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
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