[Ronja] Multiple receivers and transmitters

[J.D. Bakker]

At 23:41 +0200 23-09-2008, Karel Kulhavy wrote:
>                                                                In the
>daylight the noise is mostly created by the quantization of the ambient light.

Strictly speaking that's not what happens (or at best it's a major 
oversimplification).

The photodiode is a semiconductor in which incident light causes an 
electrical current to flow. In daylight a large part of that current 
is effectively DC (compared to the signal bandwidth). Any DC current 
flowing through a semiconductor junction induces shot noise as 
individual electrons cross the barrier, which can be likened to the 
noise of raindrops falling on a thin roof. This noise current is 
indistinguishable from the wanted signal.

Shot noise increases with the square root of current flow, and thus 
with the square root of incident light power. On a sunny day the 
receiver gets 10^5 more light energy than on a moonlit night, but the 
difference in noise level is 'only' 300x.

JDB.
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