Hi A GPSDO is nothing more than a PLL that operates at 1Hz. If you use the same criteria as they do in some systems, as soon as the OCXO pulse is within 1/8 second (sort of 10%) of the GPS, the PLL is locked / converged / doing it's thing / functioning as it should. It's a time domain system, so time is the reasonable way to look at it.
Looking at DAC voltage *assumes* that the OCXO is perfect. Consider a situation when the temperature is continuously climbing. The OCXO has a TC, the DAC will move continuously. The same thing applies to aging / warmup / what ever. Bob On Aug 30, 2013, at 9:36 PM, Bob Stewart <[email protected]> wrote: > Hi Bob, > > I'm not sure I understand what you mean by "45 degree phase error" in a > GPSDO. This isn't a situation where I'm at the right frequency but at the > wrong phase: at least not by my understanding of phase and frequency. Maybe > PLL is even the wrong term. I dunno, but it feels right for this. Let's say > I'm counting the number of OCXO pulses in each second, and in two intervals > 100 seconds apart I have a surplus of 1 count each. I think that means my > 10MHz OCXO is off by 1ppb. Is that right: the reciprocal of 1 error count > during 100 seconds times 10,000,000? And yet my DAC still has about 100 > steps to go before it reaches best solution. So, OK, what is the "magic > number" in ppb or ppt I should consider for convergence? Maybe I should > collect 24 hours of data and see what the average is for the time between DAC > changes and work from there. I suppose 5 counts from best solution would be > fine. By my math that would be 240ppt, or about 417 > seconds between DAC updates. I'm out of my depth on this project. > > > Thanks for your continued help. > > Bob > > > > >> ________________________________ >> From: Bob Camp <[email protected]> >> To: Discussion of precise time and frequency measurement >> <[email protected]> >> Sent: Friday, August 30, 2013 7:31 PM >> Subject: Re: [time-nuts] GPSDO PLL convergence question >> >> >> Hi >> >> Convergence in a PLL has no absolute definition. In some cases 45 degree >> phase error = all is running fine. >> >> I would simply track GPS time vs OCXO time and declare it ok when it's >> inside some magic number of nanoseconds. >> >> Bob >> >> > _______________________________________________ > 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. _______________________________________________ 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.
