Amplitude modulation (Communication Electronics )

Tulsiramji Gaikwad-Patil College of
Engineering & Technology
Wardha Road, Nagpur-441 108
Department of Electronics and
Communication Engineering
B.E. Third Year (V Semester)
Course: BEECE504T: Communication
Electronics
By
Mr. Rahul Dhuture
Assistant Professor
Electronics and Communication Engineering Department
TGPCET, Nagpur.
1
TGPCET/20-21/BEECE504T

Syllabus
Unit I: Amplitude (Linear) Modulation MARKS- (08)
• Base band & Carrier communication, Introduction of amplitude modulation, Equation of AM,
• Generation of AM (DSBFC) and its spectrum, Modulation Index , Power relations applied to
sinusoidal signals,
• DSBSC – multiplier modulator, Non linear generation, switching modulator, Ring modulator &
its spectrum,
• SSBSC, ISB & VSB, their generation methods & Comparison, AM Broadcast technical
standards.
Unit II: Angle Modulation MARKS-(12)
• Concept of Angle modulation, Types of Angle Modulation, frequency spectrum, Narrow band
& wide
• band FM, Modulation index, Bandwidth, Phase Modulation, Bessel’s Function and its
mathematical analysis,
• Generation of FM (Direct & Indirect Method), Comparison of FM and PM.
TGPCET/20-21/BEECE504T/U-I 2

Unit III: Pulse Modulation MARKS- (10)
• Band limited & time limited signals, Narrowband signals and systems, Sampling theorem in
time
• domain, Nyquist criteria, Types of sampling- ideal, natural, flat top, Aliasing & Aperture effect.
• Pulse Analog modulation: PAM PWM & PPM.
Unit IV: Noise MARKS- (10)
• Sources of Noise, Types of Noise, White Noise, Thermal noise, shot noise, partition noise,
Low
• frequency or flicker noise, burst noise, avalanche noise, Signal to Noise Ratio, SNR of tandem
Connection,
• Noise Figure, Noise Temperature, Friss formula for Noise Figure, Noise Bandwidth.
Unit V: AM and FM Receivers MARKS-(10)
• Communication Receiver, Block Diagram & special Features
• Block diagram of AM and FM Receivers, Super heterodyne Receiver, Performance
characteristics:
• Sensitivity, Selectivity, Fidelity, Image Frequency Rejection, Pre-emphasis, De-emphasis
• AM Detection: Rectifier detection, Envelope detection, Demodulation of DSBSC:
Synchronous detection,
• Demodulation of SSBSC.
• FM Detection: Foster Seelay FM Detector & FM detection using PLL

Unit VI: Broadband Communication Links & Multiplexing MARKS- (10)
• Multiplexing: Frequency Division Multiplexing, Time Division Multiplexing, Code Division
Multiplexing.
• Short and Medium Haul Systems: Coaxial Cables, Fiber optic links, Microwave Links,
Tropospheric scatter Links.
• Long Haul Systems: Submarine cables.
Books:
• Text Books:
• 1. Kennedy & Devis : Electronic Communication Systems , Tata McGraw Hills
Publication(Fourth Edition)
• 2. Dennis Roddy & Coolen - Electronic Communication, PHI (Fourth Edition)
• 3. B. P. Lathi: Modern Digital and Analog. Communication Systems: Oxford Press Publication
(Third Edition)
• Reference Books:
• 1. Simon Haykin: Communication Systems, John Wiley & Sons (Fourth Edition)
• 2. Taub & Schilling: Principles of Communication Systems, Tata McGraw-Hill

UNIT - 1
Amplitude Modulation
5
TGPCET/20-21/BEECE504T/U-I

Block Diagram of Communication
Systems

Classification of Communication
System

What is Modulation
• Modulation
In the modulation process, some characteristic of a high-
frequency carrier signal (bandpass), is changed according to the
instantaneous amplitude of the information (baseband) signal.
Why Modulation is used
• Suitable for signal transmission (distance…etc)
• Multiple signals transmitted on the same channel
• Capacitive or inductive devices require high
frequency AC input (carrier) to operate Stability and
noise rejection

About Modulation….
• Application Examples
• Broadcasting of both audio
and video signals.
• Mobile radio communications, such as cell phone.
• Basic modulation types
–Amplitude Modulation: changes the amplitude.
– Frequency Modulation: changes the frequency.
– Phase Modulation: changes the phase

