Shooting Down Shot Noise 1

[antakamatics]

Citizens of Eng-tips. I would really appreciate your advice.

I've been wrestling with a pesky noisy phototransistor for quite a long time, and am at wit's end.

The sensor is being used to transuce an audio signal in the 1-5kHz range. Unfortunately the shot noise is introducing a broadband "hiss" into the audio that cannot be filtered, or at least not completely because it overlaps the audio signal.

Can anyone recommend a simple circuit to effectively "Dolby" filter such noise?

Thanks in advance for sharing your ideas!

Most obliged,

Jim

 

[VEBill]

Hello and season's greetings.

Use a phototransistor that is less noisy - I acknowledge in advance this is not likely easy to find.

Increase the signal amplitude to get out of the noise. If you're already at the headroom, then this won't work. Perhaps compression (dynamic gain adjustment) could be used, if applicable.

Double check that the noise source is as you assume. Make sure it's not your first amplifier stage.

Is there any way to reduce the currents in the phototransistor to reduce the noise? You might want to post an extract of that small section of the schematic.

Digitize the audio before transmitting it (digitally) over the link.

 

[IRstuff]

Is your signal sufficiently strong? Why are you trying to detect a supposedly audio signal with a photodetector?

TTFN
faq731-376

 

[antakamatics]

Thanks very much for your replies!

The signal I am transducing is surprisingly enough, a harmonica reed.

I'm relatively sure of the horse source as I experimented with an isolated sensor with just a pull up resistor, illuminated by mini incandescent light.

I've tried a compressor but was actually contemplating an expander. . . reasoning that I want to emphasize the high amplitude signal over the baseline noise.

I'll post a segments of schematic tomorrow, but forewarn you that it is embarrassingly simply. Thanks again for taking the time to share your ideas.

Jim

 

[antakamatics]

sorry for failure to spell check
hourse = noise
simply = simple