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FE-5680A Rubidium Frequency Standard FAQ

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  1. Frequency Electronics FE-5680A Rubidium Frequency Standard FAQ
  2. culled from "time-nuts" mailing list, and web sites.    Jan. 19 2012  J.Beale
  3. --------------------------------------------------------------------------------
  4.  
  5. Time Nuts mailing list archives:
  6. http://www.mail-archive.com/time-nuts@febo.com/info.html
  7. http://www.febo.com/pipermail/time-nuts/
  8.  
  9. Frequency Electronics official FE-5680A product page
  10. http://www.freqelec.com/rb_osc_fe5680a.html
  11.  
  12. FE-5680A Notes, links, and control software:
  13. http://www.vk3um.com/Rubidium%20Standard.html
  14.  
  15. "Further Information on Rubidium Std. FE-5680A"
  16. http://www.vk3um.com/Fe-5680A4.pdf
  17.  
  18. FE-5680A Technical Manual
  19. http://www.wa6vhs.com/Test%20equipment/FREQUENCY%20STANDARDS/FE-5680A/5680%20TECH%20MANUAL.pdf
  20.  
  21. Fhotos of unit with some signals marked
  22. https://plus.google.com/photos/109928236040342205185/albums/5680473650837554113
  23.  
  24. Misc FE-5680A photos
  25. http://www.yeyudo.cn/article.asp?id=230
  26.  
  27. Atomic Nixie Clock project, using 5680A as timebase
  28. http://www.amug.org/~jthomas/atomicnixie.html
  29.  
  30. =================================================================
  31. NOTE: there are MANY different varieties of "FE-5680A" with many options so any specific
  32. feature, output, or repair procedure may not apply to your unit. See for example,
  33. the options listed in the Technical Manual (link above).
  34.  
  35. =================================================================
  36. from FE-5680 Tech Manual Table 3, Sheet 9 (PDF page 10/19):
  37.  
  38. PARAMETER SPECIFICATION
  39. Frequency 10 MHz*   Type Sinusoidal
  40. Amplitude (minimum) 0.5 Vrms into 50Ω(+7dBm)
  41. Adjustment Resolution <1 x 10-12 over range of 3.8 x 10-5
  42. C-field potentiometer Resolution 1 x 10-11 over range of 3 x 10-9
  43. Drift:
  44.         2 x 10-9/year
  45.         2 x 10-11/day
  46. Short Term Stability: 1 sec ≤ 100 sec 1.4 x 10-11 t
  47. Retrace 5 x 10-11
  48. Phase Noise (fo=10 MHz)
  49.         @ 10 Hz: -100 dBc
  50.         @ 100Hz: -125 dBc
  51.         @ 1000 Hz: -145 dBc
  52. Input Voltage Sensitivity 2 x 10-11/(15V to 16V)
  53. Frequency vs.Temperature (-5℃ to +50℃) ±3 x 10-10
  54. Spurious Outputs -60 dBc
  55. Harmonics -30 dBc
  56. Loop Lock Indication
  57.         > 3Vdc=Unlocked
  58.         < 1Vdc=Locked
  59. Input Power (@25 C) 11 watts steady state, 27 watts peak
  60. DC Input Voltage/Current
  61. 15V to 18V @ 1.8A peak and 0.7A steady-state
  62. except Opt 25: +22V to +32V @ 1.25 peak, 0.5A steady-state
  63. Ripple +15V: < 0.1Vrms
  64. Warm-up Time < 5 minutes to lock @ 25℃
  65. Size: 25 x 88 x 125 mm, .98 x 3.47 x 4.92 inches
  66. Weight 434 grams, 15.3 oz.
  67.  
  68. Full manual available here:
  69. http://www.wa6vhs.com/Test%20equipment/FREQUENCY%20STANDARDS/FE-5680A/5680%20TECH%20MANUAL.pdf
  70.  
  71. =================================================================
  72. Why doesn't the C-field pot on the side do anything?
  73.  
  74. Some versions of the 5680A (most/all available on auction sites as of Jan. 2012) have this
  75.  
  76. circuit disconnected. So, it doesn't do anything. You can program the frequency offset via
  77.  
