FE-5680A Rubidium Frequency Standard FAQ

Frequency Electronics FE-5680A Rubidium Frequency Standard FAQ
culled from "time-nuts" mailing list, and web sites.    Jan. 19 2012  J.Beale
--------------------------------------------------------------------------------

Time Nuts mailing list archives:
http://www.mail-archive.com/time-nuts@febo.com/info.html
http://www.febo.com/pipermail/time-nuts/

Frequency Electronics official FE-5680A product page
http://www.freqelec.com/rb_osc_fe5680a.html

FE-5680A Notes, links, and control software:
http://www.vk3um.com/Rubidium%20Standard.html

"Further Information on Rubidium Std. FE-5680A"
http://www.vk3um.com/Fe-5680A4.pdf

FE-5680A Technical Manual
http://www.wa6vhs.com/Test%20equipment/FREQUENCY%20STANDARDS/FE-5680A/5680%20TECH%20MANUAL.pdf

Fhotos of unit with some signals marked
https://plus.google.com/photos/109928236040342205185/albums/5680473650837554113

Misc FE-5680A photos
http://www.yeyudo.cn/article.asp?id=230

Atomic Nixie Clock project, using 5680A as timebase
http://www.amug.org/~jthomas/atomicnixie.html

=================================================================
NOTE: there are MANY different varieties of "FE-5680A" with many options so any specific
feature, output, or repair procedure may not apply to your unit. See for example,
the options listed in the Technical Manual (link above).

=================================================================
from FE-5680 Tech Manual Table 3, Sheet 9 (PDF page 10/19):

PARAMETER SPECIFICATION
Frequency 10 MHz*   Type Sinusoidal
Amplitude (minimum) 0.5 Vrms into 50Ω(+7dBm)
Adjustment Resolution <1 x 10-12 over range of 3.8 x 10-5
C-field potentiometer Resolution 1 x 10-11 over range of 3 x 10-9
Drift:
	2 x 10-9/year
	2 x 10-11/day
Short Term Stability: 1 sec ≤ 100 sec 1.4 x 10-11 t
Retrace 5 x 10-11
Phase Noise (fo=10 MHz)
	@ 10 Hz: -100 dBc
	@ 100Hz: -125 dBc
	@ 1000 Hz: -145 dBc
Input Voltage Sensitivity 2 x 10-11/(15V to 16V)
Frequency vs.Temperature (-5℃ to +50℃) ±3 x 10-10
Spurious Outputs -60 dBc
Harmonics -30 dBc
Loop Lock Indication
	> 3Vdc=Unlocked
	< 1Vdc=Locked
Input Power (@25 C) 11 watts steady state, 27 watts peak
DC Input Voltage/Current
15V to 18V @ 1.8A peak and 0.7A steady-state
except Opt 25: +22V to +32V @ 1.25 peak, 0.5A steady-state
Ripple +15V: < 0.1Vrms
Warm-up Time < 5 minutes to lock @ 25℃
Size: 25 x 88 x 125 mm, .98 x 3.47 x 4.92 inches
Weight 434 grams, 15.3 oz.

Full manual available here:
http://www.wa6vhs.com/Test%20equipment/FREQUENCY%20STANDARDS/FE-5680A/5680%20TECH%20MANUAL.pdf

=================================================================
Why doesn't the C-field pot on the side do anything?

Some versions of the 5680A (most/all available on auction sites as of Jan. 2012) have this

circuit disconnected. So, it doesn't do anything. You can program the frequency offset via

RS-232 serial commands though, see "control software" link above.

=================================================================
What if your 5680A does not lock up after several minutes?
John Beale, Dec. 2 2011

Several minutes after powering up (apply +15V on DB9 pin 1 and +5V on pin 4) the unit should
indicate lock (pin 3 voltage drops low).  If it does not, the internal VCXO frequency may have

shifted enough so the loop does not pass through 10 MHz and achieve lock. If you have a

frequency counter, look at the 10 MHz output signal before
lock to see if it sweeps through 10 MHz or not. One of my units would only reach 9.99998 MHz as

I received it, but I fixed it with C217 adjustment, as below.

Open up the top cover by removing screws (some hidden underneath labels on top)
Gently move aside the insulating foam blocks (they are fragile).
Rotate trimmer cap C217 slightly with a screwdriver.
Try again and see if it locks. Repeat as needed.
See also these illustrations:
https://plus.google.com/u/0/photos/109928236040342205185/albums/5680473650837554113/568068300849

0223330
https://plus.google.com/u/0/photos/109928236040342205185/albums/5680473650837554113/568171579937

7076466

=================================================================
Can I get a square wave 10 MHz output instead of a sine wave?

Yes, by removing the bottom plate, moving one 15 ohm resistor and installing a cable.
See the modification shown here:

https://plus.google.com/u/0/photos/109928236040342205185/albums/5680473650837554113/569329680997

6448530
https://plus.google.com/u/0/photos/109928236040342205185/albums/5680473650837554113/569329503400

6787730

=================================================================
What is the typical drift with temperature (tempco)?

