rickman 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.

Are you referring to the voltage used to control the VCXO? Not sure
what parts you can tweak, but I don't see why the dithering can't be
sped up (say 1 kHz) and low pass filtered to produce an actual value of
32000.2.

Probably what the box does is check every second what the frequency
error is and change the DAC output. It works out as I described.
But indeed, it would be better to add some extra precision that way.
Those Datum units are over 20 years old...

I Have seen and considered these before. That is indeed a GPS referenced
rubidium standard. Of course we always prefer stuff that is either
cheap or available as surplus (like the Datum and Trimble) :-)

One man's "cheap" is another man's "precious". You'd have to use a
number for me to know what you consider "cheap".

We run a co-channel diversity repeater with 5 transmitter sites (and three
times as much receiver sites), from donations made by individual hams.
We get lots of surplus equipment donated, that is how we got a large
pile of Datum 9390 units from a decommissioned pager network.

Spending $5000 on new GPSDOs is a bit difficult for us, we also have other
expenses. Fortunately Trimble Thunderbolts are cheaper and look a lot
better on the scope. I need to finish a bit of monitoring code (that we
use together with "Lady Heather's Disciplined Oscillator Control Program")
and we can start testing how much difference there is in practice.

We also want to try software-defined FM modulation of the signal instead
of the analog modulator that of course also introduces awful frequency
differences because of imperfect alignment.