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CHAPTER 5
5.1
Noise
5.2
Transmission Media & EM Propagations
Introduction
Define as
• undesired random variations that interface with
the desired signal and inhibit communication.
Where does noise originate in a communication
system?
• Channel @ transmission medium
• Devices @ Equipments
Cont’d...
Noise Effect
• One of the main limiting factor in
obtaining high performance of a
communication system.
• Decrease the quality of the receiving
signal.
Block Diagram of Communication
System With the Existence of Noise
Cont’d...
• Noise, interference and distortion
– Noise
• Refers to random and unpredictable
electrical signals produced by natural
process.
• Superimposed on information bearing
signal, the message partially corrupted or
totally erased.
• Can be reduced by filtering but can’t
totally eliminated.
Cont’d...
– Interference
• A contamination by extraneous signals
from human sources (e.g. from other
Tx, power lines, machineries)
• Often occurred in radio system whose
Rx antenna intercept several signals
at the same time.
Cont’d...
– Distortion
• The signal perturbation caused by
imperfect response of the system to the
desired signal.
• Disappear when the signal us turned-off.
• Can be corrected by the equalizers.
Noise Remedies?
REDUCE BANDWIDTH
INCREASE TRANSMITTER’S
POWER
LOW NOISE AMPLIFIERS
Types of NOISE
NOISE
INTERNAL
EXTERNAL
THERMAL NOISE
-transistor
-diode
-resistors
MAN MADE NOISE
-automobile engine
-electric motor
-computer
SHOT NOISE
-electronic system
-equipment
SPACE NOISE
-solar noise
-sky noise
FLICKER NOISE
-tubes
ATMOSPHERIC NOISE
-Noise blanking
-lighting
Cont’d...
– Noise generated outside the electronic
equipment used.
– Source can be terrestrial or
extraterrestrial (E.g. the earth, the
moon, the sun, the galaxies).
– Do not effect the entire
communication frequency spectrum
but affect certain frequencies at
certain times and locations.
– Types: Man made noise, space noise,
atmospheric noise.
Cont’d...
a. Man made noise
o Produced by mankind
o Source : Spark-producing
mechanisms
o Impulsive in nature & contains a
wide range of frequencies
propagated through space.
o Sometimes called industrial noise
(metropolitan & industrial area).
Cont’d...
b. Space noise
o The sun is a powerful source of
radiation.
o Stars also radiate noise called
cosmic, stellar or sky noise.
o Important at higher frequencies
(VHF and above) because
atmospheric noise dominates at
lower frequencies.
Cont’d...
c. Atmospheric noise
o The principle source is lightning ( a
static electricity discharge.
oCan propagate for a long distances
through space.
oThe lightning energy relatively low
frequency (up to several MHz).
Cont’d...
- Electronic noise generated by the
passive and active components
incorporated in the designs of
communications equipment.
- Types : Shot noise, flicker noise,
thermal noise.
Cont’d...
• Shot Noise
o Caused by a random arrival of carriers
(holes and electrons) at the output of an
electronic devices.
o Randomly varying & superimposed onto
any signal present.
o Sometimes called transistor noise.
Cont’d...
• Flicker noise
o Excess noise that related to dc current
flow through imperfect conductors.
o The real nature of flicker noise not yet
fully understood.
Thermal Noise
• This type of noise arise due to the
random motion of free electrons in the
conducting medium such as resistor.
• Each free electron inside a resistor is in
motion due to its thermal energy.
• The path of electron motion is random
and zig-zag due to collision with the
lattice structure.
Cont’d...
• The net effect of the motion of all
electrons constitutes an electric current
flowing through the resistor.
• It causes the rate of arrival of electron at
either end of a resistor to vary randomly
and thereby varies the resistor’s potential
difference. That is the direction of current
flow is random and has a zero mean
value.
Cont’d...
• Resistors and the resistance within all
electronic devices are constantly
producing noise voltage Vn(t).
• Since it is dependent on
temperature, it is also referred to as
thermal noise.
• Thermal noise also known as Johnson noise or white
noise.
• In 1928, J.B. Johnson founded that Noise Power is
direct proportionally with temperature and bandwidth.
P =kTB
n
Where
Pn
k
T
B
= noise power (Watt)
= Boltzman constant (1.38 x 10-23 J/K)
= conductor temperature (K) [Add 273 to C]
= Bandwidth of system (Hz)
• Noise spectrum density is constant for all value of
frequency to 1012 Hz.
•
• From the study of circuit theory, the
relationship between source resistor and
matched load under maximum power transfer
is when Rn = RL .
• The total of noise source power is Pn.
Known as Rn = RL = R,
Therefore voltage at RL is
Vn
RL
VL 
Vn 
Rn  RL
2
 Vn 


