Hi Bob, You've given me a lot to work with. Perhaps the best answer is to put in a user parameter for how many seconds between updates to consider a lock condition. For warmup, I had planned to put in a 5 minute holdover period, but that could easily be user configurable, as well.
Thanks again, Bob >________________________________ > From: Bob Camp <[email protected]> >To: Discussion of precise time and frequency measurement <[email protected]> >Sent: Friday, August 30, 2013 8:55 PM >Subject: Re: [time-nuts] GPSDO PLL convergence question > > >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. > > > _______________________________________________ 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.
