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{{about|fundamental noise processes in electronics|electronic noise arising from outside sources|electromagnetic compatibility|and|electromagnetic interference|acoustic noise due to electromagnetic fields|electromagnetically excited acoustic noise and vibration||Noise (disambiguation)}}

[[File:Pink.noise.png|thumb|Analog display of random fluctuations in voltage in [[pink noise]].]]

In electronics, '''noise''' is an unwanted disturbance in an electrical signal.<ref name=noise>{{cite book |first=C. D. |last=Motchenbacher |first2=J. A. |last2=Connelly |title=Low-noise electronic system design |publisher=Wiley Interscience |year=1993 |isbn=0-471-57742-1 }}</ref>{{rp|5}} Noise generated by electronic devices varies greatly as it is produced by several different effects.

In [[communication system]]s, noise is an error or undesired random disturbance of a useful information [[signal]]. The noise is a summation of unwanted or disturbing energy from natural and sometimes man-made sources. Noise is, however, typically distinguished from [[Interference (communication)|interference]],{{efn|e.g. [[cross-talk]], deliberate [[Radio jamming|jamming]] or other unwanted [[electromagnetic interference]] from specific transmitters}} for example in the [[signal-to-noise ratio]] (SNR), [[signal-to-interference ratio]] (SIR) and [[signal to noise plus interference|signal-to-noise plus interference ratio]] (SNIR) measures. Noise is also typically distinguished from [[distortion]], which is an unwanted systematic alteration of the signal waveform by the communication equipment, for example in [[signal-to-noise and distortion ratio]] (SINAD) and [[total harmonic distortion plus noise]] (THD+N) measures.

While noise is generally unwanted, it can serve a useful purpose in some applications, such as [[random number generation]] or [[dither]].

==Noise types==
Different types of noise are generated by different devices and different processes. [[Thermal noise]] is unavoidable at non-zero temperature (see [[fluctuation-dissipation theorem]]), while other types depend mostly on device type (such as [[shot noise]],<ref name=noise/><ref name=shot>{{cite journal |first=L. B. |last=Kish |first2=C. G. |last2=Granqvist |title=Noise in nanotechnology |journal=Microelectronics Reliability |volume=40 |issue=11 |date=November 2000 |pages=1833&ndash;1837 |publisher=Elsevier |doi=10.1016/S0026-2714(00)00063-9 }}</ref> which needs a steep potential barrier) or manufacturing quality and [[semiconductor]] defects, such as conductance fluctuations, including [[1/f noise]].

===Thermal noise===
{{Main article|Johnson–Nyquist noise}}

Johnson–Nyquist noise<ref name=noise/> (sometimes thermal, Johnson or Nyquist noise) is unavoidable, and generated by the random thermal motion of charge carriers (usually [[electron]]s), inside an [[electrical conductor]], which happens regardless of any applied [[voltage]].

Thermal noise is approximately [[white noise|white]], meaning that its [[power spectral density]] is nearly equal throughout the [[frequency spectrum]]. The amplitude of the signal has very nearly a [[Gaussian probability density function]]. A communication system affected by thermal noise is often modeled as an [[additive white Gaussian noise]] (AWGN) channel.

===Shot noise===
{{Main article|Shot noise}}

Shot noise in electronic devices results from unavoidable random statistical fluctuations of the [[electric current]] when the charge carriers (such as [[electron]]s) traverse a gap. If electrons flow across a barrier, then they have discrete arrival times. Those discrete arrivals exhibit [[shot noise]]. Typically, the barrier in a diode is used.<ref>{{Citation |last=Ott |first=Henry W. |title=Noise Reduction Techniques in Electronic Systems |date=1976 |publisher=John Wiley |isbn=0-471-65726-3 |pp=208, 218}}</ref> Shot noise is similar to the noise created by rain falling on a tin roof. The flow of rain may be relatively constant, but the individual raindrops arrive discretely.

The root-mean-square value of the shot noise current ''i''<sub>n</sub> is given by the Schottky formula.
:<math>i_n = \sqrt{2 I q \Delta B} </math>

where ''I'' is the DC current, ''q'' is the charge of an electron, and Δ''B'' is the bandwidth in hertz. The Schottky formula assumes independent arrivals.

[[Vacuum tube]]s exhibit shot noise because the electrons randomly leave the cathode and arrive at the anode (plate). A tube may not exhibit the full shot noise effect: the presence of a [[space charge]] tends to smooth out the arrival times (and thus reduce the randomness of the current).

