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

[Bob kb8tq]

Hi

If simulation is going to be precise, you need a pretty good noise
model for all of the various things that get in the way. You can come
up with those models, but it isn’t going to be easy. Generally, once
you get the device running, you switch to working with it rather than 
doing simulation. 

Either way, you still are tracking down nutty little issues. What your
issues are will be different than what somebody else’s are.  Your DAC
sum doesn’t quite do things right. The sensitivity of the TCXO changes
with EFC voltage ( or frequency offset, whichever you prefer ). The 
GPS module does something weird every 1219 seconds. Lots of fun
stuff to chase down. This *is* why you need to be able to run comparison
tests and likely a lot of comparison tests. 

Getting 1x10^-9 frequency accuracy out of a TCXO based GPSDO is 
not easy. 1x10^-9 ADEV at 1 second is a bit easier. The “best of the best”
with a TCXO is not a lot better than your frequency goal. That “best of the
best” likely does not use a $1 TCXO. ( Which is in no way saying that 
you don’t have a pretty good TCXO….).

Taking care of the corner cases is a part of GPSDO design. It needs to 
be done simply to keep the device from going nuts far more often than a 
reasonable customer would tolerate. 

What this or that design will run into is somewhat unique to that design. This 
BOM / spec will have a different set than another BOM and another spec. 
They both will bump into bothersome corners pretty often. 

So what to do:

1) Start looking at the DAC voltage with something that gets you into a 
voltage resolution equivalent to <= 1x10^-10 frequency. 

2) Toss a beach towel over the breadboard and see if that helps. ( = 
take out random drafts ).

3) Monitor the PPS out of the GPS module against your Rb to see how
it’s doing. ( = does it jump right at the points the frequency goes nuts …)

4) Look at the TCXO in “free run” in the breadboard. How stable *is* it? 
Phase and frequency plots are the thing to look at. ADEV comes later. 
Manually tune the DAC to get it close. 

Bob

> On Mar 5, 2022, at 3:48 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> 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.
> 
> 
> <TCXO_drift.csv><GPS_good.csv><Freq_diff.png><Phase_diff.png><ADEV_tuning.png><Phase_diff_residue.png>_______________________________________________
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