MIT-WPU

SY ECE(AIML) and SY ECE

Communication Systems
(ECA2PM02A/ECE2PM02A)
 SY ECE AI-ML (2023-27)Structure Sem.IV

January 25 DoEEE, MITWPU 2

 SY ECE (2023-27) Structure Sem.IV

January 25 SECE, MITWPU 3

Course Objectives
1. Knowledge:
(i) The need of modulation.
(ii) Concept of analog communication systems and digital communication system with
pulse modulation.
(iii) Noise, noise parameters and random processes.
2. Skills:
(i) Hands on to explore the generation and reception of different analog and
digital pulse modulation techniques.
3. Attitude:
(i) To explain and demonstrate the methods of generation and reception of different
analog and digital communication systems.

January 25 SECE, MITWPU 4

 Course Outcomes
1. Describe the basic concepts of communication systems and amplitude
modulation technique (CL-II).
2. Analyse the methods of angle modulation and understand the concept
of radio receivers (CL-IV).
3. Understand noise and random processes and describe the importance
of their various parameters in communication systems(CL-II).
4. Illustrate pulse and digital modulation schemes (CL-II).

January 25 SECE, MITWPU 5

Syllabus
1. Amplitude Modulation: Elements of communication system,
need of modulation, baseband and carrier communication,
Amplitude Modulation (AM), modulation index, spectrum of AM
wave, power, transmission efficiency and bandwidth, generation of
AM and its variants such as DSB-SC, SSB, ISB, VSB, RF wave
propagation. [12 hours]

2. Angle Modulation and Radio Receivers: Concept of angle

modulation, mathematical analysis of FM and PM, frequency
spectrum, NBFM and WBFM, FM generation methods, TRF receiver,
superheterodyne receiver for AM and FM, performance
characteristics of receiver, AM and FM detectors, FM stereo receiver.
[11 hours]

January 25 SECE, MITWPU 6

Syllabus
3. Noise and Random Processes:
Sources of noise, types of noise, noise figure, noise factor, noise resistance,
noise temperature, bandwidth, SNR, Friis’s formula, noise calculations.
Introduction to random process, Stationary processes, Mean, Correlation &
Covariance functions, Ergodic processes, Transmission of a random process
through a LTI filter, Power spectral density, Gaussian process. [10 hours]

4. Digitization of Analog Signal: Sampling Theorem, sampling

techniques, analog pulse modulation methods: PAM, PWM and PPM,
Digital Communication system block diagram, Digital pulse modulation
methods: PCM, Non-uniform Quantization and Companding, Differential
Pulse Code Modulation, Delta Modulation, Adaptive Delta Modulation. [12
hours]

January 25 SECE, MITWPU 7

 Laboratory work
1. Generation, detection and spectral analysis of AM.
2. Generation, detection and spectral analysis of DSB-SC.
3. Generation, detection and spectral analysis of SSB.
4. Generation, detection and spectral analysis of FM.
5. Verification of Sampling Theorem and sampling techniques.
6. Study of linear PCM and Companded PCM system.
7. Study of Delta Modulation Technique.
8. Study of Adaptive Delta Modulation Technique.

January 25 SECE, MITWPU 8

 Internal Assessment
CCA LCA
Assessment CCA1 Midterm CCA2 LCA1 LCA2 LCA3 Term Total
exam end
exam
Marks 10 25 10 5 5 5 40 100

Tentative Distribution
CCA1 unit 1 open book assignment
Midterm exam Unit 1 &2
CCA2 Unit 4 &3 Quiz
LCA1 Practical exam
LCA2 Oral /Journal
LCA3 Oral
Term end exam Unit 1,2,3,4

January 25 SECE, MITWPU 9

 Books
Text Books:
1. B.P. Lathi, Modern Digital and Analog Communication Systems. Oxford
University Press, 3rd Edition.
2. Simon Haykin, Communication Systems, John Wiley & Sons, 4th Edition
Reference Books:
1. Taub, Herbert, and Donald L. Schilling. Principles of communication
systems. McGraw-Hill Higher Education, 3rd Edition.
2. Kennedy George, Brendan Davis, Electronic communication systems. Tata
McGraw-Hill Publishing Co. Ltd., 4th Edition.

