Noise
Noise is defined as an unwanted form of signal that disturbs, interferes and affects the wanted signal. In
general, any undesired signal becomes noise.
Types of Noise
(1) External Noise
External noise is normally introduced in the transmitting medium or the channel. I Ins noise type
is difficult to quantify and to solve for their sources are not within the system and usually of
natural origin. Examples of this noise type are:
(a) Industrial Noise or Man-made Noise
The industrial or man-made noise can occur randomly at frequencies up to around 600 MHz.
The common sources are man-made devices used in the industry and the home such as
fluorescent lights, ignition system of engines, switching equipments, commutators of
electric motors and leakage from high voltage transmission lines. This noise is usually,
considered mainly from the point of view of environmental health and safety, rather than
nuisance, as sustained exposure can cause permanent hearing damage (long-term
exposures of over 85 decibels).
(b) Atmospheric Noise or Static Noise
Atmospheric noise is mainly caused by lightning discharges during thunderstorms and other
natural electrical disturbances occurring in the earth's atmosphere. The intensity of this
noise is inversely proportional with frequency and therefore it becomes less severe at
frequencies above 30 MHz or in general as frequency increases, the effect of this noise
decreases until it will become negligible.
(c) Extraterrestrial Noise or Space Noise
Space noise can be observed at frequencies in the range from about 8 MHz to somewhat
above 1.43 GHz (up to a maximum of 1.5 GHz). Some sources are the sun (solar noise) in the
solar system, the stars (cosmic noise) and galaxies (galactic noise) in the universe.
The solar noise is a constant noise radiation from the sun. This is an electrical disturbance
due to solar cycle activities that has a 1-year cycle like corona flares and sunspots.
The cosmic noise is an rf noise radiated by distant stars although very far from our solar
system but the distance is compensated by their number in effect still can affect
frequencies, used for communications.
(2) Internal Noise
Internal noise is produced and introduced at the receiver by the components that make up the
receiver itself like resistors, diodes, transistors and even wires or conductors within. The
following are examples of internal noise of receivers:
(a) Thermal Noise
Thermal noise was discovered by a Scottish botanist Robert Brown (hence the name
Brownian noise) in 1827 and was first thoroughly studied and measured by J.B. Johnson at
Bell Labs (also known as Johnson noise) in 1928. It is dependent on temperature (Thermal
noise) and its frequency content is spread equally throughout the usable spectrum (White
noise). This is also called Gaussian noise. [he primary source is the rapid and random motion
of charge carriers inside a resistive component when heated.
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𝑉𝑛 = √4𝐾𝑇𝐵𝑊𝑅
k = Boltzmann's constant =1.38 x 1023 J/K
T = noise temperature (K) = 290 K or 17°C (if not given- this value is considered the standard
noise temperature)
BW = bandwidth (Hz)
R = resistance generating the noise
The maximum available noise power in watts (W) is given by
N = kTBW
Total thermal noise voltage generated by several resistors
For several resistors, the total generated noise voltage is given by
𝑉𝑛 = √4𝐾𝑇𝐵𝑊𝑅𝑇
𝑊ℎ𝑒𝑟𝑒 ∶ 𝑅𝑇 = 𝑅1 + 𝑅2 + 𝑅3 + ⋯ (𝑠𝑒𝑟𝑖𝑒𝑠 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑)
1
1
1
1
=
+
+
+ ⋯ (𝑝𝑎𝑟𝑎𝑙𝑙𝑒𝑙 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑)
𝑅𝑇
𝑅1 𝑅2 𝑅3
Alternately, the total noise voltage and current can be found using these formulas:
For series resistors,
√𝑉𝑛 = 𝑉𝑛1 2 + 𝑉𝑛2 2 + 𝑉𝑛3 2 + ⋯
For parallel resistors,
√𝐼𝑛 = 𝐼𝑛1 2 + 𝐼𝑛2 2 + 𝐼𝑛3 2 + ⋯
(b) Shot Noise
The shot noise is consisting of random fluctuations of the electric current in an electrical
conductor which is caused by the fact that the current is carried by discrete charges
(electrons).
