Noise (electronic)
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Electronic noise exists in any electronic circuit, as a result of random variations in current or voltage caused by the random movement of the electrons carrying the current as they are jolted around by thermal energy. The lower the temperature the lower is this thermal noise.
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[edit] Measurement
Electronic noise is often measured in volts per root hertz 
. This term is derived from the fact that doubling the bandwidth of the measurement doubles the power level measured, but voltage is proportional to the square root of power. Integrated circuit devices, such as op-amps commonly quote equivalent input noise level in these terms (at room temperature).
[edit] Effects of thermal (Johnson) noise
This phenomenon limits the minimum signal level that any radio receiver can usefully respond to, because there will always be a small but significant amount of thermal noise arising in its input circuits. This is why radio telescopes, which search for very low levels of signal from stars, use front-end ciruits, usually mounted on the aerial dish, cooled in liquid nitrogen to a very low temperature.
[edit] See also
- Noise - Root page
- Thermal noise
- Generation-recombination noise
