[time-nuts] Testing frequency using NTP

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Oct 1 20:44:48 EDT 2008

Steve Rooke 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

Save yourself a counter and just divide the frequency down to about 1Hz 
and time stamp the 1Hz transitions with the Linux box.
As long as you know the division factor its easy enough to calculate the 


More information about the time-nuts mailing list