Modulation Theory
• A sine wave is represented as follows
c(t)= Ac cos(2πfct)
• Here Ac, fc all represent parameters that can be
modulated in the carrier waveform in order to
carry information.
• The modulation schèmes are known as :
Ac -> Amplitude Modulation
fc -> Frequency Modulation

Basic Amplitude
Modulation (A.M.)
• Amplitude Modulation is the simplest and earliest
form of transmitters
• The information signal varies the instantaneous
amplitude of the carrier

Modulation permits the use of
multiplexing
• Multiplexing means allowing simultaneous
communication by multiple users on the same
channel.
• For instance, the radio frequency spectrum must
be shared and modulation allows users to separate
themselves into bands.

AMPLITUDE MODULATION
(AM)
• In amplitude modulation, the message signal m(t) is
impressed on the amplitude of the carrier signal
c(t) = Ac COS (2∏fct)
• This results in a sinusoidal signal whose amplitude is a
function of the message signal m(t)
• There are several different ways of amplitude
modulating the carrier signal by m(t)
• Each results in different spectral characteristics for the
transmitted signal

• Mainly these methods are used for AM:
(a) Double Sideband with Large carrier AM
(DSB-LC AM)
(b) Double sideband, suppressed-carrier AM
(DSB-SC AM)
(c) Single-sideband AM (SSB AM)
(d) Vestigial Sideband (VSB) modulation

Full AM modulation ( DSB-LC)
1 The carrier signal is
Sc (t) = Ac cos(Ꞷc t) where Ꞷ c =2∏fc
2 In the same way, a modulating signal
(information signal) can also be expressed as
Sm ( t)= Am cos (Ꞷ m t)
3 The amplitude-modulated wave can be
expressed as
s(t) =[ Ac + Sm (t)] cos(Ꞷ c t)

4. By substitution
s(t)=[Ac + Am COS(Ꞷ m t) ] COS (Ꞷ c t)
5. The modulation index.
m=Am / Ac
6. Therefore The full AM signal may be
written as
S(t) = Ac (1+m COS(Ꞷ m t) ) COS Ꞷ c t

• By using mathematical formula
COSA COSB = 1/2 [COS(A+B) +COS (A-B)]
Final mathematical expression of AM. wave
S(t) = Ac ( COS Ꞷ c t) +
𝒎 𝑨𝒄
𝟐
COS (Ꞷ c + Ꞷ m)t +
𝒎 𝑨𝒄
𝟐
COS (Ꞷ c - Ꞷ m)t

Double-Sideband Suppressed-Carrier
AM
A double-sideband, suppressed-carrier (DSB-SC) AM
signal is obtained by multiplying the message signal
m(t) with the carrier signal c(t) = Ac cos (2∏fct)
Amplitude-modulated signal:
u(t) = m(t) * c(t) = Ac m(t) cos(2∏fct)

• An example of the message signal m(t), the
carrier c(t), and the modulated signal u (t) are
shown in fig in next slide.
• This figure shows that a relatively slowly
varying message signal m(t) is
• changed into a rapidly varying modulated
signal u(t), and due to its rapid changes with
time, it contains higher frequency components

Double-Sideband Suppressed-Carrier
AM
• Figure : An example of message, carrier, and DSB-SC modulated signals

Single-Sideband AM
• The two sidebands of an AM signal are mirror
images of one another
• As a result, one of the sidebands is redundant
• Using single-sideband suppressed-carrier
transmission results in reduced bandwidth and
therefore twice as many signals may be
transmitted in the same spectrum allotment

Single-Sideband AM
• A method, illustrated in
Figure, generates a
DSB-SC AM signal and
then employs a filter
that selects either the
upper sideband or the
lower sideband of the
double-sideband AM
signal
v TGPCET/20-21/BEECE504T/U-I 24
Figure : Generation of a singlesideband AM
signal by filtering one of the sidebands
of a DSB-SC AM signal.

Advantages/disadvantages
Advantages of Amplitude Modulation, AM
• It is simple to implement and having frequency range from750khz to 1250 khz
• it can be demodulated using a circuit consisting of very few components
• AM receivers are very cheap as no specialized components are needed.
Disadvantages of amplitude modulation
• It is not efficient in terms of its power usage
• It is not efficient in terms of its use of bandwidth, requiring a
bandwidth equal to twice that of the highest audio frequency.
• It is prone to high levels of noise because most noise is amplitude
based and obviously AM detectors are sensitive to it.

