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
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