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Model PRS10 Rubidium Frequency Standard

1. 0 O GAS 8 002 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 GA will write 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 0 lt 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 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 PRSIO Rubidium Frequency Standard 28 RS 232 Instruction Set and restart and may be queried by the PH command Example PH could return a typical value of 24 Frequency Synthesizer Control A frequency synthesizer which uses the 10 MHz OCXO as a frequ
2. 1PPS input re synchronized to 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 1 input and the next E CLK Measuring the duration of INTERPOLATE allows the position ofthe I PPS input to be measured to about 400ns 2000 0 2ns PRSIO Rubidium Frequency Standard 52 Circuit Descriptions Schematic RB F4 Sheet 4 of 7 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 noise 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
3. 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 command 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 8 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 PF PF value 0 lt value lt 4 value C 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 G of the 1 PLL The stability factor is equal to 201 The default value is 2 which provides a stability PRS10 Rubidium Frequency Standard 36 RS 232 Instruction Set factor of 20 2 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 i
4. The varactor for the crystal is tuned by the dual modulus frequency synthesizer U400 an MC145190 which compares the divided reference 10 00MHz 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 sensitivity 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 50Q source U404 the VCO and to the 500 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
5. 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 Appendix A details the values for R N and A for the range of frequencies needed SP SP R N A 1500S 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 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 PRS10 Rubidium Frequency Standard RS 232 Instruction Set 29 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 co
6. 01059 462 249 Thin Film 1 50 ppm MELF Resistor R 250 4 01479 461 1 0K R 252 4 01479 461 1 0K R 256 4 01503 461 10K Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor R 261 4 01355 462 301K Thin Film 196 50 ppm MELF Resistor R 262 4 01355 462 301K Thin Film 196 50 ppm MELF Resistor R 263 4 01309 462 100K Thin Film 1 50 ppm MELF Resistor R 264 4 01309 462 100K Thin Film 1 50 ppm MELF Resistor R 265 4 01098 462 634 Thin Film 1 50 ppm MELF Resistor R 266 4 01309 462 100K Thin Film 1 50 ppm MELF Resistor R 267 4 01309 462 100K Thin Film 1 50 ppm MELF Resistor R 268 4 01405 462 1 00M Thin Film 1 50 ppm MELF Resistor R 269 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 270 4 01347 462 249K Thin Film 1 50 ppm MELF Resistor R271 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 272 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 273 4 01455 461 100 Thick Film 5 200 ppm Chip Resistor R 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 R277 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R278 4 01305 462 90 9K Thin Film 1 50 ppm MELF Resistor R279 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 281 4 01213 462
7. 01213 462 4 01405 462 4 01242 462 4 01146 462 4 01088 462 4 01230 462 4 01117 462 4 01251 462 4 01447 461 4 01479 461 4 01242 462 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 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 VALUE 47 1 0M 22 249 100 49 9K 10 0K 10 10 10 0K 10 0K 100K 100K 249K 49 9K 1 0K 249 3 32K 2 2K 10 0K 1 00M 20 0K 2 00K 499 15 0K 1 00K 24 9K 47 1 0K 20 0K 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 47 DESCRIPTION PRS10 Parts List 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 Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm Thin Film 1 50 ppm MELF Resistor MELF Resistor Thick Film 5 20
8. 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 Film 1 50 ppm MELF Resistor PRS10 Rubidium Frequency Standard PRS10 Parts List 71 REF SRS PART VALUE R 288 4 01294 462 69 8K R 289 4 01280 462 49 9K R 290 4 01479 461 1 0K R291 4 01575 461 10M R 292 4 01479 461 1 0K DESCRIPTION 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 R 293 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 294 4 01251 462 24 9K Thin Film 196 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 R 304 4 01479 461 1 0K R 305 4 01527 461 100K Thick Film 5 200 ppm
9. 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 5 003 18 569 5 003 18 569 5 00387 552 5 003 18 569 5 003 18 569 5 003 18 569 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 5 00487 574 5 00479 574 5 00479 574 5 00487 574 3 00803 360 3 00648 360 3 00648 360 3 00538 360 VALUE 1000P 01U 4 7P 2 7P 10 128 18 10 10 10 01U 10 2 2U T35 2 2U T35 1000P 2 2U T35 2 2U T35 2 2U T35 1000P 2 10008 10 120P 10 1000 10 10 10 10 10 10 10 10 10 5 6P 500V 10 10 1000 1000 10 1000 2 20 68 500V 18 500V 18P 500V 68P 500V MMBV609 MBRD660CT MBRD660CT MMBD352L PRS10 Parts List DESCRIPTION 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 Capacitor
10. 596 All Sizes C202 5 00299 568 LU ap Ceramic 50V SMT 1206 10 X7R C 203 5 00387 552 1000P apacitor Chip SMT1206 50V 5 NPO C 204 5 00466 572 1U SMT Film Capacitors 50V 5 Sizes C C C 205 5 00299 568 AU Cap Ceramic 50V SMT 1206 10 X7R C 206 5 00387 552 1000P Capacitor Chip SMT1206 50V 5 NPO C 207 5 00466 572 LU SMT Film Capacitors 50V 5 All Sizes C 208 5 00299 568 LU Cap Ceramic 50V SMT 1206 10 X7R C210 5 00387 552 1000P Capacitor Chip SMT1206 50V 5 NPO PRSIO Rubidium Frequency Standard 66 PRS10 Parts List REF C212 C216 C217 C 218 C219 C 220 C 221 222 C 223 C 224 C 226 227 228 229 C 230 301 302 304 306 308 C 309 310 C311 C 400 C 401 C 402 C 403 C 404 C 405 C 406 C 407 C 408 C 409 C 410 411 413 414 415 416 417 418 419 C 420 421 422 423 424 425 426 427 SRS PART 5 00299 568 5 00387 552 5 00466 572 5 00299 568 5 00375 552 5 00466 572 5 00454 572 5 00454 572 5 00299 568 5 00299 568 5 00466 572 5 00466 572 5 00299 568 5 00299 568 5 00299 568 5 00299 568 5 00299 568 5 00299 568 5 00299 568 5 00298 568 5 00375 552 5 00375 552 5 00472 569 5 00299 568 5 00299 568 5 00387 552 5 00387 552 5 00299 568 5 00299 568 5 00387 552 5 00466 572 5 00298 568 5 00466 572 5 00462 572 5 00299 568 5 00299 568 5 00373 552 5 00375 552 5 003
11. Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor R 324 4 01249 462 23 7K Thin Film 1 50 ppm 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 331 4 01073 462 348 Thin Film 1 50 ppm MELF Resistor R 332 4 01117 462 1 00K Thin Film 1 50 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 R 337 4 01455 461 100 R 338 4 01503 461 10K R 339 4 01479 461 1 0K R 340 4 01479 461 1 0K R 341 4 01503 461 10K R 342 4 01479 461 1 0K R 343 4 01503 461 10K R 344 4 01464 461 240 R 345 4 01464 461 240 R 346 4 01464 461 240 R 347 4 01464 461 240 R 348 0 00000 000 UNDECIDED PART R 349 4 01464 461 240 R 350 4 01464 461 240 R351 4 01464 461 240 R 352 4 01464 461 240 R 353 0 00000 000 UNDECIDED PART R 354 4 01493 461 3 9K R 355 4 01527 461 100K R 356 4 01493 461 3 9K R 357 4 01527 461 100K R 358 0 00000 000 UNDECIDED PART Thick Film 5 200 ppm Chip Resistor Thick Film 596 200 ppm Chip Resistor Thick Film 596 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 596 200 p
12. EEPROM see the SP and MO commands The command sets the frequency offset in units of parts in 10 corresponding to a frequency resolution of 10 wHz 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 external frequency control voltage an SF command or from the external 1 phase lock loop control algorithm However SF set command is ignored if the unit is phase locked to an external Ipps 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 is 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 b
