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 (SNR),
signal-to-interference ratio (SIR) and 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, which needs a steep potential
barrier) or manufacturing quality and semiconductor
defects, such as conductance fluctuations,
including 1/f noise.
=== Thermal noise ===
Johnson–Nyquist noise (sometimes thermal,
Johnson or Nyquist 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 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 ===
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. 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.
i
n
=
2
I
q
Δ
B
{\displaystyle i_{n}={\sqrt {2Iq\Delta B}}}
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 (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
mesoscopic 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.
=== 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
Modification or disruption of a signal travelling
along a mediumAtmospheric 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 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 constant radiation from the Sun due to
its high temperature. Electrical disturbances
such as corona discharges, as well as sunspots
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
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.
=== Mitigation ===
In many cases noise found on a signal in a
circuit is unwanted. When creating a circuit,
one usually wants a true output of what the
circuit has accomplished. There are many different
noise reduction techniques that can change
a noisy altered output signal to a more theoretical
output signal.
Faraday cage – A Faraday cage is a good
way to reduce the overall noise in a complete
circuit. The Faraday cage can be thought of
as an enclosure that separates the complete
circuit from outside power lines and any other
signal that may alter the true signal. A Faraday
cage will usually block out most electromagnetic
and electrostatic noise.
Capacitive coupling – A current through
two resistors, or any other type of conductor,
close to each other in a circuit can create
unwanted capacitive coupling. If this happens
an AC signal from one part of the circuit
can be accidentally picked up in another part.
The two resistors (conductors) act like a
capacitor thus transferring AC signals. There
may be other reasons for which capacitive
coupling is wanted but then it would not be
thought of as electronic noise.
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. Since ground
is thought of as 0 V, the presence of a voltage
is undesirable at any point of a ground bus.
If this is the case, it would not be a true
ground. A good way to fix this is to bring
all the ground wires to the same potential
in a ground bus.
Shielding cables – In general, using shielded
cables to protect the wires from unwanted
noise frequencies in a sensitive circuit is
good practice. A shielded wire can be thought
of as a small Faraday cage for a specific
wire as it uses a plastic or rubber enclosing
the true wire. Just outside the rubber/plastic
covering is a conductive metal that intercepts
any noise signal. Because the conductive metal
is grounded, the noise signal runs straight
to ground before ever getting to the true
wire. It is important to ground the shield
at only one end to avoid a ground loop on
the shield.
Twisted pair wiring – Twisting wires very
tightly together in a circuit will dramatically
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. Even if the wires are twisted
very tightly, there may still be small loops
somewhere between them, but because they are
twisted the magnetic field going through the
smaller loops induces a current flowing in
opposite ways in each wire and thus cancelling
them out.
Notch filters – Notch filters or band-rejection
filters are essential when eliminating a specific
noise frequency. For example, in some countries
(notably the USA and Canada) power lines within
a building run at 60 Hz. Sometimes a sensitive
circuit will pick up this 60 Hz noise through
some unwanted antenna (could be as simple
as a wire in the circuit). Running the output
through a notch filter at 60 Hz will amplify
the desired signal without amplifying the
60 Hz noise. So in a sense the noise will
be lost at the output of the filter.
== Quantification ==
The noise level in an electronic system is
typically measured as an electrical power
N in watts or dBm, a root mean square (RMS)
voltage (identical to the noise standard deviation)
in volts, dBμV or a mean squared error (MSE)
in volts squared. 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 signal-to-noise ratio (SNR or S/N),
signal-to-quantization noise ratio (SQNR)
in analog-to-digital conversion and compression,
peak signal-to-noise ratio (PSNR) in image
and video coding, Eb/N0 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 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 (
V
/
H
z
{\displaystyle \scriptstyle \mathrm {V} /{\sqrt
{\mathrm {Hz} }}}
). 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 decibels
(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(f1 − f2),
dBrn(144-line).
Noise levels are usually viewed in opposition
to signal levels and so are often seen as
part of a signal-to-noise ratio (SNR). Telecommunication
systems strive to increase the ratio of signal
level to noise level in order to effectively
transmit data. In practice, if the transmitted
signal falls below the level of the noise
(often designated as the noise floor) in the
system, data can no longer be decoded at the
receiver. 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 (normalized signal-to-noise
ratio) 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.
== 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
