Analog Communication Systems
Amplitude Modulation
By
Dr. Eng. Omar Abdel-Gaber M. Aly
omar.aly@aun.edu.eg
Assistant Professor
Electrical Engineering Department
College of Engineering Al-Majmaa
Al-Majmaa University
Outline
 Overview
of Modulation
 What is modulation?
 Why modulation?
 Classifications of modulations
 Definitions of Bandwidth
 Linear Modulation:
 Amplitude modulation
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Overview of Modulation

What is modulation?

The process of varying a carrier signal in order to
use that signal to convey information.
 Why

modulation?
1. Efficient transmission of signals using antennas
of practical size:
 The
optimal antenna size is related to wavelength:
 Voice signal: 3 kHz
 Wavelength: λ = c / f = 3 x 108 /(3000) = 300 Km
 If modulated by a 100 MHz carrier:
 Wavelength λ = 3×108 / (108) = 3m
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Overview of Modulation
 Why
modulation?

2. Utilizing the channel for transmission
of more than one signal (multiplexing)

3. Modulation allows us to get better
trade-off between bandwidth and
signal-to-noise ratio (By choosing the
suitable modulation technique)
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Overview: Types of Modulation
 Analog
modulation
The input message is continuous in time and value
 Continuous-wave modulation (focus of this
course)

A
parameter of a high-freq carrier is varied in
accordance with the message signal
 If a sinusoidal carrier is used, the modulated carrier is:
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Overview: Types of Modulation
 Linear
modulation: A(t) is linearly related to
the message.

AM, DSB, SSB
 Angle
modulation:
Phase modulation: Φ(t) is linearly related to the
message.
 Freq. modulation: dΦ(t)/dt is linearly related to the
message.

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Overview: Types of Modulation
Linear modulation
(Amplitude modulation)
Message
Carrier
Angle modulation:
Phase modulation
Freq modulation
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Overview: Types of Modulation
 Analog
pulse modulation
Message value is continuous. Transmission
happens at discrete times.
 Transmitted signal is a sequence of pulses
 The amplitude, width, or position of the pulse can
be varied over a continuous range according to the
message value at the sampling instant.
 PAM: Pulse amplitude modulation
 PWM: Pulse width modulation
 PPM: Pulse position modulation

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Overview: Types of Modulation
Message
PAM: Pulse amplitude modulation
PWM: Pulse width modulation
PPM: Pulse position modulation
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Problems to be studied
 For
each modulation scheme, we will
study the following topics:
 How
does the modulator work?
 How does the demodulator work?
 What is the required bandwidth?
 What is the power efficiency?
 What is the performance in the presence of
noise?
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Definitions of Bandwidth
A
measure of the extent of significant spectral
content of the signal for positive frequencies.
 Band-limited signal:
Bandwidth is (BW) = W.
 When
BW = 2W.
the signal is not band-limited:
Different
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definitions
exist
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Definitions of Bandwidth
 Null-to-null


bandwidth:
Null: A frequency at which the spectrum is zero.
3dB bandwidth:
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Definitions of Bandwidth
 Radio
spectrum is a scarce and expensive
resource:


US license fee: ~ $77 billions / year
Communications systems should provide
the desired quality of service with the
minimum bandwidth.
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AM and FM Radio

AM radio ranges from 535 to 1605 kHz



The bandwidth of each station is 10 kHz.
AM signals can travel quite long distance due to ionospheric refraction
The FM radio band goes from 88 to 108 MHz



FM stations are 200 kHz apart
FM has much better quality than AM
Ionospheric refraction doesn't affect FM or TV signals too much (lineof-sight propagation, need tall antenna)
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http://www.cybercollege.com/frtv/frtv017.htm
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Amplitude Modulation: Double
Sideband (DSB)

Amplitude modulation is characterized by
the fact that the amplitude A of the carrier
is varied in proportional to the message
signal m(t)
m(t)
m(t)cos(wct)
(Modulating signal)
(Modulated signal)
cos(wct) (Carrier)
m(t )  M (w )
1
m(t ) cos(wc t )  M (w  wc )  M (w  wc )
2
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Double Sideband (DSB-SC)
m(t)
M(w)
BW = B
m(t)cos(wct)
1
M (w  wc )  M (w  wc )
2
wc ≥ 2pB
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BW = 2B
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(DSB-SC) Demodulation
The demodulation consists of multiplication of the
incoming modulated signal m(t)cos(wct) by a carrier
cos(wct) followed by a low pass filter
m(t)cos(wct)
e(t)
0.5m(t)
Low Pass Filter

cos(wct)
1
e(t )  m(t ) cos (wc t )  m(t )  m(t ) cos( 2wc t )
2
1
1
E (w )  M (w )  M (w  2wc )  M (w  2wc )
2
4
2
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(DSB-SC) Demodulation
E(w)
LPF
 This
method of demodulation is called
synchronous detection or coherent detection,
we use a carrier of exactly the same frequency
and phase as the carrier used for modulation
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(DSB-SC) Demodulation

In the time domain:
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Modulator Implementations
 Modulation

can be achieved in several ways
Multiplier Modulators:
 Modulation
achieved by using an analog multiplier.
 Nonlinear Modulators:
 Modulation
can be achieved by using nonlinear device
NL

Switching Modulators:
 Modulation
can be achieved by using a simple switching
operation.
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Nonlinear Modulators:

Let the input-output characteristics of the
NL elements be approximated as:
y(t )  ax(t )  bx (t )
2
z (t )  2am(t )  4bm(t ) cos(wct )
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Switching Modulators:
A modulated signal can be obtained by multiplying
m(t) by any periodic signal f(t) of the fundamental
radian frequency wc.
 Using Fourier series f(t) can be expressed as


f (t )   Cn cos( nwc t )
n 0

m(t )f (t )   Cn m(t ) cos( nwc t )
n 0

If the signal is passed through a BPF of BW=2B and
centered at wc we can get the modulated signal
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Switching Modulators:

Consider the square pulse train w(t)
whose Fourier series is
1 2
1
1
w(t )   (cos wc t  cos 3wct  cos 5wc t  ...)
2 p
3
5
 The signal m(t)w(t) is given by:
1
2
1
m(t ) w(t )  m(t )  (m(t ) cos wc t  m(t ) cos 3wc t  ...)
2
p
3

Multiplication of a signal by a square pulse
train is a switching operation
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Diode-bridge modulator
+
-
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-
+
27
Ring modulator
wo (t ) 
4
1
1
(cos wc t  cos 3wc t  cos 5wc t  ...)
p
3
5
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