[time-nuts] Ebay FE-5680A Rb: Are they that good?

Bob Smither smither at c-c-i.com
Sun Dec 11 11:31:47 EST 2011

Bob Camp wrote:
> Hi
> If you dig into the circuit, the resolution of the DDS is indeed 7x10^-13 per LSB.
> Bob

Thanks for the confirmation.

> On Dec 9, 2011, at 9:51 PM, Bob Smither wrote:
>> Bob Smither wrote:
>> <snip>
>>> My documentation states that the full range of adjustment is:
>>>  7f ff ff ff => +383 Hz
>>>  80 00 00 00 => -383 Hz
>> This scale factor is clearly wrong - using it and applying an appropriate correction changes the frequency way too much.
>> From:
>>  http://article.gmane.org/gmane.comp.time.nuts/9473/match=fe+5680+pin+outs
>> (thanks Jose!) a better estimate is 140000 / Hz.
>> Using data after 20 hours:
>>  http://www.c-c-i.com/node/155
>> the estimated frequency offset is 1.36E-3 Hz.  Using the above scale factor gives a correction of -190 counts.  Here is the command I used to (temporarily) program the new offset:
>>  2e 09 00 27 ff ff ff 42 bd
>> (the bd is the exclusive-or checksum of the offset)
>> Now I wait ...

After 13 hours with the above correction the offset appeared to be ~ +3E-4 Hz or 
  3E-11.  This is close to the expected aging of the unit (2E-11 / day).

A plot showing the reduced drift from the offset correction of -190 is here:


Adding a correction for the +3E-4 Hz to the previous correction gives a new 
offset of -229 counts.  Shortly after I got this set up, a power line glitch 
reset things so I took the opportunity to adjust the time interval to ~ 100 
usec.  I then applied the -229 counts as:

   2e 09 00 27 ff ff ff 1b e4

(the e4 is the checksum) and starting measuring again.

It looks like this is chasing the aging of the unit.  More later.

Bob Smither                                              Smither at c-c-i.com
A well regulated Militia, being necessary to the security of a free State,
the right of the people to keep and bear Arms, shall not be infringed.
        -- Second amendment to the Constitution of the united States
Circuit Concepts, Inc.                                        281-331-2744
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