13. Film 5 200 ppm Chip Resistor R219 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 220 4 01376 462 499K Thin Film 1 50 ppm MELF Resistor R221 4 01230 462 15 0K Thin Film 1 50 ppm MELF Resistor PRS10 Rubidium Frequency Standard 70 PRS10 Parts List REF SRS PART VALUE DESCRIPTION R 222 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 223 4 01230 462 15 0K Thin Film 1 50 ppm MELF Resistor R 224 4 01278 462 47 5K Thin Film 1 50 ppm MELF Resistor R 226 4 01238 462 18 2K Thin Film 1 50 ppm MELF Resistor R 227 4 01305 462 90 9K Thin Film 1 50 ppm MELF Resistor R 228 4 01363 462 365K Thin Film 1 50 ppm MELF Resistor R 229 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 230 4 01557 461 1 8M Thick Film 5 200 ppm Chip Resistor R231 4 01575 461 10M Thick Film 5 200 ppm Chip Resistor R 232 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 233 4 01309 462 100K Thin Film 1 50 ppm MELF Resistor R 234 4 01376 462 499K Thin Film 1 50 ppm MELF Resistor R 235 4 01251 462 24 9K Thin Film 1 50 ppm MELF Resistor R 236 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 237 4 01251 462 24 9K Thin Film 1 50 ppm MELF Resistor R 238 4 01278 462 47 5K Thin Film 1 50 ppm MELF Resistor R 240 4 01479 461 1 0K Thick Film 5 200 ppm Chip Resistor 241 4 01117 462 1 00K Thin Film 1 50 ppm MELF Resistor R 248 4 01439 461 22 Thick Film 5 200 ppm Chip Resistor R 249 4
14. SOURCE to set the GATE 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 in 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 PRSIO 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 ofthe 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 10MHz oven oscillator front vertical PCB Crystal heater PCB front vertical Analog servos an
15. 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 R 153 4 00899 43 1 P1H104 T NTC Thermistor various R 154 4 00899 43 1 P1H104 T NTC Thermistor various R 200 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 201 4 01305 462 90 9K Thin Film 1 50 ppm MELF Resistor R 202 4 01295 462 71 5K Thin Film 1 50 ppm MELF Resistor R 203 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 204 4 01557 461 1 8M Thick Film 5 200 ppm Chip Resistor R 205 4 01575 461 10M Thick Film 5 200 ppm Chip Resistor R 206 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 207 4 01376 462 499K Thin Film 1 50 ppm MELF Resistor R 208 4 01230 462 15 0K Thin Film 1 50 ppm MELF Resistor R 209 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R210 4 01230 462 15 0K Thin Film 1 50 ppm MELF Resistor R211 4 01278 462 47 5K Thin Film 1 50 ppm MELF Resistor R213 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R214 4 01305 462 90 9K Thin Film 1 50 ppm MELF Resistor R215 4 01309 462 100K Thin Film 1 50 ppm MELF Resistor R216 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R217 4 01557 461 1 8M Thick Film 5 200 ppm Chip Resistor R218 4 01575 461 10M Thick
16. 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 frequency offset is quadratic in the field strength with a fractional frequency resolution of about 1x10 at 3mA and of 2 5x10 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 PRSIO Rubidium Frequency Standard Circuit Descriptions 51 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 rate 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 P
17. 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 Examples VB0 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 ofthe PRS10 Example ID will return the identification string PRSIO 3 15 SN 12345 model firmware version serial number PRSIO 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 1PPS output pin 1 on the main connector J100 The LOCK IPPS pin may be configured per the fo
18. 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 an 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 7 Resonance Cell and Lamp Heaters The heater and control circuits for the lamp and resonance cell are identical to the circuit described for the crystal oscillator See Sheet 1 of 1 The resonance cell heaters U600 and Q600 are located on the back of the resonance cell The lamp heaters U800 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
19. 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 on 24 CLEAN and 24 HEAT The supplies are filtered by L104 and L105 ferrite beads with about 34H and Q 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 10MHz the network L100 L101 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
20. 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 installing a 10 kQ resistor for another R353 on the microcontroller PCB Pin 5 IPPS IN PHOTO The default configuration uses this pin as a 1 input to allow time tagging or phase locking to an external 1pps 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 All of the pins on the interface connector are protected against continuous connection to any potential up to 24 Vdc 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 conne
21. diagram of this PLL is shown in Figure 3 The phase detector is the time tagging circuit and firmware which has a gain of Kae 1bit ns The loop filter is a digital filter consisting of an optional pre filter and a standard proportional integral controller PRSIO 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 d Time tag circuit Figure External Ipps Phase Locking Block Diagram The PI controller is programmed by choosing an appropriate integrator time constant and a stability factor 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
22. followed by a question mark or ASCII 6310 is 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 used 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 PRSIO Rubidium Frequency Standard 6 Abridged Command List Set Value or Write Query
23. 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 operating 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 15 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 1005 source from a GPS or LORAN receiver for example by applying a 1pps pulse to the 1 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
24. 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 I PPS outputs and to receive the IPPS input A DB9 female allows direct connection to a computer usually via COM2 A 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 a 50Q load The output will be about 0 5Vrms about 1 41 Vpp The RS 232 interface uses CMOS logic levels OV and 5V 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 5 V 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 CD DSR and CTS are all pulled high via 10kQ resistors An XON XOFF protocol is used to pause communications as needed PRSIO Rubidium Frequency Standard Circuit Descriptions 59 The LOCK IPPS function may be configured via RS 232 The factory default is a low level to indicate lock with a 10us pulse to 5V at 1PPS 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 PRSIO Rubidium Frequency S
25. 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 1 input The time constant for pot adjustments depend on the setting ofthe frequency lock loop gain see GA 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 PC COM 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 PRSIO Rubidium Frequency Standard 42 Circuit Descriptions Circuit Descriptions Schematic RB F1 sheet 1 of 7 Components shown on this schematic are located 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
26. 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 1 PLL and is in the range 2000 lt SF lt 2000 SLOPE 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 Example MR would return a value of 3450 if the magnetic offset is at 3000 the SF command requested an offset of 2000 x 107 and the SS CAL factor has the nominal value of 1450 PRSIO Rubidium Frequency Standard RS 232 Instruction Set 31 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 10 All of the channels for calibrating the unit are linearized so t
27. of 20Vdc the drain will have about 40V peak to peak so there will be a circulating current of 2 94 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 0 030 for the 56pF part at 1SOMHz 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 quench 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 Schematic RB F7 Sheet 7 of 7 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
