[ntp:questions] Performance estimation

William Unruh unruh at invalid.ca
Mon Jun 15 19:02:22 UTC 2020

On 2020-06-15, David Woolley <david at ex.djwhome.demon.invalid> wrote:
> On 15/06/2020 15:38, David Taylor wrote:
>>> https://www.eecis.udel.edu/~mills/ntp/html/cluster.html
> What is the clock resolution?  If you try and measure jitters that 
> aren't several times the resolution, they are not going to be 
> particularly valid.
> If the hardware clock is almost dead on, and the peak to peak dither is 
> just less than the resolution, there will be long periods in which it 
> will read as as zero, even though it is actually close to one resolution 
> unit.  You could also get cases where dither was very low but read as 
> one resolution unit for long periods.  In fact, if it was possible to 
> find tune the actual clock oscillator, during an ideal lock you would 
> have peak to peak dither, as measured, of one or two resolution units, 
> even though the actual phase noise was much less.

>From his graphs this sure seems to be what is going on. The changes in
the clock seem to all have 1/2 us resolution, while the clock seems to
have 1us resolution.

> (Arguably, a jitter that is less than the clock resolution will result 
> in worse time accuracy than one that a few times it, as the clock 
> resolution will not be dithered out.
> That makes the, normally unrealistic, assumption that the systematic 
> error is less than the clock resolution.)

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