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R.M.K COLLEGE OF
ENGINEERING AND
TECHNOLOGY

21EC401
COMMUNICATION
SYSTEMS
Department : ECE

Batch/Year : 2021-2025 / II year

Created by : Dr. C Arun, Dr. K Kannan,

Ms. Sivalakshmi P, Ms. Komathi B J

Date : 28.01.2023
 Table of Contents
S.No Contents Page
Number

1 Course Objectives 7

2 Pre Requisites 8

3 Syllabus 9

4 Course outcomes 10

5 CO- PO/PSO Mapping 11

6 Unit 1 – Amplitude Modulation 12

6.1 13
Lecture Plan
6.2 14
Activity based learning
6.3 15
Lecture Notes
6.3.1 Amplitude Modulation 16

6.3.2 AM Generation - Square law and Switching 29

modulator
6.3.3 33
AM Detection – Envelope detector
6.3.4 34
DSB-SC-AM

6.3.5 37
Generation of DSB-SC-AM
6.3.6 41
SSB - SC

6.3.7 44
Generation of SSB - SC

6.3.7 50
Vestigial Side Band - VSB

6.3.8 51
VSB Generation – Filter Method

6.3.9 54
Comparison of AM Techniques

5
S.No Contents Page
Number
6.4 62
Assignments
6.5 64
Part A Q & A

6.6 69
Part B Questions
6.7 71
Supportive online Certification courses
6.8
Real time Applications in day to day life and
72
to Industry
6.9 73
Content beyond the Syllabus
7 Assessment Schedule 76

8 Prescribed Text Books & Reference Books 77

9 Mini Project suggestions 78

6
1. COURSE OBJECTIVES

OBJECTIVES:

To discuss the concepts of various analog modulation schemes and their spectral
characteristics
To summarize various types of noises in communication system
To analyze the effect of noise in communication systems
To describe the concepts of sampling and quantization
To discuss the concepts of pulse modulation techniques
2. PRE REQUISITES

1. 21EC301 – SIGNALS AND SYSTEMS

By learning this course the student will have deep insight in to Fourier
transform, Trigonometric identities, Sampling and quantization.

2. 21EC202 – ELECTRONIC DEVICES

By learning this course the student will understand the operation of electronic
devices such as diodes, Transistors, FET, etc

21EC401- COMMUNICATION
Semester IV
SYSTEMS

21EC202 - ELECTRONIC 21EC301 - SIGNALS

DEVICES AND SYSTEMS

Semester II Semester III

 3. SYLLABUS

21EC401 COMMUNICATION SYSTEMS LTPC

3 003
UNIT I AMPLITUDE MODULATION 9

Amplitude Modulation- DSBSC, DSBFC, SSB, VSB - Modulation index, Spectra, Power
relations and Bandwidth – AM Generation – Square law and Switching modulator, AM
detection - Envelope detector, DSBSC Generation – Balanced and Ring Modulator, DSBSC
detection – Coherent detector, SSB Generation – Filter, Phase Shift and Third Methods, VSB
Generation – Filter Method –comparison of different AM techniques.

UNIT II ANGLE MODULATION 9

Phase and frequency modulation, Narrow Band and Wide band FM – Modulation index,
Spectra, Power relations and Transmission Bandwidth - FM modulation –Direct and Indirect
methods, FM Demodulation – FM to AM conversion, FM Discriminator - PLL as FM
Demodulator.

UNIT III NOISE CHARACTERIZATION 9

Noise sources – Noise figure, noise temperature and noise bandwidth – Noise in cascaded
systems. Hilbert Transform, Pre-envelope & complex envelope - Representation of Narrow
band noise –In- phase and quadrature components, Envelope and Phase components.

UNIT IV NOISE PERFORMANCE IN AM AND FM SYSTEMS 9

AM Super heterodyne Receiver - Noise performance analysis in AM systems, AM Threshold

effect, FM Super heterodyne Receiver, Noise performance analysis in FM systems – FM
Threshold effect, Pre-emphasis and de-emphasis for FM, Comparison of noise performance
of AM and FM Systems.

UNIT V PULSE MODULATION TECHNIQUES 9

Baseband sampling – Aliasing-Quantization - Uniform & non-uniform quantization -

quantization noise - Logarithmic Companding – PAM, PPM, PWM, PCM – TDM, FDM.

TOTAL: 45 PERIODS
4. COURSE OUTCOMES

OUTCOMES:

On successful completion of this course, the student will be able to

CO1: Compare different Amplitude Modulation Schemes for their efficiency

and bandwidth

CO2: Summarize the concepts of Angle Modulation Systems

CO3: Explain different types of Noise in Communication Systems

CO4: Analyze the behaviour of a communication system in presence of

noise

CO5: Summarize the principles of Sampling and Quantization

CO6: Describe the concepts of Pulse modulation Techniques

5. CO- PO/PSO MAPPING

MAPPING OF COURSE OUTCOMES WITH PROGRAM OUTCOMES:

Program
Program Outcomes Specific
Course Level
Outcomes
Outcom of
es CO K3,
K3 K4 K4 K5 K5, A3 A2 A3 A3 A3 A3 A2 K5 K5 K3
K6
PO-1 PO-2 PO-3 PO-4 PO-5 PO-6 PO-7 PO-8 PO-9 PO-10 PO-11 PO-12 PSO-1 PSO-2 PSO-3
C212.1 K2 2 1 - - - - - - - - - - - - 2
C212.2 K2 2 1 - - - - - - - - - - - - 2
C212.3 K3 3 2 2 2 - - - - - - - - - 2 3
C212.4 K3 3 2 2 2 - - - - - - - - - 2 3
C212.5 K2 2 1 - - - - - - - - - - - - 2
C212.6 K3 3 2 2 2 - - - - - - - - - - 3

11
6 UNIT I – AMPLITUDE
MODULATION

12
6.1 LECTURE PLAN

UNIT I – AMPLITUDE MODULATION

Mode of Delivery
Taxonomy level
Proposed Date
No. of Periods

Pertaining CO
Actual Date

Reason for
Deviation
S.No

Topic

Black Board
1 Amplitude Modulation 1 CO1
K2 Teaching
Black Board
2 DSBSC, DSBFC 1 CO1 K2
Teaching
Black Board
3 SSB, VSB 1 CO1 K2
Teaching
AM Generation –
Black Board
4 Square law and 1 CO1 K2
Teaching
Switching modulator
AM Detection –
5 Envelope detector
1 CO1 K2 PPT

DSBSC Generation –
Black Board
6 Balanced 1 CO1
and Ring Modulator K2 Teaching

DSBSC Detection – Black Board

7 Coherent Detector
1 CO1
K2 Teaching
SSB Generation – Filter, Black Board
8 Phase Shift and Third 1 CO1
Methods K2 Teaching
VSB Generation – Filter
method, comparison of
9 different AM 1 CO1 K2 PPT
techniques

Total No. of Periods : 9

13
6.2 ACTIVITY BASED LEARNING
1 2 3 4
5
6
7

8
9
10

ACROSS

1. Application of VSB

6. The transform that is used in the generation of SSB-SC

7. Angle of carrier Wave is varied according to the message

signal
8.Third method of generating SSB-SC wave
9. Amplitude of carrier wave is varied according to the
message signal
10. Ability of the receiver to select a signal of desired

frequency. while rejecting all others.

DOWN
2. The frequency other than desired frequency

3. The type of modulation with the condition 𝑉𝑚> 𝑉𝑐

4. The modulator used to generate DSB-SC

5. The modulator used to generate DSB-SC which is arranged

in ring structure.

14
6.3 Lecture Notes

15
AMPLITUDE MODULATION

1.1 INTRODUCTION:
Communication is the process of conveying or transferring message from
one point to another point. Two of the main barriers in human
communication are “distance and language”.

Fig 1.1 General Block Diagram of Communication System

BASIC COMPONENTS

Transmitter

Channel or medium

Receiver

Noise degrades or interferes with transmitted information.

TRANSMITTER

The transmitter is a collection of electronic components and circuits that

converts the electrical signal into a signal suitable for transmission over a
given medium.

Transmitters are made up of oscillators, amplifiers, tuned circuits and filters,

modulators, frequency mixers, frequency synthesizers, and other circuits.
 COMMUNICATION CHANNEL

The communication channel is the medium by which the electronic signal is sent

from one place to another.

Types of media include

Electrical conductors

Optical media

Free space

System-specific media (e.g., water is the medium for sonar).

RECEIVERS

A receiver is a collection of electronic components and circuits that accepts the

transmitted message from the channel and converts it back into a form

understandable by humans.

Receivers contain amplifiers, oscillators, mixers, tuned circuits and filters, and a

demodulator or detector that recovers the original intelligence signal from

the modulated carrier.

TRANSCEIVERS

A transceiver is an electronic unit that incorporates circuits that both send and

receive signals.

Examples

Telephones, Fax machines, Handheld CB radios, Cell phones, Computer modems

ATTENUATION

Signal attenuation, or degradation, exists in all media of wireless transmission.

It is proportional to the square of the distance between the transmitter and

receiver.
 Noise

Noise is random, undesirable electronic energy that enters the

communication system via the communicating medium and interferes with
the transmitted message.

Drawbacks of Baseband Transmission [Without Modulation]

Excessively large antenna heights.

Signals get mixed up.

Short range of communication.

Multiplexing is not possible.

Poor quality reception.

The above drawbacks can be overcome by means of Modulation techniques.

1.2 Modulation

Modulation is the process by which some characteristics of a carrier wave is

varied in accordance with the instantaneous value of a modulating signal or
message signal.

Fig.1.2 General diagram for modulation

 Fig.1.3 Representation of carrier and modulating wave

Carrier Signal

Actually carrier signal does not contain any information but it only carrier
information.

