Noise
Lecture 6
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Definition
Sources of noise
Noise Calculations
Definition:

Electrical noise may be said to be the
introduction of any unwanted energy,
which tend to interfere with the proper
reception
and
reproduction
of
transmitted signals.
Sources of noise
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External
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Atmospheric
Industrial
Extraterrestrial
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Solar noise
Cosmic noise
Internal
Atmospheric
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Atmospheric noise also known as static
It is caused by naturally occurring
disturbances in the earth’s atmosphere
SOURCES
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lightening discharges,
thunderstorms and other natural
electric disturbances.
Nature and Form
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It comes in the form of amplitude
modulated impulses.
Such impulse processes are random and
spread over the whole of the RF
spectrum used for broadcasting.
It consists of spurious radio signals with
many frequency components.
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It is propagated in the same way as
ordinary radio waves of the same
frequency.
Any radio station will therefore receive
static from thunderstorms both local
and distant.
It affects radio more than it affects
television. The reason, field strength is
inversely proportional to frequency.
At 30MHz and above atmospheric noise is less
severe for two reasons:
•Higher frequencies are limited to line of
sight propagation
•Very little of this noise is generated in the
VHF range and above.
Industrial
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Noise made by man easily outstrips any
other between the frequencies of 1 to
600 MHz.
This includes such things as car and
aircraft ignition, electric motors,
switching equipment, leakage from high
voltage lines etc.
Extraterrestrial
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Solar noise
This is the noise that originates from
the sun.
The sun radiates a broad spectrum of
frequencies, including those, which are
used for broadcasting.
•The sun is an active star and is
constantly changing
•It undergoes cycles of peak activity from
which electrical disturbances erupt.
•The cycle is about 11 years long.
Cosmic noise
•Distant stars also radiate noise in much the
same way as the sun.
•The noise received from them is called
black body noise.
•Noise also comes from distant galaxies in
much the same way as they come from the
milky way.
Extraterrestrial noise is observable at
frequencies in the range from about 8MHz
to 1.43GHz.
Apart from man made noise it is strongest
component over the range of 20 to
120MHz.
Not much of it below 20MHz penetrates
below the ionosphere
Internal Noise
This is the noise generated by any of
the active or passive devices found in
the receiver.
This type of noise is random and
difficult to treat on an individual basis
but can be described statistically.
Random noise power is proportional to
the bandwidth over which it is
measured.
Gaussian Noise
This is the cumulative effect of all random noise generated
both external and internal to the communication system
and averaged over a period of time. This includes all
frequencies.
For electronic circuits this more specifically called white
noise, Johnson noise or Thermal noise.
Thermal Noise
The noise generated by the agitation and interaction
of electrons is called thermal noise.
The internal kinetic energy of a particle can be
expressed through its temperature.
The kinetic energy of a body is zero at a temperature
of absolute zero.
The noise generated by a resistor, for example, is
proportional to its absolute temperature as well as
the bandwidth over which the noise is to be
measured.
Pn  Tf
Pn  kTf
where k = Boltzmann’s constant J/K (joules per Kelvin)
T = absolute temperature in Kelvin, K = 273 + oC
f
Pn
= frequency bandwidth of system
= maximum noise power output
Any ordinary resistor not connected to a voltage
source will have a voltage associated with it in such
a case the resistor may be represented
diagrammatically as shown.
R
V
RLoad
If the load is noiseless and is receiving the
maximum noise power generated by our
noisy resistor then the following is true:
2
2
2
2
V
V
(Vn 2) Vn
Pn 



RLoad R
R
4R
Vn  4kTfR
Observations
For maximum power transfer:
V =Vn/2
That is t the voltage across the load is half the voltage of
the noise generating resistor.
Also
Pn  kTf
Example
Determine the noise voltage produced by a
1 M resistor at room temperature (17 oC)
over a 1 MHz bandwidth.
Example 2
An amplifier operating over the frequency range
from 18 to 20 MHz has a 10 kilo ohm input
resistor. What is the rms noise voltage at the
input to the amplifier if the ambient temperature
is 27 oC?
Shot noise
In a transistor the major contributor to noise is
called shot noise. The formula for shot noise in
a diode is given as:
in  2qIdcf
= rms shot noise current
q = charge of an electron = 1.6 1019 C
I dc = direct diode current
f = frequency bandwidth of system
in
Example
Find the shot noise current for a diode with a
forward bias of 1mA over a 100 kHz
bandwidth.
Noise Calculations
Addition of Noise due to several sources in series
Given two sources of thermal agitation,
Vn1  4kTfR1
Vn 2  4kTfR2
The sum of their effect is given by
Vn ,tot  V  V  4kTfR1  4kTfR2
2
n1
2
n2
Vn,tot  4kTf ( R1  R2 )
Example
Calculate the noise voltage at the input of a
television RF amplifier using a device that has 200
ohm equivalent noise resistance and a 300 ohm
input resistor. The bandwidth of the amplifier is 6
MHz and the temperature is 17oC.
Noise Figure
Signal to noise Ratio
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Two main reasons why we calculate equivalent
noise of a device
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to compare two devices in order to evaluate their
performance
to compare the signal and the noise at the same
point to ensure that noise is not excessive
The measure for this calculation is the signal to
noise ratio. This is a relative measure of the
desired signal power to the noise power
Signal to noise Ratio
signal power Ps
S/N 

noise power Pn
In decibel
Ps
S / N  10 log10
Pn
Example
An amplifier operating over a 4 MHz bandwidth has a
100-ohm input resistance. It is operating at 27 oC, has
voltage gain of 200 and has an input signal of 5mV rms.
Determine the rms output signals (desired and noise)
disregarding any external sources of noise. Calculate the
signal to noise ratio at the output.
Noise Figure
This term is used to describe how noisy a device
is. It is a ratio of the signal to ratio at the input to
the signal to noise ratio at the output.
input S/N
F
output S/N
input S/N
F  10 log10
output S/N
dB
Example
A transistor amplifier has a measured S/N power of 10 at its
input and 5 at its output. Calculate the noise figure.
Show that the equation
can be written as
F  10 log10
input S/N
output S/N
F  10 log 10 input S/N - 10log 10output S/N
Example
Two resistors, 5 kohm and 20 kohm, are at 27oC.
Calculate the thermal noise power and voltage for a 10
kHz bandwidth
for each resistor
for their series combination
for their parallel combination