[Laser] discrete vs op amp sensitivity

[Art]

>
>
>I agree with Yves that the cound card noise level
>should not be a major factor as long as there is
>sufficient gain in the preamp that the preamp noise
>was dominant.  The higher the preamp gain the lower
>the overall dynamic range.  But this would not seem to
>me to be much of an issue for most optical
>communications purposes (unless perhaps in an urban
>area with a large area detector).  For an HF receiver,
>on the other hand it is very important to minimize the
>gain in order to achieve the necessary dynamic range,
>so the sound card noise would be important.


I'm not sure I agree with this,  although I do see your point.

But, let's look at a fictitious example........

Let's say you and I use the same optical detector and we are 
attempting to detect the same weak signal.

Your sound card has 6 db more noise than mine does (with the inputs 
shorted and looking at relative output).

I think my SYSTEM would hear (detect) a signal that was 6 db weaker 
than your system would hear. Correct or not?

I wouldn't argue that the preamp gain is much more significant as a 
noise source, but to treat the final output stage as though it didn't 
matter much is not correct either.

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

I think it's allot easier for me to put in a db quieter sound card 
than it is for my QSO partner to increase their output by 6 db. Every 
db matters::>

Regards,

Art