[time-nuts] Two GPSDO 10 MHz phase drift

[Doug Ronald]

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