communication system by prof. praveen chitti

7/28/2019 Communication System by Prof. Praveen Chitti

1/25

JCE BELGAUM

COMMUNICATION SYSTEM

Block Diagram of Communication System

Modulation; need for modulation

Types of modulation

Amplitude Modulation

Frequency Modulation

Super heterodyne receiver

Radio telephony & telegraphy

Communication: Communication is exchange of information between 2

points i.e., means of conveying the information from one point to other. The point

from where the information is transmitted is called transmitter and the point where

the information is received is called receiver.

Basic Block Diagram of a Communication System:

The elementary block diagram of a communication system is as shown

above.

Notes By Prof. PRAVEEN CHITTI, HOD ECE

Information

SourceTransmitter Channel Receiver Destination

Voice, VideoText

Modulation Noise Demodulation Loud SpeakerCRT Printer


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JCE BELGAUM

The information source produces the information which will be in the form of

voice, video and text. This information stored is electrical in nature (transmitter

converts non-electrical signal into electrical one). The O/P of the information source

is fed to the transmitter where a process called modulation is carried out in which the

information signal is superimposed on the carrier signal, the modulation is then sent

through the channel to the destination. The channel is medium through which the

information signal travels. The communication through the channel; may be wired or

wireless communication.

If the information signal flows through wire or transmission lines, the

communication system is said to be line or wired communication system. In line

communication the transmitter and receiver are connected through cables.

Ex: Telephony, Telegraphy

If the input signal flows through the open space in the form of electromagnetic

or radio waves it is called wireless or radio communication.

Ex: TV, Radio, Mobile

Maximum amount of noise interfere with the information signal in the channel;

Noise is some unwanted electromagnetic energy that interfere with information

signal and tries to corrupt it, due to noise the quality of information transmission will

degrade. Once the noise is added it cannot be separated from the information. The

noise can be either natural (lighting & radiating from sun & stars) or man made

(ignitions, welding, electric motion).Even though noise cannot be completely

eliminated, its effect can be reduced by using various techniques.

The output of channel is then given to receiver where a process called

demodulation or detection is carried to extract the information signal from modulated

carrier. The information signal is then fed to the output devices, Such as loud

speaker, monitor & printer etc.



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JCE BELGAUM

CLASSIFICATION OF COMMUNICATION SYSTEM

Modulation

Modulation is defined as the process of changing certain characteristics of

high frequency signal called carrier signal according to the instantaneous value of

message signal or modulating signal.

In modulation the high frequency signal called carrier signal is made to carry

the information or message signal. Using modulation the information signal can be

superimposed on the carrier by changing certain characteristics of carrier.


Communication System

Unidirectional or

Bidirectional

Based on the technique of

transformation

Based on the Information

Signal

Simplex Half

Duplex

Full

Duplex

Radio Walkie

talkie

Telephone

Analog DigitalBase Band

Transmission

Modulated

Communication

System


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JCE BELGAUM

Information can be introduced in the carrier signal by changing one of three

characteristics of the carrier. Based on which characteristic of the carrier is changed,

there are 3 different type of modulation.

Amplitude modulation (AM)

Frequency Modulation (FM)

Phase Modulation (FM)

Need For Modulation

The low frequency signal such as voice, video and text cannot be directly

transmitted using antenna because these low frequency signals are heavily

attenuated in the space and are corrupted by noise. So the message signals are

always transmitted using a technique called modulation.

1). It reduces the length of the antenna

According the communication theory the efficient transmission of signal

through the space the length l of the antenna should be almost quarter wave

line.

i.e., l = 4

l = c . = c4f f

Hence

l 1f

This equation clearly shows that if the frequency of the transmitted

signals is small the length will be large and vice versa.

Thus, using modulation, frequency is increased to reduce the length l.



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JCE BELGAUM

2). Mixing of Signals is Avoided

If several audio signals are transmitted through the space at the time they will

mix up and becomes difficult to separate them at the receiving end.

If modulation is used, separate carriers can be assigned to each message

signal so hat they do not mix up and can easily be separated.

3). Long Distance Communication is Possible

As energy associated with high frequency signals is more, long distance

communication is possible in modulation.

