What is 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
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.
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
Modulation is the
simplest and earliest
form of transmitters
The information signal
amplitude of the
Benefits of Modulation
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.
Modulation permits the use of
Multiplexing means allowing simultaneous
communication by multiple users on the same
For instance, the radio frequency spectrum must be
shared and modulation allows users to separate
themselves into bands.
AMPLITUDE MODULATION (AM)8
In amplitude modulation, the message signal m(t) is impressed
on the amplitude of the carrier signal c(t) = Accos(2fct)
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
Mainly these methods are used for AM:
Double Sideband with Large carrier AM (DSB-LC AM)
Double sideband, suppressed-carrier AM (DSB-SC AM)
Single-sideband AM (SSB AM)
Vestigial Sideband (VSB) modulation
Full AM modulation ( DSB-LC) 11
1 The carrier signal is
sc (t ) Ac cos( c t ) where c 2f c
In the same way, a modulating signal (information
signal) can also be expressed as
sm (t ) Am cos m t
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.
Therefore The full AM signal may be
s(t ) Ac (1 m cos( mt )) cos( c t )
cos A cos B 1 / 2[cos(A B) cos(A B)]
s(t ) Ac (cos ct )
cos( c m )t
cos( c 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) = Accos(2fct)
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
Figure : An example of message, carrier, and DSB-SC modulated
two sidebands of an AM signal are
mirror images of one another
As a result, one of the sidebands is
Using single-sideband suppressed-carrier
transmission results in reduced bandwidth
and therefore twice as many signals may be
transmitted in the same spectrum allotment
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
Figure : Generation of a singlesideband AM signal by filtering one of
the sidebands of a DSB-SC AM signal.
Sideband and carrier power
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
of m(t ) cos c t which is m (t ) / 2
The carrier power
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.