Conductors and resistors typically do not exhibit shot noise because the electrons [[thermalize]] and move diffusively within the material; the electrons do not have discrete arrival times. Shot noise has been demonstrated in [[wikt:mesoscopic|mesoscopic]] resistors when the size of the resistive element becomes shorter than the electron–phonon scattering length.<ref>{{cite journal|last1=Steinbach|first1=Andrew|last2=Martinis|first2=John|last3=Devoret|first3=Michel|title=Observation of Hot-Electron Shot Noise in a Metallic Resistor|journal=Phys. Rev. Lett.|date=1996-05-13|volume=76|issue=20|pages=38.6–38.9|bibcode = 1996PhRvL..76...38M |doi = 10.1103/PhysRevLett.76.38 }}</ref>

===Flicker noise===
{{Main article|Flicker noise|1/f 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 noise|pink]] spectrum. It occurs in almost all electronic devices and results from a variety of effects.

===Burst noise===
{{Main article|Burst noise}}
Burst noise consists of sudden step-like transitions between two or more discrete voltage or current levels, as high as several hundred [[microvolt]]s, at random and unpredictable times. Each shift in offset voltage or current lasts for several milliseconds to seconds. It is also known a ''popcorn noise'' for the popping or crackling sounds it produces in audio circuits.

===Transit-time noise===
If the time taken by the electrons to travel from emitter to collector in a transistor becomes comparable to the period of the signal being amplified, that is, at frequencies above [[Very high frequency|VHF]] and beyond, the transit-time effect takes place and noise input impedance of the transistor decreases. From the frequency at which this effect becomes significant, it increases with frequency and quickly dominates other sources of noise.<ref>{{cite book |url=https://books.google.com/?id=D30ACAhesdIC&pg=SA3-PA6&lpg=SA3-PA6&dq=Transit-time+noise#v=onepage&q=Transit-time%20noise&f=false |title=Communication Theory |page=3–6 |publisher=Technical Publications |isbn=9788184314472|year=1991 }}</ref>

==Coupled noise==
{{See also|Electromagnetic compatibility}}

While noise may be generated in the electronic circuit itself, additional noise energy can be coupled into a circuit from the external environment, by [[inductive coupling]] or [[capacitive coupling]], or through the [[antenna (radio)|antenna]] of a [[radio receiver]].

===Sources===
;[[Intermodulation]] noise
: Caused when signals of different frequencies share the same non-linear medium.

;[[Crosstalk]]
:Phenomenon in which a signal transmitted in one circuit or channel of a transmission systems creates undesired interference onto a signal in another channel.

;[[Interference (communication)|Interference]]
:Modification or disruption of a signal travelling along a medium

;[[Atmospheric noise]]
:This noise is also called static noise and it is the natural source of disturbance caused by [[lightning]] discharge in thunderstorm and the natural (electrical) disturbances occurring in nature.

;Industrial noise
:Sources such as automobiles, aircraft, ignition electric motors and switching gear, High [[voltage]] wires and [[fluorescent lamp]]s cause industrial noise. These noises are produced by the discharge present in all these operations.

;Solar noise
:Noise that originates from the [[Sun]] is called ''solar noise''. Under normal conditions there is constant [[radiation]] from the Sun due to its high temperature. Electrical disturbances such as [[corona discharge]]s, as well as [[sunspot]]s can produce additional noise. The intensity of solar noise varies over time in a [[solar cycle]].

;[[Cosmic noise]]
:Distant stars generate noise called cosmic noise. While these stars are too far away to individually affect terrestrial [[communications system]]s, their large number leads to appreciable collective effects. Cosmic noise has been observed in a range from 8&nbsp;MHz to 1.43&nbsp;GHz, the latter frequency corresponding to the 21-cm [[hydrogen line]]. Apart from man-made noise, it is the strongest component over the range of about 20 to 120&nbsp;MHz. Little cosmic noise below 20MHz penetrates the ionosphere, while its eventual disappearance at frequencies in excess of 1.5 GHz is probably governed by the mechanisms generating it and its absorption by hydrogen in interstellar space.{{cn|date=September 2018|reason=Detail and speculation not supported in [[Cosmic noise]]}}

===Mitigation===
In many cases noise found on a signal in a circuit is unwanted. There are many different noise reduction techniques that can reduce the noise picked up by a circuit.
#Faraday cage – A [[Faraday cage]] enclosing a circuit can be used to isolate the circuit from external noise sources. A faraday cage cannot address noise sources that originate in the circuit itself or those carried in on its inputs, including the power supply.
#Capacitive coupling – [[Capacitive coupling]] allows an AC signal from one part of the circuit to be picked up in another part through interaction of electric fields. Where coupling is unintended, the effects can be addressed through improved circuit layout and grounding.
#Ground loops – When grounding a circuit, it is important to avoid [[Ground loop (electricity)|ground loops]]. Ground loops occur when there is a voltage difference between two ground connections. A good way to fix this is to bring all the ground wires to the same potential in a ground bus.
#Shielding cables – A [[shielded cable]] can be thought of as a Faraday cage for wiring and can protect the wires from unwanted noise in a sensitive circuit. The shield must be grounded to be effective. Grounding the shield at only one end can avoid a ground loop on the shield.
#Twisted pair wiring – [[Twisted pair|Twisting wires]] in a circuit will reduce electromagnetic noise. Twisting the wires decreases the loop size in which a magnetic field can run through to produce a current between the wires. Small loops may exist between wires twisted together, but the magnetic field going through these loops induces a current flowing in opposite directions in alternate loops on each wire and so there is no net noise current.
#Notch filters – Notch filters or [[Band-stop filter|band-rejection filters]] are useful for eliminating a specific noise frequency.  For example, power lines within a building run at 50 or 60&nbsp;Hz [[line frequency]]. A sensitive circuit will pick up this frequency as noise. A notch filter tuned to the line frequency can remove the noise.

