Joe,
> I log the output of the TIC, in nanoseconds, and use that file to
generate an ADEV plot.
Good. That's what you need. During normal operation those readings are
bounded by the PLL. So it's essentially a measurement of how well the
PLL is working, how aggressive the OCXO is steered, etc. ADEV isn't the
best way to process that kind of data because it's a boring, even
misleading, straight line going down forever.
> Maybe my setup is in fact comparing itself with itself?..!
Yes. Oops. But, here's an idea for you.
One useful technique is to have your GPSDO running fine and then
*disable* the disciplining. If you designed the GPSDO you'll know the
exact spot in the h/w or s/w to do this. From this point forward your
OCXO is still running, your GPS/1PPS receiver is still receiving, the
TIC is still comparing, and you are still logging TIC readings every
second. But now the DAC is frozen and the OCXO is free-running.
When you plot this data you will see phase slowly wandering away from
zero, you may see a slight drift in frequency, and mostly what you will
see is the "bathtub" ADEV shape that you were looking for. This method
works because as soon as your disable disciplining your OCXO becomes
independent of GPS and so the ADEV plot will be a measurement of an
oscillator instead of a measurement of a PLL.
This is not something you would do everyday, but especially now that you
are understanding how a GPSDO works and playing with Allan statistics
it's a educational exercise.
The Trimble Thunderbolt (aka TBolt) GPSDO has this disable-disciplining
feature. Note it's not "holdover"; that's something else entirely.
/tvb
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