[time-nuts] Testing frequency using NTP

[Tom Van Baak]

> 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

An "all-PC" frequency counter is a great idea. True, the
performance is limited compared to GPS time sources and
hardware frequency counters, but there's something to be
said for an all (or almost all) software approach.

Time-stamping from an external pin transition should work fine.

Consider also using the sound card as a time comparator.
If you put your divided down UUT the left channel and an
NTP sync'd parallel or serial port tick into the right channel
you can compare the relative phase by looking at the stereo
input data. Phase change over time gives you frequency error.

The instability of the sound card's own LO drops out of the
equation. With 44.1 kHz sampling you could handle tick rates
of 1 kHz without any problem. This should be quite a bit more
accurate than 1 Hz sampling, so you might have to wait less
for your goal of 1 ppm resolution. If the limiting factor is the
quality of your NTP connection, then you've done a good job.

/tvb