Need for Modulation[Advantage of Modulation]

Easy of Radiation

Adjustment of bandwidth

Reduction in height of antenna

Avoid mixing of signals

Increases the range of communication

Multiplexing

Improves quality of reception

To reduce interference

Reduction of noise

Demodulation

Demodulation is defined as the process of extracting a modulating or

baseband signal from the modulated signal.
Classification of Modulation

Fig.1.4 Classification of Modulation Techniques

AMPLITUDE MODULATION
Amplitude modulation is the process by which amplitude of carrier signal is
varied in accordance with instantaneous value (amplitude) of the modulating
signal, but phase and frequency of carrier wave is remains constant.

Fig.1.5 Representation of Amplitude Modulated wave

 Mathematical Representation of AM wave:

Let the Modulating signal 𝑽𝒎 𝒕 = 𝑽𝒎 𝒔𝒊𝒏𝝎𝒎 𝒕 −−−− −(1)

Carrier signal 𝑽𝒄 𝒕 = 𝑽𝒄 𝒔𝒊𝒏𝝎𝒄 𝒕 −−−− −(𝟐)

Where, 𝑽𝒎 − 𝐴𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 𝑜𝑓 𝑚𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑛𝑔 𝑠𝑖𝑔𝑛𝑎𝑙
𝑉𝑐 − 𝐴𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 𝑜𝑓 𝑐𝑎𝑟𝑟𝑖𝑒𝑟 𝑠𝑖𝑔𝑛𝑎𝑙

𝝎𝒎 − 𝐴𝑛𝑔𝑢𝑙𝑎𝑟 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑜𝑓 𝑚𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑛𝑔 𝑠𝑖𝑔𝑛𝑎𝑙,

𝝎𝒄 − 𝐴𝑛𝑔𝑢𝑙𝑎𝑟 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑜𝑓 𝑐𝑎𝑟𝑟𝑖𝑒𝑟 𝑠𝑖𝑔𝑛𝑎𝑙

The amplitude of the carrier signal is changed after modulation

𝑽𝑨𝑴 = 𝑉𝐶 + 𝑉𝑚 𝑡 −−−− −(3)

Sub Eq(1) in Eq(3)

𝑽𝑨𝑴 = 𝑉𝐶 + 𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡

𝑉𝑚
= 𝑉𝐶 [1 + 𝑠𝑖𝑛𝜔𝑚 𝑡]
𝑉𝐶
𝑽𝑨𝑴 = 𝑉𝐶 1 + 𝑚𝑎 𝑠𝑖𝑛𝜔𝑚 𝑡 −−−− −(4)
where, 𝒎𝒂 − Modulation index or Depth of modulation

Hence, AM wave is given by,

= 𝑉𝐴𝑀 𝑠𝑖𝑛𝜔𝑐 𝑡 −−−− − 5

Sub Eq(4) in Eq(5)

𝑽𝑨𝑴 = 𝑉𝐶 1 + 𝑚𝑎 𝑠𝑖𝑛𝜔𝑚 𝑡 𝑠𝑖𝑛𝜔𝑐 𝑡 −−−− −(6)
 MODULATION INDEX (OR) DEPTH OF MODULATION

The ratio of maximum amplitude of modulating signal to the maximum

amplitude of carrier signal.

𝑽𝒎
𝒎𝒂 =
𝑽𝑪

PERCENTAGE OF MODULATION

𝑉𝑚
% modulation = 𝑚𝑎 × 100 = × 100
𝑉𝐶

Eg.ma = 0.5, then 50% modulation.𝑚𝑎 = 0.5

AM FREQUENCY SPECTRUM AND BANDWIDTH

FREQUENCY SPECTRUM

The AM wave is given by

𝑽𝑨𝑴 = 𝑉𝐶 [1 + 𝑚𝑎 𝑠𝑖𝑛𝜔𝑚 𝑡]𝑠𝑖𝑛𝜔𝑐 𝑡
= 𝑉𝐶 𝑠𝑖𝑛𝜔𝑐 𝑡 + 𝑚𝑎 𝑉𝐶 𝑠𝑖𝑛𝜔𝑚 𝑡𝑠𝑖𝑛𝜔𝑐 𝑡

WKT,

𝑚𝑎𝑉𝐶
𝑽𝑨𝑴 = 𝑉𝐶 𝑠𝑖𝑛𝜔𝑐 𝑡 + [cos 𝜔𝑐 − 𝜔𝑚 𝑡 − cos 𝜔𝑐 + 𝜔𝑚 𝑡]
2

𝐜𝐨𝐬 𝑨 − 𝑩 − 𝐜𝐨𝐬 𝑨 + 𝑩
[𝒔𝒊𝒏𝑨𝒔𝒊𝒏𝑩 = ]
𝟐
𝑚𝑎 𝑉𝐶 𝑚𝑎 𝑉𝐶
𝑽𝑨𝑴 = 𝑉𝐶 𝑠𝑖𝑛𝜔𝑐 𝑡 + cos 𝜔𝑐 − 𝜔𝑚 𝑡 − cos 𝜔𝑐 + 𝜔𝑚 𝑡
2 2

Carrier LSB USB

𝒎𝒂 𝑽𝑪
where, = amplitude of lower and upper sideband
𝟐
 BANDWIDTH
𝐵𝑊 = 𝑓𝑈𝑆𝐵 − 𝑓𝐿𝑆𝐵
𝐵𝑊 = (𝑓𝑐 + 𝑓𝑚 ) − (𝑓𝑐 − 𝑓𝑚 )
𝐵𝑊 = 2𝑓𝑚

PHASOR REPRESENTATION

From the AM Generation wave,

 𝑉𝑚𝑎𝑥 = 𝑉𝑚 + 𝑉𝑐
𝑉𝑚𝑖𝑛 = 𝑉𝑐 − 𝑉𝑚
𝑉𝑚𝑎𝑥 − 𝑉𝑚𝑖𝑛 = 2𝑉𝑚

𝑉𝑚𝑎𝑥 − 𝑉𝑚𝑖𝑛
𝑉𝑚 =
2
𝑉𝑐 = 𝑉𝑚𝑎𝑥 − 𝑉𝑚

𝑉𝑚𝑎𝑥 − 𝑉𝑚𝑖𝑛
= 𝑉𝑚𝑎𝑥 − [ ]
2
𝑉𝑚𝑎𝑥 𝑉𝑚𝑖𝑛
= 𝑉𝑚𝑎𝑥 − +
2 2
𝑉𝑚𝑎𝑥 + 𝑉𝑚𝑖𝑛
𝑉𝑐 =
2
But modulation index,
𝑉𝑚
𝑚𝑎 =
𝑉𝐶

𝑉𝑚𝑎𝑥 − 𝑉𝑚𝑖𝑛 /2 𝑉𝑚𝑎𝑥 − 𝑉𝑚𝑖𝑛

𝑚𝑎 = =
𝑉𝑚𝑎𝑥 + 𝑉𝑚𝑖𝑛 /2 𝑉𝑚𝑎𝑥 + 𝑉𝑚𝑖𝑛

𝑉𝑚𝑎𝑥 − 𝑉𝑚𝑖𝑛
%𝑚𝑎 = × 100
𝑉𝑚𝑎𝑥 + 𝑉𝑚𝑖𝑛
 AM POWER DISTRIBUTION

The total power in modulated wave will be

𝑃𝑡 = 𝑐𝑎𝑟𝑟𝑖𝑒𝑟 𝑝𝑜𝑤𝑒𝑟 + 𝑝𝑜𝑤𝑒𝑟 𝑖𝑛 𝐿𝑆𝐵 + 𝑝𝑜𝑤𝑒𝑟 𝑖𝑛 𝑈𝑆𝐵
𝑃𝑡 = 𝑃𝐶 + 𝑃𝐿𝑆𝐵 + 𝑃𝑈𝑆𝐵

𝑉𝑐 2 𝑉𝐿𝑆𝐵 2 𝑉𝑈𝑆𝐵 2
= + +
𝑅 𝑅 𝑅

(𝑅𝑀𝑆)2 (𝑉𝑐 / 2)2 𝑉𝑐 2

WKT, 𝑃𝐶 = 𝐿𝑜𝑎𝑑 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 = =
𝑅 2𝑅

𝑉𝑐 2
𝑃𝐶 =
2𝑅

(𝑉𝑈𝑆𝐵 / 2)2
lllly Power in sidebands, 𝑃𝐿𝑆𝐵 = 𝑃𝑈𝑆𝐵 = 𝑅

𝑚𝑎 𝑉𝑐
but, 𝑉𝑈𝑆𝐵 =
2

𝑚𝑎 2 𝑉𝑐 2 𝑚𝑎 2
𝑃𝐿𝑆𝐵 = 𝑃𝑈𝑆𝐵 = = 𝑃
8𝑅 4 𝐶
Total Power, 𝑃𝑡 = 𝑃𝐶 + 𝑃𝐿𝑆𝐵 + 𝑃𝑈𝑆𝐵

𝑉𝑐 2 𝑚𝑎 2 𝑉𝑐 2 𝑚𝑎 2 𝑉𝑐 2
= + ( )+ ( )
2𝑅 4 2𝑅 4 2𝑅

𝑉𝑐 2 𝑚𝑎 2 𝑚𝑎 2
= + 𝑃𝐶 + 𝑃
2𝑅 4 4 𝐶

𝑚𝑎 2
𝑃𝑡 = 𝑃𝐶 [1 + ]
2

𝑚𝑎 2
𝑃𝑡 = 𝑃𝐶 [1 + ]
2
 CURRENT RELATION IN AM WAVE
𝑚𝑎 2
𝑃𝑡 = 𝑃𝐶 [1 + ]
2
WKT, 𝑃𝑡 = 𝐼𝑡 2 𝑅

𝑃𝐶 = 𝐼𝐶 2 𝑅

𝑚𝑎 2
𝐼𝑡 2 𝑅 = 𝐼𝐶 2 𝑅[1 + ]
2

𝑚𝑎 2
𝐼𝑡 = 𝐼𝐶 [1 + ]
2

TRANSMISSION EFFICIENCY
It is the ratio of power contained in both sidebands to total transmitted power.
𝑃𝑜𝑤𝑒𝑟 𝑖𝑛 𝑠𝑖𝑑𝑒𝑏𝑎𝑛𝑑
%%𝜂 = × 100
𝑇𝑜𝑡𝑎𝑙 𝑝𝑜𝑤𝑒𝑟
(𝑃𝐿𝑆𝐵 + 𝑃𝑈𝑆𝐵 )
= × 100
𝑃
2 𝑡
𝑚 2𝑉 𝑚 2𝑉 2
( 𝑎 𝑐 + 𝑎 𝑐 )
= 8𝑅 8𝑅 × 100
2
𝑉𝑐 𝑚𝑎 2
[1 + ]
2𝑅 2
𝑉𝑐 2 𝑚𝑎 2 𝑚𝑎 2
( + )
= 2𝑅 4 4 × 100
𝑉𝑐 2 𝑚𝑎 2
[1 + ]
2𝑅 𝑚 22 𝑎
%𝜂 = 2 × 100
𝑚𝑎 2
1+ 2

