[time-nuts] Optimizing GPSDO for phase stability

[Erik Kaashoek]

The GPSDO/Timer/Counter I'm building also is intended to have a 
stabilized PPS output (so with GPS jitter removed).
The output PPS is created by multiplying/dividing the 10MHz of a 
disciplined TCXO up and down to 1 Hz using a PLL and a divide by 2e8. No 
SW or re-timing involved.
The 1 PPS output is phase synchronized with the PPS using a SW control 
loop and thus should be a good basis for experiments that require a time 
pulse that is stable and GPS time correct.
As I have no clue how to specify or evaluate the performance of such a 
PPS output I've done some experiments.
In the first attached graph you can see the ADEV of the GPS PPS (PPS - 
Rb) and the 1 PPS output with three different control parameters (Tick - RB)
As I found it difficult to understand what the ADEV plot in practice 
means for the output phase stability I also added the Time Deviation 
plot as I'm assuming this gives information on the phase error versus 
the time scale of observation.
Lastly a plot is added showing the Phase Difference. All plots where 
created using the linear residue as the Rb used as reference is a bit 
out of tune.
Also the TIM files are attached
The "PPS - RB" and "Tick - RB Kp=0.04" where measured simultaneously and 
should show the extend to which the GPS PPS is actually drifting in 
phase versus the Rb and how this impacts the output phase of the 
stabilized output PPS.
My conclusion is that a higher then expected Kp of 0.1 gives the most 
stable output phase performance where the best frequency performance is 
realized with a Kp = 0.04
I welcome feedback on the interpretation of these measurements and the 
application of output phase stabilization.








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