Noise (electronics)

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Random fluctuations of voltage in pink noise

In electronics, noise is an unwanted disturbance in an electrical signal.[1]: 5 

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

In particular, noise is inherent in physics and central to

cryogenically, and even then quantum noise
would remain inherent.

Electronic noise is a common component of noise in signal processing.

In

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.

1/f noise
.

Thermal noise

Johnson–Nyquist noise[1] (more often thermal noise) is unavoidable, and generated by the random thermal motion of charge carriers (usually electrons), inside an electrical conductor, which happens regardless of any applied voltage.

Thermal noise is approximately

Gaussian probability density function. A communication system affected by thermal noise is often modelled as an additive white Gaussian noise
(AWGN) channel.

Shot noise

Shot noise in electronic devices results from unavoidable random statistical fluctuations of the electric 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.[3] 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.[4]

The root-mean-square value of the shot noise current in is given by the Schottky formula.

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

triodes
because the cathode current splits randomly between the screen grid and the anode.

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 mesoscopic resistors when the size of the resistive element becomes shorter than the electron–phonon scattering length.[5]

Partition noise

Where current divides between two (or more) paths,[6] 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

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.

Burst noise

Burst noise consists of sudden step-like transitions between two or more discrete voltage or current levels, as high as several hundred

microvolts
, at random and unpredictable times. Each shift in offset voltage or current lasts for several milliseconds to seconds. It is also known as 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 VHF 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.[7]

Coupled noise

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 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 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 by lightning discharges in thunderstorms and other electrical disturbances occurring in nature, such as corona discharge.
Industrial noise
Sources such as automobiles, aircraft, ignition electric motors and switching gear, High voltage wires and fluorescent lamps 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 approximately constant radiation from the Sun due to its high temperature, but solar storms can cause a variety of electrical disturbances. 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 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-cm hydrogen 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.[citation needed]

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 – 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.
  2. Capacitive coupling – Capacitive coupling allows 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 avoid 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.
  4. 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.
  5. Twisted pair wiring – 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.
  6. Notch filters – Notch filters or
    line 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

The noise level in an electronic system is typically measured as an electrical

line). Noise may also be characterized by its probability distribution and noise spectral density
N0(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

analog-to-digital conversion and compression, peak signal-to-noise ratio (PSNR) in image and video coding and noise figure in cascaded amplifiers. In a carrier-modulated passband analogue 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 (normalized signal-to-noise ratio) would result in a certain bit 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 by stochastic properties such as its variance, 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 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 ().

equivalent input noise
level in these terms (at room temperature).

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

Notes

  1. ^ E.g. crosstalk, deliberate jamming or other unwanted electromagnetic interference from specific transmitters

References

Further reading

External links