4). Multiplexing is Possible

Multiplexing is a technique using which several signals can be transmitted

through the same channel at a time without allowing them to mix up.Since separate carriers are used for every information signal hence

multiplexing is possible.

5). Effect of Noise is Reduced

6). Improves Quality of Reception

7). Power Requirement reduces



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JCE BELGAUM



VM

(t)

VM

O

-VM

VC(t)

VC

O

-VC

VAM

(t)

O

VC

-VC

-(VC

+V

M)

wt

Modulating (Message)

Signal

wt

Carrier Signal

wct

Amplitude Modulated

Signal

+ve Envelope

- ve Envelope


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JCE BELGAUM

It is the process of changing amplitude of the carrier according to the

instantaneous value of message signal keeping frequency and phase of the carrier

constant. A.M. is used in radio and picture transmission in T.V.

For sinusoidal message signal and carrier signal the amplitude modulated signal is

as shown in the fig above.

Expression for AM signal

Let Vc (t) = Vc sinwct represents the carrier signal,

Vm (t) = Vm sinwmt be the message signal.

In AM since amplitude of carrier varies according to amplitude variations of

message signal therefore, amplitude of AM signal is a function of time.

Let A(t) represents the instantaneous amplitude of AM signal, then we can write.

A(t) = Vc + Vm sin wmt

Since frequency of the AM signal is same as that of carrier signal the expression for

AM is given by

Vam(t) = A(t) sin wct

Vam (t) = [Vc + Vm sin wmt]. Sin wct

= Vc (1 + Vm / Vc sin wmt). Sin wct

Let Vm / Vc = m then

Vam (t) = Vc (1 + m sin wmt) Sin wct

This is the required expression for amplitude modulated signal

Where Vc = carrier signal amplitude

Wm = 2fm is the modulating signal frequency

Wc = 2fc is the carrier signal frequency &

m = VmVc

is the modulation index or depth of modulation or modulation factor of AM.



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JCE BELGAUM

Note: If Vm (t) = Vm cos wmt

& Vc (t) = Vc cos wct then

Vam (t) =Vc [1+mcos wmt] coswct

Modulation Index of AM

Modulation index is defined as the ratio of modulating signal amplitude to the

carrier signal amplitude.

That is Vm = = ma =mVc

M.I. in case of AM lies in range of 0 < m < 1. M.I. specifies the extent by which the

amplitude of the carrier is varied from its un-modulated values.

Effect of Modulation Index on A.M. signal

Case 1 When m < I

i.,e Vm < IVc

Or Vm < Vc

The AM signal is called as under modulated which appears as shown below.


O

VC

- Vc

Wc

t


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JCE BELGAUM

Case 2 When m = 1

Vm = 1Vc

Vm = Vc

The AM signal is called as 100% modulated or perfectly modulated which appears

as shown in fig.

Case 3 When m >1

Vm > 1

Vc

Vm > Vc

The AM signal is said to be over modulated or extra modulated. Hence a

distorted AM result, because during every cycle of AM part of the information is lost,

hence over modulation is not used in practice. The over modulated AM signal is as

shown in the fig above.

Hence the practical value of m used is 0.3 to 0.4; higher value of m is not

used to reduce the transmitter power.

Expression for Modulation Index In terms of Vmax & Vmin


O

VC

- Vc

Wt

O

VC

- Vc

Loss of Information


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JCE BELGAUM

By definition m = Vm __________ (1)Vc

From the above figure

Vmax =Vc + VmVmin =2 Vm + VminVm = Vmax Vmin ------------------- (A)

2Vmin = Vc- Vm therefore Vc = Vmin + Vm


O

VC

- Vc

wct

-(Vc + Vm)

Vc + Vm

Vmax

Vmin


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JCE BELGAUM

Substituting from (A) we get

Vc = Vmin + Vmax Vmin2

Vc = 2Vmin + Vmax Vmin

2= Vmax + Vmin2 2

Vc = Vmax + Vmin ------------------- (B)2

Substitutingeqn(A) & (B) in (1) we get

m = (Vmax Vmin ) / 2(Vmax + Vmin ) / 2

m = Vmax VminVmax + Vmin

m = Vmax Vmin x 100%Vmax + Vmin

Spectrum of AM signal

Consider the expression for AM Signal

Vam (t) = Vc [1+m sin wmt] sin wct

=[Vc+m Vc sin wmt] sin wct

=Vc sin wct +m Vc sin wmt. Sin wct

Using the trignometrical relation

Sin A. sin B = [cos (A-B)-cos (A+B)]