𝑚𝑎 2
%𝜂 = × 100
2 + 𝑚𝑎 2

If ma = 1 then %η = 1/3 x 100 = 33.3%

Only 33.3% of energy is used and remaining power is wasted by carrier information
along with sidebands.
 Advantages of AM wave:

AM is a relatively inexpensive

AM wave can travel a long distance

It covers larger area than FM

Disadvantages of AM wave:

Poor performance in presence of noise

Inefficient use of transmitted power

Wastage in bandwidth

Applications:

Used for commercial broadcasting of audio and video signals.

Two-way mobile radio communication.

Aircraft communication in VHF frequency range.

DEGREE OF MODULATION

The modulating signals preserved in the envelope of amplitude modulated

signal only if Vm<Vc then ma< 1 𝑉𝑚 < 𝑉𝐶 then 𝑚𝑎 < 1

Where Vm - maximum amplitude of modulating signal

Vc - maximum amplitude of carrier signal

There are three types of degree of modulation

Under Modulation, ma< 1 i.e., Vm<Vc

Critical Modulation, ma = 1 i.e., Vm = Vc

Over Modulation, ma> 1 i.e., Vm>Vc

DEGREE OF MODULATION

Under Modulation
The envelope of the amplitude modulated signal does not reach the zero
amplitude axis.
An envelope detector can recover the message signal without any distortion.

Fig.1.6 Under Modulation

Critical Modulation
The envelope of the modulated signal just reaches the zero amplitude axis.
An envelope detector can recover the message signal without any distortion.

Fig.1.7 Critical Modulation

Over Modulation
The envelope of the modulated signal crosses the zero amplitude axis.
Both positive and negative extensions of modulating signals are cancelled or
clipped out. The envelope of message signals is not same. This is called
envelope distortion.

Fig.1.8 Critical Modulation

GENERATION(MODULATION) OF AM WAVE

Generation of AM
Wave

Linear Non-Linear
Modulation Modulation

Switching Square law Balanced

Modulation Modulator Modulator

Fig.1.9 Generation of AM

LINEAR MODULATION – SWITCHING MODULATION

Fig.1.10 Linear Modulation- Switching Modulation

Carrier signal c(t) and message signal m(t) is connected in series,
V1(t) = c(t) + m(t)
V1(t) = Vc cosωct + Vmsinωmt
Amplitude of c(t) is very greater than m(t). So diode is ON and OFF by c(t).

A simple diode used for an AM switching modulator. The diode is forward biased
for every positive half cycle of the carrier and behaves like short circuit switch.
The signal appears at the input of bandpass filter.

For negative half cycle of the carrier the diode is reverse biased and behaves like
open switch. The signal does not reach the filter and no output is obtained. Thus
signal is modulated at the rate of carrier frequency.

V2(t) = V1(t) , c(t) > 0

= 0 , c(t) < 0

The output is periodically change

V2(t) = V1(t) gp(t) ----- (2)
where gp(t) – periodic pulse train of duty cycle equal to half period T0/2
 In Fourier series, ∞
1 2 (−1)𝑛−1
𝑔𝑝 𝑡 = + ෍ cos[(2𝑛 − 1)𝜔𝑐 𝑡]
2 𝜋 2𝑛−1
𝑛=1
1 2
𝑔𝑝 𝑡 = + 𝑐𝑜𝑠𝜔𝑐 𝑡 + 𝑜𝑑𝑑 ℎ𝑎𝑟𝑚𝑜𝑛𝑖𝑐
2 𝜋

1 2
𝑔𝑝 𝑡 = + 𝑐𝑜𝑠𝜔𝑐 𝑡 −−− −(3)
2 𝜋

1 2
𝑉2 𝑡 = 𝑉𝑐 𝑐𝑜𝑠𝜔𝑐 𝑡 + 𝑉𝑚 𝑐𝑜𝑠𝜔𝑚 𝑡 [ + 𝑐𝑜𝑠𝜔𝑐 𝑡]
2 𝜋

𝑉𝑐 2𝑉𝑚 𝑉𝑚 2𝑉𝑐
𝑉2 𝑡 = 𝑐𝑜𝑠𝜔𝑐 𝑡 + 𝑐𝑜𝑠𝜔𝑚 𝑡𝑐𝑜𝑠𝜔𝑐 𝑡 + 𝑐𝑜𝑠𝜔𝑚 𝑡 + 𝑐𝑜𝑠 2 𝜔𝑐 𝑡
2 𝜋 2 𝜋

AM signal Modulating Signal 2 nd harmonic signal

After BPF
The output voltage is given by,

𝑉𝑐 2𝑉𝑚
𝑦(𝑡) = 𝑐𝑜𝑠𝜔𝑐 𝑡 + 𝑐𝑜𝑠𝜔𝑚 𝑡𝑐𝑜𝑠𝜔𝑐 𝑡
2 𝜋

NON-LINEAR MODULATION – SQUARE LAW MODULATOR

In general, any device operated in non-linear region of its output characteristics is

capable of producing AM waves when the carrier and modulating signals are fed to the
input.

Thus a transistor, triode tube, a diode etc., may be used as a square law modulator. The
above circuit is common emitter configuration. The modulating signal is applied to the
emitter and RF carrier at the base of the transistor.

A square law modulator circuit consists of

A non linear device

A bandpass filter

A carrier source and modulating signal

The modulating and carrier signal are connected series with each other.
𝑉1 𝑡 = 𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡 + 𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡 −−− −(1)
The input – output relation for non-linear devices as follows,
𝑉2 𝑡 = 𝑎1 𝑉1 𝑡 + 𝑎2 𝑉1 2 𝑡 + … … … . .−−− −(2)
Where a1 and a2 are constants
Sub eq(1) in eq(2)
𝑉2 𝑡 = 𝑎1 [𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡 + 𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡] + 𝑎2 [𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡 + 𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡]2 + … … … . .

= 𝑎1 𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡 + 𝑎1 𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡 + 𝑎2 𝑉𝑚 2 𝑠𝑖𝑛2 𝜔𝑚 𝑡 + 𝑎2 𝑉𝑐 2 𝑠𝑖𝑛2 𝜔𝑐 𝑡

+2𝑎2 𝑉𝑚 𝑉𝑐 𝑠𝑖𝑛𝜔𝑚 𝑡𝑠𝑖𝑛𝜔𝑐 𝑡 … … … . .
1st term – Modulating signal
2nd term – Carrier signal
3rd term – Square modulating signal
4th term – Square carrier signal
5th term – AM wave with only side bands

The LC tuned circuit acts as a bandpass filter. This circuit is tuned to frequency f c
and its bandwidth is equal to 2fm
𝑉2 𝑡 = 𝑎1 𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡 + 2𝑎2 𝑉𝑚 𝑉𝑐 𝑠𝑖𝑛𝜔𝑚 𝑡𝑠𝑖𝑛𝜔𝑐 𝑡
AM Detection – Envelope detector
 Envelope detector is used to detect (demodulate) high level AM wave. Following is
the block diagram of the envelope detector.

 This envelope detector consists of a diode and low pass filter. Here, the diode is
the main detecting element. Hence, the envelope detector is also called as the
diode detector. The low pass filter contains a parallel combination of the resistor
and the capacitor.
 The AM wave s(t) is applied as an input to this detector. We know the
standard form of AM wave is
s(t) = Ac[1+ka m(t)] cos(2πfct)]

 In the positive half cycle of AM wave, the diode conducts and the capacitor
charges to the peak value of AM wave.
 When the value of AM wave is less than this value, the diode will be reverse
biased. Thus, the capacitor will discharge through resistor R till the next positive
half cycle of AM wave.
 When the value of AM wave is greater than the capacitor voltage, the diode
conducts and the process will be repeated.
 We should select the component values in such a way that the capacitor charges
very quickly and discharges very slowly.
 As a result, we will get the capacitor voltage waveform same as that of the
envelope of AM wave, which is almost similar to the modulating signal.
DSB-SC [Double Sideband Suppressed Carrier]

The transmitted wave consists of only the upper and lower sidebands.
Transmitted power is saved here through suppression of carrier wave because it
does not contain any useful information.

