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

[Bruce Griffiths]

Bruce Griffiths wrote:
> Bruce Griffiths wrote:
>   
>> 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
>>
>>   
>>     
> 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