[time-nuts] Re: GPSDO Control loop autotuning

[Bob kb8tq]

Hi

If you want “best” jitter compared to the GPS PPS, put in a very wide 
band loop and move the OCXO very quickly to match the PPS. That
will give you best performance by that measure. If you go to every 
tenth sample, the same process will occur, just at a slightly different
point. 

Since the purpose of the loop is to *attenuate* the jitter passed from 
the GPS PPS to the OCXO, this sort of tuning is normally counter 
productive. 

The best way to work out the constants is to measure the noise on the 
GPSDO output. Then select sets of loop constants that give reasonable
performance at various time constants.

With an ideal OCXO, the longest time constant would pretty much always 
be best. If we had zero drift / zero aging “ideal” OCXO’s there would 
be less demand for GPSDO’s :) 

With a real (as opposed to ideal) OCXO, a long time constant will have
it wandering all over the place. The DAC will be cycling like crazy as a
result. This stuff is what most constant switching algorithms focus
on. Not quite the same as auto tune since the constants have already
been worked out and you are simply switching from set A to set B.

Bob

> On Mar 20, 2022, at 1:58 PM, Pluess, Tobias via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi all,
> 
> I am still working on my GPSDO also now with added NTP time server. This
> one is still work in progress, though.
> In the meantime I found another interesting problem. Currently, my GPSDO
> uses a PI controller to steer the DAC voltage of the OCXO. This proved to
> work very well. (see my source code at https://github.com/tcpluess/gpsdo).
> Currently, I use manually determined KP and KI constants for the control
> loop. I can adjust them via serial port, of course.
> I am thinking about whether it would be possible to implement some sort of
> autotuning to this controller to find the "best" set of control
> parametersthat give the least jitter.
> 
> How could I do this?
> 
> one idea is the following:
> since I have a TIC that measures the time interval between the GPS-PPS and
> the OCXO-derived PPS, I could collect, for example, the last 1000 samples
> of this time interval and calculate the 1sec, 10sec, 100sec and 1000sec
> averages of this time interval. If the control loop operates "well", the
> time intervall should become 10 times smaller when the averaging time is 10
> times increased. On the other hand, if the control loop is too slow and the
> OCXO is drifting, this assumption will not hold. In the first case, we can
> increase the integration time for the control loop (make the loop time
> constant larger), in the second case we have to decrease the integration
> time (make the control loop faster).
> I wouldn't run this autotuning all the time, just maybe once to determine
> the optimal loop parameters. However the averaged time interval samples
> probably provide some interesting info....
> Is there a way to estimate the ADEV on-the-run without having to collect
> millions of samples?
> I have enough RAM left on my microcontroller to collect maybe 1000, or
> probably even 2000 samples. It would be interesting to do something with
> this info.
> 
> 
> best
> Tobias
> HB9FSX
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