[time-nuts] Allan Deviation -> more data: GPS 1PPS against OCXO/128

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Oct 28 17:46:23 EDT 2006


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
>
>
>
>
>
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>   
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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