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?

I did something of that sort when building a VE2ZAZ GPSDO. This has
only a 10-bit DAC (or 12, I can't remember for sure) which is dithered
fairly rapidly to add a couple of additional bits of precision. I
low-pass-filtered the DAC output pretty heavily and then used some
resistor scaling to reduce its effect to a few percent of the total
5-volt control range that my OCVCXO allows. The "coarse" setting is
via an external potentiometer, driven from a good 5-volt precision
reference. In addition to the oven for the crystal, I added a
separate thermistor/heater board which levels out the temperature in
the whole enclosure, which I then insulated pretty heavily.

This approach wouldn't change the period of the wobble but would
reduce its magnitude by whatever divide-down ratio you chose to
implement.