Noise

In electronics and communication systems, noise is a random fluctuation or variation of an electromagnetic analog signal such as a voltage or a current.

Electronic noise is a characteristic of all electronic circuits. Depending on the circuit, the noise generated by electronic devices can vary greatly. Noise can be produced by several different effects. Thermal noise and shot noise are inherent to all devices. The other types depend mostly on manufacturing quality and semiconductor defects.

The noise level in an electronic system is typically measured as an electrical power N in Watt or dBm, a root mean square (RMS) voltage (identical to the noise standard deviation) in Volt or dBμV, a mean squared error (MSE) in Volt². The noise may also be characterized by its probability distribution and noise spectral density N₀(f) in Watt/Hertz.

A noise signal is typically but not always considered as a linear addition to a useful information signal. Typical signal quality measures involving noise are signal-to-noise ratio (SNR or S/N), signal-to-quantization noise ratio (SQNR) in analog-to-digital coversion and compression, peak signal-to-noise ratio (PSNR) in image coding and video coding, E_b/N₀ in digital transmission, carrier to Noise Ratio (CNR) before the detector in carrier-modulated systems, and noise figure in cascaded amplifiers.

In communication systems, the noise is an error or undesired disturbance of a useful information signal, before or after the detector and decoder. The noise is a summation of unwanted or disturbing energy introduced into a communications system from man-made and natural sources. Noise is however often distinguished from interference, i.e. cross-talk, deliberate jamming and other unwanted signals from specific transmitters, added to the useful signal. See for example signal-to-noise plus interference (SNIR). Noise is also typically distinguished from distortion, i.e. unwanted alteration of the waveform. See for example signal-to-noise and distortion ratio (SINAD).

While noise is generally unwanted, it can serve a useful purpose in some applications, such as random number generation or dithering. A dithered representation of signals below the minimum strength, or between two quantization levels may be advantageus in some cases. This is true especially for signals intended for human appreciation, since the brain seems to expect signals to contain a degree of "neural noise"^[1][2], or the phenomenon of stochastic resonance, where small amount of noise improves the detection of signals in non-linear sensors.

Types

Thermal noise

Johnson-Nyquist noise (sometimes thermal, Johnson 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 (usually 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.

Thermal noise is characterized by Gaussian or Normal distribution. If it is unfiltered, it can be described as white noise, implying a constant noise spectral density in watts per hertz of N_o = kT, where k is Boltzmann's constant in joules per kelvin, and T is the receiver system noise temperature in kelvins. The total noise power N detected in a receiver with bandwidth B is BN_o. A communication system affected by thermal noise is often modelled as an additive white Gaussian noise (AWGN) channel. Note that AWGN also may be caused by other electromagnetic interference and sources of noise. According to the central limit theorem, a large amount of sumarized stochastic signals results in Gaussian distribution.

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 minimum signal, together with the background noise of the universe, remaining since Big bang, can be referred to as the noise floor. 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 to a very low temperature in liquid nitrogen.

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).

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 relative to a reference, such as 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 (W/Hz). Since the power in a resistive element is proportional to the square of the voltage across it, noise voltage (density) can be described by taking the square root of the noise power density, resulting in volts per root hertz (V/√Hz). Integrated circuit devices, such as op-amps commonly quote equivalent input noise level in these terms (at room temperature).

In telecommunication engineering, noise level is usually measured in decibels (dB) for relative power or Watts for absolute power. A suffix is added to denote a particular reference base or specific qualities of the measurement. Examples of electrical noise-level measurement units are dBu, dBm0, dBrn, dBrnC, and dBrn(f₁ − f₂), dBrn(144-line).

Noise levels are usually viewed in opposition to signal levels and so are often seen as part of a signal-to-noise ratio (SNR). Telecommunication systems strive to increase the ratio of signal level to noise level in order to effectively transmit data. In practice, if the transmitted signal falls below the level of the noise (often designated as the noise floor) in the system, data can no longer be decoded at the receiver. Noise levels in telecommunication systems are a product of both internal and external sources to the system including shot noise, thermal noise, and ambient electromagnetic interference.

Dither

Intentional introduction of additional noise (called dither) can reduce noise in the bandwidth of interest. By convolving the Probability Density Functions (PDFs) of the noise and the dither, the noise energy is spread throughout the spectrum, reducing the amplitude of noise obscuring the signal. This technique allows retrieval of signals buried in noise. While commonly applied to reduce quantisation error, dithering can be used to reduce any form of noise correlated to the signal.

See also

• Noise
• Generation-recombination noise
• Quantization noise
• Phase noise
• Noise music

References

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (March 2008)

• White noise calculator, thermal noise - Voltage in microvolts, conversion to noise level in dBu and dBV and vice versa

External links

• Active Filter (Sallen & Key) Noise Study

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:10.1109/TIM.2006.884117. 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:10.1103/PhysRevB.62.8068. 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. 

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).