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

kd7ts kd7ts at ispwest.com
Sat Oct 28 11:30:06 EDT 2006

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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