From: John Larkin
Subject: Re: reverse-biased diode noise source?
Date: Mon, 16 Sep 2002 13:57:47 -0700
Organization: Posted via Supernews, http://www.supernews.com
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On Mon, 16 Sep 2002 12:53:40 -0400, Phil Hobbs
>Now that amplifiers are so quiet, it's easy to make a well-calibrated
>low frequency noise source by shining the light from a LED on a
>photodiode--_NOT_ a phototransistor or photodarlington. Drive the LED
>from a quiet current source (e.g. a battery and metal film resistor or a
>well-designed current source with a big voltage drop across its (metal)
>sense resistor). Provided the incident light is really unmodulated, the
>photocurrent exhibits exactly full shot noise. Shot noise is white and
>Gaussian to very high accuracy, so you can calibrate the noise power
>spectral density by measuring the DC and applying the shot noise
>i sub N = sqrt(2*e*I sub dc) (1-Hz BW)
>where e is the charge on the electron (1.602e-19 coulombs).
>This method is much better than the Zener approach, because it's based
>on fundamental physics rather than device parameters that have to be
>calibrated from unit to unit. Since it's easy to generate a
>photocurrent that has exactly full shot noise--shine a flashlight on
>your photodiode--it's easy to test by comparison whether your LED drive
>current source is quiet enough.
that's neat, except that it's not a lot of signal... something like 18
pa / rootHz for a 1 mA photodiode current. A simple 50 ohm resistor
coupled into a TIA would make about that. If you run the pd's 1 mA
into a good TIA with, say, 1K transimpedance, you're still only up to
I do know that 1950-vintage radar jammers used 931 photomultiplier
tubes as noise sources.