[time-nuts] Testing frequency using NTP
bruce.griffiths at xtra.co.nz
Thu Oct 2 22:37:52 EDT 2008
Bruce Griffiths wrote:
>> 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.
The time constant should have been 22ms and the phase shift 1.16us with
a tempco of 580ps/C.
Since both channels have the same nominal low frequency cutoff the
differential phase shift tempco will be somewhat smaller.
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