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