[Laser] Sensitivity to Sound Card Noise

[Andrew T. Flowers, K0SM]

I think I should make some comments about noise and dynamic range from 
some of my experiments.  The dynamic range of these detectors can easily 
exceed that of the soundcard, particularly at night when the flash from 
any man-made lights can put the system into clipping.  This can easily 
happen in urban environments.  This is a situation that you want to 
avoid if you are doing weak-signal DSP detection.  From my experience, 
this seems to be more limited by the soundcard than the PGP front-end 
circuit.  Many sound-editing programs have a real-time LED display of 
the input level to help you make this adjustment.

Andy


Art wrote:

> OK, thanks to Steve, Yves, and the group for their patience and for 
> getting me straightened out.
>
> I think I've got it now::>
>
> When looking at the power levels as Steve suggested below, it becomes 
> much clearer-at least in my mind.
>
> I'll rest easier knowing that sound card selection isn't as critical 
> as I had thought it to be previously.
>
> As to the performance of the PGP detector relative to the op amp 
> transimpedance front end, I think the book is still open though.
>
> I think the only way to really know the answer is to test them side by 
> side in a physical environment that accounts for the difference in the 
> active area of the photodiode in both types of preamps. Or, stated 
> another way....if both preamps under test have the same size 
> photodiodes, they can be tested side by side. If not, we have to 
> adjust the intensity of the light input so that the same amount of 
> photons falls on each detector.
>
> Also, the bandwidth of both detectors needs to be similar for the test 
> to be fair. Fortunately, the bandwidth of the typical 
> photodiode/transimpedance op amp based front end is easily modified.
>
> Signed,
>
> Curious in Maine
>
>
>
>
>> 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.
>
>
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