Fig.1.11 Generation of DSB-SC

Expression for DSB-SC

Let the Modulating signal 𝑉𝑚 𝑡 = 𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡 −−− −(1)

Carrier signal 𝑉𝑐 𝑡 = 𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡 −−− −(2)

𝑉(𝑡)𝐷𝑆𝐵−𝑆𝐶 = 𝑉𝑚 𝑡 𝑉𝑐 𝑡
= 𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡
= 𝑉𝑚 𝑉𝑐 𝑠𝑖𝑛𝜔𝑚 𝑡𝑠𝑖𝑛𝜔𝑐 𝑡
𝑉𝑚 𝑉𝑐
𝑉(𝑡)𝐷𝑆𝐵−𝑆𝐶 = [cos 𝜔𝑐 − 𝜔𝑚 𝑡 − cos 𝜔𝑐 + 𝜔𝑚 𝑡]
2

LSB USB
SPECTRUM OF DSB – SC AM

Bandwidth, BW = 2fm

PHASOR DIAGRAM

DSB-SC AM WAVEFORM
POWER CALCULATION DSB – SC AM

Total power transmitted in AM is

𝑃𝑡 = 𝑃𝐶 + 𝑃𝐿𝑆𝐵 + 𝑃𝑈𝑆𝐵

𝑉𝑐 2 𝑚𝑎 2 𝑉𝑐 2 𝑚𝑎 2 𝑉𝑐 2
= + ( )+ ( )
2𝑅 4 2𝑅 4 2𝑅

𝑚𝑎 2 𝑚𝑎 2
= 𝑃𝐶 + 𝑃 + 𝑃
4 𝐶 4 𝐶

𝑚𝑎 2
𝑃𝑡 = 𝑃𝐶 [1 + ]
2
If the carrier is suppressed, then the total power transmitted in DSB-SC AM is

𝑃𝑡 𝐼 = 𝑃𝐿𝑆𝐵 + 𝑃𝑈𝑆𝐵

𝑚𝑎 2 𝑉𝑐 2 𝑚𝑎 2 𝑉𝑐 2
= +
8𝑅 8𝑅

𝐼 𝑚𝑎 2 𝑉𝑐 2 𝑚𝑎 2
𝑃𝑡 = = 𝑃𝐶
4𝑅 2
𝑃𝑡 − 𝑃𝑡 𝐼
𝑃𝑜𝑤𝑒𝑟 𝑠𝑎𝑣𝑖𝑛𝑔 =
𝑃𝑡

𝑚 2 𝑚 2
𝑃𝐶 [1 + 2𝑎 ] − 𝑃𝐶 2𝑎
=
𝑚 2
𝑃𝐶 [1 + 2𝑎 ]
1
=
𝑚𝑎 2
[1 + ]
2
2
% 𝑃𝑜𝑤𝑒𝑟 𝑠𝑎𝑣𝑖𝑛𝑔 = 𝑥 100
[2 + 𝑚𝑎 2 ]

if ma = 1 % 𝑃𝑜𝑤𝑒𝑟 𝑠𝑎𝑣𝑖𝑛𝑔 = 66.67% , if [𝑚𝑎 = 1]

In DSB-SC, 66.67% of power is saved due to the suppression of carrier wave.
Generation (or) Modulation of DSB-SC AM

Generation of DSB-SC AM
Wave

Balanced Modulator Ring Modulator

(or)
Diode Balanced Modulator
(or)
Double Balanced Modulator

Fig.1.12 Generation of DSB-SC AM Wave

BALANCED MODULATOR:
In balanced modulator, two non-linear devices are connected in the balanced
mode, so as to suppress the carrier wave.

It is assumed that the two transistors are identical and the circuit is
symmetrical. Since the operation is confined in non-linear region of its
transfer characteristics.

Fig.1.13 Balanced Modulator

 The modulating voltages across the two winding of a centre tap transformer are
equal and opposite inphase. i.e., 𝑉𝑚 = −𝑉𝑚 𝐼

The input voltage to Transistor T1 is given by

𝑉𝑏𝑒 = 𝑉𝑐 + 𝑉𝑚
𝑉𝑏𝑒 = 𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡 + 𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡 −−− −(1)

Similarly, the input voltage to Transistor T2 is given by

𝑉𝑏𝑒 𝐼 = 𝑉𝑚 𝐼 + 𝑉𝑐
𝑉𝑏𝑒 𝐼 = −𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡 + 𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡 −−− −(2)

By the non-linearity relationship the collector current can be written as,

2
𝑖1 = 𝑎1 𝑉𝑏𝑒 + 𝑎2 𝑉𝑏𝑒 −−− −(3)
𝐼 𝐼 2
1
𝑖1 = 𝑎1 𝑉𝑏𝑒 + 𝑎2 𝑉 𝑏𝑒 −−− −(4)

Sub eq(1) &eq(2) in eq(3) &eq(4)

𝑖1 = 𝑎1 [𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡 + 𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡] + 𝑎2 [𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡 + 𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡]2
𝑖1 𝐼 = 𝑎1 [𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡 − 𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡] + 𝑎2 [−𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡 − 𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡]2

The output AM voltage Vo is given by,

𝑉0 = 𝑘(𝑖1 − 𝑖1 𝐼 )

This is because currents i1 and i11 flow in opposite direction in a tuned circuit.
𝑉0 = 2𝑘𝑎1 𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡 + 4𝑘𝑎2 𝑉𝑐 𝑉𝑚 𝑠𝑖𝑛𝜔𝑐 𝑡𝑠𝑖𝑛𝜔𝑚 𝑡

2𝑎2 𝑉𝑐
𝑉0 = 2𝑘𝑎1 𝑉𝑚 [1 + 𝑠𝑖𝑛𝜔𝑐 𝑡]𝑠𝑖𝑛𝜔𝑚 𝑡
𝑎1
𝑉0 = 2𝑘𝑎1 𝑉𝑚 [1 + 𝑚𝑎 𝑠𝑖𝑛𝜔𝑐 𝑡]𝑠𝑖𝑛𝜔𝑚 𝑡

2𝑎2 𝑉𝑐
𝑤ℎ𝑒𝑟𝑒, 𝑚𝑎 = = 𝑚𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑖𝑛𝑑𝑒𝑥
𝑎1

The carrier is applied to the centre tap of input transformer and is inphase at base
of T1& T2. The modulated signal is antiphase at the two bases.
RING MODULATOR (OR) DIODE BALANCED MODULATOR

It is one of the most popular methods of generating a DSB-SC wave. The circuit
employs diodes as non-linear devices and the carrier signal is connected between
centre tap of the input and output transformers. The four diodes are controlled by
a carrier Vc(t) of frequency fc.

Fig.1.14 Ring Modulator

POSITIVE HALF CYCLE OF CARRIER

Diodes D1 and D2 are forward biased. At the time D3 and D4 are reverse
biased and acts like open circuits. The current divides equally in the upper
and lower portions of the primary windings of Tr2.

The current in upper part of the winding produces a magnetic field that is
equal and opposite to the magnetic field produced by the current in lower
half of the secondary winding.

These magnetic fields cancel each other act and no output is induced in the
secondary winding. Thus the carrier is effectively suppressed.

NEGATIVE HALF CYCLE OF CARRIER

When the polarity of the carrier reverses Diodes D1 and D2 are reverse
biased and diodes D3 and D4 conduct. Again the current flows in the
secondary winding of Tr1 and the primary winding of Tr2.

The equal and opposite magnetic fields produced in Tr2 cancel each other
out and thus result is zero carrier output. The carrier is effectively balanced
out.
PRINCIPLE OF OPERATION
When both the carrier and modulating signals are present, during positive
half cycle of the carrier, Diodes D1 and D2 conduct, while diodes D3 and D4
does not conduct.

When polarity of modulating signal changes, the result is a 180o phase

reversal. At the time, during the negative half cycle of the carrier, diodes D 3
and D4 are in forward bias and negative half cycle of the carrier, diodes D 1
and D2 are in reverse bias.
𝑉0 𝑡 = 𝑉𝑚 𝑡 . 𝑉𝑐 (𝑡)

𝑉𝑚 𝑉𝑐
𝑉0 𝑡 = [cos 𝜔𝑐 − 𝜔𝑚 𝑡 − cos 𝜔𝑐 + 𝜔𝑚 𝑡]
2

LSB USB

The ring modulator circuit is also known as double balanced modulator

because comparing to balanced modulator, here two more diodes are used.

Case (i) : When modulating signal alone present, Diodes D1 , D2 or D3, D4 will

conduct depending on signal polarity.

Case (ii) When carrier signal alone present, the flow of current in two halves of

output transformer is equal and opposite and no output can

develop across the load.

Case (iii) When both signals are present, the resultant potential in one-half of

the output transformer becomes larger than other.

 Fig.1.15 Waveforms of Ring Modulator

It is more efficient in transmitted power and better signal to noise ratio compared
to DSBFC & SSB transmission.

Even though carrier is suppressed the bandwidth of DSBSC remains same as

DSBFC. The output is free from carrier and contains upper and lower sidebands
only, hence ring modulator is also called double balanced modulator.

SINGLE SIDEBAND SUPPRESSED CARRIER [SSB-SC]

In DSB signal, the basic information is transmitted twice, once in each

sideband. The sidebands are the sum and difference of the carrier and
modulating signals the information must be contained in both of them.
So, either one sideband is enough for transmitting as well as recovering the
useful message. One sideband may be suppressed. The remaining sideband
is called a single sideband suppressed carrier (SSB-SC) signal.
 Fig.1.16 Block diagram of SSB generation

SSB requires half of the bandwidth of the DSB-SC uses considerably less transmitted
power. i.e., BW = fm

In order to suppress one of the sidebands, the input signal fed to the modulator
‘1’ is 90o out of phase with that of the signal fed to the modulator ‘2’.

