Objective:
- To study and learn the amplitude modulation.
- To learn the MATLAB coding for amplitude modulation.
- To observe the amplitude modulation wave form.
Theory:
The amplitude
modulation is a process in which the amplitude of carrier c(t) is varied about a mean value with the massage signal m(t). Amplitude modulation (AM) is the family of modulation schemes in
which the amplitude of a sinusoidal carrier is changed as a function of the
modulating message signal.
Let, the massage signal
is, m(t)= Amcos (2 πfmt)
And the carrier signal
is, c(t)= Accos(2πfct)
So the modulated signal
is,
s(t)= Ac
cos(2πfct)+ Am cos(2πfmt)
= Ac (1+ka
m(t))sin(2πfct)
Here Ac and Am is the amplitude of carrier and message
signal fc and fm is the frequency of carrier and message signal
and ka is a constant called the amplitude sensitivity of the
modulated signal.
If
k a m ( t ) <1
and f c >> ω m
then
modulation will be perfect. In amplitude modulation only the amplitude of the
carrier wave is changed in accordance the signal. However the frequency of the
modulated wave remains the same.
Require instrument:
1. Mat lab software
2. Computer
MATLAB code:
Fc= input('Carrier signal Frequency=');
Fm=input('Message Signal Frequency=');
Ac=input('Carrier signal Amplitude=');
Am=input('Message Signal Amplitude=');
t=0:0.1:100;
A=2*pi*Fc*t;
Ct=Ac*cos(A);
subplot(3,1,1);
plot(t,Ct,'r')
xlabel('Time');
ylabel('Amplitude');
title('Carrier Signal');
grid on;
B=2*pi*Fm*t;
Mt=Am*cos(B);
subplot(3,1,2);
plot(t,Mt,'g')
xlabel('Time');
ylabel('Amplitude');
title('Message Signal');
grid on;
M=Fm/Fc;
Mod=M*Mt.*Ct;
AM=Ct+Mod;
subplot(3,1,3);
plot(t,AM,'b')
xlabel('Time');
ylabel('Amplitude');
title('Amplitude Modulated Signal');
grid on;
Figure 01: The Carrier Signal,
Message Signal and Amplitude Modulated Signal.
Discussion:
1. The
carrier frequency f c
is much greater than the message frequency
f m
.
2. The
amplitude of the carrier and message signal should take the same value.
3. It
can send massage without interference.
4. The
condition k a m ( t ) <1
should be satisfied.
Conclusion:
Amplitude modulated
signals are generated by multiplying message signals with a carrier and adding
the carrier to this product. The amplitude of the message signal is varied to
produce different values of the modulation index. The MATLAB environment can be
an effective tool for viewing the effects of various forms of modulation.