January 25 SECE, MITWPU 10

 Module I

Amplitude Modulation
History
Evolution of
Electronic
Communication
System
 Evolution of electronic communication system

https://youtu.be/oxTUC5I22LU

https://www.youtube.com/watch?v=cBskroqaKkI
 Importance of communication systems

Significance of Human Communication:

⚫ Communication is the process of exchanging information.

⚫ Main barriers are language and distance.

January 25 SECE, MITWPU 15

 Significance of Human Communication:

Methods of communication:
1. Face to face
2. Signals
3. Written word (letters)
4. Electrical innovations:
⚫Telegraph
⚫Telephone
⚫Radio
⚫Television
⚫Internet (computer)
January 25 SECE, MITWPU 16
Block diagram of a communication System

Figure 1-1: A general model of all communication systems.

 Communication System

⚫Basic components:
⚫Transmitter
⚫Channel or medium
⚫Receiver

⚫Noise degrades or interferes with transmitted

information thru channel

January 25 SECE, MITWPU 18

 Communication System model

Figure 1-2: A general model of all communication systems.

 Input transducer
• Transducer: Converts one form of energy into another form
(physical to electrical)
• Message/Baseband/Modulating/Information signal: LF (fm)
• Audio, Video, Data (triple-play services)

20
 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
21
Communication media or channel
 Channels
The communication channel is the medium by which the electronic signal is sent
from one place to another.
Electrical Conductors
Simplest form -Coaxial cable, twisted pair cable

Optical Media
Light pipe that carries message on a light wave
Used in Long distance calls and all Internet Communications.

Free Space or Radio or Wireless

Makes use of Electromagnetic Spectrum
Intelligence signals are converted into EM waves(Electro Magnetic Waves).
Used in Cellular Communication, WiFi, WiMax etc.

Other types of Media

In SONAR (SOundNavigationAndRanging), Water is used as the medium.
 Noise

Noise is random, undesirable electronic energy that enters the

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

• The measure of noise is usually expressed in terms of the signal to

noise ratio (S/N or SNR)
• SNR is Signal power divided by the noise power.
• High SNR is preferred for better performance

24
 Communication Systems
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
⚫A demodulator or detector that recovers the original intelligence signal from the
modulated carrier.
⚫The output is original signal.
⚫i.e. Voice signal sent to speaker, Video signal is fed to an LCD screen for display, or
binary data received by a computer later printed or displayed on monitor.
January 25 SECE, MITWPU 25
 Transceivers
A transceiver is an electronic unit that incorporates circuits that
both send and receive signals.
Examples
• Telephones
• Fax machines
• Handheld radios
• Cell phones
• Computer modems

26
 Output Transducer
It converts electrical signal into physical waveform.

Audio
Signal

Video
Signal

27
Classification of electronic communication systems
Electronic Communication System

Unidirectional/ Nature of Channel/Medium

Technique of
Bidirectional information transmission
Communication Signal

Wired Wireless
Digital Baseband Communication
Simplex Analog
Half Transmission using
System Full
Duplex Modulation
Duplex

January 25 SECE, MITWPU 28

 Simplex Communication- One way
⚫The simplest method of electronic communication is referred to as simplex.
⚫This type of communication is one-way. Examples are:
⚫Radio
⚫TV broadcasting
⚫Beeper (personal receiver)
⚫Remote Control

January 25 SECE, MITWPU 29

 Two Way Communication- Half Duplex
⚫The form of two-way communication in which only one party transmits at a
time is known as half duplex. Examples are:
⚫Police, military, etc. radio transmissions
⚫Citizen band (CB)
⚫Family radio
⚫Amateur radio

January 25 SECE, MITWPU 30

 Two Way Communication- Full Duplex
⚫Most electronic communication is two-way and is referred to as duplex.
⚫When people can talk and listen simultaneously, it is called full duplex.
The telephone is an example of this type of communication.