In electronic devices, the shot noise is due to shot effect caused by random variations in the
arrival of electrons or holes at the output electrode of an amplifying device and appearing as
a randomly varying noise current superimposed on the output. The name is derived from
the fact that when amplified it is supposed to sound as though a shower of lead shots were
falling on the metal sheet.
The shot noise formulas for a diode are given below:
A. Temperature-limited vacuum-tube diode
𝐼𝑛 = √2𝑞𝑒 𝐼𝑑𝑐 𝐵𝑊
B. PN-Junction semiconductor diode
𝐼𝑛 = √2𝑞𝑒 (𝐼𝑑𝑐 + 𝐼𝑠 )𝐵𝑊
I„ = rms shot noise current for a diode (A)
qe = charge of an electron
ldc = direct diode current (A)
Is = reverse saturation current (A)
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𝐼𝑑𝑐 = 𝐼𝑠 (𝑒
𝑞𝑒 𝑉𝑑
𝐾𝑇
− 1)
(c) Transit-time Noise
This noise is caused by the transit-time effect that is when the time taken by an electron to
travel from the emitter to the collector of a transistor becomes comparable to the period of
the signal being amplified. Its greatest effect is at higher frequencies particularly in the
microwave region. It is otherwise known as the "high-frequency noise".
(d) Flicker noise
The flicker noise is a poorly understood form of noise found at low audio frequencies in
transistors. it is proportional to emitter current and junction temperature. It is also inversely
proportional to frequency and may be completely negligible at about above 500 Hz. It is
sometimes considered the "low-frequency noise" and otherwise known as modulation noise
or excess noise or 1/f noise or pink noise.
(3) Miscellaneous noise
(a) Partition noise
Partition noise occurs whenever current is divided between two electrodes and results to
random fluctuations in the process.
(b) Burst noise
Burst noise is another low-frequency noise found in transistors. Its name arises because it
appears as a series of bursts at two or more levels. It is also called "pop-corn" noise.
(c) Avalanche noise
Avalanche noise is large noise spikes present in the avalanche current due to oscillations
that result in the avalanching action.
The Equivalent Noise Resistance
𝑅𝑒𝑞 = 𝑅1 +
𝑅2
𝑅3
𝑅𝑛
+
+ ⋯+
2
2
2
2
(𝐴1 )
(𝐴1 ) (𝐴2 )
(𝐴1 ) (𝐴2 )2 … (𝐴𝑛−1 )2
𝑅𝑒𝑞 = 𝑅1 +
𝑅2
𝑅3
𝑅𝑛
+
+ ⋯+
𝐺1 𝐺1 𝐺2
𝐺1 𝐺2 … 𝐺𝑛−1
A = voltage gain
G = power gain
The Signal-to-Noise Ratio
Signal-to-noise ratio is a relative measure of the desired signal power to the undesired noise power at a
particular point in a system. The higher the value the better the system is.
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𝑆
𝑆𝑖𝑔𝑛𝑎𝑙 𝑃𝑜𝑤𝑒𝑟
=
𝑁
𝑁𝑜𝑖𝑠𝑒 𝑃𝑜𝑤𝑒𝑟
𝑆𝑝
𝑆
𝑆𝑣
(𝑑𝑏) = 10 log
= 20 log
𝑁
𝑁𝑝
𝑁𝑣
The Noise Factor (NF) or Noise Ratio (NR) and Noise Figure (F)
The noise factor is used to specify how noisy a device is. It is defined as the ratio of input signal-to-noise
ratio to the output signal-to-noise ratio usually determined at the standard noise temperature of 290 K
(17°C). The noise factor is an absolute value or unitless value while the noise figure is its dB value.
𝑆𝑖
𝑁𝑖 𝑖𝑛𝑝𝑢𝑡
𝑁𝐹 =
𝑆𝑜
𝑁𝑜 𝑜𝑢𝑡𝑝𝑢𝑡
𝐹 = 10𝑙𝑜𝑔𝑁𝐹
Note that for an ideal noiseless device, the input S/N is equal to the output S/N.
𝑁𝐹 = 1 𝑎𝑛𝑑 𝐹 = 0𝑑𝑏
The Equivalent Noise Temperature (𝑻𝒆 )
The equivalent noise temperature is not the actual operating temperature of the device.
