counter clockwise extreme or 5 VDC at the clockwise extreme and will read 2 50 VDC when the potentiometer is centered This voltage should be recorded before any calibration is at tempted This will allow you to return to the previous calibra tion if necessary and serves as a record of the frequency aging of the unit Connect the RS 232 serial cable between the SIM900 mainframe and the PC or laptop computer Verify that you can talk to the SIM940 with RbMon by typing the ID command in the RS232 Communications tab see procedure in section 3 4 Be certain that the SIM940 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 PT8 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 SIM940 has been off for more than o
29. in 1 This output will have a CMOS high level 5 V 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 0 V and pulse high for 10 us at 1 pps An RC filter R11 amp C12 together with the Schmitt trigger U3D recovers the lock status and gates the 1 pps output off when the unit is unlocked The Schmitt trigger also controls the Unlocked and Locked LEDs via U4A and U4B The 1 pps output from the PRS10 is gated by U4C and buffered by the octal driver U10 The 8 outputs of U10 are ganged together through two 100 O resistors R36 amp R37 to drive the rear panel 1 pps OUT via a balun T5 The 1 pps output is intended to drive 50 O loads to 2 5 VDC requiring 50 mA of current during the 10 us 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 330 uF capacitor C45 and a 1 uF capacitor C44 the system s 5 VDC supply is isolated by a 1 KQ resistor R35 and the balun T5 requires that all of the output current returns to the local ground The PRS10 can receive serial data on Pin 7 and transmit serial data on Pin 4 The serial interface uses 8 bits at 9600 baud between OV amp 5 V with one stop bit no parity and X on X off protocol Conveniently the default baud rate for communications between the SIM900 Mainframe and installed SIM mod
30. ion An asynchronous serial interface allows for direct communication with internal PRS10 frequency standard via the SIM900 mainframe This interface is used for frequency calibration 1pps timing and status determination A Windows application program is provided The SIM940 will time tag the rising edge of a user supplied 1pps input which usually comes from a GPS receiver The module can phase lock to this input to cancel long term aging The time tag results may be read back via the serial interface Timing is referenced to the leading edge of the 1 pps output pulse The 1pps output pulse will be moved into coincidence with the user supplied 1 pps input after 256 input pulses Thereafter the SIM940 will adjust its frequency to maintain the phase lock between the 1 pps input and the lpps output The phase lock algorithm uses an adjustable natural time constant from 8 minutes to 18 hours which may be adjusted via the serial interface Several thousand PRS10 s have been operating in various OEM appli cations with a demonstrated MTBF several hundred thousand hours and so a malfunction of the PRS10 is very unlikely In the event of a failure of the physics package 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 SIM940 will find application as an autonomous frequency stan dard in virtually any calibration or R amp D laboratory where precision time or freque
31. m range 2 x 107 0 to 5 VDC 0 5 ppm remote interface Warm up time 6 minutes time to lock lt 7 minutes time to 1 x 10 ASRS SIM940 Rubidium Frequency Standard General Information vii Interfaces Front panel LEDs Rear Panel BNCs Operating General Characteristics Locked Indicates frequency is lock to rubidium Unlocked Indicates frequency is unlocked 1 pps input Blinks with each 1 pps reference input applied to rear panel 1 pps sync Frequency Adjust Indicates 1 pps output is synchronized within 1 us of 1 pps input 0 to 5 VDC adjusts frequency by 0 002 ppm 1 pps input 100 kQ input Requires CMOS level pulses 0 to 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 time constant from 8 minutes to 18 hours 1 pps output 50 Q pulse output 10 MHz outputs Three 50 ohm isolated 10 MHz sine outputs SIM DB 15 male Temperature power and communications 10 C to 40 C Temperature stability A f f lt 1 x 10 19 410 C to 40 C Storage temperature 55 C to 85 C Magnetic field A f f lt 2 x 10 for 1 Gauss field reversal Relative humidity 95 non condensing Interface Serial RS 232 through SIM interface Power 24 VDC Supply current 2 2 A at start up 0 6 A after warm up period Weig