Let 𝑉1 𝑡 = 𝑉𝑚 𝑠𝑖𝑛(𝜔𝑚 𝑡 + 90𝑜 )𝑉𝑐 𝑠𝑖𝑛(𝜔𝑐 𝑡 + 90𝑜 )

𝑉1 𝑡 = 𝑉𝑚 𝑐𝑜𝑠𝜔𝑚 𝑡 𝑉𝑐 𝑐𝑜𝑠𝜔𝑐 𝑡
𝑉2 𝑡 = 𝑉𝑚 𝑠𝑖𝑛𝜔𝑚 𝑡 𝑉𝑐 𝑠𝑖𝑛𝜔𝑐 𝑡
𝑉𝑆𝑆𝐵 𝑡 = 𝑉1 𝑡 + 𝑉2 𝑡
= 𝑉𝑚 𝑉𝑐 [𝑠𝑖𝑛𝜔𝑚 𝑡 𝑠𝑖𝑛𝜔𝑐 𝑡 + 𝑐𝑜𝑠𝜔𝑚 𝑡𝑐𝑜𝑠𝜔𝑐 𝑡]

𝑉𝑚 𝑉𝑐
𝑉𝑆𝑆𝐵 𝑡 = cos 𝜔𝑐 − 𝜔𝑚 𝑡
2

Spectrum of SSB – SC AM
 PHASOR DIAGRAM

POWER CALCULATION
Power in SSB-SC AM is given by
1 2
𝑃𝑡 𝐼𝐼 = 𝑈𝑆𝐵 𝑜𝑟 𝐿𝑆𝐵 = 𝑚 𝑃
4 𝑎 𝑐
Power savings with respect to AM with carrier
𝑃𝑡 − 𝑃𝑡 𝐼𝐼
𝑃𝑜𝑤𝑒𝑟 𝑠𝑎𝑣𝑖𝑛𝑔 =
𝑃𝑡 2
𝑚𝑎 2 𝑚
𝑃𝐶 [1 + 2 ] − 𝑃𝐶 4𝑎
=
𝑚 2
𝑃𝐶 [1 + 2𝑎 ]
𝑚 2 𝑚 2
1 + 2𝑎 − 4𝑎 4 + 𝑚𝑎 2
= =
𝑚 2 4 + 2𝑚𝑎 2
[1 + 2𝑎 ]
4 + 𝑚𝑎 2
% 𝑃𝑜𝑤𝑒𝑟 𝑠𝑎𝑣𝑖𝑛𝑔 = 𝑥 100
4 + 2𝑚𝑎 2
If 𝑚𝑎 = 1, then η = 83.33%. In addition to carrier, one of the sidebands also
suppressed.
ADVANTAGES
Since only single sideband is transmitted, the bandwidth of transmitter and
channel is only fm.

Power of suppressed carrier and sideband is saved.

Because of narrow bandwidth of SSB, the effect of noise at the receiver

circuits is reduced. This gives better quality of reception in SSB.
Application
Point to point radio telephone communication

SSB Telegraph system.

Police wireless communication.

VHF and UHF Communication.

GENERATION (OR) MODULATION OF SSB-SC

Generation of SSB-SCAM
Wave

Frequency Phase Discrimination

Discrimination
(or)
Filter method
Phase Shift Method Weavers Method
(or) (or)
Phasing Method Modified Phase
Method

Fig.1.17 Generation of SSB-SC Techniques

FILTER METHOD (OR) FREQUENCY DISCRIMINATION
In this method basically consists of a balanced modulator (to generate DSB-SC
signal) and suppression filter (to remove unwanted sidebands).

Fig.1.18 Filter method – SSBSC Generation

As shown in the block diagram, the balanced modulator produces DSB output. This
DSB signal contains both the sideband.

The filter must have a flat pass band and extremely high attenuation outside the
pass band.

In order to have this type of response the Q of the tuned circuits must be very
high.

The required value of Q factor increases as the difference between modulating

frequency and carrier frequency increases.

Carrier frequency is usually same as the transmitter frequency increases.

For higher transmitting frequencies them required value of Q is so high that there
is no practical way of achieving it.

In such situation, initial modulation is carried out at a low frequency carrier say 100
kHz by the balanced modulator. Then the filter suppresses one of the sidebands.

The frequency of the SSB signal generated at output of filter is very low as
compared to the transmitter frequency.

The frequency is boosted up to the transmitter frequency by the balanced mixer

and crystal oscillator. This process of frequency booting is also called as up
conversion.

The SSB signal having frequency equal to the transmitter frequency is then
amplified by the linear amplifiers.
PHASE SHIFT METHOD (OR) PHASING METHOD
The modulating signal and carrier signals are fed into balanced modulator 1
in the usual manner. The balanced modulator 2 is given these signals after a
phase shift of 90o.

The unwanted sidebands filters can be removed by generating the

components of sidebands out of phase. If the undesired sideband is USB and
then two USB’s are generated such that they are 180o out of phase with
each other. So that USB’s add with each other cancel out each other.

Two balanced modulators and two phase shifters are used in this phasing
method. The carrier signal is cancelled out in this circuit by both of the
balanced modulator and unwanted sidebands cancel at the output of
summing amplifiers.

Fig.1.19 Phase shift method – SSBSC Generation

Input signal for modulator 1,

𝑉𝑐 𝑡 = 𝑉𝑐 sin 𝜔𝑐 𝑡 −−− −(1)
𝑉𝑚 𝑡 = 𝑉𝑚 sin 𝜔𝑚 𝑡 −−− −(2)
Input signal for modulator 2,
𝜋
𝑉𝑐 𝑡 = 𝑉𝑐 sin(𝜔𝑐 + )𝑡 = 𝑉𝑐 cos 𝜔𝑐 𝑡 −−− −(3)
2𝜋
𝑉𝑚 𝑡 = 𝑉𝑚 sin(𝜔𝑚 + )𝑡 = 𝑉𝑚 cos 𝜔𝑚 𝑡 −−− −(4)
2
Output for modulator 1,
𝑉1 𝑡 = 𝑉𝑐 sin 𝜔𝑐 𝑡 . 𝑉𝑚 sin 𝜔𝑚 𝑡
1
= 𝑉𝑚 𝑉𝑐 cos 𝜔𝑐 − 𝜔𝑚 𝑡 − cos 𝜔𝑐 + 𝜔𝑚 𝑡 −−− −(5)
2
Output for modulator 2,
𝑉2 𝑡 = 𝑉𝑚 𝑉𝑐 cos 𝜔𝑐 𝑡 cos 𝜔𝑚 𝑡
1
= 𝑉𝑚 𝑉𝑐 cos 𝜔𝑐 − 𝜔𝑚 𝑡 + cos 𝜔𝑐 + 𝜔𝑚 𝑡 −−− −(6)
2
The output from linear summer is 𝑉1 𝑡 +𝑉2 𝑡
𝑉𝑜 = 𝑉1 𝑡 +𝑉2 𝑡 = 𝑉𝑚 𝑉𝑐 cos 𝜔𝑐 − 𝜔𝑚 𝑡

The output of two balanced modulator are summed to produce lower sideband
signal. Thus one of the sideband is cancelled, where as other is reinforced.
ADVANTAGES
It provides the easy of switching from one sideband to the other.

It does not require any sharp cut off filter.

It has ability to generate SSB at any frequency.

DISADVANTAGES

Since we are using two balanced modulators, each should have equal
sensitivity and give exact same output.

The carrier phase-shift network must provide an exact 90o phase –shift at
carrier frequency.
Modified Phase Shift Method (or) Weaver’s Method

During 1950’s , D.K.Weavers was developed the method of generating SSB-SC

modulation. This method is overcome the limitation of phasing method.

The disadvantage of phase shift method is the requirement of an AF phase-shift

circuit which should operate over large audio frequencies.

But it also retains the advantage like its ability to generate SSB at any frequency
and use of low audio frequency but the circuit is complex.

Balanced modulator BM1 , BM2 both have the unshifted modulating signal as
inputs once BM take the audio frequency subcarrier with a 90o shift from
oscillator.

BM2 receives the subcarrier signal directly from the oscillator. This method tries to
avoid the phase shift of audio frequencies.

The lowpass filter at the output of BM1 & BM2 cut off frequency ensures the input
to the BM3 & BM4 output of BM3 & BM4 gives the desired sideband suppression.

Fig.1.20 Third method (or ) Weavers method– SSBSC Generation

Output of SSB signal,

Modulating signal, 𝑉𝑚 𝑡 = 𝑉𝑚 sin 𝜔𝑚 𝑡 −−− −(1)

AF Carrier signal, 𝑉𝑜 𝑡 = 2𝑉𝑜 sin 𝜔𝑜 𝑡 −−− −(2)

RF Carrier signal, 𝑉𝑐 𝑡 = 2𝑉𝑐 sin 𝜔𝑐 𝑡 −−− −(3)

Output of Balanced modulator 1,

= 2𝑉𝑜 sin(𝜔𝑜 𝑡 + 90) 𝑉𝑚 sin 𝜔𝑚 𝑡

= 𝑉𝑚 𝑉𝑜 [cos 𝜔𝑜 𝑡 − 𝜔𝑚 𝑡 + 90 − cos[( 𝜔𝑜 𝑡 + 𝜔𝑚 𝑡) + 90]] −−− −(4)

Output of Balanced modulator 2,

= 2𝑉𝑜 sin 𝜔𝑜 𝑡 𝑉𝑚 sin 𝜔𝑚 𝑡

= 𝑉𝑚 𝑉𝑜 [cos( 𝜔𝑜 𝑡 − 𝜔𝑚 𝑡)] − cos( 𝜔𝑜 𝑡 + 𝜔𝑚 𝑡)] −−− −(5)

The lowpass filters in the BM1& BM2 eliminates the upper sidebands of modulator

Output of LPF 1 is 𝑉𝑚 𝑉𝑜 cos 𝜔𝑜 𝑡 − 𝜔𝑚 𝑡 + 90

Output of LPF 2 is 𝑉𝑚 𝑉𝑜 cos( 𝜔𝑜 − 𝜔𝑚 )𝑡

𝐴𝑠𝑠𝑢𝑚𝑒, 𝑉𝑚 = 𝑉𝑜 = 𝑉𝑐 = 1

Output of Balanced modulator 3,

= 2𝑠𝑖𝑛𝜔𝑐 𝑡 cos(𝜔𝑜 𝑡 − 𝜔𝑚 𝑡 + 90) −−− −(6)

