Steve Rooke wrote: > Mark, > > 2008/12/30 Mark Sims <[email protected]>: > >> A long time ago, I did something very similar with an early release Magellan >> 5000 board (and a couple of other models of GPS cards) that did not >> implement the 1PPS signal. I think the best time sync that I ever got was >> around 1 mS (most boards were around 10 mS). >> > > I'm not actually looking at time syncing, only making a GPSDO. > > >> True 1PPS signals are implemented and synchronized in hardware. The NMEA >> messages are generated in software. Due to the vagaries of things like >> software interrupt latencies, serial port bit synchronization, etc I doubt >> that you could ever get better than 1 serial bit time of accuracy out of any >> board and 10-100 bit times is more likely. At 4800 bits/sec the >> jitter/offset of even 1 bit time would be huge at best. Perfectly OK for >> things like clock displays, but totally inadequate for things like >> oscillator locking. >> > > Ok, so I can see what you mean about jitter but would that not be > taken care of by a low pass filter with a reasonable time constant > following phase-frequency detection between the PPS and a divided down > ocxo? Agreed the short term stability (ADEV for small tao?) would be > poor but would it not improve significantly in the longer term (ADEV > for large tao?)? As for offset against an absolute PPS synced to GPS > time, well this is not important to me. > > 73, Steve > Steve
Instead of waxing lyrically try doing some estimates of the averaging time required for a given accuracy: You need to measure the ADEV of the reference source (ie the relevant NMEA sentence). As a crude model say the jitter is 10 millisec of white phase noise and you want the GPSDO to be within 1ppm for averaging times from 1 sec to a time equal to the loop time constant then the loop time constant will have to be about 3 hours. If you hope to achieve 1E-9 averaging over something like 3000 hours will be required. Whilst one can achieve lower noise by averaging the GPSDO over a day or so, this may not be what you want. Aging and thermal drift of the OCXO over 3000 hours will make it impractical to achieve a stability of 1E-9. Bruce _______________________________________________ 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.
