[time-nuts] Allan Deviation -> moredata: GPS1PPS against OCXO/128
Ulrich Bangert
df6jb at ulrich-bangert.de
Sun Oct 29 19:50:03 UTC 2006
Hi Bruce,
> 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.
Please allow me to ask: Did you get it from my homepage or did you have
a printed version of it??
Best regards
Ulrich Bangert
> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Dr Bruce Griffiths
> Gesendet: Sonntag, 29. Oktober 2006 14:37
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] Allan Deviation -> moredata: GPS1PPS
> against OCXO/128
>
>
> 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: time-nuts-bounces at febo.com
> >> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Dr
> Bruce Griffiths
> >> Gesendet: Samstag, 28. Oktober 2006 23:46
> >> An: kd7ts at ispwest.com; 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
> >>> time-nuts at febo.com
> >>> 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
> >> time-nuts at febo.com
> >> https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts
> >>
> >>
> >
> >
> > _______________________________________________
> > time-nuts mailing list
> > time-nuts at febo.com
> > 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
> time-nuts at febo.com
> https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts
>
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