[time-nuts] Testing frequency using NTP

[Scott McGrath]

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

On Wed, Oct 1, 2008 at 8:07 PM, Steve Rooke <sar10538 at gmail.com> wrote:
> I'm wondering about the possibility of checking the frequency of my
> oscillator by using a NTP synced RT Linux system. What I'm thinking of
> doing is building a long chain of dividers feeding a standard freq
> counter in totallise mode such that I can count the number of cycled
> over a long time period. What I would then do is to gate this with the
> Linux system to measure the number of cycles over that period. I
> figure that although the exact point of gate switching would not be
> very accurate, due to clock jitter and uncertain delays in the OS,
> this error could be made insignificant, in terms of the possible
> stability of the oxco, when the sampling period is large. Even
> watching the NTP stats on my workstation, OpenSUSE 11, it seems to
> remain stable within a few ms, now that it has been stabilised for
> months, and on a dedicated real time Linux system I should be able to
> switch the gate within ms of the correct time so these errors should
> only affect the least significant bits of the counting chain. If I
> make the sampling period long, say hours, this should push those
> errors well down the ppm.
>
> Anyone have comments on this approach please? Feel free to blow holes in it.
>
> Thanks,
> Steve
> --
> Steve Rooke - ZL3TUV & G8KVD
> Omnium finis imminet
>
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