CHAPTER 5
5.1 Noise
5.2 Transmission Media & EM
Propagations
Announcement
Test #1 set 2
4 April 2008 (Friday)
Test #2 set #2
9 April 2008 (Wednesday)
8.30 p.m
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 turnedoff.
 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.
o Can propagate for a long distances
through space.
o The 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
o
o
Caused by a random arrival of carriers
(holes and electrons) at the output of an
electronic devices.
Randomly varying & superimposed onto
any signal present.
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



Ans:
An electronic device operating at a
temperature of 17 degree Celsius with a
bandwidth of 10 kHz, determine the
following:
i) Thermal noise power in watts and dBm.
ii) rms noise voltage for a 100 ohms load
resistance.
N=KTB = (1.38x10-23 J/K)(290)(1x104)=4x10-17 W , N(dBm)=10 log[(4x10-17
W)/(1mW)]
= -134dBm.
rms noise voltage, VN=(4RKTB)1/2 = 0.1265µV.
How to Quantify 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
Terms and definition




Noise factor, F is the ratio of the noise produced by a
real resistor to the simple thermal noise of an ideal
resistor.
Noise figure, NF (noise factor in dB) is a frequently used
measure of an amplifier's goodness, or its departure from
the ideal.
Noise temperature, Te is a means for specifying noise
in terms of an equivalent temperature.
Thermal noise – In any object with electrical resistance
the thermal fluctuations of the electrons in the object will
generate noise
Noise Calculation

SNR is ratio of signal power, S to noise power, N.
SNR  10 log

Noise Factor, F
F

Noise Figure, NF
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
Na - Noise inside the amplifier
Analysis of Noiseless Amplifier Model
S 0  GSi and
N 0  G ( N i  N ai )
Nai - Noise at the input of the amplifier
SNR0 <<< SNRi
   noiseless amplifier model
Noise Factor, F
SNRi
F

SNR0
Si
Ni
N i  N ai
N ai

 1
GSi
Ni
Ni
G( N i  N ai )
As known, noise power
(sometimes referred as Pn )
Noise Factor,
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 kBTe 3

G3G2G1kBTi
Ti  Te1 Te 2
Te 3



Ti
G1Ti G2G1Ti
Known as the overall noise factor, FTOTAL
FTOTAL
Ti Te1
Te 2
Te 3




Ti
Ti
G1Ti G1G2Ti
FTOTAL
Te1
Te 2
Te 3
 1


Ti
G1Ti G1G2Ti
Te
If F  1 
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
From F=[1+Te/Ti]=[1+Te/To]
TeTOTAL
1
T0
 Te 2   Te3 
1 
 1 1 
 1
T0
Te1  T0



 1


T0
G1
G1G2
TeTOTAL Te1 Te 2
Te3



T0
T0 G1T0 G1G2T0
TeTOTAL
Te 2
Te3
 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:
General equation for F and Te
( Fn  1)
( F2  1) ( F3  1)
F  F1 

 ... 
G1
G1G2
G1G2 ...Gn 1
Te 2
Te 3
Ten
Te  Te1 

 ... 
G1 G1G2
G1G2 ...Gn 1
Transmission Loss, Attenuator


Every transmission medium will produce power
loss.
Pout < Pin. (ideally Pout = Pin )
Power loss or attenuated is given by the following
equation:
L
Pin
1

Pout G
 Pin 
  GdB
LdB  10 log 10 
 Pout 
Cont’d...

We also can calculate by using this
following equation;
LdB  
Where
ℓ = transmission medium length
α = attenuated constant
Example 2



For an amplifier with an output signal voltage
of 4V, an output noise voltage of 0.005 V, and
input and output resistance of 50 ohms,
determine the signal to noise power ratio.
Ans :
[S/N](dB)=20 log [Vs/Vn]=20 log
[4/0.005]=58.06dB.
Example 3
For a non-ideal amplifier and the following parameters,
determine

a)
b)
c)
Input S/N ratio (dB)
Output S/N ratio (dB)
Noise Factor and Noise Figure, given:
input signal power = 2 x 10-10W.
input noise power = 2 x 10-18W.




Ans :
[S/N]i=100,000,000 [S/N]o=25,000,000
F=[100,000,000/25,000,000]=4
NF = 10 log 4 = 6dB.
Example 5
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 6
For three cascaded amplifier
stages, each with noise figure of
3dB and power gain of 10 dB,
determine the total noise figure.
Example 7
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.
Transmission Media / Channels
Introduction

Provides the connection between
the transmitter and receiver.
1.
2.
3.
Pair of wires – carry electric signal.
Optical fiber – carries the information on a
modulated light beam.
Free space – information-bearing signal is
radiated by antenna
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Cont’d...

Signal transmission problem


additive noise – generated internally by
components used to implement the
communication system.
Interference from other users of the
channel.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Cont’d...

Minimizing noise effects


Increasing the power of
transmitted signal.
Constraint


Limited power level
Channel bandwidth availability
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Cont’d...
Wire lines
Underwater
acoustic
Channels
Fiber optics
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Wireless
electromagnetic
Cont’d...

Wire Lines Channel
 Signals transmitted are distorted in
both amplitude and phase.
– corrupted by noise.
 Carry a large percentage of daily
communication around the world.
http://en.wikipedia.org/wiki/Coaxial_cable
http://en.wikipedia.org/wiki/Twisted_pair
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Cont’d...
Twisted pair
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Coaxial cable
Cont’d...

Fiber Optics Channel







Low signal attenuation
Highly reliable photonic devices
Large bandwidth available
Services : voice, data facsimile and video
Tx – light source
(e.g. LED, laser)
Rx – photodiode
Noise source : photodiodes & amplifiers
http://en.wikipedia.org/wiki/Fiber_optics
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Cont’d...
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Cont’d...

Wireless Electromagnetic Channels



Electromagnetic energy is coupled to the
propagation medium by antenna (radiator)
Antenna size & configuration – Frequency of
operation
Efficient radiation – antenna longer than 1/10 λ
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Various frequency bands of the electromagnetic spectrum
Cont’d...
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Cont’d...
Mode of propagation of EM waves

i.
ii.
iii.
Ground-wave propagation
Sky-wave propagation
Line-of-sight (LOS)
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
GROUND-WAVE PROPAGATION





Surface-wave propagation
Dominant mode of propagation
Frequency band: 0.3 – 3 MHz
Applications: AM broadcasting,
maritime radio broadcasting
Disturbances for signal transmission:
atmospheric noise, man-made noise,
thermal noise.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Cont’d...
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
SKY-WAVE PROPAGATION





Transmitted signals being reflected
from ionosphere
Frequency : above 30 MHz
Little loss
Problem : Signal Multipath
Application : Satellite communications
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Cont’d...
> fc
Antenna at
different angles
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
LINE-OF-SIGHT (LOS) PROPAGATION



VHF band and higher
Limited by curvature of earth
Problem : Thermal noise (Rx front end)
Cosmic noise (pick-up by antenna)

Application: A TV antenna mounted on
a tower of 300 m height to provide a
broad coverage area (67km)
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Cont’d...
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Cont’d...

Underwater acoustic channels



EM waves do not propagate over long
distances under water except at extremely
low frequencies
Expensive – because of the large and
powerful transmitters required
Problem : Attenuation – skin depth
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
Cont’d...


Multipath channel – signals reflections
from the surface and the bottom of the
sea.
Noise : ambient ocean acoustic noise,
man-made acoustic noise
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 5 : TRANSMISSION MEDIA / CHANNEL
THE END