  78. RS-232 serial commands though, see "control software" link above.
  79.  
  80. =================================================================
  81. What if your 5680A does not lock up after several minutes?
  82. John Beale, Dec. 2 2011
  83.  
  84. Several minutes after powering up (apply +15V on DB9 pin 1 and +5V on pin 4) the unit should
  85. indicate lock (pin 3 voltage drops low).  If it does not, the internal VCXO frequency may have
  86.  
  87. shifted enough so the loop does not pass through 10 MHz and achieve lock. If you have a
  88.  
  89. frequency counter, look at the 10 MHz output signal before
  90. lock to see if it sweeps through 10 MHz or not. One of my units would only reach 9.99998 MHz as
  91.  
  92. I received it, but I fixed it with C217 adjustment, as below.
  93.  
  94. Open up the top cover by removing screws (some hidden underneath labels on top)
  95. Gently move aside the insulating foam blocks (they are fragile).
  96. Rotate trimmer cap C217 slightly with a screwdriver.
  97. Try again and see if it locks. Repeat as needed.
  98. See also these illustrations:
  99. https://plus.google.com/u/0/photos/109928236040342205185/albums/5680473650837554113/568068300849
  100.  
  101. 0223330
  102. https://plus.google.com/u/0/photos/109928236040342205185/albums/5680473650837554113/568171579937
  103.  
  104. 7076466
  105.  
  106. =================================================================
  107. Can I get a square wave 10 MHz output instead of a sine wave?
  108.  
  109. Yes, by removing the bottom plate, moving one 15 ohm resistor and installing a cable.
  110. See the modification shown here:
  111.  
  112. https://plus.google.com/u/0/photos/109928236040342205185/albums/5680473650837554113/569329680997
  113.  
  114. 6448530
  115. https://plus.google.com/u/0/photos/109928236040342205185/albums/5680473650837554113/569329503400
  116.  
  117. 6787730
  118.  
  119. =================================================================
  120. What is the typical drift with temperature (tempco)?
  121.  
  122. The official spec says +/- 3E-10 from -5C to +50C (5680 Tech Manual, Table 3
  123. One of my units shows -7E-12 frequency change per degree C increase in case temperature
  124.  
  125. (measured at case temp. of 45C and 55 C). For this reason it is beneficial to stabilize the
  126.  
  127. temperature, for example with a thermostatically controlled fan blowing on the unit's heasink.
  128.  
  129. See also
  130. https://plus.google.com/u/0/photos/109928236040342205185/albums/5680473650837554113/568563667393
  131.  
  132. 0493138
  133.  
  134.  
  135. =================================================================
  136. Long Term Frequency stability
  137. Bert Kehren, Jan 17 2012
  138.  
  139. After eight weeks of monitoring to 1 E-12, I still se no aging. Waiting for
  140.  a change so I can do other tests. One thing I clearly se is a 4 Hz filter  
  141. response changing the output by +- 3 E-11. It may be more, will have to
  142. find a  way to check it more accurately, because of limited response time.
  143.  
  144.  
  145. =================================================================
  146. Input Voltage Requirements (official spec is 15 - 18 volts DC)
  147.  
  148. from Pete Bell, Jan. 18 2012:
  149. One of  the regulators (the one that runs the lamp and lamp heater) is
  150. running at  13.8V and has a 500mV dropout voltage, so there is very
  151. little (~ 700mV)  headroom. The one that runs the cell heater is about
  152. a volt lower and the  one that runs the analog circuits is set to 8.8V,
  153. so has plenty of  headroom.
  154.  
  155. I suspect the lamp regulator is mostly there to reduce the  start-up
  156. current, though - the FE-5650 has a very similar lamp circuit with  no
  157. regulator and the only significant difference seems to be that  the
  158. current pulled when the unit is cold is  higher.
  159. ======================================================
  160.  
  161. FE-5680A attachment to PCB (some, not all ebay units, which come screwed onto a PCB)
  162. 1/16" hex screws
  163.  
  164.  
  165. ======================================================
  166. Undocumented RS-232 commands
  167. from Scott Newell N5TNL, Jan 18 2012
  168.  
