[GPS_Standard] Trimble 34310-T oscillator stability?

[Dave Platt]

> Hello the list.  I just built a VE2ZAZ GSPDO.  This is my first GPSDO, so I don't really know what to expect.  I chose (randomly) a Trimble 34310-T as my oscillator.  It seems to wander around a bit, and I was wondering what I should expect from this oscillator.  What I am seeing is up to a +/- 4 count delta per 16 second sample.  The DAC is chasing it around quite a bit, as well.  I'd have to look, but over a 24 hour period, it seems to be +/- 0x0020 or so.  Is this normal?  I'm driving things with a Motorola Oncore UT+ in timing mode (or whatever the term is) with an indoor active puck antenna.  The SynTac software doesn't show anything wrong with the UT+, and it maintains lock with enough satellites that it should be good.

I suspect that you are seeing jitter from the GPS.  I see similar sample-period
deviations using an Efratom oscillator, and if I compare the oscillator output
with a rubidium oscillator there's negligible drift or jitter.  As I understand it,
GPS reception can be expected to jitter as a result of atmospheric disturbances,
reflections in the signal path, etc.  In tracking mode this shows up as
positional inaccuracy, in timing mode it shows up as jitter.

Hence, the need for a fairly long time averaging period... average out the
jitter so that your tracker is compensating for actual oscillator inaccuracy
and not chasing jitter.

DAC wander over a day can be due to temperature variations, either
in the oscillator itself or in the voltage regulators which create the
reference for the DAC voltage circuit.

Take a look at the data for my GPSDO... you can see both the sample-
to-sample error jittering around, and also the DAC compensating for
temperature change in my garage (the DAC voltage correlates very
well to our local weather).

http://snulbug.mtview.ca.us/dave/fll.png