==Quantification==
The '''noise level''' in an electronic system is typically measured as an electrical [[Power (physics)|power]] ''N'' in [[watt]]s or [[decibel|dBm]], a [[root mean square]] (RMS) voltage (identical to the noise [[standard deviation]]) in volts, [[decibel|dBμV]] or a [[mean squared error]] (MSE) in volts squared. Noise may also be characterized by its [[probability distribution]] and [[noise spectral density]] ''N''<sub>0</sub>(''f'') in watts per hertz.

A noise signal is typically 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 converter|analog-to-digital conversion]] and compression, [[peak signal-to-noise ratio]] (PSNR) in image and video coding, [[Eb/N0|''E''<sub>b</sub>/''N''<sub>0</sub>]] in digital transmission, [[carrier to noise ratio]] (CNR) before the detector in carrier-modulated systems, and [[noise figure]] in cascaded amplifiers.

Noise is a random process, characterized by [[Stochastic process|stochastic]] properties such as its [[variance]], [[Probability distribution|distribution]], and [[spectral density]]. The spectral distribution of noise can vary with [[frequency]], so its power density is measured in watts per hertz (W/Hz). Since the power in a [[resistor|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 (<math>\scriptstyle \mathrm{V}/\sqrt{\mathrm{Hz}}</math>). [[Integrated circuit]] devices, such as [[operational amplifiers]] commonly quote [[equivalent input noise]] level in these terms (at room temperature).

Noise power is measured in watts or [[decibel]]s (dB) relative to a standard power, usually indicated by adding a suffix after dB. Examples of electrical noise-level measurement units are [[dBu]], [[dBm0]], [[dBrn]], [[dBrnC]], and dBrn(''f''<sub>1</sub> − ''f''<sub>2</sub>), dBrn(144-[[line (electrical engineering)|line]]).

Telecommunication systems strive to increase the ratio of signal level to noise level in order to effectively transfer data. Noise in telecommunication systems is a product of both internal and external sources to the system.

In a carrier-modulated passband analog communication system, a certain [[carrier-to-noise ratio]] (CNR) at the radio receiver input would result in a certain [[signal-to-noise ratio]] in the detected message signal. In a digital communications system, a certain [[Eb/N0|''E''<sub>b</sub>/''N''<sub>0</sub>]] (normalized signal-to-noise ratio) would result in a certain [[bit error rate]].<!--[[User:Kvng/RTH]]-->

==Dither==
If the noise source is correlated with the signal, such as in the case of [[quantisation error]], the intentional introduction of additional noise, called [[dither]], can reduce overall noise in the bandwidth of interest. This technique allows retrieval of signals below the nominal detection threshold of an instrument. This is an example of [[stochastic resonance]].

==See also==
* [[Colors of noise]]
* [[Generation–recombination noise]]
* [[Phonon noise]]
* [[Noise reduction]] and [[noise cancellation]] for audio and images
* [[Matched filter]] for noise reduction in modems
* [[Error correction]] for digital signals subject to noise.
* [[Discovery of cosmic microwave background radiation]]

==Notes==
{{noteslist}}

==References==
{{reflist}}
{{More footnotes|date=September 2011}}
* [http://www.sengpielaudio.com/calculator-noise.htm White noise calculator, thermal noise - Voltage in microvolts, conversion to noise level in dBu and dBV and vice versa]
* {{FS1037C MS188}}
* Scherz, Paul. (2006, Nov 14) ''Practical Electronics for Inventors''. ed. McGraw-Hill.

==Further reading==
* {{cite book
 | author=Sh. Kogan
 | title=Electronic Noise and Fluctuations in Solids
 | publisher=Cambridge University Press
 | year=1996
 | isbn=0-521-46034-4}}

==External links==
* [http://www.c-c-i.com/node/77 Active Filter (Sallen & Key) Noise Study]

{{Noise}}
{{Analogue TV transmitter topics}}

[[Category:Electrical parameters]]
[[Category:Noise (electronics)| ]]