1. Amplitude
1
Modulation
By: Durgesh Suthar
En. No: 11115027

2. What is Modulation
2

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

3. About Modulation

3
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.

4. Modulation Theory

4
A sine wave is represented as follows

c(t)= Ac cos(2πfct +φ(t)
 Here Ac, fc and φ(t) 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
Φ(t) -> Phase Modulation

5. Basic Amplitude
Modulation

Amplitude
Modulation is the
simplest and earliest
form of transmitters

The information signal
varies the
instantaneous
amplitude of the
carrier

6. Benefits of Modulation

6
Modulation can shift the spectral content of a message signal
into a band which is better suited to the channel

Antennas only efficiently radiate and admit signals whose
wavelength is similar to their physical aperture.

Hence, to transmit and receive, say, voice, by radio we need to shift
the voice signal to a much higher frequency band.

7. 7

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.

8. AMPLITUDE MODULATION (AM)8

In amplitude modulation, the message signal m(t) is impressed
on the amplitude of the carrier signal c(t) = Accos(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

9. 9

10. 10

11. Full AM modulation ( DSB-LC) 11
1 The carrier signal is
sc (t )  Ac cos( c t ) where  c  2f c
2
In the same way, a modulating signal (information
signal) can also be expressed as
sm (t )  Am cos  m t

12. 13
3 The amplitude-modulated wave can be expressed as
s(t )  Ac  sm (t )cos(c t )
4 By substitution
s (t )  Ac  Am cos( mt )cos( c t )
5 The modulation index.
Am
m 
Ac

13. 13
6
Therefore The full AM signal may be
written as
s(t )  Ac (1  m cos( mt )) cos( c t )
cos A cos B  1 / 2[cos(A  B)  cos(A  B)]
mAc
mAc
s(t )  Ac (cos ct ) 
cos( c   m )t 
cos( c   m )t
2
2

14. Double-Sideband Suppressed-Carrier AM

14
A double-sideband, suppressed-carrier (DSB-SC) AM signal is
obtained by multiplying the message signal m(t) with the carrier
signal c(t) = Accos(2fct)

Amplitude-modulated signal
u (t )  m(t )c(t )  Ac m(t ) cos(2 f c t )

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

At the same time, the modulated signal retains the main characteristics
of the message signal; therefore, it can be used to retrieve the message
signal at the receiver

15. Double-Sideband Suppressed-Carrier
15
AM

Figure : An example of message, carrier, and DSB-SC modulated
signals

16. 16
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

17. Single-Sideband AM

A method, illustrated in
17
.
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
Figure : Generation of a singlesideband AM signal by filtering one of
the sidebands of a DSB-SC AM signal.

18. Sideband and carrier power

18
Carrier term does not carry information, and hence the carrier
power is wasted
 AM (t )  A cos ct  m(t ) cos ct  carrier  sidebands
Pc is the mean sq. value of
A cos c t which is A2 / 2
 The sideband power P is the mean sq. value
s
2
of m(t ) cos  c t which is m (t ) / 2

The carrier power

19. Advantages/disadvantages
20
Advantages of Amplitude Modulation, AM
There are several advantages of amplitude modulation, and some of these
reasons have meant that it is still in widespread use today:

It is simple to implement

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
Amplitude modulation is a very basic form of modulation, and although its
simplicity is one of its major advantages, other more sophisticated systems
provide a number of advantages. Accordingly it is worth looking at some of
the 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.