[time-nuts] Re: Phase coherence with 2X GPSDO

[ed breya]

Just an interesting observation. Last night, shortly after I wrote about 
my dual GPSDO system, I decided to take a look at it, with the usual 
scope setup looking at both 10 MHz outputs. It looked like one of those 
reasonably good times, where the phase between them seemed very stable 
during fairly long observation time. They were nearly in quadrature, 
drifting very slightly back and forth, over maybe an hour of occasional 
observation. This is akin to the proverbial "watching paint dry" - 
nothing new to report, over a long time, blah blah blah.

I went inside for at most a couple minutes to look at something, and 
when I returned to the garage, I noticed they were nearly in-phase, all 
of a sudden. Over the next half hour or so, I could see the phase drift 
back to about where it was before. During that brief moment when I was 
gone, I had missed what was probably a DAC value update, when one of the 
units decided it was time to make a change.

The change of one or some LSBs must have had a relatively instantaneous 
effect in the time scale involved, and I missed it, but I did see the 
aftermath. So, you can add that sort of effect in the time/frequency 
situation. The temperatures and drifts of various parts, and the SW 
figuring out what to do about it, tend to go quite slowly, but once the 
decision is made to change a value, there's a step function involved.

As I understand, the tuning DAC may be updated once a second at most (if 
comparing to the 1PPS), in small increments, as the DO part tries to 
keep everything right according to the GPS part. With my simple setup 
and observations, I can't tell really how big, or how often these 
adjustments are made, but whatever this was, it was quite obvious. The 
most interesting things tend to happen when you're not looking.

Ed