I agree: better using a cheap counter (using the GPSDO to sync it). However to do a time interval measurement your sampling frequency must be stable enough between the reference PPS and the PPS-under-measure. The trick is to offset the PPS-under-mesure enough (say 100mS) to gether some stable samples from your sampling clock. Moreover, to get the best from averaging, I think that the sampling clock must not be synchronous with either the reference or the PPS-under-measure.
On Fri, Dec 2, 2011 at 8:39 AM, mike cook <[email protected]> wrote: > Le 02/12/2011 01:53, Bob Camp a écrit : > > Hi >> >> Ok, 62.5 ppb at 1 second would be 62.5 ns. that sounds right for a 16 MHz >> clock. >> >> Your accuracy will be related to the offset between the two 1 ops events >> (divided TCXO and GPS PPS) and the accuracy of your crystal. >> >> With some luck you can get the crystal frequency to< 1 ppm by counting >> it against the GPS pps output. If the TCXO pps is within 100 ms of the GPS >> pps, you should get about 100 ppb accuracy. The trick is to keep the TCXO >> pps that close. Can be done, but it takes some fiddling. >> >> I don't think that the measurements have to be done concurrently. > > The GPS pps signal will have a low ns jitter, possibly defined in the > data sheet, that can in this context be ignored if measurements are take > over a long period as can that for the RTC. > > So for me, I would characterise the micro first, counting cpu cycles per > GPS PPS period. If that is done over enough periods, a good indication of > the measuring devices rate against a known standard will be known. (1) > Then measure the RTC pps using the same procedure. (2) > Finally adjust the result from 2 by that of (1). > > However, even that will be more costly than renting a counter. > > > > ______________________________**_________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/** > mailman/listinfo/time-nuts<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.
