David, I do not know whether your design enables you to measure this with the necessary precision or not, but in general questions for the sources of such effects are more easily answered if NOT the observed frequency fluctuation is specified but the sudden phase offset value that the regulation loop is reacting to by the frequency fluctuation.
I have dealt a lot with such sudden phase offsets in my DIY GPSDO because I wanted to be able to recognize outliers which should not have an influence on the loop response. The experience out of it is: 1) Sometimes there will be sudden phase offsets that are more or less due to errors in the phase comparator circuits, like a sudden glitch that makes the phase comparator measure a wrong value. Errors like that are produced for example by spikes on the power lines. It is characteristic for that kind of error that it has a certain statistical probability to appear but when it appears it has a negative influence on a SINGLE phase measurement only and does not produce a "history". Errors like this may come from the receiver circuitry as well. A analogue pll @ 10 kHz with a time constant of several seconds should not react heavily on effects like that. 2) Then there is a second source of sudden phase offsets that is more serious: If there is a divider chain after the LO which most GPSDOs will have down to 1 pps or 10 kHz in the G3RUH design then there are chances that one or more flip-flops in the divider chain may change its state due to an unwanted glitch. Because the divider chain has a "memory" in the sense that its next state always depens on the previous state, this will result in a permanent phase offset that a "normal" pll will react on (try to remove it) ALTHOUGH the LO's FREQUENCY may be perfect all over the time. It is characteristic for this kind of effect that the measured phase offset is an integer multiple of the lo's period length. 3) Depending on fundamental changes in the geometry of the observed sats, i.e. a sat dissappears below the horizont or a new one appears above the horizont, the receiver must "re-think" its strategy on which signals to rely more and on which less, which gives rise to sudden changes in its output timing in the order of typically 20-30 ns. I have observed a lot of events when the phase offset suddenly changes by 30 ns or so stays so for some minutes and then falls back by the same amount as if the receiver had now "learned" the new situation. The things that you apply to like Satellite Elevation Mask Control a.s.o. have only an impact if 3) is the issue that you observe. So you should try to check whether 2) or 3) is the case. Best regards Ulrich Bangert > -----Ursprüngliche Nachricht----- > Von: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Im Auftrag von David Smith > Gesendet: Donnerstag, 10. April 2008 13:05 > An: [email protected] > Betreff: [time-nuts] Jupiter GPS Settings For Best Timing Performance > > > I've been working on the performance of my portable GPSDO > (very similar > to G3RUH design) and have got down to a reasonable level. Testing > against an HP3815A, it will remain within better than 1x10^-10 for > periods of 10-15 mins. > > However, there are times when it seems to get a "hit" and it > will then > move up to ± 5x10^-10 before settling down again. The > movement seems to > follow the time constant of the PLL loop filter, possibly indicating > that it's coming from the Jupiter GPS? > > Reading the Jupiter doco (Zodiac GPS Receiver Family > Designer's Guide), > there seem to be several Zodiac Binary messages that can be used to > alter the GPS's performance (e.g. Satellite Elevation Mask > Control). I > already have a processor talking to the GPS card and displaying > position, time, gridsquare, status etc. It would be a simple > matter to > have it tweak the parameters as required. > > Does anyone have any experience with this? > > Regards, > David Smith > > _______________________________________________ > 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.
