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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Dec 2 18:39:18 UTC 2008


Lux, James P wrote:
>
> 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?
>
>   
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



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