Noise (electronics)

This article is about fundamental noise processes in electronics. For electronic noise arising from outside sources, seeelectromagnetic compatibilityandelectromagnetic interference.For acoustic noise due to electromagnetic fields, seeelectromagnetically induced acoustic noise and vibration.For other uses, seeNoise (disambiguation).

Random fluctuations of voltage inpink noise

Inelectronics,noiseis an unwanted disturbance in an electrical signal.

Noise generated by electronic devices varies greatly as it is produced by several different effects.

In particular, noise is inherent in physics and central tothermodynamics.Any conductor withelectrical resistancewill generate thermal noise inherently. The final elimination of thermal noise in electronics can only be achievedcryogenically,and even thenquantum noisewould remain inherent.

Electronic noise is a common component ofnoise in signal processing.

Incommunication systems,noise is an error or undesired random disturbance of a useful informationsignalin 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,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 generationordither.

Uncorrelated noisesources add according to the sum of their powers.

Noise types

Different types of noise are generated by different devices and different processes.Thermal noiseis unavoidable at non-zero temperature (seefluctuation-dissipation theorem), while other types depend mostly on device type (such asshot noise,which needs a steep potential barrier) or manufacturing quality andsemiconductordefects, such as conductance fluctuations, including1/f noise.

Thermal noise

Main article:Johnson–Nyquist noise

Johnson–Nyquist noise(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 densityis 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

Main article:Shot noise

Shot noise in electronic devices results from unavoidable random statistical fluctuations of theelectric currentwhen 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.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 currenti_nis given by the Schottky formula.

$i_{n}={\sqrt {2Iq\Delta B}}$

whereIis the DC current,qis the charge of an electron, and ΔBis the bandwidth in hertz. The Schottky formula assumes independent arrivals.

Vacuum tubesexhibit 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 chargetends to smooth out the arrival times (and thus reduce the randomness of the current).Pentodesand screen-gridtetrodesexhibit more noise thantriodesbecause the cathode current splits randomly between the screen grid and the anode.

Conductors and resistors typically do not exhibit shot noise because the electronsthermalizeand move diffusively within the material; the electrons do not have discrete arrival times. Shot noise has been demonstrated inmesoscopicresistors when the size of the resistive element becomes shorter than the electron–phonon scattering length.

Partition noise

Where current divides between two (or more) paths,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

Main articles:Flicker noiseand1/f noise

Flicker noise, also known as 1/fnoise, is a signal or process with a frequency spectrum that falls off steadily into the higher frequencies, with apinkspectrum. 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 hundredmicrovolts,at random and unpredictable times. Each shift in offset voltage or current lasts for several milliseconds to seconds. It is also known aspopcorn noisefor 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 aboveVHFand 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.

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, byinductive couplingorcapacitive coupling,or through theantennaof aradio receiver.

Sources

Intermodulationnoise

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 system creates undesired interference onto a signal in another channel.

Interference

Modification or disruption of a signal travelling along a medium

Atmospheric noise

Also called static noise, it is caused bylightningdischarges in thunderstorms and other electrical disturbances occurring in nature, such ascorona discharge.

Industrial noise

Sources such as automobiles, aircraft, ignition electric motors and switching gear, Highvoltagewires andfluorescent lampscause industrial noise. These noises are produced by the discharge present in all these operations.

Solar noise

Noise that originates from theSunis calledsolar noise.Under normal conditions, there is approximately constantradiationfrom the Sun due to its high temperature, butsolar stormscan cause a variety of electrical disturbances. The intensity of solar noise varies over time in asolar cycle.

Cosmic noise

Distant stars generate noise called cosmic noise. While these stars are too far away to individually affect terrestrialcommunications systems,their large number leads to appreciable collective effects. Cosmic noise has been observed in a range from 8 MHz to 1.43 GHz, the latter frequency corresponding to the 21-cmhydrogen line.Apart from man-made noise, it is the strongest component over the range of about 20 to 120 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.

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.

1. Faraday cage – AFaraday cageenclosing 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.
2. Capacitive coupling –Capacitive couplingallows an AC signal from one part of the circuit to be picked up in another part through the interaction of electric fields. Where coupling is unintended, the effects can be addressed through improved circuit layout and grounding.
3. Ground loops – When grounding a circuit, it is important to avoidground 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.
4. Shielding cables – Ashielded cablecan 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.
5. Twisted pair wiring –Twisting wiresin 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.
6. Notch filters – Notch filters orband-rejection filtersare useful for eliminating a specific noise frequency. For example, power lines within a building run at 50 or 60 Hzline frequency.A sensitive circuit will pick up this frequency as noise. A notch filter tuned to the line frequency can remove the noise.

Thermal noise can be reduced by cooling of circuits - this is typically only employed in high accuracy high-value applications such as radio telescopes.

Quantification

Thenoise levelin an electronic system is typically measured as an electricalpowerNinwattsordBm,aroot mean square(RMS) voltage (identical to the noisestandard deviation) in volts,dBμVor 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 distributionandnoise 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 conversionand compression,peak signal-to-noise ratio(PSNR) in image and video coding andnoise figurein 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 certainE_b/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 bystochasticproperties 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 aresistiveelement 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 ($\scriptstyle \mathrm {V} /{\sqrt {\mathrm {Hz} }}$).Integrated circuitdevices, such asoperational amplifierscommonly quoteequivalent input noiselevel in these terms (at room temperature).

Dither

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.

See also

• Active noise controlfor noise reduction through cancellation
• Colors of noise
• Discovery of cosmic microwave background radiation
• Error detection and correctionfor digital signals subject to noise
• Generation–recombination noise
• Matched filterfor noise reduction in modems
• Noise (signal processing)
• Noise reductionand for audio and images
• Phonon noise

References

• This article incorporatespublic domain materialfromFederal Standard 1037C.General Services Administration.Archived fromthe originalon 2022-01-22.(in support ofMIL-STD-188).

Further reading

• Sh. Kogan (1996).Electronic Noise and Fluctuations in Solids.Cambridge University Press.ISBN0-521-46034-4.
• Scherz, Paul. (2006, Nov 14)Practical Electronics for Inventors.ed. McGraw-Hill.

External links

• Active Filter (Sallen & Key) Noise Study.Archived2015-05-22 at theWayback Machine.
• White noise calculator, thermal noise - Voltage in microvolts, conversion to noise level in dBu and dBV and vice versa