The official spec says +/- 3E-10 from -5C to +50C (5680 Tech Manual, Table 3
One of my units shows -7E-12 frequency change per degree C increase in case temperature

(measured at case temp. of 45C and 55 C). For this reason it is beneficial to stabilize the

temperature, for example with a thermostatically controlled fan blowing on the unit's heasink.

See also
https://plus.google.com/u/0/photos/109928236040342205185/albums/5680473650837554113/568563667393

0493138


=================================================================
Long Term Frequency stability
Bert Kehren, Jan 17 2012

After eight weeks of monitoring to 1 E-12, I still se no aging. Waiting for
 a change so I can do other tests. One thing I clearly se is a 4 Hz filter
response changing the output by +- 3 E-11. It may be more, will have to
find a  way to check it more accurately, because of limited response time.


=================================================================
Input Voltage Requirements (official spec is 15 - 18 volts DC)

from Pete Bell, Jan. 18 2012:
One of  the regulators (the one that runs the lamp and lamp heater) is
running at  13.8V and has a 500mV dropout voltage, so there is very
little (~ 700mV)  headroom. The one that runs the cell heater is about
a volt lower and the  one that runs the analog circuits is set to 8.8V,
so has plenty of  headroom.

I suspect the lamp regulator is mostly there to reduce the  start-up
current, though - the FE-5650 has a very similar lamp circuit with  no
regulator and the only significant difference seems to be that  the
current pulled when the unit is cold is  higher.
======================================================

FE-5680A attachment to PCB (some, not all ebay units, which come screwed onto a PCB)
1/16" hex screws


======================================================
Undocumented RS-232 commands
from Scott Newell N5TNL, Jan 18 2012

I'm playing around with the unlockable problem child. Turns out I get a response from the unit

to several undocumented serial port commands. In particular, the replies from 0x22, 0x57, 0x59,

and 0x5A seem to vary each time I read the unit. Not all the commands respond each time; there's

certainly a bug or two lurking. (I'm probably getting the poor unit's serial input all confused

and out of sync.) Anyway, commands 0x57 and 0x59 appear to send back a lot of data: 0x56 bytes!

I'm not yet validating the data length or checksum of the FE-5680 reply.

Here's an example of the replies to the commands (in hex):

Reply to command 0x22, 0x0D bytes: [22] [0D] [00] [2F] [20] [5B] [29] [00] [A1] [C8] [4A] [1D]

[6C]
Reply to command 0x29, 0x09 bytes: [29] [09] [00] [20] [FF] [00] [00] [00] [FF]
Reply to command 0x2B, 0x09 bytes: [2B] [09] [00] [22] [20] [5B] [29] [00] [52]
Reply to command 0x2D, 0x09 bytes: [2D] [09] [00] [24] [20] [5B] [29] [00] [52]
Reply to command 0x47, 0x08 bytes: [47] [08] [00] [4F] [20] [5B] [29] [52]
Reply to command 0x53, 0x07 bytes: [53] [07] [00] [54] [79] [00] [79]

Reply to command 0x57, 0x56 bytes: [57] [56] [00] [01] [20] [20] [5B] [29] [00] [A1] [48] [4E]

[1D] [C0] [00] [D6] [72] [85] [6A] [DB] [25] [80] [14] [8A] [BD] [2E] [5E] [86] [CB] [BA] [E5]

[CA] [F5] [2D] [0F] [4F] [7F] [D3] [D1] [37] [2B] [2C] [90] [54] [45] [68] [9A] [AC] [AA] [4B]

[50] [33] [CF] [9B] [2B] [FD] [BD] [E6] [AD] [6D] [6B] [BA] [FF] [FA] [17] [03] [62] [F4] [E8]

[79] [00] [83] [23] [C1] [17] [69] [37] [18] [8E] [E9] [13] [A3] [FB] [1F] [1B] [F8]

Reply to command 0x59, 0x56 bytes: [59] [56] [00] [0F] [20] [20] [5B] [29] [00] [A1] [C8] [4A]

[1D] [40] [00] [D6] [72] [95] [6A] [DB] [25] [80] [54] [8A] [BD] [3C] [5E] [86] [C9] [BA] [E5]

[CA] [F5] [2D] [0D] [4F] [7F] [D3] [D5] [37] [2B] [2C] [90] [54] [45] [68] [BA] [BC] [EA] [4B]

[50] [33] [CF] [9B] [29] [FD] [BD] [E6] [AD] [6C] [6B] [BE] [FF] [FA] [15] [02] [62] [F4] [E8]

[D8] [57] [83] [23] [C1] [17] [68] [37] [18] [86] [E9] [13] [AB] [FB] [5F] [1B] [79]

Reply to command 0x5A, 0x0D bytes: [5A] [0D] [00] [57] [00] [00] [40] [08] [9E] [07] [00] [00]

[D1]
Reply to command 0x61, 0x09 bytes: [61] [09] [00] [68] [20] [5B] [29] [00] [52]
Reply to command 0x65, 0x07 bytes: [65] [07] [00] [62] [20] [20] [20]
Reply to command 0x67, 0x08 bytes: [67] [08] [00] [6F] [01] [F4] [B8] [4D]
Reply to command 0xF0, 0x0E bytes: [F0] [0E] [00] [FE] [33] [2E] [34] [00] [00] [00] [00] [00]

[00] [29]