28. 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 1s required so as not to degrade the signal 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 PRSIO Rubidium Frequency Standard 12 PRS10 Overview 22 bit DAC 10MHz Low Noise Matching 10 bit Ovenized Oscillator Transformer C 500 Output DE x gt thesizer Reference Dual Modulus 22 4825MHz VCXO Synthesizer 359 72MHz Af 1E 9 step 1 359 72MHz Temperature Controlled 150MHz Lamp Oscillator wes 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 nat
29. 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 Ipps PLL algorithm will reduce the PRS10 s frequency of operation by adjusting its SF parameter to eliminate the 100ns phase shift between the 1 pps reference input and the 1 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 pps output to the 1 pps input The table below shows the integral and proportional gain terms For the nominal PT value of 8 the integral PRSIO Rubidium Frequency Standard RS 232 Instruction Set 35 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 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 1pps input and 1 output will be gradually eliminated Phase jumps of 100ns are quite common on 1pps outputs from GPS receivers which are a likely 1 reference to the PRS10
30. the 19th harmonic ofthe 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 A 39 The microwave frequency is generated by frequency multiplication ofthe 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 low noise flywheel We only need to tune over a range of 0 1ppm to accommodate for fill pressure variations of 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 48MHz 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 PRSIO Rubidium Frequency Standard Circuit Descriptions 53
31. 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 1pps inputs have been qualified 129 both the lamp and unit have been restarted STI Power supplies and Discharge Lamp STI bit Condition which sets bit Corrective Action 0 424 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 6 Gate voltage too low check SD2 setting PRSIO Rubidium Frequency Standard RS 232 Instruction Set 39 ST2 RF Synthesizer ST2 bit Condition which sets bit Corrective Action 0 RF synthesizer PLL unlocked Query SP verify values RF crystal varactor too low Query SP verify values RF crystal varactor too high Query SP verify values 6 _ RFAGC control too high SD0 values ST3 Temperature Controllers ST3 bit Condition which sets bit Corrective Action O _ Lamp temp below set point Wait for warm up 6 Case temperature toolow for warm up ST4 Frequency Lock Loop Control ST4 bit Condition which sets bit Corrective Action 0 Frequency lock
32. 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 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 21 10 maintains the low noise inherent to the SC cut resonator yet the full scale range is sufficient to compensate for crystal aging over the lifetime ofthe 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
33. 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 drive level with a shunt capacitor about IpF 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 is 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 PRSIO Rubidium Frequency Standard 54 Circuit Descriptions 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 Co
34. 0 ppm Chip Resistor Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor Thick Film 5 200 ppm Chip Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm MELF Resistor MELF Resistor MELF Resistor MELF Resistor Thick Film 5 200 ppm Chip Resistor Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm Thin Film 1 50 ppm MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor MELF Resistor Thick Film 5 200 ppm Chip Resistor 73 PRS10 Rubidium Frequency Standard 74 PRS10 Parts List REF R 548 R 550 R 551 R 552 R 553 R 600 R 601 R 602 R 603 R 604 R 605 R 800 R 801 R 803 R 804 R 900 RX249 T 100 100 101 102 150 200 201 202 20
35. 1000s r Thus the PRS10 provides natural time constants ranging from 506 seconds to 18 0 hours While the integrator time constant t determines the natural time constant v it is the natural time constant which characterizes the loop response 2 The natural time constant is given by 7 Kia vco The PRS10 accepts stability factors ranging from 0 25 to 4 0 in powers of 2 The default value of 1 0 corresponds to a critically damped response lt 1 0 and gt 1 0 correspond to under damped and over damped responses respectively PRSIO Rubidium Frequency Standard 16 PRS10 Overview With t and specified the proportional gain Aj of the controller is given by the equation A 26 K 26 0 0015 With the default time constant ti of 65 536 seconds and a stability factor G 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 160 The PRS10 also provides an optional pre filter The pre filter is enabled by default but it can be disabled by sending the command LMO 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
36. 2 SD2 value SD2 SD2 Set DAC lamp intensity SD3 SD3 value SD3 SD3 Set DAC lamp temperature SD4 SD4 value SD4 SD4 Set DAC crystal temperature SD5 SD5 value SDS 05 Set DAC cell temperature SD6 SD6 value SD6 SD6 Set DAC 10 MHz amplitude SD7 SD7 value SD7 SD7 Set DAC RF deviation Analog Test 12 bit values Spare 7204 ADI 24V heater supply 10 AD2 24V electronics supply 10 AD3 Drain voltage to lamp FET 10 AD4 Gate voltage to lamp FET 10 ADS 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 11 Crystal thermistors AD12 Cell thermistors AD13 Lamp thermistors AD14 Frequency calibration pot AD15 Analog ground Analog Test 8bit values AD16 VCXO varactor voltage 17 varactor voltage AD18 AGC for RF AD19 RF PLL lock signal PRS10 Rubidium Frequency Standard 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 electroni
37. 201 1 s for the signal at its PRSIO Rubidium Frequency Standard Circuit Descriptions 45 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 30kO 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 0 995 So the servo will settle when the temperature of the block increases by 0 9 3 0 3 C There is a small temperature offset between the temperature sensor and the device whose temperature we wish to control Since the 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 compensat
38. 32 interface which allows closed case calibration of the PRS10 This capability may also be used to servo the 10 MHz or 1005 outputs to another frequency or time source in a system For example this would allow the PRS10 to be locked to the 1 from a GPS receiver with a long time constant to eliminate aging PRSIO 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 or the FRS N 1A8A4B 10 MHz sine output 55 C to 65 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 Ipps output and 1pps input and a for the 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 RG174 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 Descriptio
39. 4 SPLIT 6 LOCK 4 SHOULDER TO 220 4 40X1 4 SOCKET 4 40X1 4 BUTTON 4 40X1 2 SOCKET 6 32X1 4 BUTTON 6 32X5 8 SOCKET FOIL CU 1 2 22 INSULATING 4 40X3 16 M F 4 40X3 8 M F 4 40X3 16PF UND 4 40X1 4PF UNDR 34AWG MAGNET S S SEAMLESS PROTO MATERIAL CU TUBING 1 16 64 PIN STRIP 64 HDR PIN R A PHOTODIODE RB 10 RB 1 RB 4 RB 6 RB 7 RB 9 PRS10 Parts List DESCRIPTION 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 Capacitor Chip SMT0805 50V 10 NPO Crystal Crystal Nut Mini Washer Split Washer lock Washer nylon Insulators Screw Allen Head Screw Allen Head 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 C
40. 447 461 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 MELF 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 PRS10 Rubidium Frequency Standard R 448 R 449 R 450 R 500 R 501 R 502 R 503 R 504 R 505 R 506 R 507 R 508 R 509 R 510 R511 R 512 R 513 R 514 R 515 R 516 R 517 R 518 R 519 R 520 R 521 R 522 R 523 R 524 R 525 R 526 R 527 R 528 R 529 R 530 R531 R 532 R 533 R 534 R 536 R 537 R 538 R 539 R 540 R 541 R 542 R 543 R 544 R 545 R 546 R 547 SRS PART 4 01447 461 4 01551 461 4 01439 461 4 01059 462 4 01021 462 4 01280 462 4 01213 462 4 00925 462 4 00925 462 4 01213 462 4 01213 462 4 01309 462 4 01309 462 4 01347 462 4 01280 462 4 01479 461 4 01059 462 4 01167 462 4 01487 461 4
