[ntp:questions] Re: NTP precision
David J Taylor
david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid
Sat Sep 24 07:40:53 UTC 2005
Tom Smith wrote:
> Richard B. Gilbert wrote:
>> If I understand the definition of jitter, it has units of frequency
>> (1/T) and cannot meaningfully be added to or subtracted from time
>> (T).
>
> Regrettably, even that's not clear. See the definitions that Brad
> pointed out:
>
> Jitter
> Short-term variations in Frequency with components greater than
> 10 Hz. The estimated time error of the system clock measured as
> an exponential average of RMS time differences.
>
> and the 4.2.0 documentation for ntpq:
>
> jitter
> The estimated time error of the system clock measured as an
> exponential average of RMS time differences.
>
> -Tom
Perhaps the second definition should read (with an explanation in
brackets):
---------------------------------------
jitter
The estimated time error of the system clock measured as an
exponential average of RMS time differences. (Jitter errors are caused by
short-term variations in frequency with components greater than 10Hz).
---------------------------------------
Jitter would (in this case) be measured in time, not frequency, so it's
unfortunate that the first definition mentioned frequency first,
potentially confusing folk.
BTW: what are the errors caused by longer term variations called?
David
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