2
VL
2 

Power at VL , PL 

R
R
and Pn  PL  kTB
therefore
2
Vn
 kTB
4R
2
Vn  4kTBR
Vn 
4kTBR
2

Vn
2
4R
Example 1
•
A receiver has a BW of 10 kHz
with the 4.14 x 10-17 W noise
power. A resistor that matches the
receiver input impedance is
connected across its antenna
terminals. Calculate the resistor’s
temperature in Celsius.
Example 2
• A 1 kΩ resistor is connected across
1 kΩ antenna input of a television
receiver. The BW of the receiver is 5
MHz and the resistor at the room
temperature 293 K. Calculate the
noise power and noise voltage
applied to the receiver input.
How to Quantifying the Noise?
• The presence of noise degrades the
performance of analog and digital
communication.
• The extent to which noise affects the
performance of communication systems is
measured by the output signal to noise power
ratio or SNR (for analog communication
systems) and probability of error (for digital
communication systems).
Cont’d...
• The signal quality at the input of the receiver is
characterized by the input signal to noise ratio.
Because of the noise sources within the receiver,
which is introduced during the filtering and
amplification processes, the SNR at the output of
the receiver will be lower than at the input of the
receiver.
• This degradation in the signal quality is
characterized in terms of noise equivalent
bandwidth, N0, effective noise temperature, Te.
and noise figure,F
Noise Calculation
•
SNR is ratio of signal power, S to noise power, N.
SNR  10 log
•
Noise Factor, F
•
Noise Figure, NF
F
S
dB
N
Si N i
So N o
NF  10 log F
Si N i
 10 log
(dB)
So N o
Noise Calculation In Amplifier
o Two types of model
- Noise amplifier Model.
- Noiseless amplifier model.
Analysis of Noise Amplifier Model
S0  GSi and
Na
N 0  GNi  N a  G( N i 
)  G( N i  N ai )
G
Analysis of Noiseless Amplifier Model
S 0  GSi and
N 0  G ( N i  N ai )
SNR0 <<< SNRi
SNRi
F

SNR0
As known as
Noise Factor,
Si
Ni
N i  N ai
N ai

 1
GSi
Ni
Ni
G ( N i  N ai )
Ni  kTi B and N ai  kTe B
N ai
kTe B
Te
F  1
 1
 1
Ni
kTi B
Ti
Noise Temperature,
Te  ( F  1)Ti
Analysis of Cascade Stages
•
Consider three two ports in cascade
antenna
F1, Te1
Si
N
S2
N2
G1
i
Ti
F3, Te3
S1
N1
Nai1
pre-amplifier
Stage 1
F2, G2, Te2
Nai2
G3
Nai3
demodulator
Stage 2
amplifier
Stage 3
So
No
Stage 1
Signal Power, S1  G1Si
Noise Power, N1  G1 ( N i  N ai1 )
 G1 (kTi B  kTe1 B)
 G1kB(Ti  Te1 )
Stage 2
Signal Power, S 2  G2 S1  G2G1Si
Noise Power, N 2  G2 ( N1  N ai 2 )
 G2 N1  G2 N ai 2
 G2G1kB(Ti  Te1 )  G2 kTe 2 B
Stage 3
Signal Power, S0  G3 S 2  G3G2G1Si
Noise Power, N 0  G3 ( N 2  N ai3 )
 G3 N 2  G3 N ai3
 G3G2G1kB(Ti  Te1 )  G3G2 kTe 2 B  G3kTe3 B
Noise Factor, F
SNRi
Ftotal 

SNRO

Si
SO
Ni
NO
Si
G3G2G1S i
kTi B
G3G2G1kB (Ti Te1 )  G3G2 kBTe 2  G3 kBTe 3
G3G2G1kB(Ti  Te1 )  G3G2 kBTe 2  G3 kBTe3

G3G2G1kBTi
Ti  Te1 Te 2
Te3



Ti
G1Ti G2G1Ti
Known as the overall noise factor, FTOTAL
FTOTAL 
Ti
T
T
Te 3
 e1  e 2 
Ti
Ti
G1Ti
G1G2Ti
FTOTAL  1 
If F  1 
Te1
T
Te 3
 e2 
Ti
G1Ti
G1G2Ti
Te
Ti
and Ti  T0  290 K
therefore Te  ( F  1)T0
 FTOTAL
( F2  1) ( F3  1)
 F1 

G1
G1G2
And we can calculate noise temperature, Te
FTOTAL
( F2  1) ( F3  1)
 F1 

G1
G1G2
TeTOTAL
1
T0
 Te 2
  Te 3

1 
 1 1 
 1
T0
T0
Te1 



 1


T0
G1
G1G2
TeTOTAL Te1 Te 2
Te 3



T0
T0 G1T0 G1G2T0
TeTOTAL
Te 2
Te 3
 Te1 

G1 G1G2
It can also be shown that the overall noise figure, F
and the effective noise temperature, Te of n networks
in cascade is given by:
( Fn  1)
( F2  1) ( F3  1)
F  F1 

 ... 
G1
G1G2
G1G2 ...Gn 1
Te 2
Te3
Ten
Te  Te1 

 ... 
G1 G1G2
G1G2 ...Gn 1
Transmission Loss, Attenuator
• Every transmission medium will produce power
loss.
Pout < Pin.
 Power loss or attenuated is given by the
following equation:
Pin
1
L

Pout G
LdB
 Pin
 10 log 10 
 Pout

  GdB

Cont’d...
 We also can calculate by using this
following equation;
LdB  
Where
ℓ = transmission medium length
α = attenuated constant
Example 3
Determine:
a. Noise Figure for an equivalent
temperature of 75 K (use 290 K
for the reference temperature).
b. Equivalent noise temperature
for a Noise Figure of 6 dB.
Example 4
For three cascaded amplifier stages,
each with noise figure of 3dB and
power gain of 10 dB, determine the
total noise figure.
Example 5
An amplifier consists of three identical
stages in tandem. Each stage having
equal input and output impedances. For
each stages, the power gain is 8 dB when
correctly matched and the noise figure is
6dB. Calculate the overall power gain
and noise figure of the amplifier.