𝑇𝑒 = 𝑇𝑜 (𝑁𝐹 − 1)
𝑇𝑒 = equivalent noise temperature (K)
𝑇𝑜 = reference temperature = 290 K
NF = noise factor (unitless)
𝑇𝑒𝑞 = 𝑇1 +
𝑇2
𝑇3
𝑇𝑛
+
+ ⋯+
2
2
2
2
(𝐴1 )
(𝐴1 ) (𝐴2 )
(𝐴1 ) (𝐴2 )2 … (𝐴𝑛−1 )2
𝑇𝑒𝑞 = 𝑇1 +
𝑇2
𝑇3
𝑇𝑛
+
+ ⋯+
𝐺1 𝐺1 𝐺2
𝐺1 𝐺2 … 𝐺𝑛−1
Overall Noise Factor
𝑁𝐹 = 𝑁𝐹1 +
𝑁𝐹2 − 1
𝑁𝐹3 − 1
𝑁𝐹𝑛 − 1
+
+
⋯
+
(𝐴1 )2
(𝐴1 )2 (𝐴2 )2
(𝐴1 )2 (𝐴2 )2 … (𝐴𝑛−1 )2
𝑁𝐹 = 𝑇1 +
𝑁𝐹2 − 1 𝑁𝐹3 − 1
𝑁𝐹𝑛 − 1
+
+ ⋯+
𝐺1
𝐺1 𝐺2
𝐺1 𝐺2 … 𝐺𝑛−1
Friis Formula
the term Friis' formula can refer to either of two formulas used in telecommunications engineering. The
first is used to compute the noise figure or noise temperature of a receiver composed of a number of
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cascaded stages. The second, called the Friis Transmission Equation, is used in computing transmission
of signals between two antennas using electromagnetic waves, and is discussed in a separate article.
Other Noise Problems
Environmental noise
Environmental noise is the collection of offending sounds to which humans are involuntarily exposed.
The principal sources of environmental noise are motor vehicles and aircraft. This noise form is
commonly referred to as noise pollution.
Acoustic noise
comparing noise in relation to sound, what is commonly meant is meaningless Hound of greater than
usual volume. Thus, a loud activity may be referred to as noisy. However, conversations of other people
may be called noise for people not involved in any of them and noise can be any unwanted sound such
as the noise of aircraft, neighbors playing loud music, or road noise spoiling the quiet of the countryside.
Industrial noise
Industrial noise is usually considered mainly from the point of view of environmental health, rather than
nuisance, as sustained exposure causes permanent hearing damage. A-weighted measurements are
commonly used for this as well, and special exposure meters are available that integrate noise over a
period of time to give an equivalent sound pressure level, defined by standards. In the case of industrial
noise affecting nearby residences or other sensitive receptors, the phenomenon is considered noise
pollution.
Audio noise
In audio recording and broadcast systems, audio noise refers to the residual low level sound (usually hiss
and hum) that is heard in quiet periods of program. In audio engineering. audio noise can also refer to
the unwanted residual electronic noise signal that gives rise to acoustic noise heard as 'hiss'. This signal
noise is commonly measured using A-weighting or ITU-R 468 weighting.
Radio noise
Radio noise is interference picked up between transmitter and receiver output, often referred to as
static. Radio noise can be caused by virtually any electromagnetic source, from lightning to man-made
electronics, including the receiver itself. Transmitter power must be increased to overcome radio noise
over long distances.
Video noise
In video and television, video noise refers to the random dot pattern that is superimposed on the
picture as a result of electronic noise, the 'snow' that is seen with poor (analog) television reception or
on VHS tapes. Interference and static are other forms of noise, in the sense that they are unwanted,
though not random, which can affect radio and television signals.
