[time-nuts] GPS 1PPS, phase lock vs frequency lock, design (Bob kb8tq) (life speed)

[ed breya]

I think I have a setup that exemplifies this situation, and some 
anecdotes. A while back, I acquired two "identical" GPSDO boards, and 
boxed them up together, with common environment, power supply, and GPS 
signal via a splitter. I've mentioned this thing a couple of times here, 
and had planned to do some experimenting to see how they track each 
other, if crosstalk at the front-ends may have effects, etc. I haven't 
done any of this yet beyond looking at the relative phase of the 10 MHz 
outputs on a scope, over various periods from minutes to days.

I had expected them to agree quite closely after enough running time, 
and be quite stable, but was disappointed. The phase drifts up and down, 
sometimes very, slowly, over an hour or so, and sometimes quickly, 
noticeable over a few minutes observation time. After some pondering on 
why identical units with the same GPS signal should drift like this, I 
realized that besides possible front-end interactions, and noise, that 
this was likely mostly from the sawtooth effect - the discreteness of 
phase comparison of the 1 PPS vs 10 MHz counting, and discreteness of 
OCXO tuning voltage via the DACs. They each responded differently, for a 
number of reasons. More time and voltage resolution would help, of 
course, but they will never perfectly agree, even in this idealized 
setup with identical units. Virtually identical, that is - there's no 
such thing as truly identical units, and operating in identical conditions.

Ed