Vam (t) = Vc sin wct +m Vc [cos (wct-wmt)-cos(wct+wmt)]2

Vam (t) = Vc sin wct + m Vc cos (wc-wm)t - m Vc cos(wc+wm)t2 2

Where Vc sin wct =carrier

m Vc cos (wc-wm) t =L.S.B2

m Vc cos(wc+wm)t =U.S.B2



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JCE BELGAUM

From the above expression it is seen that AM signal consist of 3 components

the component Vc sin wct is called as carrier component having frequency fc and

amplitude Vc.

The component m Vc cos (wc-wm) t

2is called as the lower side band having frequency fc fm and amplitude m Vc

2

The component m Vc cos (wc+wm)t2

is called as upper side band having frequency fc+fm and amplitude m Vc2

That is the amplitude of both the side bands is same hence the same information is

available in the side bands, carrier does not convey any information.

Hence the line spectrum of AM signal appears as shown.

B.W = f USB - fLSB= fc+fm fc+fm

BW = 2fm Hz

Expression for total power in an AM signal

Consider the spectrum of AM signal

Pt = P carrier+ PLSB + PUSB


Amplitud

e

Vc

mVc/2

0

L.S.

B

U.S.

B

Carrie

r

fc fm

fc fc +fm

Frequency

Amplitude

Vc

mVc/2

0

L.S.B U.S.B

Carrier

fc fm fc fc + fm

BW = 2fm Hz

Frequency


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JCE BELGAUM

Pt = Pc + PLSB + PUSB

We haveP = Vrms . Irms

P = Vrms2

= (Vmax /2 )2 = Vmax2

R R 2R

Pc = (Vc )2 = Vc 2 .(a)2R 2R

PLSB = (mVc /2)2 = m2 Vc

2 (b)

2 8R R

PUSB = m2 Vc

2 (c)

8R

Pt = Vc2 + 2m2 Vc2

2R 8R

Pt = Vc2 (1 + m2 )

2R 2

Pt = Pc (1 + m2 /2 )

The variation of AM transmitter power with modulation index m is as shown below.

Ptmax = 1.5 Pc


1.5

125

Pc

0.5 1.0


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JCE BELGAUM

Expression for total antenna current in an AM Transmitter

Let It be the total antenna current fed to the antenna and Ic be the carrier

current and R be the resistance of the antenna used.

Then the transmitter power Pt is given by

Pt = It2 , R ..(a)

Pc = Ic2 , R(b)

Substituting eqn. (a) & (b) in Pt = Pc ( 1 + m2 ) we get.

2

It2 , R = Ic

2 , R ( 1 + m2 )2

It = Ic(1 + m2 /2)It is the required expression for the total antenna current

Expression for transmission efficiency of AM

The transmission efficiency of an AM is defined as the ratio of Power in the side

bands to the total transmitted power i,e.

= PSB x 100%Pt

Pt = Pc (1 + m2/2)

Pt = Pc + m2 /2 Pc

PSB =m2/2 Pc is called side band power

PSSB =m2/4 Pc is single side band power.

= m2/2 Pc x 100%Pc (1+m2/2) = m2 x 100% m2 +2

if m = 1



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JCE BELGAUM

max = 1 x 100 %1+2

max = 33.333%.

Only 33.33% of power is used and 66.666% is present in carriers.

Various forms of AM Signals

1. Double side band full carrier (DSB FC)

Here, the carrier & both the side bands are transmitted.

2. Double side band suppressed carriers ( DSB SC)

Here, the two side bands are transmitted without carriers.

3. Single side band full carrier (SSB FC)

Here, along with the carrier only one side band is transmitted.

4. Single side band suppressed Carrier (SSB SC)

Here, only one side band is transmitted by suppressing the other side

band and carrier. It is the universally used method of AM

Transmission.



