Noise (electronics)


In electronics, noise is an unwanted disturbance in an electrical signal. Noise generated by electronic devices varies greatly as it is produced by several different effects.
In communication systems, 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, for example in the signal-to-noise ratio, signal-to-interference ratio and signal-to-noise plus interference ratio 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 and total harmonic distortion plus noise 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, while other types depend mostly on device type or manufacturing quality and semiconductor defects, such as conductance fluctuations, including 1/f noise.

Thermal noise

Johnson–Nyquist noise is unavoidable, and generated by the random thermal motion of charge carriers, inside an electrical conductor, which happens regardless of any applied voltage.
Thermal noise is approximately 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 modelled as an additive white Gaussian noise channel.

Shot noise

Shot noise in electronic devices results from unavoidable random statistical fluctuations of the electric current when the charge carriers 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 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.
Vacuum tubes exhibit shot noise because the electrons randomly leave the cathode and arrive at the anode. A tube may not exhibit the full shot noise effect: the presence of a space charge tends to smooth out the arrival times. Pentodes and screen-grid tetrodes exhibit more noise than 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 resistors when the size of the resistive element becomes shorter than the electron–phonon scattering length.

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 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 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.

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.

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 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 band-rejection filters are useful for eliminating a specific noise frequency. For example, power lines within a building run at 50 or 60 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 N in watts or dBm, a root mean square voltage in volts, dBμV or a mean squared error in volts squared. Noise may also be characterized by its probability distribution and noise spectral density N0 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, signal-to-quantization noise ratio in analog-to-digital conversion and compression, peak signal-to-noise ratio in image and video coding, Eb/N0 in digital transmission, carrier to noise ratio before the detector in carrier-modulated systems, and noise figure in cascaded amplifiers.
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. Since the power in a resistive element is proportional to the square of the voltage across it, noise voltage can be described by taking the square root of the noise power density, resulting in volts per root hertz. Integrated circuit devices, such as operational amplifiers commonly quote equivalent input noise level in these terms.
Noise power is measured in watts or decibels 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, dBrn.
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 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 would result in a certain bit error rate.

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.