[ntp:questions] Performance estimation

David Woolley david at ex.djwhome.demon.invalid
Mon Jun 15 18:00:21 UTC 2020


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.

(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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