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The Colors of Noise
The color names for these different types of sounds are derived from an analogy between the spectrum
of frequencies of sound wave present in the sound and the equivalent spectrum of light wave
frequencies
White noise
White noise is a signal with a flat frequency spectrum in linear space. In other words, the signal has
equal power in any linear band, at any center frequency, having a given bandwidth. For example, the 20
Hz frequency range between 40 and 60 Hz contains the same amount of power as the range between
4000 and 4020 Hz. An infinite-bandwidth white noise signal is purely a theoretical construct. By having
power at all frequencies. the total power of such a signal would be infinite. In practice. a signal is
"white" if it has a flat spectrum over a defined frequency band.
Pink noise
Pink noise has a flat frequency spectrum in the logarithmic space. It has equal power in bands that are
proportionally wide. This means that pink noise would have equal power in the frequency range from 40
to 60 Hz as in the band from 4000 to 6000 Hz. Since humans hear in such a proportional space, where a
doubling of frequency is seen as the same size regardless of actual frequency (40 - 60 Hz is heard as the
same interval and distance as 4000 - 6000 Hz). every octave contains the same amount of energy and
thus pink noise is often used as a reference signal in audio engineering. That is, the human auditory
system perceives approximately equal magnitude on all frequencies. The power density, compared with
white noise, decreases by 3 dB per octave (density proportional to 1/f).
Brown (or red) noise
Brown noise is similar to pink noise, but with a power density decrease of 6 dB per octave with
increasing frequency (density proportional to 102) over a frequency range which does not include DC. It
can be generated by an algorithm which simulates Brownian motion or by integrating white noise.
Brown noise is not named for a power spectrum that suggests the color brown; rather, the name is a
corruption of Brownian motion. Also known as "random walk" or "drunkard's walk" noise.
Blue (or azure) noise
The power density of blue noise increases 3 dB per octave with increasing frequency (density
proportional to f) over a finite frequency range. In computer graphics, the term "blue noise" is
sometimes used more loosely as any noise with minimal low frequency components and no
concentrated spikes in energy. This can be good noise for dithering; retinal cells are arranged in a bluenoise-like pattern for this reason.
Purple (or violet) noise
The power density of purple noise increases 6 dB per octave with increasing frequency (density
proportional to f2) over a finite frequency range. It is also known as differentiated white noise or violet
noise.
Grey noise
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Grey noise is noise subjected to a psychoacoustic equal loudness curve (such as an inverted A-weighting
curve) over a given range of frequencies, so that it sounds like it is equally loud at all frequencies. Some
people say that this would be a better definition of "white noise" than the "equal power at all
frequencies" definition, since white light never has an equal power spectrum, but rather can have a
range of spectra, for example that of a 5400K black body.
Red noise
Red noise has more than one definition:
(a) Red noise is a synonym for brown noise.
(b) Red noise is a synonym for pink noise.
The oceanic ambient noise (that is, noise from distant sources) is often described as "red" due to the
selective absorption of higher frequencies by the ocean.
Orange noise
Orange noise is quasi-stationary noise wither finite power spectrum with a finite number of small bands
of zero energy dispersed throughout a continuous spectrum. These bands of zero energy are centered
about the frequencies of musical notes in whatever scale is of interest. Since all in-tune musical notes
are eliminated, the remaining spectrum could be said to consist of sour, citrus. or "orange" notes.
Green noise
Green noise is supposedly the background noise of the world. A really long term power spectrum
averaged over several outdoor sites. Rather like pink noise with a hump added around 500 Hz. The midfrequency component of white noise, used in halftone dithering.
Black noise
Black noise, or silent noise has several different definitions.
(a) Noise with a 1/fβ spectrum, where β > 2. Used in modeling various environmental processes. It
is said to be a characteristic of "natural and unnatural catastrophes like floods, droughts, bear
markets, and various outrageous outages, such as those of electrical power." Further, "because
of their black spectra, such disasters often come in clusters."
(b) Noise that has a frequency spectrum of predominately zero power level over all frequencies
except for a few narrow bands or spikes. An example of black noise in a facsimile transmission
system is the spectrum that might be obtained when scanning a black area in which there are a
few random white spots. Thus, in the time domain, a few random pulses occur while scanning.
Black noise is a noise with a power density that is constant for a finite frequency range above 20 kHz. it
is more accurately called ultrasonic white noise. This black noise is like the so-called black light with
frequencies too high to be sensed but still capable of affecting the environment.
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