Online Assignment -1
Q.1 What is Communication system and give
the classification of the same.
Q.2 Define and explain with mathematical
expression of the A.M. modulation wave.

Thank you….

• Angle modulation is the process of varying the frequency and
phase of a carrier wave in proportion to the frequency and
phase of a base band signal. The amplitude of the carrier
remains constant.
• It is of two types
Frequency Modulation
Phase Modulation
Angle Modulation

Frequency modulation is the process of varying the frequency
of a carrier wave in proportion to the amplitude of a
baseband signal. The amplitude and phase of the carrier
remains constant.
Frequency Modulation

Theoretically, an infinite number of sidebands
produced, but most of power is contained in first
(m+1) sidebands
Thus transmission requires a bandwidth of
approximately 2 (m+1) fm Hz
Frequency Modulation Index

Another term common to FM is the modulation index, as
determined by the formula:
Frequency Modulation Index
m
f
f
m


Sideband structure is more complicated than for AM;
many sidebands produced
Complexity depends on m However, spacing between carrier and
sidebands (and between adjacent sidebands) is
equal to fm, just as for AM

• For FM, the bandwidth varies with both deviation and modulating
frequency
• Increasing modulating frequency reduces modulation index so it reduces
the number of sidebands with significant amplitude
• On the other hand, increasing modulating frequency increases the
frequency separation between sidebands
• Bandwidth increases with modulation frequency but is not directly
proportional to it
Bandwidth

• There are no theoretical limits to the modulation index or the
frequency deviation of an FM signal
• The limits are a practical compromise between signal-to-noise
ratio and bandwidth
• Government regulations limit the bandwidth of FM
transmissions in terms of maximum frequency deviation and
the maximum modulation frequency
Narrowband and Wideband FM

• The maximum frequency deviation of an FM transmitter is
restricted by law, not by any physical constraint
• Traditional oscilloscope displays are not useful in analyzing FM
signals
• A spectrum analyzer is much more useful in determining the
qualities of an FM signal
FM Measurements

Unit-3
Pulse Modulation
TGPCET/19-20/BEELE304T/U-III 41

• Pulse Modulation
• Sampling Theorem
• Pulse Amplitude Modulation
• Pulse Width Modulation
• Pulse Position Modulation
• Pulse Code Modulation
• Delta Modulation
CONTENTS

Sampling Theorem
Fig 1:Impulse sampling of an analog voltage
A sampler is a mixer with a train of very
narrow pulses as the local oscillator input.
If the analog input is sampled
instantaneously at regular intervals at a
rate that is at least twice the highest
analog frequency
fs > 2fa(max)
then the samples contain all of the
information of the original signal.

• The analog signal v(t) has a signal spectrum represented by the Fourier transform
V(f), and the sampling signal
consists of instantaneous impulses every nTs sec, where n = 0, +1, +2, …
• The Fourier transform of s(t) is
Sampling Theorem
   s
ns
nff
T
fS  



1
   



n
snTtts 
The time-domain product performed by the sampler produces a
sampled output spectrum given by
   s
ns
s nffV
T
fV  


1

• where this spectrum consists of replicas of the analog signal spectrum V(f),
translated in frequency by each of the sampling frequency harmonics
• The sampler is a wideband (harmonic) mixer producing upper and lower
sidebands at each harmonic of the sampling frequency.
• Figure (2a) illustrates the correct way to sample: if sampling is done at fs >
2fA(max) the upper and lower sidebands do not overlap each other,
• and the original information can be recovered by passing the signal
through a low-pass filter (see Figure 2c and d).
Sampling Theorem

• A PAM waveform consists of a sequence of flat-topped pulses.
The amplitude of each pulse corresponds to the value of the
message signal x(t) at the leading edge of the pulse.
Pulse Amplitude Modulation
Fig 3: Pulse Amplitude Modulation waveform

• The circuit of Figure (4)is used to illustrate pulse amplitude
modulation (PAM). The FET is the switch used as a sampling
gate.
• When the FET is on, the analog voltage is shorted to ground;
when off, the FET is essentially open, so that the analog signal
sample appears at the output.
• Op-amp 1 is a noninverting amplifier that isolates the analog
input channel from the switching function.
Pulse Amplitude Modulation –
Natural and Flat-Top Sampling

