[time-nuts] Re: Two GPSDO 10 MHz phase drift

[Bob Camp]

Hi

If you see multiple cycle slips and they are in the same direction, then yes, at least one unit 
is unlocked. 

~40 minutes is about 2,400 seconds. One cycle at 10 MHz should be 100 ns. More or less
you are slipping 1/24 ns per second. It’s not a lot.

A way to double check things would be to take a look at the PPS outputs. Even a pretty
basic setup should be able to resolve < 100 ns. An old HP 5336 era counter off eBay might
be a sub $100 sort of option. Any of a number of MCU based boards could do the job
with a bit of code. A digital scope would be anoter option. 

Bob



> On Mar 3, 2023, at 2:51 PM, Doug Ronald via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> I'm back for a new (to me) question:
> I have two TrueTime XL-DC receivers, one with an ovenized 10 MHz crystal oscillator and the other with a rubidium
> oscillator. Both are receiving from the same GPS antenna, and both receivers are tracking 5 or 6 satellites all the
> time. I repaired the rubidium clock itself where the oven was too hot to maintain internal lock, and has an external VCO
> line to pull the unit to the GPS 1 PPS. That line has a correction voltage that changes, then stabilizes to about 2.8
> VDC.
> With an oscilloscope, I monitor the phase of the 10 MHz from one unit to the other, and much to my surprise, the phase
> drifts, always in the same direction, but not linearly. It takes about 40 minutes to complete a 360 degree cycle. I
> expected there would maybe be some tiny jitter in the phase, but not a continuous error.
> My fear is that one or both of the 10 MHz references is not being corrected to the GPS signal, but I thought I would ask
> here to see if this drift might be expected. Probably others have done this experiment, so I think I'll get a good
> answer.
> Thanks,
> Doug Ronald, W6DSR
> 
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