Ulrich Bangert wrote: > 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 > > Ulrich
I read your paper in the AMSAT Journal and believe that an English translation of this would be very informative to those who cant read German. Bruce _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
