[time-nuts] Testing frequency using NTP

[Bruce Griffiths]

Tom Van Baak wrote:
>> 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:
> http://www.leapsecond.com/pages/sound-1pps/
>
> 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
>
> /tvb
> http://www.LeapSecond.com
>   
Tom

With  a high end sound card one can lock the LO to a synthesized SPDIF 
source with a stable frequency reference which should minimise long term 
sound card LO frequency variations when these are significant.

The major source of interchannel phase shift instability will be the 
phase shift instabilities of the sound card input RC coupling network 
particularly if electrolytic capacitors are used and the input 
frequencies are too close to the high pass RC filter 3dB frequency. 
Amplifier phase shift instabilities are much smaller and the 
interchannel group delay mismatch will be relatively small as the group 
delay for each channel is determined by the digital filters used by the ADC.

With a high end (24 bit) sound card it is relatively easy to achieve a 
measurement noise comparable to that of the traditional zero crossing 
detector used in a dual mixer sytem.

Bruce