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Electronic noise is an unwanted signal characteristic of all electronic circuits. Depending on the circuit, the noise put out by electronic devices can vary greatly. This noise comes from many different electronic effects.
Thermal noise and shot noise are inherent to all devices. The other types depend mostly on manufacturing quality and semiconductor defects.
In some applications, electronic noise can serve a useful purpose. A common example of this is in random number generation.
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Types
Shot noise
Shot noise in electronic devices consists of random fluctuations of the electric current in an electrical conductor, which are caused by the fact that the current is carried by discrete charges (electrons).
Thermal noise
Johnson-Nyquist noise (sometimes thermal noise, Johnson noise or Nyquist noise) is the noise generated by the equilibrium fluctuations of the electric current inside an electrical conductor, which happens regardless of any applied voltage, due to the random thermal motion of the charge carriers (the electrons).
The charges may be bound (for a dielectric material) or free (for a conductor). Free charges generate kinetic energy from their motion according to the equation E = (mv2)/2. This kinetic energy results in noise. Bound charges generate kinetic energy when the direction of polarity changes.
This noise is characterized as Additive white Gaussian noise (AWGN) with a noise spectral density in watts per hertz of No = kT, where k is Boltzmann's constant in joules per kelvin, and T is the receiver system noise temperature in kelvins. Since thermal noise can be considered as white noise, the total noise power N detected in a receiver with bandwidth B is BNo.
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 circuits, usually mounted on the aerial dish, cooled in liquid nitrogen to a very low temperature.
Flicker noise
Flicker noise, also known as 1/f noise, is a signal or process with a frequency spectrum that falls off steadily into the higher frequencies, with a pink spectrum. It occurs in almost all electronic devices, and results from a variety of effects, though always related to a direct current.
Burst noise
Burst noise consists of sudden step-like transitions between two or more levels (non-Gaussian), as high as several hundred millivolts, at random and unpredictable times. Each shift in offset voltage or current lasts for several milliseconds, and the intervals between pulses tend to be in the audio range (less than 100 Hz), leading to the term popcorn noise for the popping or crackling sounds it produces in audio circuits.
Avalanche noise
See Avalanche diode and Avalanche breakdown.
Measurement
Electronic noise is properly measured in watts of power. Because noise is a random process, it can be characterized by stochastic properties such as its variance, distribution, and spectral density. The spectral distribution of noise can vary by frequency, hence its power density is measured in watts per hertz 
. Since the real power in a resistive element is proportional to the square of the voltage across the element, noise voltage (density) can be described by taking the square root of the noise power density, resulting in volts per root hertz 
. Integrated circuit devices, such as op-amps commonly quote equivalent input noise level in these terms (at room temperature).
See also
References
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This article or section includes a list of references or external links, but its sources remain unclear because it lacks inline citations. You can improve this article by introducing more precise citations where appropriate. (March 2008) |
Further reading
- Constable, J. H. (December 2006). "Investigation of Environmental Noise in Small Electrical Conductors". IEEE Transactions on Instrumentation and Measurement (IEEE) 55 (6): 2045–2054. doi:, http://ieeexplore.ieee.org/xpls/abs_all.jsp?isnumber=4014679&arnumber=4014691&count=64&index=23.
- G. Gomila and L. Reggiani (September 2000). "Anomalous crossover between thermal and shot noise in macroscopic diffusive conductors" (subscription required). Physical Review B (APS) 62 (12): 8068–8071. doi:, http://prola.aps.org/abstract/PRB/v62/i12/p8068_1.
- Sh. Kogan (1996). Electronic Noise and Fluctuations in Solids. Cambridge University Press. ISBN 0521460344.
- M. V. Kunnavakkam (1996). Experimental investigation of current dependent noise in metal wires (M.S. Thesis). Binghamton University.
Wikipedia content modification information:
- This page was last modified on 17 November 2008, at 23:14.
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