[time-nuts] Tuning a GPSDO loop for optimal disturbance handling

[Erik Kaashoek]

For any good GPSDO you may want to eliminate all kind of disturbances 
but in a cheap portable GPSDO this may not be possible.
To learn about tuning the parameters of such a GPSDO the phase versus a 
Rb of a 10MHz TCXO exposed to realistic temperature changes was 
recorded. (attached)
The changes in temperature caused frequency errors up to 2e-8 with 
frequency changing up to 5e-9 in one minute, far above the target 
frequency stability of 1e-9.
The phase error ranged between +1.2e-5 s and -1.8e-5 s, also much larger 
than the maximum phase error target of 1e-7 s
Next a recording was made of the phase versus a Rb of the PPS of a GPS 
with good antenna location (attached)
Using these recordings a simulation for various loop parameters was done 
using the gpsim1 tool made by Tom Van Baak.
Timelab plots of the frequency difference, phase difference and ADEV 
where made showing the TCXO in red and the simulated performance of the 
controller for various kp and ki values
Although experimenting with different parameter tuning will for sure 
require more time the following learning's where collected
A kp of 0.01 sufficiently reduced the impact of the jitter in the PPS
A ki of 0.0001 was not enough to reduce the phase error below 1e-7, a ki 
of 0.001 was needed but this led to some instability.
Increasing the kp to 0.05 removed the instability in the phase but the 
impact of jitter in the PPS on the frequency error grew to just below 1e-9.
Decreasing the kp below 0.01 caused too much instability when a ki of 
0.001 was used.
No useful value of kd was found as increasing the kd caused the PPS 
jitter to directly impact the frequency error.
The value of kii set to to zero for all simulations.
Next step is to measure the actual performance of the GPSDO using these 
parameters and to simulate and test the impact of a not so good PPS.
Any suggestions for directions of further tuning exploration?
Erik.


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