Bruce Griffiths wrote: > Bruce Griffiths wrote: > >> 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 >> >> >> > Oops I omitted a factor of 100. > The differential sampling jitter would need to be less than about 10ns. > Practically all high end sound cards should easily meet this spec. > > The sampling clock jitter spec for a given SNR and oversampling ration is > > jitter < SQRT(OSR)/(2*PI*SNR) > OSR = oversampling ratio > SNR = signal to noise ratio > > e.g. with a 100dB SNR a 10KHz input and an OSR of 9.6 (192kHz) > The sampling jitter needs to be < 500ps rms. > > with a 100dB SNR a 1kHz input and an OSR of 96 > the sampling jitter needs to be < 15.6 ns rms. > > Bruce > > > Correction: After rereading the ADC chip datasheet I realised that the actual OSR for 192kHz output sample rate is 128 (independent of the input signal frequency).
Thus for a 1kHz input and 100dB SNR the input clock jitter should be < 18ns rms. for a 10kHz input and 100dB SNR the input clock jitter should be < 1.8ns rms. It is very likely that the differential sampling clock jitter contribution of a high end sound card to the ADEV noise floor will in around 1E-15 or less at Tau = 100sec. 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.
