Bruce, > If GPSDO did some statistical filtering instead of just blindly > accepting all PPS signals as valid and usable such dropouts > would cease > to be much of a problem. > There's no substitute for a a correctly engineered design with an > appropriate tracking loop bandwidth and statistical filtering > of outliers. A good crystal will drift very little over half > an hour or so when the > GPS derived PPS signal may be unreliable.
I have read lots of intelligent stuff from you in the last weeks that makes you a brother in mind, but let me explicitely say THANKS for this one. I have been using robust statistical methods in my own GPSDO design since years now. Every new second I compute the median over some hundred seconds of past phase data and after that i compute the MAD (median absolute deviation) over the same period. The MAD is is a measure for the width of the statistical distribution as is the standard deviation. Unlike the standard deviation, is it completely insensible to outliers itself. 99% of "normal" data are within +/-5 MAD around the median so once you have performed the math it is really easy to detect outliers. Since the algorithm needs a certain amount of RAM and sheer processing power this is not easily done with single-chip-processors. Thank you for pointing at the fact that sometimes a certain complexity of hardware and software is necessary to get a job done and that the quality of a GPSDO cannot be measured in term of low&cheap parts count as seems to be a quite common opinion. Best regards Ulrich Bangert > -----Ursprüngliche Nachricht----- > Von: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Im Auftrag von Dr Bruce Griffiths > Gesendet: Samstag, 28. Oktober 2006 23:46 > An: [EMAIL PROTECTED]; Discussion of precise time and > frequency measurement > Betreff: Re: [time-nuts] Allan Deviation -> more data: > GPS1PPS against OCXO/128 > > > kd7ts wrote: > > Didier Juges wrote: > > > >> There are sudden increases in noise (bursts that last from > seconds to > >> minutes) on the plots I posted. > >> > >> I believe the sudden and drastic increase in noise at times comes > >> from > >> the GPS loosing lock. At the moment, I cannot hook up the > computer to > >> the GPS and verify, but I will do that later. > >> > > > > -------- > > > > I have a Brooks Shera GPSDO that exhibited similar symptoms. > > The phase showed huge jumps around 4:00 - 4:30 every > > morning. The PLL loop might, or might not recover, but > > usually didn't. I didn't have the time to spend > > troubleshooting, and we seldom ran tests overnight, so I > > just lived with it for more than 5 years. > > > > I retired recently and finally had the time to devote to > > finding the problem. It was so easy, it is almost > > embarassing. I picked up another GPSDO system based on a > > Jupiter GPS engine and an Isotemp ovenized 10 MHz oscillator > > with EFC. It was the antenna I purchased to go with this, > > that turned out to be the useful missing piece of the puzzle. > > > > I swapped antennas between the two units to compare the SS > > numbers reported by the Motorola UT+. They appeared to be > > about the same, so I swapped them back. This continued for > > another week or so, and I exhausted all remaining > > possibilities. I swapped the two patch antennas again, but > > this time I let it run for a week. I never observed the > > problem during this time, so I replaced the patch antenna > > (cheap) with a Symmetricomm antenna that is commonly used on > > Cell sites. The system has been 100% for about 3 weeks now. > > > > I beleve the Symmetricomm antenna has much better filtering, > > and because it has an "N" connector, I was able to use a > > longer cable, with lower loss and better mounting location. > > > > Watching the SS numbers reported by the UT+ did not provide > > any insight. They were generally between 43 and 47 and > > tracking 8 with the patch antenna. I have been watching the > > numbers for about 2 weeks with the Symmetricomm antenna > > connected, and they show between 47 and 52 and tracking 8. > > > > I can only speculate on the exact mechanism, but it appears > > that the system is functioning properly. > > > > It is the station reference for 10 and 24 GHz transverters > > and a DSP-10 IF rig. > > > > We have 5 of these GPSDO units in the area, and all I ever > > heard was, "well mine runs just fine !" > > > > > > > > Mike KD7TS > > > > > > > > > > > > _______________________________________________ > > time-nuts mailing list > > [email protected] > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > > Good timing antennas have built in ceramic or equivalent bandpass > filters to minimise the effect of interference. > A patch antenna is not as satisfactory as a quadrifilar helix > or a choke > ring ground plane antenna for accurate timing purposes. > > If GPSDO did some statistical filtering instead of just blindly > accepting all PPS signals as valid and usable such dropouts > would cease > to be much of a problem. > There's no substitute for a a correctly engineered design with an > appropriate tracking loop bandwidth and statistical filtering > of outliers. A good crystal will drift very little over half > an hour or so when the > GPS derived PPS signal may be unreliable. > > Bruce > > _______________________________________________ > time-nuts mailing list > [email protected] > https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts > _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