January 25 SECE, MITWPU 31

 Communication System classification Based on
Nature of an Information Signal

January 25 SECE, MITWPU 32

 Analog Communication
Analog Signals
An analog signal is a smoothly and continuously varying voltage or current.
Examples are:
⚫Sine wave
⚫Voice
⚫Video (TV)

Figure 1.3: Analog signals (a) Sine wave “tone.” (b) Voice. (c) Video (TV) signal.

January 25 SECE, MITWPU 33

 Digital Communication
Digital Signals
⚫Digital signals change in steps or in discrete increments.
⚫Most digital signals use binary or two-state codes. Examples are:
⚫Telegraph (Morse code)
⚫Continuous wave (CW) code
⚫Serial binary code (used in computers)

Figure 1.4: Digital signals (a) Telegraph (Morse code). (b) Continuous-wave (CW) code. (c) Serial binary code.
January 25 SECE, MITWPU 34
 Based on the technique of transmission
Based on the technique used for the signal transmission, we can categories the
electronic communication system as under:
1. Baseband transmission system
The Baseband signal (original information signals) are directly transmitted.
Example: Telephone network, Computer data transmission over the coaxial cable in
computer network.
Limitations:
⚫Can not be used for Radio transmission
⚫Can not travel long distance
Therefore, for the radio communication of baseband signals, a technique
called modulation is used
2. Communication system using modulation (Broadband Transmission)

January 25 SECE, MITWPU 35

 What is Modulation?

✔ Modulation is the process of having a baseband voice, video, or digital signal

modify another, higher-frequency signal, the carrier

✔ In the modulation process, a parameter of the carrier wave (such as amplitude,

frequency or phase) is varied in accordance with the modulating signal

✔ Baseband to passband Transmission

January 25 SECE, MITWPU 36

 Modulation

Figure 1.5: Modulation at the transmitter.

January 25 SECE, MITWPU 37
Baseband and Passband(carrier modulated) Communication
⚫The term baseband is used to designate the band of
frequencies of the signal delivered by the source or transducer.
⚫ In telephony baseband is the audio band of 0 to 3.5 KHz. In
TV the baseband is video band occupying 0 to 3.5 MHz .
⚫In baseband communication message signals are directly
transmitted without any modification.
⚫Baseband signals are suitable for transmission over a pair of
wires, coaxial cables, optical fibers but can not be transmitted
over radio link.
January 25 SECE, MITWPU 38
 ⚫Baseband signals have overlapping bands, they would interfere
severely if sharing a common channel.
⚫Thus baseband communication leave much of the channel
spectrum unused.
⚫Also long haul communication require modulation.
⚫Local telephone communication, short haul PCM and long
distance PCM over optical fiber are examples of baseband
communication

January 25 SECE, MITWPU 39

 ⚫The communication that uses modulation to shift the frequency
spectrum of a signal is known as carrier communication.
⚫In this mode one of the basic parameter (Amplitude, Frequency or
Phase) of a sinusoidal carrier of high frequency is varied in
proportion to the baseband signal. This result in AM, FM or PM.

⚫Modulation is used to transmit analog as well as digital baseband

signals.

January 25 SECE, MITWPU 40

Need of Modulation
 NEED of MODULATION
1.Ease of Radiation- Reduction In Heights of Antenna

λ—is the wavelength, λ = c/f ,where f is the frequency of the signal to be

transmitted and c is the velocity of light ( 3×108 m/s)
Practicality of Antenna height is possible with modulation.
Height, h=λ/4, for efficient transmission.
For f=30Hz h=2500km Impossible

f=3KHz h=25km Impractical/Impossible

f=3MHz h=25m Practical/ can be installed

https://www.everythingrf.com/rf-calculators/frequency-to-wavelength
2.Simultaneous Transmission of Multiple signals is possible due to
modulation- It Avoids mixing of signals

January 25 SECE, MITWPU 43

2. Multiplexing - Avoid mixing of signals

1. If the baseband sound signals are transmitted without using the modulation by
more than one transmitter, then all the signals will be in the same frequency range
i.e. 0 to 20 kHZ.

2. Therefore all the signals will get mixed together and a receiver cannot separate
them from each other.

3. So if each baseband sound signal is used to modulate a different carrier, then they
will occupy different slots in the frequency domain.

