[Laser] Sensitivity to Sound Card Noise

[Art]

Thanks Andy,

Keeping the gain in the preamp as low as possible is important. As I 
just learned, the gain of the preamp can be increased to overcome the 
noise associated with the sound card. But, in doing so, the dynamic 
range is degraded.

This is all fine and well in an environment without other strong signals::>

Even in the laser world, big signals can pop up. Strobes from 
airplanes and appear and disappear. Strobes on towers are very bright 
for brief periods. If you are trying to integrate over a long period, 
to dig out weak narrowband signals, that strobe on the tower can 
destroy your entire set of data points, rendering your entire weak 
signal unreadable.

I think the effects of sound card overload are quite similar to what 
happens when we overload a conventional rf detector-spurious noise is 
created that isn't actually present on the input signal.

Having a sound card with 24 bit resolution can allow reception of a 
signal when a 16 bit sound card would be overloading and distorting. 
Having a low noise sound card means more usable dynamic range because 
additional preamp gain isn't needed to overcome the noisy sound card.

For serious weak signal work, a quiet sound card and a 24 bit sound 
card (vs the typical 16 bit sound card)  can definitely improve the 
reception in many cases.

Regards,

Art



>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.
>>
>>
>>_______________________________________________
>>Laser mailing list
>>Laser at mailman.qth.net
>>http://mailman.qth.net/mailman/listinfo/laser
>>
>
>
>_______________________________________________
>Laser mailing list
>Laser at mailman.qth.net
>http://mailman.qth.net/mailman/listinfo/laser
>
>
>--
>No virus found in this incoming message.
>Checked by AVG Free Edition.
>Version: 7.5.432 / Virus Database: 268.16.10/625 - Release Date: 
>1/13/2007 5:40 PM