1
WKT, 𝑠𝑖𝑛𝐴𝑐𝑜𝑠𝐵 = 2 [sin 𝐴 + 𝐵 + sin(𝐴 − 𝐵)]
= sin 𝜔𝑐 + 𝜔𝑜 − 𝜔𝑚 𝑡 + 90 + 𝑠𝑖𝑛 𝜔𝑐 − 𝜔𝑜 + 𝜔𝑚 𝑡 − 90 −−− −(7)
Output of Balanced modulator 4,
= 2sin(𝜔𝑐 𝑡 + 90) cos 𝜔𝑜 − 𝜔𝑚 𝑡
= sin 𝜔𝑐 + 𝜔𝑜 − 𝜔𝑚 𝑡 + 90 + 𝑠𝑖𝑛 𝜔𝑐 − 𝜔𝑜 + 𝜔𝑚 𝑡 + 90 −−− −(8)

From eq(7) &eq(8), the output of summer circuit is,

𝑉𝑜 = sin 𝜔𝑐 + 𝜔𝑜 − 𝜔𝑚 𝑡 + 90 + 𝑠𝑖𝑛 𝜔𝑐 − 𝜔𝑜 + 𝜔𝑚 𝑡 − 90 +
sin 𝜔𝑐 + 𝜔𝑜 − 𝜔𝑚 𝑡 + 90 + 𝑠𝑖𝑛 𝜔𝑐 − 𝜔𝑜 + 𝜔𝑚 𝑡 + 90
𝑉𝑜 = 2 cos 𝜔𝑐 + 𝜔𝑜 − 𝜔𝑚 𝑡

The final RF output frequency is fc + fo- fm which essentially the lower sideband of
RF carrier is fc + fo

VESTIGIAL SIDEBAND MODULATION [VSB]

Drawback of SSB Signal Generation

Generation of an SSB signal is difficult.

Selective filtering is to be done to get the original signal back.

Phase shifter should be exactly tuned to 90°.

The required filter must have a very sharp cut-off characteristics,

particularly when the baseband signal contains extremely low
frequencies (eg. Television and Telegraphic signals).

In VSB, the desired sideband is allowed to pass completely whereas just a small
portion (called trace or vestige) of the undesired sideband is allowed. The
transmitted vestige of undesired sideband compensates for the loss of the wanted
sideband.
GENERATION OF VSB – FILTER METHOD

Fig.1.20 VSB-SC Modulation

Fig.1.21 VSB SC Synchronous Detector

The product modulator generator DSB-SC wave from message signal and carrier
signal. This DSB-SC signal is given to input of BPF, which reject or suppress any
one sideband and passes a portion of other sideband.

Fig.1.22 Representation of VSB

 The portion from fc to fc+ fm is USB. The portion from fc to fc+ fv is
suppressed partially.

The portion from fc to fc - fm is LSB. Its portion from fc- fv to fc is to be

transmitted as vestige.

The filter response is only for positive frequency. The frequency response is
1
normalized, so that carrier frequency 𝐻(𝑓𝑐 ) = 2.

If the transition interval𝑓𝑐 − 𝑓𝑣 ≤ 𝑓 ≤ 𝑓𝑐 + 𝑓𝑣 , the following two conditions

are satisfied.

Sum of values of magnitude response 𝐻(𝑓𝑐 ) at any two frequencies equally

displaced above & below 𝑓𝑐 is unity.

The phase response [𝑎𝑟𝑔 𝐻(𝑓𝑐 )] is unity, 𝐻(𝑓) satisfies the condition i.e.,
𝐻 𝑓 − 𝑓𝑐 + 𝐻 𝑓 + 𝑓𝑐 = 1 for −𝑓𝑚 ≤ 𝑓 ≤ 𝑓𝑚 .

TRANSMISSION BANDWIDTH

The transmission Bandwidth of VSB modulation is

BT = fm + fv
Where, fm − Message Bandwidth
fv − Width of vestigial sideband
The VSB modulated wave is described in time domain as,
1 1
𝑠 𝑡 = 𝑉𝑐 𝑚 𝑡 cos(2𝜋𝑓𝑐 𝑡) ± 𝑉𝑐 𝑚′ (t)sin(2𝜋𝑓𝑐 𝑡)
2 2
Where plus sign corresponds to transmission of a vestige of upper sideband and
minus sign corresponds to transmission of vestige of lower sideband.

The signal m’(t) in the quadrature component of s(t) is obtained by passing the
message signal m(t) through a filter whose frequency response H Q(f) satisfies
𝐻𝑄 𝑓 = 𝑗 𝐻 𝑓 − 𝑓𝑐 − 𝐻 𝑓 + 𝑓𝑐 𝑓𝑜𝑟 − 𝑓𝑚 ≤ 𝑓 ≤ 𝑓𝑚
ADVANTAGES AND APPLICATION
Low frequencies near fc are transmitted without any attenuation.

Bandwidth is reduced compared to DSB.

The filter required need not have sharp cut-off.

Used in television for transmission of picture signal.

COMPARISON OF AM TECHNIQUES

Fig.1.25 Comparison of AM techniques

 AUDIO AMPLIFIER
The output from the demodulator is the recovered audio. This is passed into the audio
stages where they are amplified and presented to the headphones or loudspeaker.
ADVANTAGES
Superior sensitivity, frequency stability and selectivity

It offer better stability

APPLICATION
TV Receiver

All Radio Receiver

Performance of the receiver is measured on
Selectivity

Sensitivity

Fidelity

Image Frequency rejection ratio

Selectivity

The selectivity is the ability of the receiver to select a signal of a desired frequency while
rejecting all others.

Sensitivity

The ability of the receiver to pick-up week signals and amplify them is called sensitivity.

Fidelity

The ability of the receiver to reproduce all the range of modulating frequencies equally is
called fidelity of the receiver.

Image Frequency rejection ratio (IFRR):

Image frequency : Any frequency other than the desired frequency. The image frequency is given
by 𝑓𝐼𝑀= 𝑓𝐼𝐹 + 𝑓𝐿𝑂 where 𝑓𝐿𝑂 = 𝑓𝐼𝐹 + 𝑓𝑅𝐹

𝑓𝐼𝑀= 2𝑓𝐼𝐹 + 𝑓𝑅𝐹

The image frequency is rejected by using more than one tuned circuit before the preselector
in the receiver

IFRR is the measure of ability of a pre-selector to reject the image frequency.

𝐼𝐹𝑅𝑅 = 1 + 𝑄2 𝜌2
𝑊ℎ𝑒𝑟𝑒 𝑄 − 𝑄𝑢𝑎𝑙𝑖𝑡𝑦 𝑓𝑎𝑐𝑡𝑜𝑟
𝑓𝑖𝑚 𝑓𝑅𝐹
𝜌= −
𝑓𝑅𝐹 𝑓𝑖𝑚
 Problems

1. Calculate the modulation index and percentage modulation if instantaneous voltages of

modulating signal and carrier are 40 sin ωmt and 50 sin ωct respectively.

Sol:

Vm = 40 , Vc = 50

Modulation Index, m = Vm / Vc = 40 / 50 = 0.8

% Modulation = m x 100 = 0.8 x 100 = 80%

2. An audio frequency signal 10 sin 2π x 500t is used to amplitude modulate a carrier of 50

sin 2π x 105t. Calculate

i. Modulation Index

ii. Sideband Frequency

iii. Amplitude of each sideband frequency

iv. Bandwidth required

v. Total Power delivered to the load of 600 Ω

Sol:
Vm = 10 sin 2π x 500t , Vc = 50 sin 2π x 105t

i. Modulation Index

Vm = 10, , Vc = 50

m = Vm / Vc = 10 / 50 = 0.2

ii. Sideband Frequency

ωm = 2π x 500 , fm = 500 Hz

ωc = 2π x 105 , fc = 105 Hz or 100 KHz

fUSB = fc + fm = 100 KHz + 500 Hz = 100.5 KHz

fLSB = fc - fm = 100 KHz - 500 Hz = 99.5 KHz

iii.Amplitude of each sideband frequency

Amplitude of Upper and Lower sidebands = m Vc / 2

= (0.2 x 50) / 2 = 5 V

iv. Bandwidth required

BW = 2 fm = 2 x 500 = 1000Hz (or) 1 KHz

v. Total Power delivered to the load of 600 Ω

𝑚2
𝑃𝑡 = 𝑃𝐶 [1 + ]
2

𝑉𝑐 2 𝑚2
𝑃𝑡 = [1 + ]
2𝑅 2

(50)2 (0.2)2
= 2×600 [1 + ]
2

𝑃𝑡 = 2.125 𝑊𝑎𝑡𝑡𝑠

3. A 400W carrier is modulated to a depth of 80%. Calculate the total power in the
modulated wave.

Sol:

Pc = 400W , m = 0.8

𝑚2
𝑃𝑡 = 𝑃𝐶 [1 + ]
2

(0.8)2
= 400[1 + ]
2
𝑃𝑡 = 528 𝑊𝑎𝑡𝑡𝑠
4. A broadcast transmitter radiates 20 KW when the modulation percentage is 75. Calculate carrier
power and power of each sideband.

Sol:

Pt = 20KW , m = 0.75

𝑚2
𝑃𝑡 = 𝑃𝐶 [1 + ]
2

𝑃𝑡
𝑃𝑐 =
𝑚2
[1 + 2 ]

20000
=
(0.75)2
[1 + 2 ]

𝑃𝑐 = 15.6 𝐾𝑊

𝑚2 1
𝑃𝑆𝐵 = 𝑃𝐶 ×
2 2

(0.75)2 1
= 15.6 × ×
2 2

𝑃𝑆𝐵 = 2.2 𝐾𝑊

𝑃𝑈𝑆𝐵 = 𝑃𝐿𝑆𝐵 = 2.2 𝐾𝑊

5. The total antenna current of an AM transmitter is 5A. If modulation index is 0.6.

Calculate the carrier current in antenna.