4. Thus modulation is necessary to avoid mixing of signals.

3.Increases the range of communication

1. The frequency of baseband signal is low, and the low frequency signals cannot
travel a long distance when they are transmitted. They get heavily attenuated.

2. The attenuation reduces with increase in frequency of the transmitted signals and
they travel longer distance.

3. The modulation process increases the frequency of the signal to be transmitted .

Hence modulation increases the range of communication
 4.Improves quality of reception
1. Quality of a signal is based on presence of noise.
2. Noise reduction techniques will improve the quality of a received signal.
3. With Frequency Modulation(FM), and the digital communication techniques like
PCM, the effect of noise is reduced to a great extent.

Signal-to-
Noise Ratio
(SNR)

Bit error rate

(BER)
 5. Makes multiplexing possible
1. Multiplexing is the process in which two or more signals can be transmitted over
the same communication channel simultaneously .

2. This is possible only with modulation. The multiplexing allows the same channel to
be used by many signals.

3. Therefore many TV channels can use the same frequency range, without getting
mixed with each other Or different frequency signals can be transmitted at the same
time.
 AMPLITUDE MODULATION

✔ “Amplitude modulation (AM) is the process of changing the amplitude of a high

frequency carrier signal in proportion with the instantaneous value of the modulating
signal”.

✔ In AM the instantaneous amplitude of the sinusoidal high frequency carrier is changed

in proportion to the instantaneous amplitude of the modulating signal. This is the
principle of AM.

✔ AM is used in the applications such as radio transmission, TV transmission etc.

January 25 SECE, MITWPU 48

Single Tone AMPLITUDE MODULATION

Modulating signal (sine wave) and modulated carrier

January 25 SECE, MITWPU 50
 AM is DSB-FC- Double Sideband Full Carrier

Most commonly used is AM

double-sideband full carrier
(DSBFC),
or sometimes called
conventional AM or AM.

January 25 SECE, MITWPU 51

 Multitone baseband signal as a modulating signal

January 25 SECE, MITWPU 52

January 25 SECE, MITWPU 53
Modulation Index and Percentage of Modulation

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Modulation Index and Percentage of Modulation m=0.5
Vm=0.5
Vc=1

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Amplitude Modulation Depth

January 25 SECE, MITWPU 56

Trapezoidal Method
⚫ Modulation Index,
m= A-B/A+B

January 25 SECE, MITWPU 57

Equation of AM Signal

January 25 SECE, MITWPU 58

Generation of AM signal
.

January 25 SECE, MITWPU 59

January 25 SECE, MITWPU 60
 Spectrum of AM signal
This is described as the signal in the frequency domain, as opposed to the signal in the
time domain
Frequency
Spectrum of AM
signal

January 25 SECE, MITWPU 62

 Power Relations in AM (Sine)wave

⚫As we have seen carrier component of the modulated wave has the same amplitude as
the unmodulated carrier.
⚫The modulated wave contains extra energy in the two sideband components.
⚫ So modulated wave contains more power than the carrier signal.

⚫So total power is Pt =Pc + PLSB + PUSB

i.e. Pt = V²car /R + V²LSB /R + V²USB /R -----(7)

Where R is an antenna resistance and voltage is rms voltage

January 25 SECE, MITWPU 63

⚫

VPK = VRMS x 1.414

January 25 SECE, MITWPU 64

⚫

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Current Relation in the AM Wave

January 25 SECE, MITWPU 66

Modulation by Several Sine Waves
⚫

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Modulation by Several Sine Waves ( Multitone signal)

January 25 SECE, MITWPU 68

 Amplitude Modulation
Advantages and disadvantages of AM

Advantages:
⚫AM transmitter are less complex .
⚫AM receivers are simple, detection is easy.
⚫AM receivers are cost efficient. Hence even a common person can afford to buy it.
⚫AM waves can travel a longer distance.
⚫Low bandwidth.

January 25 SECE, MITWPU 69

Disadvantages:
The AM signal is also called as “Double Sideband Full Carrier (DSBFC)”signal.
The three main disadvantages are :
1. Power wastage takes place.
2. AM needs larger bandwidth.
3. AM wave gets affected due to noise.

