Bob, that is an excellent proof by contradiction. The reason I asked is on the plot Mark shared that first rising edge is pretty sharp for a system with a 500 s time constant.
On Tuesday, 13 September 2016, Bob Camp <kb...@n1k.org> wrote: > 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 > <javascript:;>> 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 > <javascript:;>> > > 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 <javascript:;> > >> 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 <javascript:;> > > 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 <javascript:;> > 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.