41. 5 206 207 208 209 210 300 301 303 304 306 307 308 309 310 311 312 313 314 400 401 402 403 404 405 cee GO G Gm ul Se lt lt eco SRS PART 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 5 00375 552 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 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 00652 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 VALUE 390 1 00M 1 0K 10 10 1 1 1 P1H104 T NTC P1H104 T NTC 1 1 1 P1H104 T NTC P1H104 T NTC 10K 100P 10 7 MHZ AD587JR LM7171AIM LM358 7812 LMC662C LMC662C LMC662C AD8561AR OP284FS LMC662C 74HC4051 74HC4051 AD822 MAX705CSA 74HC14 74HC08 74HC14 DG211BDY LM358 OP284FS AD822 LTC1452CS8 LTC1452CS8 LTC1452CS8 LTC1452CS8 AD7896AR MC145193F AD822 SA602D OP27GS 380 MHZ MC12026AD PRSIO Rubidium Frequency Standard DESCRIPTION Thick Film 596 200 ppm Chip Resistor Thin Film 196 50 ppm MELF Resistor Thick Film 5 200 ppm Chip Resistor Thin
42. 53926 0 027455 69 4684 2632 45 0 963357 10 009431 AppendixA 61 Cm N oo en N oo 86 3826 215029 1 242576 0 014111 7223 4059 50 1 333111 0 006369 0 032616 PRSIO 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 1005 from a GPS receiver or a cesium beam standard such HP 5071A 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
43. 7R Cap Ceramic 50V SMT 1206 10 X7R Capacitor Chip SMT1206 50V 5 NPO Capacitor Chip SMT1206 50V 59 NPO Cap Ceramic 50V SMT 1206 10 X7R Cap Ceramic 50V SMT 1206 10 X7R Capacitor Chip SMT1206 50V 59 NPO SMT Film Capacitors 50V 596 All Sizes Cap Ceramic 50V SMT 1206 10 X7R SMT Film Capacitors 50V 596 SMT Film Capacitors 50V 596 All Sizes All Sizes Cap Ceramic 50V SMT 1206 10 X7R Cap Ceramic 50V SMT 1206 10 X7R Capacitor Chip SMT1206 50V 59 NPO Capacitor Chip SMT1206 50V 5 NPO Capacitor Chip SMT1206 50V 5 NPO Capacitor Chip SMT1206 50V 5 NPO Capacitor Chip SMT1206 50V 5 NPO SMT Film Capacitors 50V 5 Sizes SMT Film Capacitors 50V 5 Sizes Cap Ceramic 50V SMT 1206 10 X7R 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 NPO C 428 C 429 C 430 431 C 432 C 433 C 434 C 436 C 437 C 438 C 439 C 440 C 500 C502 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 C 602 700 800 801 802 803 804 805 900 903 904 905 906 D 100 D101 D 102 D 202 SRS PART 5 00387
44. 8 Circuit Descriptions 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 the 10MHz crystal oscillator to start A non maskable interrupt XIRQ is asserted if the SPI clock is 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 18V 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 signal passing the filtered error signal to the 22 bit D A converter and respondin
45. 87 552 5 00387 552 5 00375 552 5 00456 572 5 00456 572 5 00299 568 5 00298 568 5 00299 568 5 00387 552 5 00387 552 5 00299 568 5 00375 552 VALUE 10 1000 10 10 100P 10 01U 01U 10 10 10 10 10 10 10 10 10 10 10 01U 100P 100P 4 70 135 10 10 1000P 1000P 10 10 1000P 10 01U 10 0470 10 10 68 100 1000 1000 100 0150 0150 10 01U 10 1000 1000P 10 100P PRSIO Rubidium Frequency Standard DESCRIPTION Cap Ceramic 50V SMT 1206 10 X7R Capacitor Chip SMT1206 50V 5 NPO SMT Film Capacitors 50V 596 SMT Film Capacitors 50V 596 SMT Film Capacitors 50V 596 SMT Film Capacitors 50V 596 C C MT Film Capacitors 50V 596 MT Film Capacitors 50V 596 S S All Sizes Cap Ceramic 50V SMT 1206 10 X7R Capacitor Chip SMT1206 50V 5 NPO All Sizes All Sizes All Sizes ap Ceramic 50V SMT 1206 10 X7R ap Ceramic 50V SMT 1206 10 X7R All Sizes All Sizes 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 Capacitor Chip SMT1206 50V 5 NPO Capacitor Chip SMT1206 50V 5 NPO Cap Tantalum SMT all case sizes Cap Ceramic 50V SMT 1206 10 X
46. A faster way to make the comparison 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 10ps 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 PRSIO Rubidium Frequency Standard Appendix B 63 Set up for an SR620 Described here is the set up for an SR620 Time Interval C
47. Description Activate EEPROM 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 RC1 RC Recall factory calibration Freq Lock LO LO value Frequency lock loop status FC FC high low FC FC Frequency control values DS Read detected signals and 2 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 rna SP SP Set synthesizer parameters Magnetic Tuning MS MS value Magnetic switching MO MO value MO MO Magnetic Offset MR Magnet read PPS Lock TT Time tag 1105 input TS TS value TS TS Time slope cal 1pps input TO TO value TO TO 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 105 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 7 Query Set Value Write Query Description Value EEPROM EEPROM D A Control SD0 SDO value 00 00 Set DAC RF amplitude SD1 SDl value SD1 SD1 Set DAC 1105 delay SD
48. 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 5 Capacitor Chip SMT1206 50V 5 NPO 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 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 Integrated Circuit Surface Mount Pkg Integ
49. HASE 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 10s pulse at a rate of 1Hz This pulse is combined with the LOCK output signal on the main connector pin 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 E CLK cycle 400ns Hardware on the bottom 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 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 115 1PPS Input Time Tag The rising edge of a 1PPS input signal on pin 5 of the main connector can be time tagged with 1ns resolution The time may reported via RS 232 or used to servo the unit to another frequency standard such as GPS Hardware on the bottom board provides two signals TIME LATCH and INTERPOLATE These signals latch the value ofa free running counter clocked by the E CLK which is part of the microcontroller TIME LATCH is just
50. LF Resistor R 410 4 01309 462 100K Thin Film 1 50 ppm MELF Resistor R411 4 01527 461 100K Thick Film 5 200 ppm Chip Resistor R412 4 01088 462 499 Thin Film 1 50 ppm MELF Resistor R 413 4 01088 462 499 Thin Film 196 50 ppm MELF Resistor R414 4 01088 462 499 Thin Film 1 50 ppm MELF Resistor R415 4 01251 462 24 9K Thin Film 1 50 ppm MELF Resistor R 416 4 01251 462 24 9K Thin Film 1 50 ppm MELF Resistor R417 4 01117 462 1 00K Thin Film 1 50 ppm MELF Resistor 418 4 01117 462 1 00K Thin Film 1 50 ppm MELF Resistor R419 4 01467 461 330 Thick Film 5 200 ppm Chip Resistor R 420 4 01467 461 330 Thick Film 5 200 ppm Chip Resistor R 421 4 01465 461 270 Thick Film 5 200 ppm Chip Resistor R 422 4 01355 462 301K Thin Film 1 50 ppm MELF Resistor R 423 4 01309 462 100K Thin Film 1 50 ppm MELF Resistor R 424 4 01479 461 1 0K Thick Film 5 200 ppm Chip Resistor R 425 4 01471 461 470 Thick Film 5 200 ppm Chip Resistor R 426 4 01479 461 1 0K Thick Film 5 200 ppm Chip Resistor R 427 4 01479 461 1 0K Thick Film 5 200 ppm Chip Resistor R 428 4 01447 461 47 Thick Film 5 200 ppm Chip Resistor R 429 4 01503 461 10K Thick Film 5 200 ppm Chip Resistor R 430 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor 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 01
51. MT 18UH SMT 2 2U SMT 1 0U 1 5 WIRE 1 5 WIRE 50K 9MM SIDE RB MULTIPLES MMBRS5179 MMBR941L TIP107 NE461M02 MJD47 5087 5179 MMBTHSILTI MMBTHIOLTI TIP107 PRSIO Rubidium Frequency Standard DESCRIPTION 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 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 Misc Hardware Misc Pot Multi Turn Cermet Various sizes Printed Circuit Board 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 Integ
52. 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 04 April 29 2002 PRSIO Rubidium Frequency Standard Table of Contents 1 Introduction 3 Crystal Oscillator 41 Crystal Heater 43 Specifications 4 Schematic RB F2 Sheet 2 of 7 43 Temperature Control Servos 43 Abridged Command List 5 Conversion to 10MHz TTL 44 Photocell Amplifier 45 Theoretical Overview 8 Signal Filters for Oscillator Control 45 Rubidium Frequency Standards 8 Analog Multiplexers 46 Schematic RB F3 Sheet 3 of 7 46 PRS10 Overview 11 Microcontroller 47 Block Diagram 11 RS 232 49 Ovenized Oscillator 11 12 Bit A D Conversion 49 Frequency Synthesizer 11 12 Bit Digital to Analog Converters 49 Physics Package 13 Magnetic Field Control 49 Control Algorithm 13 Phase Modulation 50 Initial Locking 14 1PPS Output 50 Locking to External 1 14 1PPS Input Time Tag 50 CPU Tasks 18 Schematic RB F4 Sheet 4 of 7 51 High Resolution Low Phase Noise Applications 19 RF Synthesizer 51 Interface Connector 19 RF Output Amplifier 52 Configuration Notes 19 Step Recovery Diode Matching 52 Hardware Notes 20 Analog Control 53 Operating Temperature 2 Schematic RB F5 Sheet 5 of 7 53 Frequency Adjustment 2 P
