CHAPTER 1 (cont…)CHAPTER 1 (cont…)Part 2.1 Part 2.1 Noise 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

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 is 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 ois e b lan k in g

-lig h tin g

S P A C E N O IS E-so la r n o ise-s ky n o is e

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-c om p u te r

E X TE R N A L

N O IS E

Types of NOISE

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 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. 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.

• 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 J/K)T = conductor temperature (K) [Add 273 to C]B = Bandwidth of 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

kTBPPR

VR

V

RVP

VVRR

RV

n

n

n

Ln

n

n

LL

nn

Ln

LL

4

44

therefore and

42,Vat Power

2

2

2

2

2

2

L

Known as Rn = RL = R,

Therefore voltage at RL is

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.

• Noise Factor, F

• Noise Figure, NF

dBNSSNR log10

oo

ii

NSNSF

)(log10

log10

dBNSNS

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

NNGGNNGNGNN

GSS

Analysis of Noiseless Amplifier Model

)(and

0

0

aii

i

NNGNGSS

i

ai

i

aii

aii

i

i

i

i

NN

NNN

NNGGS

NS

SNRSNRF

1

)(0

SNR0 <<< SNRi

As known as BkTNBkTN eaiii and

i

e

i

e

i

ai

TT

BkTBkT

NNF 111Noise Factor,

Noise Temperature, ie TFT )1(

Analysis of Cascade Stages

• Consider three two ports in cascade

G3So

No

G1 F2, G2, Te2

antenna

pre-amplifier demodulator amplifier

F1, Te1 F3, Te3Si

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

TTkBGBkTBkTG

NNGNSGS

Stage 1

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NNGNSGGSGS

eei

ai

ai

i

22112

2212

2122

12122

)(

)( Power, Noise Power, Signal

Stage 2

BkTGBkTGGTTkBGGGNGNG

NNGNSGGGSGS

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

TGGT

TGT

TTT

kBTGGGkBTGkBTGGTTkBGGG

SNRSNRF

eeeii

ii

OO

ii

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

GGF

GFFF

TFT

KTTTTF

TGGT

TGT

TTF

TGGT

TGT

TT

TTF

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

111111

)1()1(

GGT

GTTT

TGGT

TGT

TT

TT

GGTT

GTT

TT

TT

GGF

GFFF

eeeeTOTAL

eeeeTOTAL

ee

eeTOTAL

TOTAL

And we can calculate noise temperature, Te

12121

3

1

21 ...

)1(...)1()1(

n

n

GGGF

GGF

GFFF

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

TGG

TGTTT

Transmission Loss, Attenuator

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

Power loss or attenuated is given by the following equation:

GPPL

out

in 1

dBout

indB G

PPL

10log10

Cont’d...

We also can calculate by using this following equation;

dBLWhere ℓ = transmission medium length α = attenuated constant

Example 3

Determine:a. Noise Figure for an equivalent

temperature of 75 K (use 290 Kfor 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 5An 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 shoud

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