Demodulation
Figure 10: PPM demodulator
• As illustrated in Figure 10, a narrow clock pulse sets an RS flip-flop output high,
and the next PPM pulses resets the output to zero.
• The resulting signal, PWM, has an average voltage proportional to the time
difference between the PPM pulses and the reference clock pulses.
• Time-averaging (integration) of the output produces the analog variations

• PPM has the same disadvantage as continuous analog phase modulation: a
coherent clock reference signal is necessary for demodulation.
• The reference pulses can be transmitted along with the PPM signal.
• This is achieved by full-wave rectifying the PPM pulses of Figure 11-9a, which has
the effect of reversing the polarity of the negative (clock-rate) pulses.
• Then an edge-triggered flipflop (J-K or D-type) can be used to accomplish the same
function as the RS flip-flop of Figure 10 using the clock input.
• The penalty is more pulses/second will require greater bandwidth, and the pulse
width limit the pulse deviations for a given pulse period.
Demodulation

• Like PCM, a delta modulation system consists of an encoder and a decoder
• unlike PCM, however, a delta modulator generates single-bit words that represent
the difference (delta) between the actual input signal and a quantized
approximation of the preceding input signal sample.
• This is represented in Figure 14 with a sample-and-hold, comparator, up-down
counter staircase generator, and a D-type flip-flop (D-FF) to derive the digital pulse
stream.
• The continuous analog signal is band-limited in the low-pass filter (LPF) to prevent
aliasing distortion, as in any sampling system.
• The analog signal VA is then compared to its discrete approximation VB
Delta Modulation

Delta Modulation
Fig 15 DM Demodulator

Unit-4
Noise

54
1. Introduction
Noise is a general term which is used to describe an unwanted signal
which affects a wanted signal. These unwanted signals arise from a
variety of sources which may be considered in one of two main
categories:-
•Interference, usually from a human source (man made)
•Naturally occurring random noise
Interference
Interference arises for example, from other communication systems
(cross talk), 50 Hz supplies (hum) and harmonics, switched mode
power supplies, thyristor circuits, ignition (car spark plugs) motors
… etc.

55
1. Introduction (Cont’d)
Natural Noise
Naturally occurring external noise sources include atmosphere disturbance
(e.g. electric storms, lighting, ionospheric effect etc), so called ‘Sky Noise’
or Cosmic noise which includes noise from galaxy, solar noise and ‘hot
spot’ due to oxygen and water vapour resonance in the earth’s atmosphere.

56
2. Thermal Noise (Johnson Noise)
This type of noise is generated by all resistances (e.g. a resistor,
semiconductor, the resistance of a resonant circuit, i.e. the real part of the
impedance, cable etc).
Experimental results (by Johnson) and theoretical studies (by Nyquist) give
the mean square noise voltage as
)(4 2
2_
voltTBRkV 
Where k = Boltzmann’s constant = 1.38 x 10-23 Joules per K
T = absolute temperature
B = bandwidth noise measured in (Hz)
R = resistance (ohms)

57
2. Thermal Noise (Johnson Noise) (Cont’d)
The law relating noise power, N, to the temperature and bandwidth is
N = k TB watts
Thermal noise is often referred to as ‘white noise’ because it has a
uniform ‘spectral density’.

58
3. Shot Noise
• Shot noise was originally used to describe noise due to random
fluctuations in electron emission from cathodes in vacuum tubes
(called shot noise by analogy with lead shot).
• Shot noise also occurs in semiconductors due to the liberation of
charge carriers.
• For pn junctions the mean square shot noise current is
Where
is the direct current as the pn junction (amps)
is the reverse saturation current (amps)
is the electron charge = 1.6 x 10-19 coulombs
B is the effective noise bandwidth (Hz)
• Shot noise is found to have a uniform spectral density as for thermal
noise
  22
)(22 ampsBqIII eoDCn 

59
Noise may be quantified in terms of
noise power spectral density, po watts per
Hz, from which Noise power N may be
expressed as
N= po Bn watts
8. Noise Evaluation (Cont’d)
Ideal low pass filter
Bandwidth B Hz = Bn
N= po Bn watts
Practical LPF
3 dB bandwidth shown, but noise does not suddenly cease
at B3dB
Therefore, Bn > B3dB, Bn depends on actual filter.
N= p0 Bn
In general the equivalent noise bandwidth is > B3dB.

