Shot noise is generated within
semiconductor junctions when electrons cross a potential barrier. Whereas thermal noise
power is proportional to temperature, shot noise power is proportional to the bias current
in the semiconductor. The nature of shot noise is also purely random and has a flat power
spectrum with frequency.
Flicker noise is also
generated in semiconductors and is proportional to the dc bias current, but differs in
that the noise power decreases with frequency. Because this power variation is almost
directly proportional to 1/f, it is sometimes called 1/f
noise.
Atmospheric noise is a general term given to noise arising
from electromagnetic radiation from solar and galactic sources. Certain stars, for
example, emit definite and regular amounts of noise which are best avoided by pointing the
antenna away from the noise source. The compound effect of this noise is usually expressed
as an equivalent sky noise temperature and is generally much
less than thermal noise. The level of noise varies considerably with frequency, with the
higher levels of noise occurring in the microwave region of the spectrum.
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