[time-nuts] Allan Deviation -> more data: GPS1PPS against OCXO/128
Ulrich Bangert
df6jb at ulrich-bangert.de
Sun Oct 29 05:49:15 EST 2006
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
>
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> time-nuts at febo.com
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