  169. I'm playing around with the unlockable problem child. Turns out I get a response from the unit
  170.  
  171. to several undocumented serial port commands. In particular, the replies from 0x22, 0x57, 0x59,
  172.  
  173. and 0x5A seem to vary each time I read the unit. Not all the commands respond each time; there's
  174.  
  175. certainly a bug or two lurking. (I'm probably getting the poor unit's serial input all confused
  176.  
  177. and out of sync.) Anyway, commands 0x57 and 0x59 appear to send back a lot of data: 0x56 bytes!
  178.  
  179. I'm not yet validating the data length or checksum of the FE-5680 reply.
  180.  
  181. Here's an example of the replies to the commands (in hex):
  182.  
  183. Reply to command 0x22, 0x0D bytes: [22] [0D] [00] [2F] [20] [5B] [29] [00] [A1] [C8] [4A] [1D]
  184.  
  185. [6C]
  186. Reply to command 0x29, 0x09 bytes: [29] [09] [00] [20] [FF] [00] [00] [00] [FF]
  187. Reply to command 0x2B, 0x09 bytes: [2B] [09] [00] [22] [20] [5B] [29] [00] [52]
  188. Reply to command 0x2D, 0x09 bytes: [2D] [09] [00] [24] [20] [5B] [29] [00] [52]
  189. Reply to command 0x47, 0x08 bytes: [47] [08] [00] [4F] [20] [5B] [29] [52]
  190. Reply to command 0x53, 0x07 bytes: [53] [07] [00] [54] [79] [00] [79]
  191.  
  192. Reply to command 0x57, 0x56 bytes: [57] [56] [00] [01] [20] [20] [5B] [29] [00] [A1] [48] [4E]
  193.  
  194. [1D] [C0] [00] [D6] [72] [85] [6A] [DB] [25] [80] [14] [8A] [BD] [2E] [5E] [86] [CB] [BA] [E5]
  195.  
  196. [CA] [F5] [2D] [0F] [4F] [7F] [D3] [D1] [37] [2B] [2C] [90] [54] [45] [68] [9A] [AC] [AA] [4B]
  197.  
  198. [50] [33] [CF] [9B] [2B] [FD] [BD] [E6] [AD] [6D] [6B] [BA] [FF] [FA] [17] [03] [62] [F4] [E8]
  199.  
  200. [79] [00] [83] [23] [C1] [17] [69] [37] [18] [8E] [E9] [13] [A3] [FB] [1F] [1B] [F8]
  201.  
  202. Reply to command 0x59, 0x56 bytes: [59] [56] [00] [0F] [20] [20] [5B] [29] [00] [A1] [C8] [4A]
  203.  
  204. [1D] [40] [00] [D6] [72] [95] [6A] [DB] [25] [80] [54] [8A] [BD] [3C] [5E] [86] [C9] [BA] [E5]
  205.  
  206. [CA] [F5] [2D] [0D] [4F] [7F] [D3] [D5] [37] [2B] [2C] [90] [54] [45] [68] [BA] [BC] [EA] [4B]
  207.  
  208. [50] [33] [CF] [9B] [29] [FD] [BD] [E6] [AD] [6C] [6B] [BE] [FF] [FA] [15] [02] [62] [F4] [E8]
  209.  
  210. [D8] [57] [83] [23] [C1] [17] [68] [37] [18] [86] [E9] [13] [AB] [FB] [5F] [1B] [79]
  211.  
  212. Reply to command 0x5A, 0x0D bytes: [5A] [0D] [00] [57] [00] [00] [40] [08] [9E] [07] [00] [00]
  213.  
  214. [D1]
  215. Reply to command 0x61, 0x09 bytes: [61] [09] [00] [68] [20] [5B] [29] [00] [52]
  216. Reply to command 0x65, 0x07 bytes: [65] [07] [00] [62] [20] [20] [20]
  217. Reply to command 0x67, 0x08 bytes: [67] [08] [00] [6F] [01] [F4] [B8] [4D]
  218. Reply to command 0xF0, 0x0E bytes: [F0] [0E] [00] [FE] [33] [2E] [34] [00] [00] [00] [00] [00]
  219.  
  220. [00] [29]
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