Bob, the issue I'm talking about, is how you jostle the divider chain around when you do the phase lock.
You have 1PPS divided down from the OCXO, and 1PPS from the GPS, and in a truly cold start they are completely random in phase and usually off by a big chunk of a second. If you don't reset the OCXO divider chain, you have to steer EFC to get rid of that residual up to a second of phase. That can take a hugely long time and I doubt you are doing that. I think it's more likely, If you do reset the OCXO divider chain, the width of the reset pulse or some other detail, is giving you some systematically positive or negative residual phase to clear and I think you could be interpreting this as retrace when really it's mostly just EFC steering to get around the residual phase. Tim N3QE On Wed, Feb 10, 2016 at 3:45 PM, Bob Stewart <[email protected]> wrote: > Hi Tim, > > This is with the GPSDO that I designed. It uses a PLL which is corrected > for sawtooth. The receiver is an LEA-6T. I've noticed that a few of these > OCXOs continue to retrace upwards in DAC movement, even after a number of > days. I haven't run any of them for more than a week, as I've built only a > limited number of these units. The normal situation is to retrace downward > from the start, once initial warmup is over. > > Bob > > -------------------------------------------- > On Wed, 2/10/16, Tim Shoppa <[email protected]> wrote: > > Subject: Re: [time-nuts] OCXO Retrace > To: "Bob Stewart" <[email protected]>, "Discussion of precise time and > frequency measurement" <[email protected]> > Date: Wednesday, February 10, 2016, 2:15 PM > > I think > what you are observing, is the detail of how the control > loop transitions from frequency locked to phase locked. And > this has more to do with the divider chain reset and phase > detector than anything else. > Note that several of the > hobbyist/E-bay GPSDO's (especially the "Chinese > GPSDO") do not have a phase locked mode, at best they > are frequency locked, and many of these have a consistent > bias based on the gating of the frequency counter (typically > their counter gating adds an extra count, meaning that when > they are frequency locked to GPS they are running slightly > slow). > Tim > N3QE > On Wed, Feb 10, 2016 at > 2:17 PM, Bob Stewart <[email protected]> > wrote: > I've > been running up a number of OCXOs lately, all Trimble > 34310-T. For most of them, the DAC moves in the negative > direction after lock. But there are a few where the DAC > moves more positive. Is this an indication of a different > cut of crystal, or is retrace direction more or less > random? > > > > Bob - AE6RV > > _______________________________________________ > > 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.