UNIT-4
AM AND FM RECEIVER

• TRF Receiver
• Super heterodyne Receiver
• RF receiver
Content

• RF Stage- filters the desired station and amplifies weak antenna
signal
• DETECTOR- removes information from the carrier
• AF Amp- power amplifier to drive the speaker
TRF
RF
STAGE
RF
STAGE
DETECTOR AF
AMP

• Design of AM/FM radio receiver
• The radio receiver has to be cost effective
• Requirements:
– Has to work with both AM and FM signals
– Tune to and amplify desired radio station
– Filter out all other stations
– Demodulator has to work with all radio stations regardless of carrier frequency
• For the demodulator to work with any radio signal, we “convert” the carrier
frequency of any radio signal to
Intermediate Frequency (IF)
• Radio receiver design can be optimized for that frequency
• IF filter and a demodulator for IF frequency
AM/FM Radio Receiver

RFTuner IF Filter Demodulator Audio
Amplifier
•This is known as the “Superheterodyne” receiver
•Two stages: RF and IF (filtering and amplification)
•The receiver was designed by Armstrong
RF Section
•Tunes to the desired RF frequency,
•Includes RF bandpass filter centered around
•The bandwidth
•Usually not narrowband, passes the desired radio station and
adjacent stations

• The minimum bandwidth of RF filter:
• Passes the desired radio channel, and adjacent channels
• RF-IF converter:
– Converts carrier frequencyIF frequency
• How can we convert signals with different RF frequencies to the same IF
frequency?
• Local oscillator with a center frequency
is a function of RF carrier frequency
TRF BB 
IFcLO fff 
RFTuner IF Filter Demodulator Audio
Amplifier

UNIT-6
Broad band Communication links
&Multiplexing

 This technique combines time-domain samples from different message signals
(sampled at the same rate) and transmits them together across the same channel.
 The multiplexing is performed using a commutator (switch) as shown in Figure
3.19. At the receiver a decommutator (switch) is used in synchronism with the
commutator to demultiplex the data.
 TDM system is very sensitive to symbol dispersion, that is, to variation of
amplitude with frequency or lack of proportionality of phase with frequency.
This problem may be solved through equalization of both magnitude and phase.
 One of the methods used to synchronize the operations of multiplexing and
demultiplexing is to organize the mutiplexed stream of data as frames with a
special pattern. The pattern is known to the receiver and can be detected very
easily.
Time Division Multiplexing

Electromagnetic Wave
Electric
Field, E
Magnetic
Field, H
Direction of
Propagation
z
x
y

• These three properties are shared by light and radio waves
• For both reflection and refraction, it is assumed that the
surfaces involved are much larger than the wavelength; if not,
diffraction will occur
Reflection, Refraction, and Diffraction

• Ground-Wave propagation
• Ionosphere propagation
• Line of sight.
• Space Wave propogation
• Sky Wave
Terrestrial Propagation

• Frequencies up to 2 MHz.
• Vertically polarized in order to minimize currents induced in
the ground creating losses.
• Further from transmitter the more horizontal the wave front
becomes.
• Ground waves attenuate quickly above 2 MHz.
• Users: Military (15 KHz and 60 KHz)
Loran (100 KHz)
AM broadcast.
Ground Waves

• Three main regions: D, E, and F layers(F1 and F2)
• Ionization increases with altitude and is greater during the
day.
• D and E layers diminish at night.
• Follows 11 year sunspot cycle.
• Signal returns by a form of refraction.
• D and E layers absorb low frequencies( 8-10Mhz) during the
day therefore low frequencies propagate better at night.
Ionosphere Propagation

• Signals in the VHF and higher range are not usually returned
to earth by the ionosphere
• Most terrestrial communication at these frequencies uses
direct radiation from the transmitter to the receiver
• This type of propagation is referred to as space-wave, line-of-
sight, or troposphere propagation
Line-of-Sight Propagation

• Space Waves: travel directly from an antenna to another without
reflection on the ground. Occurs when both antennas are within line of
sight of each another, distance is longer that line of sight because most
space waves bend near the ground and follow practically a curved path.
Antennas must display a very low angle of emission in order that all the
power is radiated in direction of the horizon instead of escaping in the sky.
A high gain and horizontally polarized antenna is thus highly
recommended.
• Sky Wave (Skip/ Hop/ Ionosphere Wave) is the propagation of radio waves
bent (refracted) back to the Earth's surface by the ionosphere. HF radio
communication (3 and 30 MHz) is a result of sky wave propagation.
Space Wave, Sky Wave

Thank you….

Amplitude modulation (Communication Electronics )

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