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

[Didier Juges]

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
>
>
>
>   
That is most likely my problem. I have put the 5334B counter in parallel 
to the 5370A measuring the period of the 1 PPS and it clearly shows 
either a very stable 1 s reading pretty much down to the single shot 
resolution of the 5334B (10nS or so), or fluctuates by 100's of uS at a 
whack.

The active antenna is inside the shack, which is at the 2nd floor of my 
house, with a big roof over and plenty of pine trees around. I guess I 
will have to move the antenna and GPS receiver closer to the sky... I 
would rather bring the 1PPS and RSA-232 down and DC up than bring the 
1.6 GHz signal down...

Still, I would like to be able to detect the condition and freeze the 
DAC until the signal comes back (and set a warning light, with siren, 
cell paging, email alert and all :-)

Didier KO4BB