53. 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 1005 input A time tag value read with 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 1pps 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 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 32 do not allow user TO commands Check the firmware revision with the ID command PP value 0 lt value lt 999999999 PRSIO Rubidium Frequency Standard RS 232 Instruction Set 33 Place pulse This command is used to move the Ipps output from its current position The Ipps output can be moved earlier in time by 1 ns to 999999999 n
54. anufactured 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 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 highlow 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
55. as to eliminate sample jitter The modulation waveform has very little distortion noise or spurs and is precisely 70 Hz The photosignal is amplified and 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 PRSIO Rubidium Frequency Standard 14 PRSI10 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 is 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 15 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
56. ay A port bit on the microcontroller PA7 may be used to output a 10s 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 1IPPS SEL 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 115 The 1PPS port bit from the CPU is synchronized to E 0 by U506B then synchronized and delayed by 0500 The multiplexer U510 selects one of the four phases of the I PPS 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 V44 2 by Q503 s collector current which turns on D503 connecting C513 to the output of U512B The selected 1 PPS 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 Vaac 2 where it stays until the 1PPS pulse goes low When the 1PPS pulse goes low the process is reversed Q504 s current is reduced while Q503 s current is increased charging C513 back towards 10V 4 2 This PRSIO Rubidium Frequency Standard Circuit Descriptions 57 time
57. ay be locked to an external 1pps input A second order digital PLL is used to lock the unit s frequency both the 10 MHz and 1 outputs to an external 1 input with time constants ranging from 256 s to 65536s about 4 minutes to about 18 hours When provided with an accurate and stable 1 pps source the unit will automatically align its Ipps output to the 1 pps input and then adjust the frequency of the rubidium reference to maintain the alignment over time A typical application would lock the PRS10 to the 1pps 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 I pps inputs without any software interaction PRSIO Rubidium Frequency Standard 34 RS 232 Instruction Set PL PL 0 or 1j PL PL Phase lock control This command may be used to disable the 1 PLL or to re enable and so restart the 1005 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 1 if the PLL to the Ipps is enabled PL is used to write the current value 0 or 1 to the EEPROM fo
58. b87 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 the 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 is reduced PRS10 Rubidium Frequency Standard Theoretical Overview 9 Rb Isotopic Discharge Filter Resonance lamp Cell Photocell gt gt CU RF 6 834GHz 6 834 682 612 8Hz Rb 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 li
59. c 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 ground 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 a 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 R
60. control is off Frequency lock is disabled Enable w LO1 command 10 MHz EFC is too high SD4 SP 10MHz cal Tamb 10 MHz EFC is too low SP 10 MHz cal Analog cal voltage gt 4 9 V Int cal pot ext cal volt Analog cal voltage 0 1 Int cal pot ext cal volt ST5 Frequency Lock to External 1pps PRSIO Rubidium Frequency Standard 40 RS 232 Instruction Set ST5 bit Condition which sets bit Corrective Action PLL disabled Send PL 1 to enable lt 256 good Ipps inputs Provide stable 1pps inputs PLL active gt 256 bad 1 inputs _ 6 _ feontrol saturated Wait checklppsinputs Provide 1pps input ST6 System Level Events ST6 bit Condition which sets bit Bad command syntax Bad command parameter Unit has been reset 0 lt PRSIO 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 potentiometer position will not affect the frequency of operation if 1 it is turned to either extreme 2 an external control voltage
61. cted 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 KQ to ground and 3 9 CMOS gate inputs which have input protection diodes to 5 V and ground RS 232 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 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 PRSIO Rubidium Frequency Standard Applications 21 The PRS10 may be connected directly to a PC s COM2 port with three wires TXD and ground As the PRS10 sources only 5 0 V for the RS 232 via 1 kQ the connecting cable should be kept short PRS10 PC s COM DB9 Connector PC s COM DB25 Connector Pin7 RXD Pin 3 TXD Pin 3 TXD Pin4 TXD Pin RXD Pin 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 requirem
62. d amplifiers top PCB Microcontroller left side vertical PCB Frequency synthesizer right side vert PCB P S and IPPS circuits bottom PCB Cell heater PCB rear vertical Inside the resonance cell Lamp heater PCB center vertical Inside lamp enclosure REF SRS PART VALUE DESCRIPTION C 100 5 00318 569 2 2U T35 Cap Tantalum SMT all case sizes C101 5 00299 568 1U 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 108 5 00298 568 01U Cap Ceramic 50V SMT 1206 10 X7R C 109 4 01146 462 2 00K Thin Film 1 50 ppm MELF Resistor C110 5 00299 568 1U 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 00472 569 4 TU T35 Cap Tantalum SMT all case sizes 116 5 00472 569 4 7U T35 Cap Tantalum SMT all case sizes C151 5 00299 568 1U Cap Ceramic 50V SMT 1206 10 X7R C152 5 00299 568 1U Cap Ceramic 50V SMT 1206 10 X7R C 200 5 00387 552 1000P Capacitor Chip SMT1206 50 5 NPO C 201 5 00466 572 1U SMT Film Capacitors 50V
63. e Counters are used for the portion of a time interval longer than 400 ns The analog PRS10 Rubidium Frequency Standard 32 RS 232 Instruction Set circuit stretches the time interval between the 1 input and the next 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 TO value 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 1pps output is connected to the 1 input and the time tag is read with the
64. e 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 0 lt value lt 10 GA GA Gain This command sets the gain parameter in the frequency lock loop algorithm Higher frequency lock loop to remove 67 of the frequency error but have larger equivalent noise bandwidths which will reduce the short term stability of the 10 MHz output A gain of 0 PRS10 Rubidium Frequency Standard RS 232 Instruction Set 27 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 1 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 Po GAO Infinite
65. e 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 10M Hz TTL 0205 converts a 10MHz 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 sees a signal with 0 91Vdc offset and an amplitude of 0 91 Vpp while the inverting input is biased at 0 91Vdc PRSIO Rubidium Frequency Standard 46 Circuit Descriptions Photoc
66. e phase noise ofthe 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 resolution 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 e
67. e 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 0 Controls the amplitude of the RF to multiplier in resonance cell Controls the analog portion 0 to 99 ns of the delay for the 1003 output Controls the drain voltage for the discharge lamp s FET oscillator 6 Controls the amplitude of the 10 MHz oscillator PRS10 Rubidium Frequency Standard RS 232 Instruction Set 37 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 is at 71 this sensor indicates a temperature which is about midway betwe
