Hi The pps sync is done by resetting the counter that generates the PPS. At a 1 ppm frequency offset, it could take 500,000 seconds to steer it in with the OCXO. It unlikely people would wait for over a week for the PPS to line up ….
Bob > On Sep 13, 2016, at 5:58 PM, Scott Stobbe <scott.j.sto...@gmail.com> wrote: > > Interesting discussion about startup. At startup the phase error of the > synthesized PPS is +- 0.5 s. Is this coarsely set to the nearest ocxo cycle > once gps time is established (would make sense to do it this way), or is > the half second recovered steering the ocxo? > > On Tuesday, 13 September 2016, Charles Steinmetz <csteinm...@yandex.com> > wrote: > >> Mark wrote: >> >> I just ran a tbolt (which has been off for a couple of months) and logged >>> the state for a couple of hours... and then remembered something about the >>> initial DAC value setting that I had figured out long ago... it has little >>> to nothing to do with oscillator disciplining. The tbolt drives the GPS >>> from the 10 MHz ocxo. If the ocxo is too far off freq it can't track >>> satellites. The initial dac setting is used to speed up acquisition of >>> satellites and not to speed up the OCXO disciplining loop lock. >>> >> >> Well... by doing the one, it also does the other. >> >> As soon as a satellite is acquired (after a couple of minutes), the DAC >>> voltage jumps and the disciplining starts. A few seconds later when more >>> sats are tracked, it gets underway in earnest (and by then the OCXO is warm >>> enought to be within 0.1 Hz). After 1 hour the box temperature has >>> stabilized and the freq is within a couple of milli Hz. After two hours >>> the oscillator has settled down to the point where the DAC curve goes into >>> "wandering around" instead of following a smooth decay compensating for >>> the oscillator warm-up. The attached image show the first hour of the >>> process. >>> >> >> If you look carefully at the first 3-4 minutes, you'll see it does exactly >> what I described. The DAC reference is 0.510v, and the scale is >> 5000uV/division (=5mV/division). According to the paramaters, the initial >> DAC voltage (INIT) = 0.499v. I assume this was previously stored as the >> DAC value after the Tbolt had fully stabilized, some time in the past. >> >> Sure enough, the DAC voltage starts at just about 0.499v (it looks like >> 0.494v on the graph), and when the second satellite is acquired it jumps >> very quickly to 0.529v -- an overshoot of some 55% -- before settling back >> to ~0.518v, at which time it appears to be on frequency within 1e-8 or so. >> From that point disciplining continues as the crystal warms up. >> >> If one accepted my suggestion, the initial DAC voltage would be set to >> ~0.518v for this oscillator. In that case, it should be within a few >> millivolts of the voltage required when the second satellite is acquired >> and the huge step with its 55% overshoot should be avoided. >> >> I would be very interested to see the result of another dead cold start of >> this same Tbolt, with INIT set to 0.518v. Of course, the time at which the >> second satellite is acquired (hence, the temperature of the crystal when >> discipline begins, and thus, the exact DAC voltage required for a stepless >> transition, will be a bit different from one start to the next, so it won't >> be perfect. But it will be a hell of a lot better than starting from >> 0.499v). >> >> Now -- does what happens during the first five minutes really make any >> difference, given that no time-nut is going to do serious work with a GPSDO >> for at least several hours after a cold start? No, probably not. But we >> are time-nuts, after all, aren't we? >> >> Best regards, >> >> Charles >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/m >> ailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.