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JCE BELGAUM



VM

(t)

VM

O

-VM

VC(t)

VC

O

Vfm

(t)

wt

Message Signal

Carrier Signal

wt

wt

VC

r

-VC

fC

+

fC

-

fC

-VC

FM Signal

wtCarrier Frequency

Variation

fC

+ fC

-


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JCE BELGAUM

It is a type of angle modulation in which the frequency of the carrier is varied

according to the instantaneous value of the modulating signal keeping amplitude and

phase of the carrier constant. The FM signal appears as shown in the figure above.

An FM is used for sound transmission in T.V. and FM mobile phones etc.

In FM the information is available in frequency variation of the carrier. The

maximum deviation in the carrier frequency is denoted by orf and is called carrier

frequency deviation. And this deviation is always measured from the original

frequency of the carrier called center frequency fc.

Hence the max frequency of FM is given by fmax = fc +

And fmin = fc -

And the total carrier frequency swing = 2

Expression for FM Signal

Let Vm (t) = Vm cos wmt be the modulating signal and Vc (t) = Vc sin wct be the carrier

signal. Then the expression for fm signal is given by

VFM(t) = Vc sin ------(1)

If w is the frequency of the FM signal then the angle traced out of the FM signal in

a time t is given by

dw =

dt

= w d t -----(2)

The instantaneous frequency of the FM signal given by

w = wc + time Varying Frequency



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JCE BELGAUM

But in case of FM the time varying frequency is given by

T.V.F. Wc Vm (t)

T.V.F. = K Wc Vm cos wm t

Where k is constant of proportionality

w = wc + kwc Vm cos wmt

w = wc (1 + kvm coswmt)

Substituting the value of w in equation (2)

= [wc + wc k Vm cos wmt] dt

= wct + k wc Vm sin wmtwm

= wct + k wc . 2 fc sin wmt2 fm

= wct + k fc . Vm sin wmtfm

Substituting the value of in eqn (1)

VFM (t) = VC sin

VFM (t) =VC sin [wct + kfc. Vm sin wmt]

fm

But the term kfc. Vm = called carrier frequency deviation

Hence

VFM (t) = Vc sin (wct + sin wmt]

fm

Let = mf

fm

Then



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JCE BELGAUM

VFM (t) = Vc sin (wct + mf sin wmt)

Vc = Carrier Signal Amplitude

wc = 2 fc Carrier Signal Frequencywm = 2 fm Carrier Signal Frequency

& mf = modulation index of FM.

fm

Modulation Index (mf):

The modulation index of FM denoted by mf is defined as the ratio of carrier

frequency deviation to the modulation signal frequency.

i.e., mf =

fm

The value of mf is always greater than 1.

Its value lies between 0 < mf <

Practical value varies from 1 < m < 2500

Spectrum of FM Signal

The expression for FM signal is given by

VFM (t) = Vc sin (wct + mf sin wmt)

Since FM is a sine of sine function it cannot be directly expanded using power

serried expansion. The only way of expanding is by using Bessells function as

shown below.

VFM (t)= Vc [J0 (mf) sin wct + J1 (mf) [sin ((wc + wm) t sin (wc wm) t] + J2 (mf) [sin

(wc + 2wm) t + sin (wc 2wm) t] + J3 (mf) [sin ((wc + 3wm) t sin (wc 3wm) t sin (wc

3wm) t] + J4 .



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JCE BELGAUM

From the above expanded expression it seen that FM contains infinite no. of

side bands so theoretically speaking transmission of FM requires infinite band width

which is the draw back of FM transmitter.

Hence, the spectrum of FM signal appears as shown below:-

The problem of infinite band width can be solved practically by transmittingonly the significant pairs of side bands. Those pairs of the side band farthest from

the carrier care discarded and rest of pairs are transmitted.

How many pairs are significant depends upon the modulating index mf.

For ex. If mf = 5 then highest J co-efficient mentioned in the Bessells table is J8

which specifies that 8 pairs of side bands are significant.

Bandwidth of FM signal using Carsons Thumb Rule

According to this rule the bandwidth of the FM signal is approximatelycalculated using the relation.

B.W. = 2 ( + fm)Where = Carrier Frequency Deviation.

& fm= Modulation Signal Frequency.