32. mes know as a hydra cable The pin connections are given in Table 1 2 DB 15 F to SIM940 Name DB 9 F 10 3 TxD 11 2 RxD 5 Computer Ground to P S 15 424VDC 1 89 Ground P S return current Table 1 2 SIM940 Direct Interface Cable Pin Assignments The serial port settings at power on are 9600 baud 8 bits no parity 1 stop bit and no flow control The serial settings cannot be changed on the SIM940 Although the serial interface lines on the DB 15 do not satisfy the minimum voltage levels of the RS 232 standard they are typically compatible with desktop personal computers ASRS SIM940 Rubidium Frequency Standard 2 Operation In This Chapter Direct local control of SIM940 is described 2 1 2 2 2 3 2 4 2 5 2 6 Powis naa Soi HS GR ee eh eee d ES 2 2 Watm up o dg e ee 2 2 Connection to other instruments 2 2 Connection to SRS Instruments 2 3 2 4 1 SIM900 Mainframe 2 3 2 4 2 SR620 Time Interval Counter 2 3 2 4 3 DS345 Synthesized Function Generator 2 3 2 4 4 DG535 Digital Delay Generator 2 4 Direct Frequency Control of the SIM940 2 4 Using an External 1pps Reference 2 5 Operation 2 2 2 1 Power 2 2 Warm up The module is powered from the SIM900 mainframe which accom modates any voltage in the ranges 90 VAC to 260 VAC with a fre quency in the range of 47 Hz to 63 Hz In most applica
33. ncy measurements are made or in any other situations requiring an accurate frequency standard The low cost of the unit together with its extremely low aging will allow customers to deploy precision frequency references without the need to install cables or ASRS SIM940 Rubidium Frequency Standard 1 2 Quick Start Guide 1 3 antennas 1 2 Quick Start Guide 1 Turn off the SIM900 mainframe 2 Install the SIM940 in the SIM900 mainframe Use slots 7 amp 8 if available for best thermal environment and shortest connection for 10 MHz to SIM900 timebase input 3 To lock the SIM900 mainframe timebase to the SIM940 Rubid ium Frequency Standard connect a SIM940 10 MHz output to the SIM900 mainframe Timebase input with a short BNC cable 4 Turn on the SIM900 mainframe 5 The SIM940 should indicate Locked within six minutes SIM940 Rubidium Frequency Standard fs RS Overview IM940 10 MHz Rubidium Frequency Standard RUBIDIUM OSCILLATOR B Unlocked Wi Locked EXTERNAL 1 PPS SYNCHRONIZATION W 1 PPS Input B 1 PPS Sync Figure 1 1 The SIM940 front and rear panels SIM940 Rubidium Frequency Standard 1 3 SIM Interface 1 3 SIM Interface 1 3 1 The primary connection to the SIM940 Rubidium Frequency Stan dard is the rear panel DB 15 SIM interface connector Typically the SIM940 is mated to a SIM900 Mainframe via this connection either through one o
34. ncy standard used in the calibration must have an accuracy of better than 1 x 10 11 or 10 MHz 0 0001 Hz A cesium standard or a well characterized rubidium or quartz standard referenced to GPS may be used The frequency counter must have sufficient resolution and ac curacy 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 Inter val Counter in the frequency mode Usually the measurement can be done more quickly and ac curately by comparing the phase of a 10 MHz reference to the phase of the SIM940 10 MHz output A time interval counter may be used for this purpose In this case the 10 MHz reference is used asa 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 SIM940 The time 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 1ns per second then the two 10 MHz source are off by 1 x 10 or 0 001 ppm Adjus
35. ne week it should be operated continuously for at least 72 hours before the adjust ment is made Apply the 1 pps signal to the rear panel of the SIM940 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 SIM940 should be phase locked for at least 24 hours before pro ceding to the next step Query the SF parameter via the computer interface 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 1012 so 2000 corresponds to 0 002 ppm To set the SIM940 to 1 1011 you want to determine the correct value of SF to 10 so check back SIM940 Rubidium Frequency Standard fs RS Calibration in a few hours to determine if the SF parameter has settled Repeat as necessary 13 When you have a satisfactory SF value unplug the 1 pps input and power cycle the SIM940 to return control of the SF param eter to the potentiometer While monitoring the SF parameter via the computer interface adjust the calibration potentiometer to set the SF value to the value seen when the unit was phase locked to the 1pps reference Now each time the unit turns on it will be operating at the same frequency as when it did when it was phase locked to the 1 pps reference 4 3 Calibration of the time tag offset parameter The SIM940 can time tag the leading edge of a rear
36. re RbMon is documented For further details on the programming the internal PRS10 Rubidium Frequency Standard see the PRS10 Operation and Service Manual 3 1 Setting up communications cc 3 2 32 Getting connected 5 asa sco i SS Ba hen 3 2 3 3 Testing connection cc eea 3 3 3 4 Connecting to the SIM940 3 3 3 5 Terminal program 00200 eee 3 4 46 Rubidium Stats Lxx eR IR aaa 3 5 3 6 1 Factory Settings os oa ke a ER S E 3 6 3 6 2 Analog Output Values 3 7 3 6 3 Lppscontroltab 4 v ao ae eee 3 7 3 6 4 Colored boxes u d RUE Ses 3 7 3 7 RbMon MENUS v ew ve ee en eR ee eee 3 8 37 1 Fileme nu zus 3 8 372 Helpimeii S fe Aa Sad es ER I DEE 3 8 Monitor and Control A Windows based program RbMon is provided on a 31 2 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 SIM940 Rubidium Frequency Standard via the computer s COM port over a serial RS 232 interface to the SIM900 mainframe 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 re quirements include 2150 MHz Pentium or comparable Windows 95 or later and an available COM port The defaul
