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 <[email protected]> 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 <[email protected]> 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 <[email protected] >> <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 <[email protected] >> <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 -- [email protected] <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 -- [email protected] <javascript:;> >>> 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] <javascript:;> >> 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.
