[time-nuts] Allan Deviation -> more data: GPS1PPS against OCXO/128
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sun Oct 29 08:37:19 EST 2006
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
>>
>> _______________________________________________
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>> time-nuts at febo.com
>> https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts
>>
>>
>
>
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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
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