[Laser] Sensitivity to Sound Card Noise

[steve kavanagh]

> I'm not so sure that the sound card noise output is
> as trivial as you think it is.

It is.  Yves is correct.  Here is a numerical example
based on Art's scenario.  

Art's sound card has 6 dB less noise than mine, but is
otherwise identical.

We are using identical detectors and preamps, with
enough gain that the noise output from my preamp under
dark conditions is 10 dB stronger at the sound card
output than the noise generated internally in the
sound card.  Let's say that the sound card noise
generated internally produces 1 mW at the loudspeaker.
 When the detector/preamp are switched on they will
generated 10 times more noise, or 10 mW, at the
loudspeaker.  The total noise at the loudspeaker is
therefore 10 mW (from preamp) + 1 mW (from sound card)
= 11 mW.

In Art's case, the internally generated sound card
noise is 6 dB lower, or 0.25 mW at the loudspeaker. 
Now, when he turns on the preamp he will also get 10
mW of preamp/detector noise at the loudspeaker.  The
total noise is thus 10 mW (from preamp) + 0.25 mW
(from sound card) = 10.25 mW.

Art therefore has a better total noise level than I
do, by a power ratio of 10.25/11, which corresponds to
a 0.31 dB improvement in noise.  Since we are using
systems which are otherwise identical any desired
signal level will be the same for both of us, so the
signal-to-noise ratio is improved by the same amount,
just 0.31 dB.  With a higher gain preamp the
improvement due to reduction of sound card noise will
be even less.

73,
Steve

__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com