OT: S/N-Ratio measurment - help

[Magnus Danielson]

From: Tim Ressel <Tim_R1 at verifone.com>
Subject: RE: OT: S/N-Ratio measurment  - help
Date: Mon, 7 Feb 2000 14:57:32 -0800 

> I am not at all surprised to hear it. Think about the setup: here you have an
> A/D converter with the bottom bit at around 25 uV. Then you put into a PC case,
> one that is metal and sealed up to keep those nasty emissions from exiting the
> case. So you have this sound card sticking up in the noise breeze like a
> high-tech sail. And oh by the way, no shielding what so ever on the card, except
> for whatever carefull layout they may have done. Noisy? Of course it is.
> 
> Fortunately, steps can be taken. Extensive shielding if the card should help
> quite a bit. For example, a cheap shield can be made by sandwiching aluminum
> foil between two layers of shelf paper. Carefully wrap the shield around the
> card, and then ground the shield real well. Make sure nothing gets shorted by
> the shield. 
> 
> I can't guarantee this will help, but its definitely worth a try.

Shielding will help some, but I think we need to think lo frequency stuff
here. I think we need to think common impedances and current loops with low
frequencies. This cooks down to PCB layout, the use of quite ground areas etc.
It would really be an advantage to move the A/D and D/A stuff out of the
computer, since this will get it farther away from electromagnetic fields in
the box. The static shielding you proposed will work well for the RF signals
but here I think things like mu-metal shielding or equalent low frequency
magnetic isolation is really the way to go.

IMHO does analog sound gear not belong INSIDE a computer. Especially PCs and
similar machines are really poorly designed for basically any frequency.
SPDIF out of the box and then have a properly built A/D and D/A box.

Cheers,
Magnus (also spending cycles on EMC from time to time)