[time-nuts] Testing frequency using NTP

Steve Rooke sar10538 at gmail.com
Thu Oct 2 01:37:08 EDT 2008


Hmmm... If I measured a 10MHz oscillator for a 1/10 second, I could
achieve, at best, 1ppm accuracy. Now my measuring system has a non
accumulating error in the ms range, say <1s, so this would be totally
unworkable. If I sampled for 1s, best would be .1ppm accuracy, but my
measuring errors would still swamp the result. To make my measuring
errors small, I could make the 1s overhead very small compared to the
measing time period by, say, sampling for 10^6s thereby making the 1s
error 1ppm. Now the 1s error is probably considerably smaller than
that, probably by the order of at least a decade or two. If it was
just .1s error, I could get a result to 1ppm in about a day and better
if I sample of a number of days.

Cheers,
Steve

2008/10/2 Scott McGrath <scmcgrath at gmail.com>:
> It depends on how accurately you want to measure the oscillator
> frequency with your approach short term you probably would not be able
> to measure the oscillator offset any better than a few parts in 10-5
> longer term probably a few parts in 10-7 might be possible as you
> could compute the allen deviation and fit a curve through the median
> values.
>
> NTP from a stratum 3 clock is only going to be precise to a few
> milliseconds and for meaningful accuracy you need another order of
> magnitude.   This is part of the function of the drift file in xntpd
> in which the daemon attempts to compensate for the drift and offset
> inherent in cheap oscillators used in computer applications.
>
>
> - Fellow nuts am I all wet here or have I missed a technique

-- 
Steve Rooke - ZL3TUV & G8KVD
Omnium finis imminet



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