What is Modulation?
The process of varying any of the three
characteristics as the Amplitude, Frequency or the Phase of a carrier signal is
called as modulation.
We know
that the information signal to be transmitted can be of any form such as data,
music, video etc. But before transmission, it is converted into its equivalent
electrical form. The electrical equivalent form of the original signal is
called the baseband signal.
Every
electrical signal possesses basic characteristics such as Amplitude, Frequency,
and Phase. We need to change the characteristics of the signal
to make it more appropriate for the transmission.
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Need of
Modulation
We need modulation
because of the following reasons:
·
It reduces the height of the antenna used for
the transmission.
·
It increases the range of communication.
·
Using Modulation avoids the mixing of the
signal.
·
Modulation makes multiplexing of the signal
possible.
Block
Diagram of Modulation
This block
diagram shows a modulator with two inputs i.e. modulating signal and carrier
signal. And at the output, we get the modulated signal.
The
modulating signal is nothing but the information signal. This modulating signal
is primarily of two forms such as analog signal and digital signal. The design
of the modulator depends on the input signal’s
The
carrier signal is the signal upon which the modulating signal is modulated.
This carrier signal is a sinosoidal signal of a fixed frequency known as the
carrier frequency, which is very comparatively very higher than the modulating
signal.
The
modulated signal is the resultant output signal of the modulator.
Types of Modulation
Modulation is divided
into two types;
·
Analog Modulation
·
Digital modulation
Analog modulation is
further divided into three types;
·
Amplitude modulation
·
Frequency modulation
·
Phase modulation
Whereas, Digital modulation is further divided into three types;
·
Pulse Amplitude modulation (PAM)
·
Pulse width modulation (PWM)
·
Pulse code modulation (PCM)
In this article, we will
cover Analog modulation and its types.
Analog
modulation deals with an analog signal. The types of analog modulation are
briefly discussed below.
Amplitude
Modulation
Amplitude
modulation is a
type of analog modulation in which the amplitude of the
high-frequency carrier signal is changing with respect to the instantaneous
amplitude of the modulating signal.
In amplitude
modulation, the amplitude of the carrier signal changes
instantaneously with respect to the amplitude of the modulating signal.
However, the frequency and the phase of the carrier signal remains constant.
Thus the information is contained in the amplitude of the carrier signal.
Mathematical
Representation
Suppose the modulating
signal is a sinusoidal signal, it can be represented as:
m(t)
= Am cos
ωmt
ωc = 2πfc
mam(t) = Ac
(1 + (Am/Ac) cos ωmt) cos ωc t
Frequency
Spectrum
The
representation of frequency content of a signal using a graph is called
spectrum.
The
mathematical equation of AM modulated wave;
mam(t) = (Ac +
Am cos ωmt) cos ωc t
mam(t) = Ac cos
ωc t + Am cos ωmt cos ωc t
Using trigonometric identity 2 cos A
cos B = cos (A+B) + cos (A-B)
Bandwidth
= (fc – fm) – (fc + fm)
Bandwidth
= 2fm
AM
Transmitter
The basic
components of AM transmitters are oscillator and power
amplifier.
Oscillator
The oscillator is
a circuit which generates sinusoidal waveforms of different frequencies. The
crystal oscillator is generally used in AM transmitter for generating a carrier
signal of high frequency.
Power
Amplifier
Power
amplifiers are used for the amplification of the modulated signal before
feeding it to the antenna for transmission. The modulated signal has low power
and it cannot be transmitted without amplification.
Bipolar
junction transistors are used as Power amplifiers in AM transmitter.
There are
3 classes of Power Amplifiers.
Class
A Power Amplifier
The class
A amplifier operates for the whole cycles of the signal. These amplifiers
amplify the whole signal thus wasting too much power. Class A amplifier has low
efficiency.
Class
B Power Amplifier
Class B
amplifiers operate only for half of the input signal thus the efficiency of the
Class B amplifiers ranges up to 80%.
Class
C Power Amplifier
While
Class C amplifiers only operate for only 25% (positive high portion) of the
signal. Thus the efficiency of class C amplifiers is 90%.
There are
2 types of AM transmitters
Low-Level
AM Transmitter
In the
low-level transmitter, a crystal oscillator is used for generating the carrier
signal. The carrier and modulating signal is amplified using class A amplifier
before modulation. After modulation, the modulated signal is amplified again
before transmission.
The
low-level transmitter does not need high-efficiency Amplifiers thus its design
is much simpler.