68. ell 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 dc current of I amps is given by V 2qI amps VHz where 1 6 10 On a SOWA dc current the best we can do is 4pA VHz 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 10x for a photocell whose shunt resistance is at 25 C but drops to 15kQ at the operating temperature of 80 C is not important as the expected shot noise current times the transconductance gain is about 600nV VHz The transconductance amplifier is followed by a high ga
69. en the baseplate temperature and the lamp temperature Command Returned voltage Ooo AD 15 AD port 16 lt port lt 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 the 68 11 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 PRSIO Rubidium Frequency Standard 38 RS 232 Instruction Set Command Returned voltage Varactor voltage for 22 48 MHz VCXO inside RF synthesizer 4 Varactor voltage for 360 MHz VCO output of RF synthesizer 4 Gain control voltage for amplifier which drives frequency multiplier 4 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 are low and the unit is not locked Example Immediately after power is applied
70. ency 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 R to generate a frequency near 6 834 GHz The factor 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 See the MO command However
71. ent 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 DAC setting 0 lt DAC lt 4095 and Af Hz 5x10 x 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 1 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 Ifa 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 1005 input may be applied to control the frequency offset directly If either the SF command or the 1pps 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 ofthe 10 MHz output are linearized and they have a span of 2000 x 10 or 40 020 Hz PRSIO Rubidium Frequency Standard 22 RS 232 Instruction Set RS 232 Instruction Set Syntax Commands consist of a two letter mnemon
72. g to commands and queries via the RS 232 interface A description of I O from the controller follows Name Function Non maskable interrupt on watch dog time out SPI dead ee 55 PORT Mixed inputs and outputs PRSIO Rubidium Frequency Standard Circuit Descriptions 49 PA7 Spare output connected to J305 PORT Eight TTL outputs ee 2 lt 5 PORT C Chip select outputs ieu lt 55 5 PDO RS 232IN lt PE6 Spare analog input 7303 with 100kQ to ground PE7 Spare analog input 7304 with 100kQ to ground PRSIO Rubidium Frequency Standard 50 Circuit Descriptions 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 implemented 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 con
73. ght 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 also 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 PRSIO 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 PRSIO Rubidium Frequency Standard PRS10 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
74. hat 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 facilitate 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 a 1 input over the same interval and with the same resolution The ability to time tag a 1pps input allows the PRS10 to be phase locked to other clock sources such as the 1 pps 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 1pps output Returned values range from 0 to 999999999 TS TS value 7000 lt value lt 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 scal
75. ibrated 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 switching is currently enabled MO 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
76. ic 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 data is communicated in ASCII codes Commands are case insensitive and spaces ASCII 3210 are ignored Commands are processed when a carriage return ASCII1310 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 RS1 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 VB 0 or 1j
77. in 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 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 PRS10 Rubidium Frequency Standard Circuit Descriptions 47 The EFC signal has a full scale of 17Vdc and a resolution of 22 bits A LSB represents a step of about 41 which is a fractional frequency step of about 1 107 We would like for noise on the EFC to be less than one LSB To arrange this the DAC22 signal is filtered with a time cons
78. ion 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 1 OMHz 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 10MHZ 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 1 414 Vpp or 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 control signals Typical phase noise is 125dBc Hz 10Hz 145dBc Hz 100Hz and 155dBc Hz gt 1kHz PRS10 Rubidium Frequency Standard 44 Circuit Descriptions Circu
79. 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 is 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 PRSIO 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 frequency 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 Nea
80. it 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 All ofthe 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 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 u
81. llowing table LM Description of LOCK 1pps Output i 5 5 Output goes low when locked to Rb pulses high for 10 us at 1 Hz Ipps locking pre filter disabled 1 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 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 usata 1 Hz rate To configure the unit for no 1 output the command string LM 2 followed by LM will change the unit s power on default for no 1pps output PRSIO 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 m
82. lue 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 Goa and at twice the modulation frequency 2Wmod The first number the amplitude of the signal at moa is the error signal 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 Goa 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 PRSIO Rubidium Frequency Standard 26 RS 232 Instruction Set SF value 2000 value 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
83. matching network mounted on an SMB connector and D701 the photodiode Another SMB connector J701 is used to pick up some ofthe 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 150MHZz 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 during normal operation is 0 5 W The power dissipated in the MRF134 is probably about 1 2 the total input power PRSIO Rubidium Frequency Standard 58 Circuit Descriptions 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
84. mmand 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 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 N and A which are currently in use to the unit s EEPROM SP will return the values 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 0 030 Hz at 10 MHz The output frequency at 10 MHz tunes quadratically with field strength and Af Hz 0 08 DAC 4096 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 A DAC value of 1000 correspo
85. n POT WIPER Ext freq calibration Nom 2 50 V 0 5V for 2 10 3 4 TXD PHOTO RS 232 data output or photo monitor output 6 24 HEAT 24 supply for discharge lamp and heaters 8 POT 5 00 reference output for external freq cal pot 9 24 CLEAN 24 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 1 output via the LM command via RS 232 PRSIO Rubidium Frequency Standard 20 Applications Pin 4 TXD PHOTO The default configuration uses this pin as an output 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
86. ncreasing 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 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 pps 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 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 availabl
87. nds to about 6 of the full scale frequency tuning range 3000 corresponds to about 53 while 4095 is 100 of the full scale range To help cancel frequency shifts due to external magnetic fields the current in the coil is switched at a 5 Hz rate The frequency lock loop averages over a full period ofthe 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 calibration they may choose to use the SF commands which disable the analog control and PRSIO Rubidium Frequency Standard 30 RS 232 Instruction Set allow the frequency to be adjusted over a range of 2000 10 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 cal
