Communication System

August 31, 2023

INTRODUCTION TO NOISE
Any electrical signal transmitted from one point to another point is classified as having two
parts, the desirable part and undesirable part, which is noise.
Definition: Electrical noise is defined as any undesirable electrical signal energy that falls
within the
passband of the desired electrical signal.

• Electrical noise contains a wide range of amplitude levels and frequency components
that can interfere with the signal waveform.

• The amplitude level of noise is random in nature and thus unpredictable.

• The noise may arise from many sources and take many forms.

 The magnitude of noise is directly related with the ability to recover the original signal
without error. Below are the figures how the signal is corrupted by the noise:

Classification and sources of Noise

Unwanted electric signals come from a variety of sources, generally classified as either human
interference or naturally occurring noise. Human interference is produced by other
communication systems, ignition and commutator sparking, AC hum, and so forth. Natural
noise-generating phenomena include atmospheric disturbances, extra-terrestrial radiation, and
random electron motion.
Noise can be classified based on its source of generation such as

• External Noise

• Internal Noise
 Communication System
August 31, 2023

• External Noise. Noise is generated external to the communication system/ outside the device
or circuit
Below are the primary sources of external noise:
a) Atmospheric Noise (Static noise)
b) Extra-terrestrial Noise (Solar noise and Cosmic Noise)
c) Man-made noise (Industrial Noise)

a) Atmospheric Noise It occurs irregularly and is mainly caused by lightning, electrical

storms, and other atmospheric disturbances. It is more severe at frequencies up to 30 MHz
and thus affects both broadcast and shortwave radio frequencies
b) Extra-terrestrial Noise
It is also created by natural disturbances such as solar cycle and distant stars. This noise affects
the frequency range from about 8 MHz to 1.43 GHz approximately.
i. Solar Noise
It is caused by ionized gases of the sun produce broad range frequencies that penetrate Earth’s
atmosphere at frequencies used by communication systems. These disturbances are especially
intense when sunspot activity peaks approximately every 11 years.
ii. Cosmic Noise
It is caused by distant stars in our universe, like the sun, also radiate. intense levels of noise at
frequencies that penetrate Earth’s atmosphere. This radio-frequency noise, called black-body
noise.
c) Man-made Noise
It is simply noise that is produced by mankind The man-made noise is due to

• undesired radiation pick-ups from electrical appliances such as automobiles, switch

gears, electric motors, and aircraft ignitions.

• leakages from high-voltage electrical transmission lines, fluorescent lights, etc.

The man-made noise is random in nature and it can only be analyzed statistically.
Man-made noise is more effective in the frequency range of 1 MHz—500 MHz in urban,
suburban
and industrial areas. This type of noise is under human control and can be eliminated by
removing the source

• Internal Noise. Noise is created within the communication system/ within the device or
circuit
Internal Noise.
 Communication System
August 31, 2023

Internal noise is created by any of the passive or active devices used in the design of
communication receivers.
• Internal noise is also known as fluctuation noise because it is caused by spontaneous
fluctuations in the physical system.
• Some forms of internal noise occur only when there is a signal present in a circuit.
In this case, it is known as nonlinear distortion such as harmonic and intermodulation
distortion. It is produced by nonlinear amplification or/and mixing operation of electronic
circuits. It creates unwanted frequencies that interfere with the signal and degrades
performance.
• Other types of internal noise are shot noise, thermal noise, white noise (means spectrum is
constant irrespective of its source), partition noise, flicker noise, and high-frequency noise

Thermal Noise

Def. Thermal noise is the noise produced by the random motion of charged particles (usually
electrons) in conducting media.

Historically, Johnson (1928) and Nyquist (1928) first studied noise in metallic resistors—hence,
the designation Johnson noise or resistance noise. There now exists an extensive body of
theoretical and experimental studies pertaining to noise, from which we’ll freely draw.

Thermal noise is the noise produced by the random motion of charged particles (usually
electrons) in conducting media. From kinetic theory, the average energy of a particle at absolute
temperature T is proportional to kT, k being the Boltzmann constant. We thus expect thermal-
noise values to involve the product kT. In fact, we’ll develop a measure of noise power in terms
of temperature. Noise power is also proportional to the bandwidth over which it is measured.
From the above discussion we can write down.

Pn ∝ TB
Pn = KTB ------ (1)

Where

Pn = Maximum noise power output of a resistor.

K = Boltzmann’s constant = 1.38 x 10-23 joules/Kelvin.
T = Absolute temperature.
B = Bandwidth over which noise is measured.

From equation (1), an equivalent circuit can be drawn as shown in below figure
 Communication System
August 31, 2023

From equation (2), we see that the square of the rms noise voltage is proportional to the
absolute temperature of the resistor, the value of the resistor, and the bandwidth over which it
is measured. En is quite independent of the Frequency.

Shot noise
The name originates from the sound that it produces at the audio output of a receiver. The
sound is similar to that leaf shot falling on top of a tin roof. This noise is caused by the

• random behaviour of charge carriers (electrons and holes) in active devices such as
diode and transistor.

• random diffusion of minority carriers

• random generation and recombination of electron-hole pairs

• random variations in the arrival of electrons or holes at the output terminal.

In a semiconductor diode, the diode current is given by

in= (2iDneB)1/2
Where iD is the rms diode current
in is the rms shot-noise current in diode,
 Communication System
August 31, 2023

e is charge of an electron (1.6 × 10–19 Coulombs),

B is the system bandwidth in Hz.
In a semiconductor transistor, the output current i(t) is the sum of mean current Io and the
noise current i n(t) which fluctuates around Io. That is,

Transit-time noise (high frequency)

Another kind of noise that occurs in transistors is called transit time noise.
Transit time is (the duration of time that it takes for a current carrier such as a hole or current to
move from the input to the output.
This noise occurs in many microwave devices which produce more noise at frequencies
approaching their cutoff frequencies.

• This high-frequency noise occurs when the time taken by charge carriers to cross a
junction is comparable to the time period of the input signal waveform. The devices
themselves are very tiny, so the distances involved are minimal. Yet the time it takes for
the current carriers to move even a short distance is finite. At low frequencies this time
is negligible. But when the frequency of operation is high and the signal being processed
is the magnitude as the transit time, then problem can occur.

• A fluctuating current that constitutes transit-time noise is produced because some of the
charge carriers may diffuse back across the junction.

• This carrier diffusion give rise to an admittance whose conductance component

increases with frequency.
The transit time shows up as a kind of random noise within the device, and this is directly
proportional to the frequency of operation.

Flicker noise (low frequency noise)

• Flicker noise or modulation noise is the one appearing in transistors operating at low
audio frequencies. Flicker noise is proportional to the emitter current and junction
temperature. it may be neglected at frequencies above about 500 Hz and it, Therefore,
possess no serious problem.

• Its noise power varies inversely with frequency that’s why it is also referred as 1/f
noise.

• It is generated because of fluctuation in current density.

• Flicker noise is more serious in semiconductor amplifying devices.

EXAMPLE 11.1 Noise Voltage in AM Radio The front-end filter of a radio passes the broadcast AM
band from 535 kHz to 1605 kHz. The radio input has an effective resistance of 300 ohms. What is the
 Communication System
August 31, 2023

root-mean-square noise voltage that we would expect to observe due to this resistance? (Simon
hykin

Solution: