Inelectronics,noise is an unwanted disturbance in anelectrical signal.[1]: 5
Noise generated byelectronic devices varies greatly as it is produced by several different effects.
In particular, noise is inherent inphysics and central tothermodynamics. Any conductor withelectrical resistance will generatethermal noise inherently. The final elimination of thermal noise in electronics can only be achievedcryogenically, and even thenquantum noise would remain inherent.
Electronic noise is a common component ofnoise in signal processing.
Incommunication systems, noise is an error or undesired random disturbance of a usefulinformation signal in acommunication channel. The noise is a summation of unwanted or disturbing energy from natural and sometimes man-made sources. Noise is, however, typically distinguished frominterference,[a] for example in thesignal-to-noise ratio (SNR),signal-to-interference ratio (SIR) andsignal-to-noise plus interference ratio (SNIR) measures. Noise is also typically distinguished fromdistortion, which is an unwanted systematic alteration of the signal waveform by the communication equipment, for example insignal-to-noise and distortion ratio (SINAD) andtotal harmonic distortion plus noise (THD+N) measures.
While noise is generally unwanted, it can serve a useful purpose in some applications, such asrandom number generation ordither.
Uncorrelated noise sources add according to the sum of theirpowers.[2]
Different types of noise are generated by different devices and different processes.Thermal noise is unavoidable at non-zero temperature (seefluctuation-dissipation theorem), while other types depend mostly on device type (such asshot noise,[1][3] which needs a steep potential barrier) or manufacturing quality andsemiconductor defects, such as conductance fluctuations, including1/f noise.
Johnson–Nyquist noise[1] (more often thermal noise) is unavoidable, and generated by the random thermal motion of charge carriers (usuallyelectrons), inside anelectrical conductor, which happens regardless of any appliedvoltage.
Thermal noise is approximatelywhite, meaning that itspower spectral density is nearly equal throughout thefrequency spectrum. The amplitude of the signal has very nearly aGaussian probability density function. A communication system affected by thermal noise is often modelled as anadditive white Gaussian noise (AWGN) channel.
Shot noise in electronic devices results from unavoidable random statistical fluctuations of theelectric current when the charge carriers (such as electrons) 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.[4] 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.[5]
The root-mean-square value of the shot noise currentin is given by the Schottky formula.
whereI is the DC current,q is the charge of an electron, and ΔB is the bandwidth in hertz. The Schottky formula assumes independent arrivals.
Vacuum tubes 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 aspace charge tends to smooth out the arrival times (and thus reduce the randomness of the current).Pentodes and screen-gridtetrodes exhibit more noise thantriodes because the cathode current splits randomly between the screen grid and the anode.
Conductors and resistors typically do not exhibit shot noise because the electronsthermalize and move diffusively within the material; the electrons do not have discrete arrival times. Shot noise has been demonstrated inmesoscopic resistors when the size of the resistive element becomes shorter than the electron–phonon scattering length.[6]
Where current divides between two (or more) paths,[7] noise occurs as a result of random fluctuations that occur during this division.
For this reason, a transistor will have more noise than the combined shot noise from its two PN junctions.
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 apink spectrum. It occurs in almost all electronic devices and results from a variety of effects.
Burst noise consists of sudden step-like transitions between two or more discrete voltage or current levels, as high as several hundredmicrovolts, at random and unpredictable times. Each shift in offset voltage or current lasts for several milliseconds to seconds. It is also known aspopcorn noise for the popping or crackling sounds it produces in audio circuits.
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 aboveVHF and beyond, the transit-time effect takes place and the 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.[8]
While noise may be generated in the electronic circuit itself, additional noise energy can be coupled into a circuit from the external environment, byinductive coupling orcapacitive coupling, or through theantenna of aradio receiver.
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.
Thermal noise can be reduced by cooling of circuits - this is typically only employed in high accuracy high-value applications such as radio telescopes.
Thenoise level in an electronic system is typically measured as an electricalpowerN inwatts ordBm, aroot mean square (RMS) voltage (identical to the noisestandard deviation) in volts,dBμV or amean squared error (MSE) in volts squared. Examples of electrical noise-level measurement units aredBu,dBm0,dBrn,dBrnC, and dBrn(f1 −f2), dBrn(144-line). Noise may also be characterized by itsprobability distribution andnoise spectral densityN0(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 aresignal-to-noise ratio (SNR orS/N),signal-to-quantization noise ratio (SQNR) inanalog-to-digital conversion and compression,peak signal-to-noise ratio (PSNR) in image and video coding andnoise figure in cascaded amplifiers. In a carrier-modulated passband analogue communication system, a certaincarrier-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 certainEb/N0 (normalized signal-to-noise ratio) would result in a certainbit error rate. 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.
Noise is a random process, characterized bystochastic properties such as itsvariance,distribution, andspectral density. The spectral distribution of noise can vary withfrequency, so its power density is measured in watts per hertz (W/Hz). Since the power in aresistive 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 ().Integrated circuit devices, such asoperational amplifiers commonly quoteequivalent input noise level in these terms (at room temperature).
If the noise source is correlated with the signal, such as in the case ofquantisation error, the intentional introduction of additional noise, calleddither, 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 ofstochastic resonance.
This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22. (in support ofMIL-STD-188).