Dave Platt wrote:
In article ,
Rob wrote:

I think the problem is that our GPSDO has a 16-bit DAC and it is
dithering the digital value to obtain the correct frequency. So when
the correct DAC value would be 32000.2 it will do 32000 for 8 seconds
then 32001 for 2 seconds, obtaining a long-term average that is quite
good, but a wobble with 10-second period as well.

Could you modify it, to "scale down" the analog control voltage coming
out of the DAC to a smaller range which "trims" around a center
voltage that you set manually?

This is kind of risky. It may work for some of the units and not for
others. They tend to have a systematic drift, the alignment procedure
is to turn a trimmer in the oscillator so the DAC outputs a value close
to 32768 and then it tends to walk one way. When it approaches the
end of the range (say 5000 or 65000) the center has te be re-aligned.

As original, they drift about 10000 clicks in a year. Narrowing the
tuning range could make it drift too quickly to be practical.

As I said before, the first thing to do is to check the Thunderbolt
in practice. It should be much better than the aging Datums.
(they are so old that they do not even handle GPS weeknumber rollover
which first occurred in august 1999)