88. ntrol 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 DESCRIPTION RF LEVEL E F CELL TSET Controls the cell temperature Tmax 90 C Controls the RF power level to the SRD Schematic RB F5 Sheet 5 of 7 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 ofthe device Linear Power Supplies All of the power supplies operate from the 24 CLEAN input pin 9 ofthe main connector U503 an LM317 adjustable voltage regulator is used to supply 18Vdc to the system The 18 is used on the analog on the frequency synthesizer for the crystal oscillator and for the analog switches and ADC buffer on the CPU PCB 0504 an 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 PRSIO Rubidium Frequency Standard Circuit Descriptions 55 Lamp Regulator A discharge is igni
89. 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 of the reference as a function of time With the default time constant t 65 5365 and stability factor 1 the PRS10 s Ipps output will AT t for gt 1 PRSIO Rubidium Frequency Standard PRS10 Overview 17 exponentially approach the phase of the reference 1 input with a time constant 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 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 Ipps inputs 1 1005 inputs which are within 2048 ns of the first time tag result modulo 1 s are received e After receiving 256 consecutive good Ipps inputs the Ipps 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 1pps output delay to 123 456 789 ns which moves the 1pps output by 123 456 789 ns so that ne
90. onnector Male Connector Male Photodiode Deep Drawn or Stamping Machined Part Fabricated Part Machined Part Machined Part Machined Part 75 PRSIO Rubidium Frequency Standard 76 PRS10 Parts List REF SRS PART VALUE DESCRIPTION Z0 7 00862 720 RB SPACER Fabricated Part Z0 9 00571 924 SPECIALTY 56 Tape All types 4 00 3 60 gt 2 00 1 Y 1 0 20 1 637 5 gt 3 00 2 60 BOTTOM VIEW 0 144 END VIEW BASEPLATE 5 Y 0 gt m Y 0 30 A FREQUENCY ADJUSTMENT MOUNTING HOLES 1 POSITRONIC CBM11W1F2 NOTE ALL DIMENSIONS IN INCHES WITH COAX INSERT MS4104D 2 CANNON DAM11W1S WITH COAX INSERT DM53740 5000 Figure 4 Mechanical Dimensions PRSIO Rubidium Frequency Standard
91. order to obtain the maximum benefits of the averaging while simultaneously insuring that the PLL will be stable Use ofthe pre filter is recommended when locking to references that have poorer short term stability than the PRS10 but better long term stability Locking to the 1pps 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 1 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 Aia sin J1 Ct 7 AT 0 e cos 1C 1 7 lt 1 ho Sl gt AT t AT 0 AT 0 e forG 1 F amp 4 DATO 2 e 281 2 F V6 DATO s exem 2 de 1 m AT 0 is the initial
92. ounter to make precision frequency 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 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 BNC 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 50Q and LEVEL to 1 0V The TRIG LED will go on when the GATE ARM is setup properly Both A START and STOP are AC coupled and terminated into 50Q 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 are 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
93. ower Supply Lamp Control and 1PPS Timing PCB 53 RS 232 Instruction Set 22 Linear Power Supplies 53 Syntax 22 Lamp Regulator 54 Initialization 22 1PPS Input Time Tag 54 Frequency Lock loop Parameters 24 1 PPS Output Pulse Delay 55 Frequency Synthesizer Control 28 Baseplate Temperature Sensor 56 Magnetic field Control 29 Schematic RB F6 Sheet 6 of 7 56 Frequency Control 31 Resonance Cell and Lamp Heaters 56 One pulse per second control 31 Resonance Cell 56 PPS Locking Control 33 Discharge Lamp 56 Analog Control 35 Schematic RB F7 Sheet 7 of 7 57 Analog Test Voltages 36 Connector Interface Board 37 Status Bytes 37 Appendix A Frequency Synthesizer Calibration Procedures 40 Table 59 Circuit Description 41 Appendix B Precision Frequency Schematic RB Fl sheet 1 of 7 41 Measurement 61 Input Power 4 Set up for an SR620 62 Voltage Reference 4 Four input connections 62 PRSIO Rubidium Frequency Standard 2 Table of Contents Four input setups Coarse Frequency Measurements Fine Frequency Measurements Parts List for Revision H 62 62 62 64 PRSIO 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 Th
94. p 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 7 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 for the others The circuit description will refer to the crystal temperature controller The controller is a proportional integral controller The output of the error amplifier U200A is used to control the 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 R204xC
95. pm 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 596 200 ppm Chip Resistor Thick Film 596 200 ppm Chip Resistor Thick Film 596 200 ppm Chip Resistor Thick Film 596 200 ppm Chip Resistor Hardware Misc Thick Film 596 200 ppm Chip Resistor Thick Film 596 200 ppm Chip Resistor Thick Film 596 200 ppm Chip Resistor Thick Film 596 200 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 PRSIO Rubidium Frequency Standard 72 PRS10 Parts List REF SRS PART VALUE DESCRIPTION R 359 0 00000 000 UNDECIDED PART Hardware Misc R 360 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 400 4 01347 462 249K Thin Film 1 50 ppm MELF Resistor R401 4 01405 462 1 00M Thin Film 1 50 ppm MELF Resistor R 402 4 01447 461 47 Thick Film 5 200 ppm Chip Resistor R 403 4 01463 461 220 Thick Film 5 200 ppm Chip Resistor R 404 4 01527 461 100K Thick Film 5 200 ppm Chip Resistor R 405 4 01201 462 7 50K Thin Film 1 50 ppm MELF Resistor R 406 4 01561 461 2 7M Thick Film 5 200 ppm Chip Resistor R 407 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 408 4 01259 462 30 1K Thin Film 1 50 ppm MELF Resistor R 409 4 01355 462 301K Thin Film 1 50 ppm ME
96. r the plateau top the frequency deviation verses temperature is about Af Hz 0 061 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 1ppm 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 80 2 Q100 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 ofthe high speed op amp U101 by 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 be 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 operat
97. r 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 PT value 0 lt value lt 14 20199 seconds 256 512 4 194 304 PT Phase lock integrator time constant This command is used to set PLL s integrator s time constant which phase locks the PRS10 to an external 1 input The integrator time constant is equal to 2 9 seconds The default value is 8 which provides an integrator time constant of 259 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 15 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 201 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
98. rated 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 406 407 500 501 502 503 504 505 506 507 508 509 510 511 512 514 600 800 E OG iC C ere gt R Y 400 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 Z0 SRS PART 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 5 00547 504 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 0 00607 025 0 00608 025 0 00609 025 0 00629 066 0 00630 034 0 00641 031 0 00642 031 0 00643 020 0 00644 020 0 00645 055 0 00902 034 0 00908 030 0 00915 034 1 00323 130 1 00324 130 3 00668 312 7 00557 717 7 00560 721 7 00636 720 7 00638 721 7 00639 721 7 00641 721 VALUE LM358 TLC5628 74HC574 LM317T AD822 LM317T 7805 LM45CIM3 74HC74 74HC74 74HC74 LM311M 74HC153 AD822 AD822 AD790JR 7812 7808 2 2P 10 MHZ SC CUT 22 4825 MHZ 4 40 MINI
99. rated Circuit Surface Mount Pkg Voltage Reg TO 220 TAB Package PRS10 Parts List 69 REF SRS PART VALUE Q 800 3 00325 329 TIP107 Q 900 3 00665 360 MRF134 DESCRIPTION Voltage Reg TO 220 TAB Package Integrated Circuit Surface Mount Pkg R 100 4 01242 462 20 0K Thin Film 1 50 ppm MELF Resistor R101 4 01184 462 4 99K Thin Film 1 50 ppm MELF Resistor R 102 4 01309 462 100K Thin Film 1 50 ppm MELF Resistor R 103 4 00954 462 20 Thin Film 1 50 ppm MELF Resistor R 104 4 01280 462 49 9K Thin Film 1 50 ppm MELF Resistor R 105 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R 106 4 01447 461 47 Thick Film 5 200 ppm Chip Resistor R 107 4 01088 462 499 Thin Film 1 50 ppm MELF Resistor R 108 4 01184 462 4 99K Thin Film 1 50 ppm MELF Resistor R 109 4 01213 462 10 0K Thin Film 1 50 ppm MELF Resistor R110 4 01184 462 4 99K Thin Film 1 50 ppm MELF Resistor R 111 4 01067 462 301 Thin Film 1 50 ppm MELF Resistor R112 4 01447 461 47 Thick Film 5 200 ppm Chip Resistor R113 4 01146 462 2 00K Thin Film 1 50 ppm MELF Resistor R114 4 01096 462 604 Thin Film 1 50 ppm MELF Resistor R115 4 01251 462 24 9K Thin Film 1 50 ppm Resistor R 116 4 01251 462 24 9K Thin Film 1 50 ppm Resistor R 117 4 01479 461 1 0K R 118 4 01503 461 10K R 119 4 01503 461 10K R 120 4 01503 461 10K R 121 4 01213 462 10 0K R 123 4 00925 462 10 R 150 4 01407 461 1 R151 4 01407 461 1
