1) Modulation involves changing characteristics of a high-frequency carrier signal according to an information signal. This allows signal transmission over long distances and multiple signals over the same channel. 2) The main modulation types are amplitude modulation (AM), which changes amplitude; frequency modulation (FM), which changes frequency; and phase modulation (PM), which changes phase. 3) AM is the simplest form and varies the carrier amplitude by the information signal. It has advantages of simplicity but is inefficient in power and bandwidth usage, and susceptible to noise.

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.