Sol:

It = 5A, m = 0.6

𝑚2
𝐼𝑡 = 𝐼𝐶 [1 + ]
2
 𝐼𝑡
𝐼𝐶 =
𝑚2
[1 + 2 ]

5
=
(0.6)2
[1 + 2 ]
𝐼𝐶 = 4.6 𝐴

6. A commercial AM station is broadcasting with an unmodulated carrier power of 10

KW. The modulation index is set at 0.7 for a sinusoidal message signal. Find the total
transmitted power and the efficiency.

Sol:

Pc = 10KW , m = 0.7

𝑚2
𝑃𝑡 = 𝑃𝐶 [1 + ]
2

(0.7)2
= 10[1 + ]
2
𝑃𝑡 = 12.45 𝐾𝑊

𝑚2
%𝜂 = × 100
2 + 𝑚2

(0.7)2
= × 100
2 + (0.7)2
%𝜂 = 19.6 %
7. A transmitter radiates 9 KW without modulation and 10.125 KW after modulation.
Determine depth of modulation.

Sol:

Pt= 10.125 KW , Pc = 9 KW

𝑚2
𝑃𝑡 = 𝑃𝐶 [1 + ]
2

𝑃𝑡
𝑚= 2 −1
𝑃𝑐

10.125
= 2 −1
9

𝑚 = 0.5

8. A carrier of 20 MHz is amplitude modulated with a signal frequency of 3 KHz and

amplitude 5V. If the ma = 0.5, Sketch the spectra of the waveform.

Sol:

fc= 20 MHz , fm = 3 KHz , ma = 0.5 , Vm = 5V

Amplitude of Carrier

Vc = Vm / ma = 5 / 0.5 = 10V

Amplitude of Sideband

VSB = ma Vc/ 2 = (0.5 x 10) / 2 = 2.5V

9. Compute the Bandwidth of the amplitude modulated signal

c(t) = 23 x Cos (230000 πt) [1 + 0.8 x Cos (310 πt)]

Sol:

c(t) = Vc cos ωct [1 + m cos ωmt]

m = Vm / Vc

ωm = 310 π

2π fm = 310 π

2 fm = 310

BW of AM , BW = 310 Hz

10. Determine the bandwidth required for an AM signal obtained by modulating 2 MHz
carrier signal by a message signal of 4 KHz bandwidth.

Sol:

fc = 2 MHz

fm = 4 KHz

BW = 2 fm = 2 x 4 KHz = 8 KHz
6.4 ASSIGNMENTS

1. The total Power content of an AM signal is 1000W. Determine the power being
transmitted at carrier frequency and at each side bands when modulation percentage
is 100%

2. A 400W, 1MHz carrier is amplitude-modulated with a sinusoidal signal 0f 2500Hz.

The depth of modulation is 75%. Calculate the sideband frequencies, bandwidth, and
power in sidebands and the total power in modulated wave.

3. An audio frequency signal 10 sin (2π×500t) is used to amplitude modulate a

carrier of 50 sin (2π×105t). Calculate.

frequency of side bands

Bandwidth

Modulation index

Amplitude of each side band.

Transmission efficiency

Total power delivered to a load of 600Ω.

4. The output voltage of a transmitter is given by 500(1+ 0.4sin 3140 t) sin 6.28 x10 7𝑡

Calculate

Carrier frequency

Modulating frequency

Carrier power with a load resistance of 600W

Total power transmitted

5. A modulating signal m (t) =10cos (2π×103t) is amplitude modulated with a

carrier signal c(t)=50cos(2π×105t). Find the modulation index, the carrier power, and
the power required for transmitting AM wave.
6. The equation of amplitude wave is given by VAM(t) =20[1+0.8cos
(2π×103t)]cos(4π×105t). Find the carrier power, the total sideband power, and the
band width of AM wave.

7. A 400W carrier wave is modulated to a depth of 65%. Find the total power of
modulated wave?

8. Calculate power in each sideband, if power of carrier wave is 176W and there is
60% modulation in amplitude modulated signal?

9. Carrier wave of frequency f = 1MHz with a voltage of 20V used to modulated a

signal of frequency 1 KHz with a voltage of 10V. Find out the following

m?

Side band Frequencies of modulated wave?

Bandwidth

10.A carrier of 10 MHz is amplitude modulated with a signal frequency of 3 KHz and
amplitude 9V. If the ma = 0.2, Sketch the spectra of the waveform.
6.5 Part A Q & A (with K level and CO)

S.No PART A CO’S Bloom

s Level

Define modulation.
Modulation is a process by which some
1. characteristics of high frequency carrier
signal is varied in accordance with the CO1 K1
instantaneous value of the modulating signal.

Define depth of modulation.

It is defined as the ratio between message amplitude
2.
to that of carrier amplitude. CO1 K1
m=Vm/Vc
What are the degrees of modulation?
 Under modulation. m<1
3.  Critical modulation m=1 CO1 K1
 Over modulation m>1

What is the need for modulation?

 Ease of transmission
 Multiplexing
 Reduced noise CO1 K1
4.
 Narrow bandwidth
 Frequency assignment
 Reduce the equipments limitations

Define Amplitude Modulation.

In amplitude modulation, the amplitude of a carrier signal
5. is varied according to variations in amplitude of modulating CO1 K1
signal.

64
S.No PART A CO’S Bloom
s Level

Compare AM with DSB-SC and SSB-SC.

S.No AM signal DSB-SC
1 Bandwidth = 2fm Bandwidth = 2fm
2 Contains Contains USB,LSB
6. USB,LSB,Carrier CO1 K2
3 More Power is required Power required is less
for transmission than that of AM.

What are the advantages of VSB-AM?

 It has bandwidth greater than SSB but less than DSB
system.
 Power transmission greater than DSB but less than CO1
7. K1
SSB system.
 No low frequency component lost. Hence it avoids
phase distortion.

How will you generating DSBSC-AM?

There are two ways of generating DSBSC-AM such as
8. a)Balanced modulator CO1 K1
b)Ring modulators.

What are advantages of ring modulator?

• Its output is stable.
• It requires no external power source to activate the
9. diodes. CO1 K1
• Virtually no maintenance.
• Long life.
Define Demodulation. CO1 K1
Demodulation or detection is the process by which
modulating voltage is recovered from the modulated signal.
It is the reverse process of modulation. The devices used
10. for demodulation or detection are called demodulators or
detectors. For amplitude modulation, detectors or
demodulators are categorized as,
a) Square-law detectors
b) Envelope detectors

65
S.No PART A CO’S Bloom
s Level
What are the disadvantages of DSB-FC? CO1 K1
11. (i) Power wastage takes place in DSB-FC
(ii) DSB-FC is bandwidth inefficient system.
What is Vestigial Side Band Modulation? CO1 K1
Vestigial Sideband Modulation is defined as a modulation in
12. which one of the sideband is partially suppressed and the
vestige of the other sideband is transmitted to compensate
for that suppression.
What are the advantages of signal sideband transmission? CO1 K1
• Power consumption
13.
• Bandwidth conservation
• Noise reduction
Compare linear and non-linear modulators.
S.No Linear Modulators Non Linear
Modulators
1. Heavy filtering is not Heavy filtering is
required. required.
2. These modulators are These modulators CO1 K1
14. used in high level are used in low
modulation. level modulation.
The carrier voltage is The modulating signal
3. very much greater than voltage is very much
modulating signal greater than the carrier
voltage. signal voltage.

Draw the frequency spectrum of VSB. Where it is used?

VSB is used for TV transmission, since low frequencies near
fc represents picture details. They are unaffected by VSB.

15. CO1 K1

66
S.No PART A CO’S Bloom
s Level

Define carrier swing. CO1 K1

Carrier Swing (CS) is defined as the total variation in
16 frequency from lowest to highest point.
Carrier Swing = 2* frequency deviation of the FM signal =
2* Δω
Define sensitivity characteristics of a radio receiver. CO1 K1
The ability of the receiver to pick up weak signals and
amplify them is called sensitivity. It is often defined in
17.
terms of voltage that must be applied to the receiver input
terminals to give the standard output power measured at
output terminals.
Define fidelity and selectivity of a receiver. CO1 K1
Fidelity
The ability of the receiver to reproduce all the range of
modulating frequencies equally is called fidelity of the
receiver. For good fidelity, more bandwidth of Radio
Frequency (RF) and Intermediate Frequency (IF) stages
18.
are required.
Selectivity
The selectivity is defined as the ability of the receiver to
select a signal of a desired frequency while rejecting all
others. The selectivity depends upon tuned LC circuits used
in RF and IF stages.
What are the methods for generating SSB-SC signal? CO1 K1
SSB-SC signals may be generated by two methods as
under:
19.
(i)Frequency discrimination method or filter method.
(ii)Phase discrimination method
or phase-shift method.

67
S.No PART A CO’S Bloom
s Level

Define SSB-SC. CO1 K1

(i) SSB-SC stands for Single Side Band Suppressed
Carrier
(ii) When only one sideband is transmitted, the
20. modulation is referred to as Single side band modulation. It
is also called as SSB or SSB-SC.

What are the disadvantages of single side band CO1 K1

transmission?
a) Complex receivers: Single side band systems
require more complex and expensive receivers thn
21.
conventiaonal AM transmission.
b) Tuning difficulties: Single side band receivers
require more complex and precise tunig than conventional
AM receivers.
Draw the spectrum of AM signal. CO1 K1

22.