High-Level
AM Transmitter
The
high-level transmitter has the same design as low-level transmitter except it
uses Class C amplifier at modulation stage. Because of class C amplifier, this
transmitter has high efficiency and complex design.
In the
High-level transmitter, the carrier signal and modulating signal is amplified
using linear amplifiers. At the modulation block, both signals are amplified
with modulation and then fed to the antenna for transmission. Block diagram of
the High-level transmitter is as follows.
Advantages
of AM
·
AM transmitters have a simple design.
·
AM receivers are also simple. Envelope
detectors are the simplest receivers.
·
Due to high power, AM signals have a long
range of transmission.
·
The AM signal has low Bandwidth.
Disadvantages
of AM
·
Because the information is stored in the
amplitude of the modulated signal, which is affected by the noise in the
medium.
·
AM need high power for its transmission.
Applications
of AM
·
Because of its long range, it can be used for
Radio broadcasting.
·
It can be used for Television broadcasting.
·
Frequency Modulation (FM)
·
In frequency modulation, the
frequency of the carrier signal varies with respect to the instantaneous
amplitude of the modulating (message) signal.
·
The amplitude and phase of the carrier signal
remain unchanged. Only frequency of the carrier signal changes. Thus the
information is stored in the frequency of the FM modulated signal.
·
The frequency of the carrier signal increases
with an increase in the amplitude of the modulating signal and it decreases
with the decrease in the amplitude of the modulating signal.
·
Frequency Deviation
·
The difference between the original frequency
of the carrier signal and modulated frequency is called frequency deviation.
·
It is directly proportional to the amplitude
of the modulating signal.
·
Consider the FM modulating signal to be a
sinusoidal signal.
·
m(t) = Am cos(2πfmt)
·
The carrier signal is represented as:
·
c(t) = Ac cos (2πfct)
·
the instantaneous frequency of the modulated
signal is:
·
fi(t) = fc + Kf m(t)
Kf = constant
·
fi(t) = fc + Kf Am cos(2πfmt)
·
fi(t) = fc + δf cos(2πfmt)
·
Where:
·
δf = Kf Am = maximum frequency
deviation
·
The minimum and maximum frequency of the FM
modulated signal is:
·
Fmin = fc – δf
·
Fmax = fc + δf
And the
bandwidth of the FM modulated signal is:
B.W = 2 fc x
number of side-bands
B.W = 2 (fc +
δ) Carson’s Rule
FM
Transmitter
In FM
transmitter the modulating signal is mixed directly with the high frequency
carrier signal.
A general block diagram of FM transmitter is given below:
A general block diagram of FM transmitter is given below:
Advantages
·
FM modulated signal is immune to noise as
noise only affects the amplitude of the signal. And the information is stored
in the frequency of the signal.
·
FM signal consumes less power as compared to
the AM signal.
·
The transmitted power remains constant as the
amplitude of the signal remains constant.
·
Due to high frequency, the antenna of FM
receiver is very small.
Disadvantages
·
FM signal covers large bandwidth as compared
to the AM signal.
·
The design of FM transmitters and receivers
are very complex.
Applications:
Applications:
·
FM can be used for radio broadcasting.
·
Phase Modulation
·
The process in which the phase
of the carrier signal varies with the instantaneous amplitude of the modulating
(message) signal is called phase modulation.
·
·
Consider the message signal is
a sinusoidal signal.
·
·
·
c(t) = Ac cos(ωct
+ ϴ)
·
ϴ is the phase of the
signal and normally it is zero.
·
The Phase modulated signal is given below:
·
ϕpm(t) = Ac cos(ωct
+ ϴ + Kp m(t))
·
ϕpm(t) = Ac cos(ωct
+ ϴ + Kp Am cos(ωmt))
·
Kp = constant of
proportionality for phase modulation.
·
The instantaneous frequency of PM signal is;
·
ωi = d/dt (ωct + ϴ
+ Kpm(t))
·
ωi = {ωc+ Kp d/dt(m(t)}
·
The equation of the instantaneous frequency
shows that in PM the derivative of modulating signal is added with the
frequency of the carrier signal.
·
Thus it proves that if we take
the derivative of the modulating signal before feeding it to Frequency
modulator we get Phase modulation.
·
·
Phase deviation is the maximum
difference between the original phase of the carrier signal and the modulated
signal. The equation of phase modulated signal is;
·
·
·
·
·
·
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Source: electricaltechnology
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