Hi bob, thank you for the polite response regarding rise time, indeed I would fully agree.
The rise time I was referring to was the DAC efc value in the plot mark had previously attached. He just included a second plot as well. It would appear the tbolt is doing something else aggressively before going into a PLL, perhaps coarsely phase steering the last +-50 ns, but then runs out microseconds in error? On Tuesday, 13 September 2016, Bob Camp <kb...@n1k.org <javascript:_e(%7B%7D,'cvml','kb...@n1k.org');>> wrote: > Hi > > The rise time of the edge is not a good measure of the accuracy of the > timing It > simply is a way to look at how fast your gate can ramp a signal. > > If you do a long term comparison of the frequency vs time and the time > error vs time > you will see that a tight (small) damping keeps the time close at the > expense of > jerking the frequency around a lot. A loose (large) damping does not > change the > frequency much, but the time wanders quite a bit. > > Simply put: There is no free lunch. > > Bob > > > On Sep 13, 2016, at 9:01 PM, Scott Stobbe <scott.j.sto...@gmail.com> > wrote: > > > > 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/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/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.