3

2

1

11

2

1

3

Carrier

fm

fc 3fm fc 2fm fc fm fc fc + fm fc + 2fm fc + 3fm


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JCE BELGAUM

COMPARISON BETWEEN AM AND FM SIGNAL


Transmitter power depends upon modulation index Mf.

The effect of noise is more.

Only part of the transmitted power is used.

Modulation index of AM is always greater than 1.

Adjacent channel interference is more.

AM consists of only two side bands. Hence band width is less.

AM systems are less complex and less expensiveness.

AM broadcast operates in medium frequency & high frequency.

Area of reception is large.


Transmitter power does not depend upon the modulation index mf.

The effect of noise is less.

All the transmitted power is used.

Modulation index is always greater than 1.

Adjacent channel interference is less.

It consists of infinite side bands. Hence band width is large.

FM transmitter and receiver are complex and expensiveness.

FM broadcast operates in operate in very high frequency & ultra highfrequency.

Area of reception is smaller than AM.



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JCE BELGAUM

Radio Receiver

Radio receiver is a device which picks up the desired signal from the

numerous signals propagating at that time through the space, amplifies the desiredsignal, recovers from it the original modulating signal and displays it in the desired

manner.

Any radio receiver supposed to have 3 basic qualities.

Selectivity

Sensitivity

Fidelity

Super Heterodyne Receiver


RF

AmplifierMixer IF

Amplifier

LocalOscillator

Reciver

fs fif = fo - fs

fo

Antenna

GangedTunning

DetectorAudio Power

AmplifierLoud

Speaker


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JCE BELGAUM

The name super heterodyne is a contraction of super sonic heterodyne or the

production of beat frequencies. The process of heterodyning or beating involves the

mixing of the signal voltage with the local oscillator voltage and converting into a

signal of lower fixed frequency called the intermediate frequency.

The block diagram of super heterodyne receiver is as shown in the fig. above.

Working

The receiving antenna receives the RF signal (which carries the information) and the

RF amplifier amplifies the selected RF signal, local oscillator generates a voltage at

frequency f0, the output of the RF amplifier and local oscillator are (beats,

heterodynes) mixed using a mixer to produce a standard frequency signal called

intermediate frequency signal (commonly used I.F. is 455 kHz.)

The signal at IF contains the same modulation as the original carrier. This IFsignal is amplified using IF amplifier. Then the output of the IF amplifier is fed to the

detector which extracts the information signal.

The detected signal is then amplified using audio and power amplifier and

finally amplified message signal is fed to the O/P devices like load speaker.



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JCE BELGAUM

RADIO TELEPHONY & RADIO TELEGRAPHY

Radio Telegraphy

Telegraphy is a form of communication that employs type writer like M/cs

operating at a maximum speed of about 60 words per min. to send written messages

from one point to another.

In telegraphy the transmitting teletypewriter produces a set of coded pulses

when a given key is taped. In the receiving mode the same M/c prints out the

appropriate letter when a given code is received.

The telegraphy code used is called Morse Code. In Morse code all the letter,

numbers and signs are represented by Dots and Dashes. Usually, the time period of

dash is 3 times the time period for dot. The spacing between two dots/dashes or

between a dot and dash in the same letter is 1 unit and space between a dot/dash in

two letters in the same word is equal to 3 units and the spacing between the two

words is 7 units.

The coded message is transmitted using the telegraph transmitter and

received using receiver as shown in the figure.


Radio

Receiver

Telegraphy

Machine

TelegraphyMachine

OscillatorRF PowerAmplitude

Antenna

Antenna

Printed

Message


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JCE BELGAUM

Radio Telephony

In telephony the sound signal is converted into the electrical signal using

microphone. The electrical signal is modulated and then transmitted using radio

waves.

At the receiving end radio receivers are used to receive and demodulate the

signal and convert it to original electrical signal. This electrical signal is then fed to

speakers to generate sound signal.

The radio telephony transmitter and receiver are as shown in the figure.

Amplifier ModulationRFC Power

Amplitude

Antenna

Carrier

Wave

Transmitter

m

Microphone

Sound Signal

Amplifier ModulationRFC PowerAmplitude

Antenna

Receiver

Amplifier

Speaker

SouWav

Receiver

communication system by prof. praveen chitti

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