37. rs and will thereafter indicate that it is properly synchronized 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 Prop Gain Natural Time Constant Constant Parameter set by hours SF bits per SFbitspernsof Characterizes PT command hour per ns of time tag PLL response time tag 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 Table 2 1 Phase lock loop parameter values Further details of the phase locking algorithm are provided in the PRS10 Rubidiumn Frequency Standard manual SIM940 Rubidium Frequency Standard fs RS Operation ASRS SIM940 Rubidium Frequency Standard 3 Monitor and Control In This Chapter This chapter introduces the remote interface to the SIM940 The provided Windows softwa
38. t sdi Delay Value adi 2 240 24 Volt Heat TS 1467 Time Slope sd2 Fet Voltage Set ad2 2 260 24 Volt Elec PS 192 Pulse Slope sd3 Lamp Temp Set ad3 0 487 Lamp Drain PL 1 Phase Lock sd4 Crystal Temp Set ad4 0 298 Lamp Gate BF 14 Time Constant sd5 Cell Temp Set B acd 4 583 Crystal Heat Ctrl PF 2 Stability Factor sd6 Output Yoltage Bi ad6 4548 Cell Heat Ctrl PI D Integral Term RF Modulation B ad 3 028 Lamp Heat Ctrl LM 1 Lock Mode SP Param PLL ads 2 528 AC Photosignal TT Last Valid Tag ad3 2 588 Photocell adi 0 367 Case Temp Set Frequency Bb adi 1 401 Xtal Thermistors Set Slope W adi2 1 192 CallThermistors Mag Offset N adi3 1 531 Lamp Thermistors Mag Read adi4 2506 Frequency Pot Mag Switching ad15 0 003 Analog Ground Lock Madi6 0 020 22MHz Varactor Gain adi 2 840 360MHz Varactor Phase adl8 1 830 Auto Gain Ctrl Enable Power W di9 3810 RFLock 10MHz DAC Signal Yalues The screen is divided into three main sections labeled Factory Set tings 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 high light 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 m
39. t 1pps phase locking is disabled 4 Connect the 1 pps output directly to the 1 pps input with a 3 RG58 coax cable Do not use any 50 O terminators ASRS SIM940 Rubidium Frequency Standard 43 Calibration of the time tag offset parameter 4 7 5 11 Query the time tag with the TT command If the result is 5 ns which is the time delay of 3 of RG58 coax cable then you are done If the result of the TT is not 5ns then query the TO parameter with a TO A typical value of the TO parameter is 1700 ns This value is added to the time tag measurement before is it returned via the serial interface If the 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 15 ns and TO returned 1700 then send TO 1719 If the result of the TT less greater than 5ns then decrease the magnitude of the TO parameter making it less negative For example if TT returned Ins and TO returned 1700 then send TO 1696 Iterate as necessary adjusting TO until the TT returns 5 ns 10 Save the TO parameter to EEPROM by sending the command TO Verify that the TO was saved to EEPROM by reading it back from EEPROM with the TO command SIM940 Rubidium Frequency Standard fs RS Calibration ASRS SIM940 Rubidium Frequency Standard 5 SIM940 Circuit Description This chapter presents a brief description of
40. t COM port is COM1 If your computer uses COM1 as a mouse port you will need to change the COM port to COM2 The SIM940 uses the PRS10 rubidium frequency standard also man ufactured by Stanford Research Systems All commands sent to the SIM940 are passed directly to the PRS10 All commands received from the SIM940 come directly from the PRS10 No buffering or filtering of commands is performed by the SIM940 The PRS10 oper ation and service manual which is an addendum to this document serves as a programming reference for the SIM940 3 1 Setting up communications 3 2 Getting connected The PRS10 frequency standard can communicate operating and cali bration settings 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 con trol the operation of the PRS10 Communicating with the PRS10 is not required in most applications The PRS10 comes from the factory calibrated to 5 x 10 and the SIM940 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 PRS10 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 The
