CHAPTER 5 5.1 Noise 5.2 Transmission Media & EM Propagations EKT 231 : COMMUNICATION SYSTEM CHAPTER 5 : NOISE IN COMMUNICATION SYSTEM

CHAPTER 5

5.1 Noise5.2 Transmission Media & EM Propagations

EKT 231 : COMMUNICATION SYSTEMEKT 231 : COMMUNICATION SYSTEM

CHAPTER 5 : NOISE IN COMMUNICATION SYSTEMCHAPTER 5 : NOISE IN COMMUNICATION SYSTEM

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 mediumDevices @ 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 distortionNoise, 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 reducedreduced 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...

DistortionDistortion 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



F L IC K E R N O IS E-tu b es

S H O T N O IS E-e lec tron ic sys tem

-eq u ip m en t

TH E R M A L N O IS E-tran s is to r

-d iod e-res is to rs

IN TE R N A L

A TM O S P H E R IC N O IS E-N o ise b lan k in g

-lig h tin g

S P A C E N O IS E-so la r n o ise-sky n o ise

M A N M A D E N O IS E-au tom ob ile en g in e

-e lec tric m oto r-com p u te r

E X TE R N A L

N O IS E

Types of NOISE



Cont’d...External NoiseExternal Noise

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 noisea) Man made noise

o Produced by mankindo Source : Spark-producing mechanismso Impulsive in nature & contains a wide

range of frequencies propagated through space.

o Sometimes called industrial noise (metropolitan & industrial area).



Cont’d...b)b) Space noiseSpace 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 noisec. 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...Internal NoiseInternal Noise

• 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 NoiseShot 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 noiseFlicker 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.

Noise spectrum density is constant for all value of frequency to 1012 Hz.

Where Pn = noise power (Watt)k = Boltzman constant (1.38 x 10-23 joules per kelvin)T = conductor temperature (K) [Add 273 to C]B = Bandwidth of the system (Hz)

Pn = k T B



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.



kTBRV

kTBRV

kTBR

V

kTBPP

RV

R

V

R

VP

VV

RR

RV

n

n

n

Ln

n

n

LL

nn

Ln

LL

4

4

4

therefore

and

42

,Vat Power

2

2

2

2

2

2

L

Known as Rn = RL = R,

Therefore voltage at RL is



Example 5.1

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 5.2

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.

Noise Factor, F

Noise Figure, NF

dBN

SSNR log10

oo

ii

NS

NSF

)(log10

log10

dBNS

NS

FNF

oo

ii



Noise Calculation In Amplifier

o Two types of model

- Noise amplifier Model.

- Noiseless amplifier model.



Analysis of Noise Amplifier Model

)()(

and

0

0

aiia

iai

i

NNGG

NNGNGNN

GSS



Analysis of Noiseless Amplifier Model

)(

and

0

0

aii

i

NNGN

GSS



i

ai

i

aii

aii

i

i

i

i

N

N

N

NN

NNGGS

NS

SNR

SNRF

1

)(0

SNR0 <<< SNRi

As known as BkTNBkTN eaiii and

i

e

i

e

i

ai

T

T

BkT

BkT

N

NF 111Noise Factor,

Noise Temperature, ie TFT )1( EKT 231 : COMMUNICATION SYSTEMEKT 231 : COMMUNICATION SYSTEM


Analysis of Cascade Stages

Consider three two ports in cascade

G3So

No

G1 F2, G2, Te2

antenna

pre-amplifier demodulator amplifier

F1, Te1F3, Te3

Si

Ni

Ti Nai1 Nai2 Nai3

S1

N1

S2

N2

Stage 1 Stage 2 Stage 3



)(

)(

)( Power, Noise

Power, Signal

11

11

111

11

ei

ei

aii

i

TTkBG

BkTBkTG

NNGN

SGS

Stage 1



BkTGTTkBGG

NGNG

NNGN

SGGSGS

eei

ai

ai

i

22112

2212

2122

12122

)(

)( Power, Noise

Power, Signal

Stage 2



BkTGBkTGGTTkBGGG

NGNG

NNGN

SGGGSGS

eeei

ai

ai

i

332231123

3323

3230

123230

)(

)( Power, Noise

Power, Signal

Stage 3



Noise Factor, F

i

e

i

e

i

ei

i

eeei

kBTGkBTGGTTkBGGGSGGG

BkTS

NS

NS

O

itotal

TGG

T

TG

T

T

TT

kBTGGG

kBTGkBTGGTTkBGGG

SNR

SNRF

eeei

i

i

i

O

O

i

i

12

3

1

21

123

332231123

)(

)(332231123

123



21

3

1

21

0

0

21

3

1

21

21

3

1

21

)1()1(

)1( therefore

290 and 1 If

1

GG

F

G

FFF

TFT

KTTT

TF

TGG

T

TG

T

T

TF

TGG

T

TG

T

T

T

T

TF

TOTAL

e

ii

e

i

e

i

e

i

eTOTAL

i

e

i

e

i

e

i

iTOTAL

Known as the overall noise factor, FTOTAL



21

3

1

21

021

3

01

2

0

1

0

21

0

3

1

0

2

0

1

0

21

3

1

21

1111

11

)1()1(

GG

T

G

TTT

TGG

T

TG

T

T

T

T

T

GG

T

T

G

T

T

T

T

T

T

GG

F

G

FFF

eeeeTOTAL

eeeeTOTAL

ee

eeTOTAL

TOTAL

And we can calculate noise temperature, Te



12121

3

1

21 ...

)1(...

)1()1(

n

n

GGG

F

GG

F

G

FFF

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:

12121

3

1

21 ...

...

n

eneeee GGG

T

GG

T

G

TTT



Transmission Loss, Attenuator

Every transmission medium will produce power loss. Pout < Pin.

Power loss or attenuated is given by the following equation: GPPLoutin1

dBoutindB GPPL 10log10

GP

PL

out

in 1

dBout

indB G

P

PL

10log10



Cont’d...

We also can calculate by using this following equation;

dBLWhere ℓ = transmission medium length α = attenuated constant



Example 5.3

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.EKT 231 : COMMUNICATION SYSTEMEKT 231 : COMMUNICATION SYSTEM


Example 5.4

For three cascaded amplifier stages, each with noise figure of 3dB and power gain of 10 dB, determine the total noise figure.



Example 5.5

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.



External Atmospheric static space noise solar flare sunspot solar flux cosmic Man made

Any noise generated outside a receiverElectrical disturbances outside in the earth’s atmosphereLightning strikesSun and star disturbancesViolent storm on sun’s surfaceMassive magnetic storm on sunVaries with frequency, increases when solar flares occursFrom stars more distance than sunAny form of electromagnetic interference from non natural causes. Ignition and impulse noise from combustion engines and electrical appliances; high voltage lines; fluorescent lights.

Summary (Noise sources and Summary (Noise sources and causes)causes)



Internal Thermal

Shot

Random noises from active or passive devices inside receiver

Also termed thermal agitation, White or Johnson generated by rapid and random motion of atoms and electrons. Related directly to bandwidth.From shot effect, caused by random variation in arrival of majorities carriers at the output of an amplifying device.

Summary (Cont’d)Summary (Cont’d)



Documents

CHAPTER 5 5.1 Noise 5.2 Transmission Media & EM Propagations EKT 231 : COMMUNICATION SYSTEM CHAPTER 5 : NOISE IN COMMUNICATION SYSTEM