[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
>>> 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
>>
>>     
>
>
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>
>   
Ulrich

I just used the link you posted a few days ago.

Bruce



More information about the time-nuts mailing list