100. ration 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 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
101. s 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 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 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 Mo
102. s Since the Ipps input time tag is referenced to the 1 output changing the 1 output placement will change the report time tag values as well See the TT and TO commands Example PP 123456789 will move the Ipps 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 1 output time delay circuit This circuit is used to delay the 1005 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 m
103. s set to 2000 If the new integral term is less than 2000 it 1s 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 I pps output in line with the 1pps input PRSIO Rubidium Frequency Standard 18 PRS10 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 Ipps inputs This could happen if the 1 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 For 1175 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 1 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 Ipps pulse with Ins placement and measure the time for a 1 input pulse with 1 ns resolution There is an RS 2
104. set by R510 and R511 The drain voltage is controlled by an 8bit DAC whose output is multiplied by 6 and buffered U502 and 0500 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 25Vdc 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 1PPS input signal on pin 5 of the main connector can be time tagged with 1ns resolution The time may reported via RS 232 or used to servo the unit to another frequency standard such as GPS 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 PRSIO Rubidium Frequency Standard 56 Circuit Descriptions 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
105. stablishes 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 PRSIO Rubidium Frequency Standard 4 Specifications Amplitude 0 5 10 about 1 41V or 7 dBm 55x10 at shipment Allan variance SSB phase noise lt 130 10Hz lt 140 100Hz lt 150 1kHz dBc Hz dBc Temperature Retrace 5 10 72 hr off then 72 hr on Aff APT Trim Range 2x10 Af f Se ee PLEEEUN Other Electrical 0 i lt lt 5557 Protection t 30 to any pin except rf output Vac RS 232 9600 8 bits no parity 1 stop bit 0 5 levels with x on x off protocol measurement 10 accuracy 1 resolution Storage 55 to 85 Ibs Mates with ITT Cannon DAMI1WIS series PRSIO 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
106. tandard 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 A ffo Hz df hz i aed v 31 0 onm omm s37 7 018678 0009495 6 685 3476 23 0 032570 0 007103 5899 3315 39 0 047809 0 007800 9 5756 3235 15 0 055996 0 008187 5470 3074 31 0 073654 0 009054 4326 2431 31 0 167638 0 014379 25 7079 3978 54 0 267493 0 010875 26 pes Io 0011324 6793 sis 33 20302924 0 012308 29 6507 3657 22 0 3157730 012849 27 47 36 ss 25 0 424400 0 077875 PRSIO Rubidium Frequency Standard Number R N A ffo Hz df hz 41 7258 4079 29 0 521791 0 015526 46 0 25927 6 0813000009036 47 6686 3157 61 0 590535 10 009229 48 21810225133 0019094 5 13 0 ass D 72 0 639810 0 010295 se ines nos T oras ones so 61 0 60 7151 4019 20 0 805666 10 007200 63 S113 2873 53 0 858888 0 007920 onte ii s 29 0 66 1657357 0900709 008802 68 6293 35374 059
107. tant of 1s and buffered by a FET input op amp U210B an AD822 The FET op amp has 1 f noise of about 2u Vpp 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 61 V 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 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 A0 Amplified and filtered photocell signal _A3__ Crystal thermistor voltage _A4 Resonance cell thermistor voltage BO 1204 Spare Bl 24 HEATAIO B2 24 CLEAN 10 B6 Resonance cell heater control signal Schematic RB_F3 Sheet 3 of 7 All of the components shown on this schematic reside on the vertical PCB on the left side of the unit The large hole in this PCB allows access to an SMB connector to sample the microwave field in the resonance cell PRS10 Rubidium Frequency Standard 4
108. ted 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 20 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 ofthe lamp voltage regulator is also important The voltage provided to the lamp oscillator 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 10 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
109. 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 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 10MHz 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 va
110. times the interval between the 1 input and the next E CLK Measuring the duration of INTERPOLATE allows the position ofthe I PPS input to be measured to about 400ns 2000 0 2ns The E is synchronized to the 10MHz clock and four phases are generated by U500 an octal latch 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 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 the 1 PPS input relative to the E The width of the gate pulse is multiplied by a factor of about 2000 by the pulse stretcher circuit Initially C509 15 charged to 11 4Vdc C509 15 rapidly discharged by Q502 s collector current about 10 8mA during 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 Del
111. unt Pkg 67 PRS10 Rubidium Frequency Standard 68 PRS10 Parts List REF D 203 204 D 205 D 400 D 40 D 500 D 501 D502 503 504 D 700 J 100 J 100X J 400 J 700 J701 J 800 J801 J 802 JP500 JP501 L 100 L101 L 102 L 103 L 104 L 105 L 200 L300 L301 L302 L 400 L401 L 402 L 403 L 902 LX104 LX105 P 100 PCI Q 100 101 Q150 Q 400 Q 500 Q 501 Q 502 Q 503 Q 504 Q 600 SRS PART 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 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 VALUE ZMM5230B ZMM5230B ZMM5230B MMBV609 MMBV609 MBRD660CT BAV170LT1 BAWSGLTI BAV70LT1 BAV170LT1 47402 15 10 PIN MALE COAX INSERT STRAIGHT PLUG REAR MT JACK REAR MT JACK 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 S
112. ural frequency about 10 r s 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 the frequency of the inner loop s crystal keeping it locked to the 10 MHz reference PRSIO 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 15 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 offto one side ofthe transition the phase ofthe 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 V to start the lamp which drops to about 100 during normal ope
113. verter is used to measure the ac and dc 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 ofthe 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 a 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 ofthe DACs the upper DAC ofthe 22 bit pair and the DAC which controls the magnetic field are rarely changed and 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
114. w 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 10 is used to initialize the 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 The unit will lock the frequency ofthe PRS10 to the good Ipps input pulses Bad 1005 inputs 1pps inputs with time tags greater than 1 024 ns from the last good 1005 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 if LMO AT 1 AT n The pre filter if LM1 AT n l 1 0 At t3 AT n At AT n The integral term Int n 1 Int n AT n 1 xi KaaAt The proportional term Pro n 1 Ap AT n D Kaa 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 15 set to 2000 If the new f value is less than 2000 it 15 set to 2000 If the new integral term exceeds 2000 it i

Model PRS10 Rubidium Frequency Standard

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