Applications:
1.Radio broadcasting.
2. Picture transmission in a TV system.

January 25 SECE, MITWPU 70

Example 1B

January 25 SECE, MITWPU 72

Example
4. For a conventional AM modulator with a carrier freq of fc = 100 kHz
and the maximum modulating signal frequency of fm(max) = 5 kHz,
determine:
a) Freq limits for the upper and lower sidebands.
b) Bandwidth.
c) Upper and lower side frequencies produced when the modulating
signal is a single-freq 3-kHz tone.
d) Draw the output freq spectrum.

January 25 SECE, MITWPU 75

Examples
5. A 400 watt carrier is modulated to a depth of 75 percent. Calculate the
total power in the modulated wave.
Ans: 512.5 W
6. A broadcast radio transmitter radiates 10 kW when the modulation
percent is 60. How much of this is carrier power?
Ans: 8.47 kW

January 25 SECE, MITWPU 76

 Examples ( slide 63)
7. The antenna current of an AM transmitter is 8 A when only the carrier is sent,
but it increases to 8.93 A when the carrier is modulated by a single sine wave. Find
the percentage modulation. Determine the antenna current when the percent of
modulation changes to 0.8.
Ans: m= 70.1% It = 9.19 A

8. A certain transmitter radiates 9 kW with the carrier unmodulated, and 10.125 kW

when the carrier is sinusoidally modulated. Calculate the modulation index,
percent of modulation. If another sine wave, corresponds to 40% modulation, is
transmitted simultaneously, determine the total radiated power.
Ans: m=0.50, Pt = 10.84kW

January 25 SECE, MITWPU 77

Examples
9. A carrier signal of 1100 kHz is simultaneously modulated with 350 Hz,
850Hz, and 1 kHz audio sine waves. What are frequencies present in the
output.

10. A 320 watts carrier of 1280kHz frequency is simultaneously modulated

by two voice signals with modulation percentage of 60 and 65 respectively.
What will be the total sideband power radiated?
Ans: PSB = 125.17 watts

January 25 SECE, MITWPU 78

Examples
11. A carrier signal with 14 V amplitude and frequency of 10 MHz is applied to AM
modulator with 60% modulation. The modulating signal frequency is 1000Hz.
Write down the equation of the above wave and sketch the waveform in
frequency domain.
Ans: AM equation : v = Vc (1+ m sin wm t) sin wc t
fLSB = fc - fm
fUSB = fc + fm
VC = given
VUSB = VLSB = mVc/2
Draw a spectrum

January 25 SECE, MITWPU 79

Examples
12. Determine the η and percentage of total power carried by the sidebands
of the AM wave for tone modulation when
1) μ =0.3 2) μ =0.5 3) μ =0.7

Ans: η = μ²/ (μ²+2)

for μ =0.3 η = 0.043
hence only almost 4.3% of power in sidebands
Similarly for μ =0.5 η = 0.1111
and for μ =0.7 η = 0.1967

January 25 SECE, MITWPU 80

Examples
13. AM signal is expressed by e=(Ec+em)cos2π fct
Where Ec=peak amplitude of 10 volt of carrier signal with frequency fc =1
MHz and em=modulating signal.
If modulating signal contains 500Hz at 7 volts amplitude and 3kHz at 5
volts amplitude. Determine the total power transmitted and effective
modulation index. (DEC 2010) 8 marks
Ans: effective modulation index=0.8602
Pt=68.5/R watt.

January 25 SECE, MITWPU 81

Examples
AM signal is expressed by e=(Ec+em)cos2π fct
Where Ec=peak amplitude of 10 volt of carrier signal with frequency fc =1
MHz and em=modulating signal.
If modulating signal contains 500Hz at 7 volts amplitude and 3kHz at 5
volts amplitude. Determine the total power transmitted and effective
modulation index.

