Lux, James P wrote: > > On 12/1/08 10:58 PM, "Bruce Griffiths" <[EMAIL PROTECTED]> 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? > > The differential sampling jitter between channels would probably have to be less than a 100ps or so a little more if its random. Preliminary tests with a high end sound card appear to demonstrate a system noise level below 1E-15 at tau = 100 sec.
Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
