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

[Doug Ronald]

Thanks all for the responses.

I connected one XL-DC to the start trigger, and the other XL-DC to the stop trigger of an HP-5370B Time Interval Counter with the counter's clock on its own internal ovenized crystal oscillator. Interestingly, the two 1 PPS signals drift fairly constantly by about 300 ps/second. So, what I don't know is the source of the 1 PPS signals from the XL-DCs. Is that signal counted down from their internal references, or are they raw 1 PPS from the GPS receivers? I suspect they are simply counted down from the references, since they remain when I disconnect the GPS antenna, so I'm really not gaining any more insight than from measuring the 10 MHz oscillators - right?

I have a Lucent RFTG-U pair which I guess I'll have to get powered up for a 3rd reference to see which XL-DC is unlocked, but I strongly suspect the rubidium unit which I hate to have to attack again with such a complicated and difficult to work physical arrangement, especially in the physics box. I don't regularly use the RFTGs because they draw a lot of power, and radiate boo-coo RFI.

Thanks again for the ideas,
-Doug, W6DSR

-----Original Message-----
From: Dana Whitlow via time-nuts [mailto:time-nuts at lists.febo.com] 
Sent: Saturday, March 4, 2023 02:22
To: Discussion of precise time and frequency measurement
Cc: Dana Whitlow
Subject: [time-nuts] Re: Two GPSDO 10 MHz phase drift

Doug,

Have you tried the corresponding experiment of looking for time drift
between the
two 1 PPS signals' leading edges?  If so, what was the result?

Dana


On Fri, Mar 3, 2023 at 7:57 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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