January 25 SECE, MITWPU 82

Examples
14. The tuned circuit of the oscillator in a simple AM Tx employs a 50µH
coil and 1nF capacitor. If the oscillator output is modulated by audio
frequencies up to 10kHz, what is the range occupied by the sidebands?
Ans: 701.7kHz to 721.7kHz
15. A standard AM transmission, sinusoidally modulated to depth of 40%,
produces a sideband frequencies of 6.824 and 6.854 MHz. The amplitude
of each sideband frequency is 50 V. Determine the amplitude and
frequency of the carrier.
Ans: fc=6.839MHz, Vc=250V

January 25 SECE, MITWPU 83

Examples
16. A 45 V(rms) carrier is amplitude modulated by a 30 V(rms) sine wave.
Determine
i) The max and min values of the peak to peak voltage of modulated wave.
ii) The amplitude of the side frequency and
iii) Modulation index. (Dec 2010)
Ans: Vc=63.64 V, Vm=42.43V
i) Max value of modulated wave=2(Vc+Vm) =212.14V
Min value of modulated wave=2(Vc-Vm) =42.42V
ii) Amp of side frequency = mVc/2 =21.215V
iii) m=Vm/Vc =0.6667

January 25 SECE, MITWPU 84

 Double Sideband Suppressed Carrier (DSBSC) Technique
⚫

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 Double Sideband Suppressed Carrier (DSBSC) Technique

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 Double Sideband Suppresses Carrier (DSBSC) Technique

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Time Domain Representation of the DSBSC Wave

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Frequency spectrum of the DSBSC Wave

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Power Relations in the DSBSC Wave
⚫

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Features of DSBSC
⚫Suppressing the carrier in the basic AM yields DSBSC
⚫Power saving is achieved in DSBSC
⚫Total power in DSBSC depends on the modulation index
⚫The bandwith requirement of DSBSC is same as that of basic AM
version i.e. 2fm
⚫Simple diode detector will not recover the message
⚫To improve the bandwidth required to transmit AM signal next variant
of AM is introduced as SSB.

January 25 SECE, MITWPU 91

 Amplitude Modulation (DSBSC) cont…

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 Amplitude Modulation (DSB) cont…

Spectrum

Spectrum of the DSB-SC signal m(t)cos10,000t

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 Amplitude Modulation (DSB) cont…

Spectrum

Spectrum of the DSB-SC signal m(t)cos10,000t

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 Amplitude Modulation (DSB) cont…

Spectrum

Spectrum of the DSB-SC signal m(t)cos10,000t

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Amplitude Modulation (DSB) cont…
A modulating signal m(t) is given by
i) m(t) = cos 100t
ii) m(t) = cos 100t + 2 cos 300t.
a) In each case, sketch the spectrum of m(t) and AM if carrier is 2 cos 1000t.
b) Sketch DSBSC signal spectrum for carrier of 2 cos 1000t.
c) Sketch the USB spectrum if LSB is suppressed along with carrier.

January 25 SECE, MITWPU 96

Single Sideband suppressed carrier (SSBSC/SSB)

⚫LSB & USB carry same information

⚫Hence saving of BW can be achieved by suppressing one of the
sideband
⚫This led to the variant of AM called as Single sideband suppressed
carrier (SSBSC) or simply called as SSB

January 25 SECE, MITWPU 97

Single Sideband suppressed carrier (SSBSC/SSB)

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 Frequency spectrum of SSB

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Time Domain Representation of the SSB Wave

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Power Relations in the SSB Wave

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Advantages of Bandwidth Saving

⚫AM broadcasting : carrier frequency-540 to 1650 KHz

⚫RF bandwidth= (1650-540)KHz =1110 KHz
⚫Bandwidth per station with (fm = 5 KHz)

For AM:- 2fm = 10 KHz

For SSB :- fm = 5 KHz
⚫Number of AM stations = RF Bandwidth/BW per station

⚫Number of stations
✔For AM (or DSBFC) or DSBSC = (1110/10) =111
✔For SSB = (1110/5) = 222

January 25 SECE, MITWPU 102

Numericals on SSB
1. A SSB transmitter radiates 0.5KW when the modulation percentage is 60%.
How much of carrier power (in KW) is required if we want to transmit the same
message by an AM transmitter?
Ans: PAM= 6.56KW

2 Calculate the percentage power saving when the carrier and one of the
sidebands are suppressed in an AM wave modulated to a depth of (a) 100% and
(b) 50%.
Ans: (a) 83.3% , (b) 94.4 %