68
6.6 Part B Q & A (with K level and CO)

S.No PART B CO’S Blooms

Level
(i)Derive the equation of an AM wave. Also draw the CO1
modulated AM wave for various modulation index.
(ii)The antenna current of an AM transmitter is 8 ampere
1. K3
when only the carrier is sent. The current increases to 8.93
A when the carrier is modulated by a single sine wave. Find
the percentage modulation
Explain the operation of non linear modulator to generate CO1
2. AM. K2

Derive an expression for output voltage of a balanced CO1

modulator to generate DSB-SC and explain its working
3. principle. K2

Explain about the generation of DSB-SC using ring CO1

4. modulator K2

Explain the generation of SSB SC signal using phase shift CO1

5. method K2

How SSB can be generated using Weavers method? CO1

6. Illustrate with a neat block diagram. K2

Draw the VSB spectrum and explain the significance and its CO1
7. applications. K2

Compare the characteristics of DSBFC, DSBSC, SSBSC, VSB CO1

8. schemes. K2

69
6.6 Part B Q & A (with K level and CO)

S.No PART B CO’S Blooms

Level
An AM signal is generated by modulating the carrier fc=800 CO1
MHz by the signal m(t) = 0.5cos5000πt. The AM signal
S(t)=100[1+m(t)]cos2πfct is fed to a 50 ohm load.
9. 1) Determine the average power in the carrier and in the K2
sidebands.
2) Find the modulation index and peak power delivered to
the load.
For an AM- DSBFC wave with peak unmodulated carrier CO1
voltage Vc =10V, a load resistance RL=10Ω and a
modulation coefficient ma =1, determine

10. K2
a) Powers of the carrier and the upper sidebands
b) Total sideband power
c) Total power of the modulated wave
d) Draw the power spectrum
(i)Suggest a scheme for recovering the message signal from CO1
the signal s(t)=2m(t)cos2πfc t. Explain the same.
11. (ii)A 1000 KHz carrier is simultaneously amplitude modulated K2
with 300 Hz, 800Hz and 1.5 KHz audio sine waves. What will
be the frequencies present in the output?
An SSB is generated by modulating an 800 KHz carrier by CO1
the signal m(t) = Cos 2000πt+2Sin 1200πt.the amplitude of
12. K3
the carrier is Ac=10V. Obtain the magnitude spectrum of
LSB-SSB signal
Derive the expression for DSB-SC AM and calculate its power CO1
13. & efficiency. Explain a method to generate it. K2

70
6.7 Supportive online Certification courses (NPTEL,
Swayam, Coursera, Udemy, etc.,)

ONLINE COURSE NPTEL:

https://onlinecourses.nptel.ac.in/noc21_ee30/preview

Principles of Digital Communications

By Prof. Abhishek Dixit | IIT Delhi

This course covers introduction to digital communications: Interfaces and

channels for digital communications , Geometric representation of signals, Gram-
Schmidt orthogonalization, Geometric interpretation of signals, Review of random
variables: Introduction to random variables, joint probability density function,
characteristic functions, derived distributions, Review of random process:
Introduction to random processes, Gaussian process, Linear functional of random
process, Stationary and wide sense stationary random process, Power spectral
density, White Gaussian noise

ONLINE COURSE COURSERA:

https://onlinecourses.nptel.ac.in/noc21_ee11/preview

Modern digital communication techniques

By Prof. Suvra Sekhar Das | IIT Kharagpur

The course is covers the practical basics of digital communication systems, Source
Coding, Characterization of Communication Signals & Systems, Signal space
Representation, Representation of Memory less Modulation Methods, Nonlinear
modulation methods, Probability of error of different modulation schemes,
Fundamentals of estimation and detection theory used in digital communication,
Carrier phase and symbol timing synchronization techniques, Channel estimation
and equalization techniques, Power Adaptation methods for colored noise channel

71
6.8 Real time Applications in day to day life and to
Industry
 Entertainment – AM Radio and Television Broadcasting

• Radio broadcasting

Radio, sound communication by radio waves, usually through the transmission of music,
news, and other types of programs from single broadcast stations to multitudes of individual listeners
equipped with radio receivers. From its birth early in the 20th century, broadcast radio astonished
and delighted the public by providing news and entertainment with an immediacy never before
thought possible. From about 1920 to 1945, radio developed into the first electronic mass medium,
monopolizing “the airwaves” and defining, along with newspapers, magazines, and motion pictures,
an entire generation of mass culture.

• Television broadcasting

8VSB is the modulation method used for broadcast in the ATSC digital television
standard. ATSC and 8VSB modulation is used primarily in North America.

 Business

• Data Communication

• CB Radio to relay shipment progress, conference calls with other branches via telephone or
speaker phone

 Government

Police officers keeping contact with the station, or relaying the information and location of
suspect

 Personal

Cell phones or walkie – talkies for chatting with friends

 Military

Using undetectable radio signals to communicate with other soldiers, or voice commands
to control weapons

72
6.9 Contents beyond the Syllabus

AM DEMODULATION

Amplitude modulation refers to any method of modulating an

electromagnetic carrier frequency by varying its amplitude in accordance with the
message intelligence that is to be transmitted. This is accomplished by heterodyning
the intelligence frequency with the carrier frequency. The vector summation of the
carrier, sum, and difference frequencies causes the modulation envelope to vary in
amplitude at the intelligence frequency, as discussed in chapter 1. In this section we
will discuss several circuits that can be used to recover this intelligence from the
variations in the modulation envelope.
DIODE DETECTORS

The detection of AM signals ordinarily is accomplished by means of a diode

rectifier, which may be either a vacuum tube or a semiconductor diode.

The demodulator must meet three requirements: (1) It must be sensitive to the
type of modulation applied at the input, (2) it must be nonlinear, and (3) it must
provide filtering.

The waveform contains only three rf frequencies: (1) the carrier frequency, (2)
the sum frequency, and (3) the difference frequency. The modulating intelligence
is contained in the difference between these frequencies.

The vector addition of these frequencies provides the modulation envelope which
approximates the original modulating waveform.

It is this modulation envelope that the DIODE DETECTORS use to reproduce the
original modulating frequencies.

73
COMMON-EMITTER DETECTOR
The COMMON-EMITTER DETECTOR is often used in receivers to supply an amplified
detected output. The schematic for a typical transistor common-emitter detector is
shown in figure.

Input transformer T1 has a tuned primary that acts as a frequency-selective device.

L2 inductively couples the input modulation envelope to the base of transistor Q1.

Resistors R1 and R2 are fixed-bias voltage dividers that set the bias levels for Q1.
Resistor R1 is bypassed by C2 to eliminate rf.
This RC combination also acts as the load for the diode detector (emitter-base
junction of Q1). The detected audio is in series with the biasing voltage and controls
collector current.
The output is developed across R4 which is also bypassed to remove rf by C4. R3 is a
temperature stabilization resistor and C3 bypasses it for both rf and af.

Q1 is biased for slight conduction with no input signal applied. When an

input signal appears on the base of Q1, it is rectified by the emitter-base
junction (operating as a diode) and is developed across R1 as a dc bias
voltage with a varying af component. This voltage controls bias and collector
current for Q1.

The output is developed by collector current flow through R4. Any rf ripple in
the output is bypassed across the collector load resistor by capacitor C4. The
af variations are not bypassed. After the modulation envelope is detected in
the base circuit, it is amplified in the output circuit to provide suitable af
output.

The output of this circuit is higher than is possible with a simple detector.
Because of the amplification in this circuit, weaker signals can be detected
than with a simple detector. A higher, more usable output is thus developed.
 7. Assessment Schedule

Assessment Proposed Date Actual Date

Unit 1 Assignment
Assessment
Unit Test 1

Unit 2 Assignment
Assessment
Internal Assessment 1 27.02.2023
Retest for IA 1

Unit 3 Assignment
Assessment
Unit Test 2

Unit 4 Assignment
Assessment
Internal Assessment 2 18.04.2023
Retest for IA 2

Unit 5 Assignment
Assessment
Revision Test 1

Revision Test 2

Model Exam 11.05.2023

Remodel Exam

University Exam

76
8. Prescribed Text Books & Reference Books

TEXT BOOK

J.G.Proakis, M.Salehi, ―Fundamentals of Communication Systems‖, Pearson

Education 2014. (UNIT I-IV)

Simon Haykin, ―Communication Systems‖, 4th Edition, Wiley, 2014.(UNIT I-

V)

REFERENCES

B.P.Lathi, - Modern Digital and Analog Communication Systems‖, 3rd Edition,

Oxford University Press, 2007.

D.Roody, J.Coolen, ―Electronic Communications, 4th edition PHI 2006.

A.Papoulis, ―Probability, Random variables and Processes, McGraw Hill, 3rd

edition, 1991.

B.Sklar, Digital Communications Fundamentals and Applications, 2nd Edition

Pearson Education 2007

H P Hsu, Schaum Outline Series - ―Analog and Digital Communications

TMH 2006

77
9. Mini Project suggestions

COMMUNICATION THEORY MINI PROJECTS LIST

S.No Name of The Project

Intelligent approach to video transmission over 2.4 GHz wireless

1 technology
Grasping money game used in wireless channel resource allocation
2
Bluetooth Controlled Electronic Home Appliances
3
GSM Based Home Security Alarm System Using Arduino
4
Remote Industrial Security System
5
Centralized Monitoring System for Taxies
6
Wireless Weather Monitoring based on GSM
7
Cell Phone Detector
8
High fidelity and way intercom with musical ring alert system for point
to point communication
9
Wireless stepper motor Control using RF Communication
10
IR based escaping cock robot
11
Wireless Stepper motor control using IR Communication
12
Analog Transmitter and Receiver: Optimization Of Power Dissipation
13 And Maximization Of Bandwidth
ASK Modulation using OPAMP
14
Wireless Stepper Motor Control using IR Communication
15

78
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