[time-nuts] Soundcard sampling Re: Picking a good HP 10811

[Lux, James P]

On 12/1/08 10:58 PM, "Bruce Griffiths" <bruce.griffiths at xtra.co.nz> wrote:


> Thus it may well be worthwhile doing this as one then (in principle)
> only needs 3 mixers (plus 3 simultaneously sampled sound card input
> channels) and no offset source, however the maximum achievable offset
> will probably result in beat frequencies that are a little too low for a
> sound card ADC.

Most sound cards roll off below around 10-20 Hz, or, at least, you're
starting to get into the roll off filter characteristic where the phase is
changing rapidly.

Re: "simultaneously sampled"...  I would assume that sample jitter here
affects the measurement.  Granted, one can do a fit of many samples to a
sine wave, and get a sqrt(N) improvement (if it's random jitter, and not
systematic), but what's a typical spec for channel/channel jitter on a sound
card?  Or, more properly, if you're measuring an Allan deviation of, say,
1E-15 over 100 seconds for a couple 10MHz sources beaten down to, say, 100
Hz or thereabouts.. How good does it need to be?