41. t the calibration potentiometer until the drift between time interval measurements slows to less than 1 x 1071 or 10 ps per second Measure and record the DC voltage on the center pin of the FREQ CTL IN BNC SIM940 Rubidium Frequency Standard fs RS 4 4 Calibration 4 2 Calibration with a 1 pps reference such as from a GPS receiver The SIM940 may also be calibrated by phase locking the unit to the 1 pps signal from a GPS receiver The calibration potentiometer will be adjusted 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 SIM940 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 4 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 Fre quency Normally the SF parameter is set by the firmware rea
42. tions and for best performance the mainframe should be left on at all times The rubidium frequency standard is operating and outputs are available whenever the mainframe is on The unit may also be powered by a user supplied 24 VDC power supply capable of providing 2 2 A during warm up and about 0 6 A continuously The 24 VDC should be applied to pin 15 and ground should be applied to pins 1 8 amp 9 see section 1 3 2 1 The Rubidium Oscillator status LED either Unlocked or Locked will be on whenever power is applied to the unit The status will change from Unlocked to Locked within six minutes after power is applied if the module is started from room temperature Warm up and locking will take longer if the module is started from a lower temperature Locked indicates the unit is warmed up and its crystal oscillator has been locked to the rubidium physics package The output frequency will be about 200 ppm below nominal when power is first appllied to a cold module The frequency will grad uatlly converge on 10 MHz as the module 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 module will be locked to within 0 001 ppm of 10 MHz 2 3 Connection to other instruments All of the outputs on the rear of the SIM940 have a source impedance of 50 Q and are intended to drive 50 Q loads Since the
43. to it After applying a 10 MHz sine to the rear panel time base input a front panel indicator will turn on SIM940 Rubidium Frequency Standard fs RS 2 4 Operation 2 4 4 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 SIM940 The user should verify proper operation of the DG535 when locked to an external time base with an SR620 Time Interval Counter 2 5 Direct Frequency Control of the SIM940 The frequency of the rubidium hyperfine transition is adjusted via a small magentic 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 linearized the adjustment The microcontroller reads the position of a calibration potentiometer or responds to software com mands via the serial interface to set the magnetic field which controls the operating frequency The operating frequency may be directly controlled over the range of 0 002 ppm in four ways 1 By the adjustment of a 15 turn potentiometer accessible from the side of the module with the module cover removed Turn ing the potentiometer clockwise will increase the frequency by about 0 0025 Hz per turn The adjustment procedure is detailed in the
44. ules is 9600 baud The SIM940 is powered from the 24 VDC from the SIM900 main frame An L C filter L1 amp C1 is used to reduce ripple from the mainframe s power supply A p channel MOSFET Q100 is used to slow the inrush of the power supply to prevent a current trip in the mainframe as it charges C1 Three terminal regulators U1 and U2 provide 20 VDC and 5 VDC for 10 MHz output buffers and logic circuits SIM940 Rubidium Frequency Standard fs RS 5 4 Circuitry 5 1 6 10MHz 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 41 Vpp with very low phase noise The SIM940 s 10 MHz output drivers were designed to maintain the low phase noise of the PRS10 s output and to reduce the harmonic distortion with high output to output isolation The output amplifiers LM7171 s U7 U8 amp U9 have high band width low distortion low current noise but rather high voltage noise 14nV VHz A tuned transformer T1 is used to increase the voltage and source impedance of the PRS10 s 10 MHz output to avoid degrading the output noise The 10 7 MHz transformer is tuned down to 10 MHz by an additional capacitor on the primary side of the transformer The transformers have a 47 pF capacitor internally The 3 5 1 transformer increases the 10 MHz amplitude to about 5 Vpp the source impedance to about 600 O and reduces harmonic distortion A 10 VDC bias
45. ver2 4 Response from SIM 900 conn 7 x Command sent to SIM900 to connect to Slot 7 SIM940 PRS 10 power on message from SIM940 PRS10 id Command sent to SIM940 PRSIO PRS10 3 24 SM 15830 Response from SIM940 PRSTO 3 5 Terminal program 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 ENTER 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 over written Only the last line is available for typing in new commands SRS SIM940 Rubidium Frequency Standard 3 6 Rubidium Stats 3 5 Here is a sample screen shot pointing out the pertinent items of interest II Rubidium Communication COM2 inl x File Help R5232 Communications Rb Stats Terminal Communications Port C
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