[time-nuts] Testing frequency using NTP

[Björn Gabrielsson]

On Wed, 2008-10-01 at 20:48 -0400, Scott McGrath wrote:
> 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.
> 

You do not have to chose a bad NTP server. With better servers you can
gain a few orders of magnitude. 

$ ntpq -p timehost.lysator.liu.se

     remote           refid      st ...  delay   offset  jitter
================================================================
*GENERIC(0)      .GPS.            0 ...   0.000    0.014   0.002
+nissan.ifm.liu. .PPS.            1 ...   0.651    0.000   0.035
+ntp1.sth.netnod .PPS.            1 ...   3.667   -0.018   0.052
+ntp2.gbg.netnod .PPS.            1 ...  10.967   -0.010   0.014
-ntp1.mmo.netnod .PPS.            1 ...  13.899   -0.077   0.061

Also look at the NTP temperature compensation discussion at

    http://www.ijs.si/time/temp-compensation/


--

    Björn