January 25 SECE, MITWPU 103

Limitations of SSB

⚫Practical difficulty in suppressing the unwanted sideband

⚫Even some portion of the wanted sideband gets eliminated during the process.
⚫This happens because the wanted and unwanted components lie very close to each
other at the carrier frequency fc .
⚫This difficulty can be avoided by allowing a vestige/trace/fraction of unwanted
sideband along with the wanted sideband. (VSB technique)

January 25 SECE, MITWPU 104

Generation of Amplitude Modulated Signals
The AM signal can be generated using following methods
⚫Using Analog Multiplier
⚫Using a Nonlinear Resistance Device- Diode

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 Generation of Amplitude Modulated Signals Using Analog Multiplier
.

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Generation of Amplitude Modulated Signals Using a Nonlinear Resistance Device

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Generation of Amplitude Modulated Signals Using Nonlinear Resistance Device

The devices like Diodes, Transistors, FETs can be biased with suitable voltage
to constrain them to exhibit the negative resistance property.
NRD

AM
Signal

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⚫

January 25 SECE, MITWPU 109

.

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Generation of AM using NRD:

January 25 SECE, MITWPU 111

Generation of DSBSC Signals
The DSBSC signal can be generated using following methods
⚫Using Analog multiplier
⚫Using balanced modulator

January 25 SECE, MITWPU 112

Generation of DSBSC Signals using Analog Multiplier
.

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 Generation of DSBSC Signals using Balanced Modulator
⚫Balanced modulator can be constructed using the non-linear devices like diode and transistors.
⚫A balanced modulator may be defined as a circuit in which two non-linear devices are connected
in a balanced mode to produce a DSB-SC signal .
⚫The balanced modulator using the diodes is shown in figure below
⚫The diode use the non-linear resistance property for generating modulated signal.

D1

o/p
Transformer

D2

January 25 SECE, MITWPU 114

.

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⚫

DSB-SC signal
January 25 SECE, MITWPU 116
Generation of SSB Signals

The SSB signal can be generated using following

methods
1. Using Analog Multiplier
2. Using the Filter Method
3. Using the Phase shift Method
4.Using the Third Method

January 25 SECE, MITWPU 117

 SSB Generation

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 1. Generation of SSB Signals using Analog Multiplier

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 2. SSB generation using Filter Method

Generation of SSB Signals using Filter Method

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Filter Method:-
Advantages:
- Unwanted sideband is removed or heavily attenuated by a filter.

- The filter BPF with sharp cut-off edges, bandwidth is sufficiently flat and wide.

- Simplicity
Disadvantages:
- Due to inability of the system to generate SSB at high radio frequencies, the
frequency upconversion is necessary.

- Two expensive filters are to be used one for each sidebands.

- Low audio frequencies can not be used as the filter becomes bulky.
 3. Generation of SSB Signals using Phase Shift Method
⚫The phase shift method avoids filters and some of there inherent
disadvantages.
⚫Instead it makes use of two balanced modulators and two phase shift networks
• M1 receives 90⁰ phase shifted
carrier and in phase message signal
• M2 is fed with 90⁰ phase shifted
message and in phase carrier
signal.
• Both modulator produce two
sidebands
• In phase upper sideband and out of
phase lower sideband

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 ⚫

USB

LSB
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 Advantages of Phase Shift Method
⚫It can generate SSB at any frequency so the frequency up converter stage is not
required.
⚫It can use the low audio frequencies as modulating signal which is not possible in
Filter method.
⚫It is easy to switch from one sideband to other.

Disadvantages
⚫90 degree phase shift network for m(t) is critical.
⚫This network has to provide a correct phase shift of 90 degree at all modulating
frequencies which is practically difficult to achieve.

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RF wave propagation

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 RF wave propagation
+ EM waves are used for long distance communication.
+ Propagation of EM waves depends on:
Properties or Characteristics (frequency)
• Environment
• Waves travel in straight line, except where the earth and its atmosphere alter
their path.
+ Types:
• Ground (Surface) waves
• Sky waves
• Space waves

126
 RF wave propagation
+ Ground (Surface) waves
• It is a method of radio frequency
propagation that uses the area between the
surface of the earth & an ionosphere for
transmission.
• Propagate in the LF & MF range of EM
Spectrum
• Used to provide local radio communication
coverage.

