[time-nuts] Testing frequency using NTP

Tom Van Baak tvb at LeapSecond.com
Thu Oct 2 17:09:34 EDT 2008

> When using a sound card for frequency comparisons the zero crossing time 
> stamp resolution is improved if the slew rate of the input signal is 
> slow enough that several (3 or more) samples are taken in the vicinity 
> of the zero crossing. One can then make use of  WKS (Whittaker, 
> Kotelnikov, Shannon) interpolation to accurately calculate a high 
> resolution zero crossing time stamp.


> The instability of the sound card LO isnt completely cancelled if the 
> zero crossings of the the 2 signals aren't coincident.

That seems right for absolute event timing with a stereo sound card
but I think for a frequency measurement the delay, if any, between
channels would also cancel out (as long as the delay itself stays
relatively fixed). We'll know for sure when someone actually tries it.

A sound card timing experiment, vaguely related to this, is here:

There I used a sound card to generate a 1 PPS and measured its
ADEV. Note that this is unrelated to NTP (NTP disciplines the CPU
or system bus clock, which is typically not the same oscillator as
the sound card clock).

Win32 source code: http://www.leapsecond.com/tools/1hz.c


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