[time-nuts] FE-5680A tuning vs resonant peaks

[gandalfg8 at aol.com]

There is a lot of information in the Time-nuts archives relating to the FE5680, and all the commands you need

for reading and programming the unit are in the manual, as best I remember I just used Hyperterminal.



Just in case you're not aware of it, the Google site specific search string is......



Google site specific search for item "xxxx"......  xxxx"space"site:http://www.febo.com/pipermail/time-nuts/



Below are a couple of posts repeated that might be of immediate interest.

Firstly an important warning re power supplies from Skip Withrow, not saying it's relevant in this instance but well worth

repeating anyway, and secondly Jose Camara's very useful notes on connections and programming.

Hopefully neither will mind my reposting them, but they are in the archives anyway:-), and apologies if they come out a bit garbled,

the list doesn't seem too keen at times on AOL's formatting, especially when copying and pasting.

Note that some, but not all, FE5680As require an external +5V supply on Pin 4.



Nigel GM8PZR



-------------------------

 We recently had a customer that purchased an FEI FE-5650A (basically a
repackage version of the FE-5680A) and reported that it worked for several
hours, then died.  We promptly sent another unit, and he reported that it
died as well.  He had nothing but power hooked to the unit.

On return of the first unit, it was examined and found to have corrupted
code.  The corrupted code problem was thought to be associated with doing
bad things to the serial port (like framing errors), and we still believe
this to be the case.  However, the customer said only power was connected
to the unit.

I was asking some questions about how he was powering the unit, when he
said he turned on the power supply (a large HP variable supply) and turned
the voltage up to +15V (our 5650's are single supply).  Ah hah, slowly
ramping the voltage up on these oscillators appears to be a no no.

The second oscillator has now been examined and it too was confirmed to
have corrupted code.  So, the word of warning is - DO NOT slowly ramp the
supply voltage of FE-5680A and FE-5650A oscillators.  I can't say what
slowly is, but this guy was good at killing them.  If I get some time I may
try to repeat the results.

My advice was to set the supply at 15V and just turn it off and on.  I have
not heard from him since.



Skip Withrow



----------------------------------





 I recently bought a pair of FEI FE-5680A Rubidium Frequency Standard

units on ebay (best offer of $150 for 2, free shipping from Hong Kong -

seller tortlex2). It was relatively cheap (compared to EFRATOM units 10

years ago), compact and modern (programmable DDS), perhaps even useful to

replace internal timebase in some test equipment.

 

The first thing I found out is that it wasn't easy to find *reliable

information on them. Many conflicting pinouts, different packaging,

connectors all for the same FE-5680A model. One source (a seller on eBay)

lists pin 3 as +5V, when in fact that is the lock indication output - I ran

the unit like that initially, and it gave 10MHz out for a while (possibly

the ACT240 buffer output diodes taking all the current to back feed the

power) until the output buffer burned (if you get >100mA on the 5V, this is

a good candidate for replacement - it right by the DB-9 connector,

accessible by removing 4 bottom screws (the bottom plate only). I replaced

it and used pin 4 instead, which is the correct one.

 

 Another difficulty was to get the unit to respond to serial commands,

which was ultimately solved after finding the Rosetta stone (a technical

manual for the unit in my configuration). This particular model, part number

217400-30352-1 doesn't respond to the trimpot on the side (at least not with

any change in frequency) and uses the synthesizer for very fine adjustment

of frequency around nominal 10MHz. 32-bit value adjusts in 7E-13 increments

(way below the noise floor of the unit stability).

 

 After my little tribulation in getting to this point, I decided to post

my verified findings, clearly identifying the model number of the unit, to

help others that already have the unit or decide to spring $75 to join this

nuthouse... er... community.

 

    1. Manufacturer: FEI

    2. Model FE-5680A, part number 217400-30352-1

    3. Connector: DB-9M (male) with signals:

 

        pin 1: +15V input (1.7A max when cold starting, 0.6A typ steady

state)

        pin 2: GND (15V return) 

        pin 3: LOCKn (low=locked, high=unlocked) ACT240 output 

        pin 4: +5V input (80mA typ.) 

        pin 5: GND (signal) 

        pin 6: 1pps (about 1us positive pulse each second) 

        pin 7: 10MHz sinewave (~1Vpp on 50 ohm) 

        pin 8: RS-232 RX (receive commands into unit)

        pin 9: RS-232 TX (unit sends responses to pc)

 

    4. Trimpot with external access doesn't seem to do a thing - reportedly

C-field on other models.

    5. Instead of wide range DDS, this unit seems to only generate 10MHz,

which can be fine tuned by programming a 32-bit value through the serial

commands. Make it very fine - my units needed about 140000 counts to change

1Hz - that is about 7E-13 relative change per lsb. '

    6. Serial commands: I found a manual for a slightly different unit

(Googled "FE-5680A manual" and got the top link

<http://www.ham-radio.com/wa6vhs/Test%20equipment/FREQUENCY%20STANDARDS/FE-5

680A/5680%20TECH%20MANUAL.pdf>

http://www.ham-radio.com/wa6vhs/Test%20equipment/FREQUENCY%20STANDARDS/FE-56

80A/5680%20TECH%20MANUAL.pdf ), which listed three commands. 9600,8,N,1

worked fine.

 

        All commands have the general format [cmd] [lenght_lsb] [length_msb]

[xorchecksum] [data] [data] ... [data] [data xor checksum]



        6.1 READ CURRENT OFFSET:  2D 04 00 29

                            sample response:  2D 09 00 24 FF FF FE C8 36

              This command simply returns the current offset value. In this

example it shows offset of FFFFFEC8, which is equivalent to -312 (this unit

had to be slowed down by 2.1E-10 to match my GPS trained reference).  

 

        6.2 SET TEMPORARY OFFSET: 2E 09 00 27 aa bb cc dd cs (aabbccddee is

the 32-bit word to write, cs is the xor of aa,bb,cc,dd)

              no response is given to this command, use 2D command to

verify. Value is lost on power cycle. 

              Example command: 2E 09 00 27 FF FF FE C8 36

 

        6.3 SET PERMANENT OFFSET: 2C 09 00 25 aa bb cc dd cs (same as 2E

command but also writes to EEPROM so survives a power cycle)

 

    One would use the 2C command during a calibration (EEPROM life is 100k

cycles), and use the 2E command for uses like a GPS-trained reference.



 

   7. Notes:  Operating the unit without any heatsink gets the external case

to around 58C (keeps your coffee warm), could lead to premature failure as

ICs get even hotter inside. If you just turn it on for a couple hours at a

time, it might be OK, but a large heatsink on the bottom (or a small fan -

some of the slim hard disk coolers might be appropriate in size).

       I set the offset to -4096, 0 and 4096, measured a long average of 10s

gate readings, then fit a curve to it and calculated where it crossed

10.000000000000MHz, then sent the 2C command to burn it. It is easy to go

overboard, as the resolution offered is normally way overkill and beyond

measurement or stability capabilities of most time nuts (except perhaps for

a couple time-nutcases). For instance, after adjusting to 2e-12 error (in

theory, as this gets in the noise of my measurments), I turned the unit

upside down and got a step change of 3.7E-11 for the 2 g change.If you are a

time nutcase (the type that has 3 cesium clocks but still shows up late to

places) tuning it to the last digit, decide where to mount it, first  :-)



Jose Camara