[time-nuts] Mentorship needed in learning about Allan Deviation and variation.

[Tom Van Baak]

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