[time-nuts] GPSDO control loops and correcting quantizationerror

Bob Camp lists at rtty.us
Sun Sep 16 22:29:48 UTC 2012


HI

In some cases, the difference can be your definition of "time accuracy". If short term GPS time is what you are worried about, then indeed that's a different beast than a 30 day plot against your direct line to USNO.

Bob

On Sep 16, 2012, at 4:12 PM, Dennis Ferguson <dennis.c.ferguson at gmail.com> wrote:

> 
> 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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