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Noise
Lecture 6
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 External
Atmospheric Industrial Extraterrestrial
Solar noise Cosmic noise
Internal
Atmospheric Atmospheric noise also known as
static It is caused by naturally occurring
disturbances in the earth’s atmosphere
SOURCES lightening discharges, thunderstorms and other natural
electric disturbances.
Nature and Form 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.
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 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
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.
•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.
Cosmic noise
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
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.
Internal Noise
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.
fTPn
fkTPn where k = Boltzmann’s constant J/K (joules per Kelvin)
T = absolute temperature in Kelvin, K = 273 + oC
= frequency bandwidth of system
= maximum noise power outputnP
f
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.
VRLoadR
If the load is noiseless and is receiving the maximum noise power generated by our noisy resistor then the following is true:
R
V
R
V
R
V
R
VP nn
Load
n 4
)2( 2222
fRkTVn 4
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 fkTPn
Example
Determine the noise voltage produced
by a 1 M resistor at room
temperature (17 oC) over a 1 MHz
bandwidth.
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?
Example 2
In a transistor the major contributor to noise is called shot noise. The formula for shot noise in a diode is given as:
= rms shot noise currentq = charge of an electron = = direct diode current = frequency bandwidth of system
Shot noise
fqIi dcn 2
ni
dcIf
C19106.1
Find the shot noise current for a diode
with a forward bias of 1mA over a 100
kHz bandwidth.
Example
Addition of Noise due to several sources in series
Given two sources of thermal agitation,
The sum of their effect is given by
Noise Calculations
11 4 fRkTVn 22 4 fRkTVn
21
2
2
2
1, 44 fRkTfRkTVVV nntotn
)(4 21, RRfkTV totn
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 FigureSignal to noise Ratio Two main reasons why we calculate
equivalent noise of a device 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
n
s
P
P
powernoise
powersignalNS
/
In decibeln
s
P
PNS 10log10/
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 5V 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.
S/N
S/N
output
inputF
S/N
S/N log10 10 output
inputF dB
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
Example
S/N
S/N log10 10 output
inputF
S/N 10log-S/N log10 1010 outputinputF
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 resistorfor their series combinationfor their parallel combination