127
 RF wave propagation
❖ Sky waves:
• Radio wave propagation via ionosphere.
• Used for the propagation of EM waves with a
frequency range of 3 – 30MHz
• Make use of the ionosphere due to the presence
of charged ions in the region of about 60 to 300
km from the earth surface
• These ions provide a reflecting medium to the
radio or communication waves within a
particular frequency range

128
 Space Waves
❑ They travel in ( more or less) straight lines.
❑ However, since they depend on line-of-sight conditions, space waves are limited in
their propagation by the curvature of the earth, except in very unusual circumstances.
❑ Thus they propagate very much like EM waves in free space.

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AM Transmitter
❑ Two methods:
1. AM transmitters: High powers (complex and high efficiency)
2. Laboratory AM generator : Low power (simple and low efficiency).

❑Low Level Modulation (LLM):

❑High Level Modulation (HLM):

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Comparison between HLM and LLM

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AM Transmitter
Figure shows a typical block diagram of an AM transmitter.

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Commercial AM broadcast standards
❑ Bands:
✔ Longwave (LW)
✔ Mediumwave (MW)
✔ Shortwave (SW)
❑ Tuning range: 540 – 1600 KHz (MW) [Carrier Frequency range]
❑ Baseband: 20 Hz to 5 KHz
❑ Bandwidth of station: 10 KHz
❑ Transmit power : Region wise

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Comparison of AM Techniques
Parameter DSBFC DSBSC SSB ISB VSB

BW 2fm 2fm fm fm1+fm2 fm<BW<2fm

Power High Medium Less moderate SSB<P< DSBFC

η Minimum Moderate Maximum Moderate moderate

Carrier No Yes Yes Partially No

suppression

Sideband No No One SB One per channel One SB partially

suppression completely

Rx Complexity Simple Complex Complex Complex simple

Application Radio Commn Pt. to Pt. commn Mobile Telephony TV

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References
1. George Kennedy, Electronic Communication Systems. McGraw-Hill, 5th
Edition
2. Dr. Sanjay Sharma, Communication Systems (Analog and Digital), S. K.
Kataria and sons
3. https://www.toppr.com/guides/physics/communication-
systems/propagation-of-electromagnetic-waves/

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 Self Study Topics
⚫ Vestigial
Sideband (VSB) Modulation
⚫ Generation of SSB Signals using Third Method
⚫ Independent Sideband Transmission

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Vestigial Sideband (VSB) Modulation
⚫Limitation of SSB: Practical difficulty in suppressing the unwanted sideband
frequency components.
⚫Observed in practice: It also eliminate some portion of the wanted sideband.
⚫Reason: Video signal (0-5 MHz)
⚫Solution: To allow a vestige or fraction of unwanted sideband along with the wanted
sideband.
⚫Modulation: VSB-SC
⚫Used for : TV signal transmission

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 Fraction of LSB
.

Fraction of USB

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Generation of SSB Signals using Third Method

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Generation of SSB Signals using Third Method

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Third Method
Advantages:-
⚫As audio carrier is constant frequency, the design of phase shift network for constant
frequency is very simple.

⚫Up conversion process is not required as high frequency carrier can be used.

⚫Low frequencies can be transmitted.

Disadvantages:-
• Most complex method because of use of 4 Balanced modulators.

• Cost of entire system is more as compared to phase shift method

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 Independent Sideband Transmission
❑ It is an AM single sideband mode which is used with
some AM radio transmissions. This is used in some
kinds of AM stereo (sometimes known as the Kahn
system).
❑ ISB is a compromise between DSB and SSB, the
other is VSB.
❑ ISB transmission uses 2 sideband which are
independent to each other.
❑ It essentially consists of 2 SSB channels added to
form 2 sidebands around the reduced carrier.
❑ Each sideband is quite independent of the other.
❑ It can simultaneously convey totally different
transmission.

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