[time-nuts] GPSDO control loops and correcting quantizationerror

Dennis Ferguson dennis.c.ferguson at gmail.com
Sun Sep 16 20:12:48 UTC 2012


On 16 Sep, 2012, at 00:40 , Tom Van Baak wrote:
> I worry in your example about the long cross-over time. This may be ideal for frequency stability, but probably is not good for time accuracy. If one is using the GPSDO as a timing reference, I would think a shorter time constant will keep the rms time error down. Has anyone on the list done work optimizing the timing accuracy rather than the frequency stability?

I'm not sure there could be a difference between the goals of
frequency accuracy and time accuracy that would effect that time
constant. The time error is the time integral of the frequency
error, so anything which manages to minimize the frequency error
of the oscillator (both the magnitude of the error and its
duration) will also minimize the time error.  The time constant
is selected to be the minimum value which makes it probable that
the frequency or time error you have measured (for a PLL the data
are time errors) is in fact an error that the oscillator has
made rather than an artifact of the noise in the measurement
system.

There might be a difference in the best control action to take to
optimally achieve each of those goals.  In particular if your goal
is frequency accuracy the best control action in response to the
measurement of a frequency error might be to correct that error,
i.e. to minimize the frequency error once you know you have one.
If your goal is time accuracy, however, then the response to a
measured frequency error is going to be to intentionally make a
frequency error in the other direction for a while to correct the
accumulated time error.  In this case, though, it seems to me
that by selecting a PLL as the control discipline (rather than, say,
a FLL) you've already made the decision to take control actions
which ensure time accuracy.

Dennis Ferguson


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