Download Chapter 1 (Part 2.1) - Noise

CHAPTER 1
Part 2.1  Noise
Objectives
To differentiate the types of noise
 To calculate the thermal noise
generated by a resistor
 To calculate the signal-to-noise ratio
(SNR) and noise figure for an amplifier

Lecture overview
Types of noise
 Thermal noise
 Signal-to-noise ration (SNR) and noise
figure

Introduction
Noise can be defined 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
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.
◦ 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.
◦ Distortion
 The signal perturbation caused by
imperfect response of the system to
the desired signal.
 Disappear when the signal is 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
EXTERNAL NOISE
◦ Noise generated outside the device or circuit.
◦ Three primary sources:
 Atmospheric noise.
 Extraterrestrial noise
 Man-made noise
EXTERNAL NOISE
Atmospheric noise
o Naturally
occurring
electrical
disturbances that originate within
Earth’s atmosphere.
o Is often in the form of impulses that
spread energy throughout a wide
range of frequencies.
EXTERNAL NOISE
Extraterrestrial noise
o Consists of electrical signals that
originate from outside Earth’s
atmosphere.
o 2 categories:
o Solar noise: is generated directly from
sun’s heat.
o Cosmic noise: are continuously
distributed throughout the galaxies.
EXTERNAL NOISE
Man-made noise
o Noise that is produced by mankind.
o Predominant
sources
are
sparkproducing
mechanisms
such
as
commutators in electric motors,
automobile ignition systems, ac powergenerating and switching equipment.
o Is impulsive in nature and contains a
wide range of frequencies that are
propagated through space in the
same manner as radio waves.
INTERNAL NOISE
◦ Electrical interference generated within a
device or circuit.
◦ 3 primary kinds:
 Shot noise
 Transit time noise
 Thermal noise
INTERNAL NOISE

Shot Noise
oCaused by a random arrival of carriers
(holes and electrons) at the output of an
electronic devices such as diode, field-effect
transistor or bipolar transistor.
oRandomly varying & superimposed onto any
signal present.
oSometimes called transistor noise.
INTERNAL NOISE

Transit-time noise:
o Any modification to a stream of carriers as
they pass from the input to output of a device
produces an irregular, random variation.
Thermal Noise
Rapid and random movement of
electrons within a conductor due to
thermal agitation.
 Is present in all electronic components
and communications systems.
 Is a form of additive noise, meaning that
it cannot be eliminated and it increases
in intensity with the number of devices in
a circuit and with circuit length.

Thermal Noise
Since it is dependent on temperature, it
is also referred to as thermal noise.
 Thermal noise power is proportional to
the product of bandwidth and
temperature.

Where
PN = noise power (Watts)
B = bandwidth (Hz)
K = Boltzman’s constant (1.38 x 10-23 joules
per kelvin)
T = absolute temperature (kelvin) (room
temperature = 17oC or 290K)
To convert oC to kelvin, add 273oC
EXAMPLE 1
Convert the following temperatures to
kelvin: 100oC, 0oC and -10oC.
Solution:
T = oC + 273oC
T =100 oC + 273oC = 373K
T = 0oC + 273oC = 273K
T = -10oC + 273oC = 263K
NOISE VOLTAGE


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.
When RN = RL = R,
Therefore voltage at RL is
VN
RL
VL 
VN 
Rn  RL
2
2
 VN 
 
2
VL
2  VN 2

Power at VL , PL 


4R
R
R
and PN  PL  kTB
therefore
2
VN
 kTB
4R
2
VN  4kTBR
VN  4kTBR
EXAMPLE 2
For an electronic device operating at a
temperature of 170C with a bandwidth
of 10 kHz, determine:
(a) thermal noise power in watts and
dBm
(b) rms noise voltage for a 100Ω
internal resistance and 100Ω load
resistance.
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).
How to Quantifying the Noise?

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, S 0  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 Te1 Te 2
T


 e3
Ti Ti G1Ti G1G2Ti
FTOTAL  1 
If F  1 
Te1 Te 2
T

 e3
Ti G1Ti G1G2Ti
Te
and Ti  T0  290 K
Ti
therefore Te  ( F  1)T0
 FTOTAL
( F2  1) ( F3  1)
 F1 

G1
G1G2
And we can calculate noise temperature, Te
FTOTAL  F1 
TeTOTAL
1
T0
( F2  1) ( F3  1)

G1
G1G2
 Te 2   Te 3 
1 
 1 1 
 1
T0
Te1  T0



 1


T0
G1
G1G2
TeTOTAL Te1 Te 2
Te 3



T0
T0 G1T0 G1G2T0
TeTOTAL  Te1 
Te 2
T
 e3
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

Transmission Loss, Attenuator

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.
At the end of this chapter,
you should be able
To differentiate the types of noise
 To calculate the thermal noise
generated by a resistor
 To calculate the signal-to-noise ratio
(SNR) and noise figure for an amplifier
