[time-nuts] Testing frequency using NTP

[Bruce Griffiths]

> Tom
>
>
> 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.
>
>   
For temperatures around 20C a wet electrolytic cap has a tempco of aroun 
+500ppm/C or so.
With a 0.5dBinput frequency of 20Hz a typical sound card has a high pass 
filter input time constant of about 33ms.
At 1kHz the corresponding phase shift is about 800ns with a tempco of 
about 0.4ns/C.

Bruce