[time-nuts] Allan Deviation -> moredata: GPS1PPS against OCXO/128
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sun Oct 29 16:40:29 EST 2006
Ulrich Bangert wrote:
> 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
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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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>> time-nuts at febo.com
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>>
>>
>
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Ulrich
I just used the link you posted a few days ago.
Bruce
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