Hi Poul, I'm not really sure what you mean by the term "timeconstant" WRT my GPSDO and not some other algorithm.. So, to avoid a discussion of that, let me just post links to some plots and note that there are very few and very small corrections for the position term by the PID controller. I'm using a sort of deadband filter for the p term damping set at 10 counts summed for direction.
The first is to the last 2 hours on the GPSDO, and it has a lot of information on it. Blue is the DAC. It has a lot of bits, so it's scaled down to make it usable. True values are on the left in hex, though the resolution is multiplied by an additional 3.75 or so in hardware. The red is the phase in hundreds of ps measured by my TIC after correction for quantization errors. The green is raw ambient temperature which obviously doesn't have enough gain to be useful. The purple is the TI of the Rb to the GPSDO output in ns as a comparison for the final plot. http://evoria.net/AE6RV/TIC/GPSDOe.png The next plot is an ADEV of approximately the same timeframe for the corrected TIC from above. The green is the TIC the blue is the ADEV. http://evoria.net/AE6RV/TIC/TIC.png And finally is an ADEV of approximately the same timeframe of the Rb against the OCXO output as measured by my 5335A. Here the Rb phase is unwrapped. http://evoria.net/AE6RV/TIC/Rb.png Bob ________________________________ From: Poul-Henning Kamp <[email protected]> To: Bob Stewart <[email protected]>; Discussion of precise time and frequency measurement <[email protected]> Sent: Wednesday, August 27, 2014 12:32 PM Subject: Re: [time-nuts] OCXO Phase Noise Measurement in Primitive Conditions -------- In message <[email protected]>, Bob St ewart writes: So here is a pretty interesting way to optimize a GPSDO that I've been playing with for some years. I don't have a formal mathematical formulation of it. It is somewhat related to what Dave Mills calls "the Allan intercept" except this you can actually measure and not just estimate. You run several (long!) test-series with different timeconstants in your PLL, and you record the resultant EFC and phase offset as a function of time. If your timeconstant is too short, you will have a lot of high-frequency signal in the EFC, too long and you get too much high-frequency signal in the phase offset. The optimal timeconstant is where you have the least sum of spectral power where the two curves cross each other. My experience so far is that the curve around the optimum is very flat, getting the timeconstant wrong by a factor of two hardly changes the resultant performance. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [email protected] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
