[time-nuts] GPSDO with all-digital phase/time measurement?

Chris Albertson albertson.chris at gmail.com
Wed Feb 26 21:04:00 EST 2014


At this point the time measurement is quite crude, with 100-ns resolution.
> But because we keep the counter running, the unknown residuals will keep
> accumulating, and we should be able to average out this "quantization
> noise"


What you are saying is "With a long enough gate time you can measure
frequency to any desired level of accuracy."

For that to be true the  frequency must remain stable over the long period
you are averaging.  For example let's say you average over 10 seconds.
 What happens of the nominal 10MHz oscillator ran at  10.1 Mhz for five
seconds and then 9.9 Mhz for 5 seconds.   You'd measure 10Mhz and be happy.
  So there is the problem.  With long period averaging, you can only do it
for so long as you trust the clock to not change.

All the long gate time tells you is the AVERAGE so it only works of the
change over the period is smooth and monotonic.

If you want very good accuracy, say one part in 10^14 you can't get there
by waiting days and weeks because the frequency will move while you are
measuring it.

The quicker way is to measure the phase.  You can still do this digitally.
 Make a one bit measurement.  zero if the phase leads, 1 if it lags and
then over time you want the average to be 0.5  This works much faster
because your controller does not have to wait for an entire cycle of error
to accumulate.

Your counting system would have to wait for a one cycle error and then make
a very large and infrequent correction.   It could work for some use cases.
-- 

Chris Albertson
Redondo Beach, California


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