I still get generally about a 400 ppt swing with
the OSC, about 20 ns swing with the PPS and the
other things are just what they are. Different
filters and surveys don't seem to change anything that I've seen.
* * *
Is this acceptable?
You seem to have a reasonably decent view of the
sky, judging by the numbers of satellites you are
able to track, but the signal quality is lower
than I expect (note: my only Trimble GPSDO
experience is with TBolts). I'd expect the
signal reports to be mostly from 40 dBc upwards,
while yours run from low 30s to barely 40 (IIRC,
this hasn't changed materially as you've moved
the antenna multiple times). Your results look
consistent with those signal readings. I don't
recall -- have you tried a different
antenna? It's also possible that the front end
in that particular GPS is noisier than the TBolts I'm familiar with.
Also, I see that the oscillator loop gain is set
for 1.4 Hz/v. I have no idea what is normal for
the OCXOs used in that unit. Have you verified
this yourself? (Turn disciplining off, then
manually run the DAC voltage up and down while
watching the frequency on a counter.) Lowering
the GPSDO loop gain may improve matters. Or
not. (IIRC, this may mean setting "gain" to a
higher number -- the "gain" does not SET the loop
gain at that value, it tells the digital loop
"expect the oscillator to have this EFC characteristic.")
I did figure out the temp thing and found a link
where it's explained, but I have a new chip
coming and expect to change it, but I doubt if
anything will change except the reported temp
will most likely track to less than 1° C unlike it's doing now.
That would be my expectation, too.
Would it be worth looking into the MV89A OCXO as
a replacement to get better ppt? I haven't done
any looking into my OCXO yet to see what it's spec sheet may be.
That depends on whether or not the remaining
errors you are seeing are attributable to the
oscillator. The remaining errors can be classed
broadly as (i) noisy/wandering OCXO, and (ii)
adjustments to the DAC that the GPS thinks it
should make. In your case, they look more like
class (ii) than class (i) to me. Before you
switch out the OXCO, let the crystal that is in
the unit now settle down for a few months of
uninterrupted running. The class (i) errors
should go down by themselves. If the errors you
see do not go down, it is more evidence that your
limitation is noise in the GPS, not the OCXO.
Best regards,
Charles
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