## Shot, flicker and atmospheric noise

#### 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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