[time-nuts] Re: GPSDO Control loop autotuning

[Erik Kaashoek]

Hi Tobias,

I'm very new to all this GPSDO stuff and going through similar learning 
but maybe I have some remarks/questions that could be relevant for you.
- Developing control algorithms is an extremely long process unless done 
on a simulator. Tom Van Baak has an excellent simulator and several 
realistic data sets for testing. http://www.leapsecond.com/pages/gpsdo-sim/
- Use Timelab to look at the ADEV, the frequency error and the phase 
error to evaluate the performance.
- Why do you prefilter? In my experience this only adds a time delay in 
the input data for the loop and makes it harder to do a good job. Test 
if the prefilter works by taking one of the GPS data sets from Tom and 
run it through the prefilter and test using the simulator if the results 
improve.
- In my GPSDO temperature changes and supply voltage variations where 
the largest source of variations. You can simulate this with the 
simulator by creating LO data sets with artificial changes in frequency. 
Add a drift and test if the loop can keep up, add a jump and see how 
fast it recovers
Erik, PD0EK

On 20-3-2022 21:46, Pluess, Tobias via time-nuts wrote:
> Hi Bob,
>
> I see your point on quickly moving the OCXO. However of course this is NOT
> what I do. To be precise, my GPSDO does this exactly once after powerup, to
> quickly align the PPS. After that, the control loop takes over and steers
> the OCXO according to the error signal.
> I also have already implemented the algorithm that switches the control
> parameter sets: just after powerup, a "quick" set is used, that quickly
> brings the OCXO to the right frequency but also lets the DAC work quite
> hard. If the time error stays below 100 ns for a couple minutes, an
> "intermediate" control set is used with longer loop time constants. If the
> time error stays below 100 ns for a couple minutes, the "slow" control set
> is used which, currently, has a loop time constant of one hour. The DAC
> ouput changes very rarely, about one count up or down every couple minutes.
> What I wanted to achieve with this autotuning is to find out whether 1 hour
> is a good time constant. Should it be longer? shorter? what would be the
> best value?
>
> Since the GPSDO already has a TIC built in, that measures the time interval
> between the two PPS, I thought it must be, somehow, possible to assess the
> current performance of the GPSDO. Lets say by estimating the ADEV. Based on
> that value, it should then be possible to adapt the loop time constant,
> until some sort of optimum is found. No?
>
> Best
> Tobias
> HB9FSX
>