[time-nuts] ADEV vs MDEV - using sound card

[Bruce Griffiths]

Magnus Danielson wrote:
> Bob Camp wrote:
>> Hi
>>
>> My main concern with the low frequency pole in the sound card is the 
>> quality of the R/C used. You can certainly model what ever you have. 
>> If they used an aluminum electrolytic for the "C" it may not be the 
>> same next time you check it ....
>
> Do consider to bypass it. This is routinely done both by audio folks 
> and various other. The cap is there to remove DC offsets which can be 
> problematic in audio editing. I am quite sure you feel at home with 
> the soldering iron to do that upgrade.
>
>> On a 10 Hz system, a 1 Hz pole is probably not an issue. It might get 
>> in the way with a 1 Hz beat note.
>> Another thing I have only seen in passing: "Sigma Delta's have poor 
>> low frequency noise characteristics". I haven't dug into it to see if 
>> that's really true or not. If you buy your own ADC's, you certainly 
>> would not be restricted to a Sigma Delta.
>
> Strange, most Sigma Delta's I have seen would have the opposite said 
> about them. It's the upper end that is problematic.
>
>> Even with a cheap pre-built FPGA board, you could look into higher 
>> sample rates than a conventional sound card. You would drop back to 
>> 16 bits, but it might be worth it. 
>
> In fact, one of my FPGA demo-board does 3 MS at 14 bit. Crappy for 
> audio, but maybe good enough for this application.
>
> Cheers,
> Magnus
>
> _
A pity that there is a relatively low bandwidth PGA used to drive the 
inputs of that dual channel simultaneous sampling ADC.

Bruce