Usman Institute of Technology
Department of Electrical Engineering
COMMUNICATION SYSTEMS (EL-321)
Fall- 2016
Engr. Waseem Zeeshan
Engr. Tahzeeb Jamal
Engr. Muhammad Umair
EXPERIMENT # 7: Observe the Characteristics of Amplitude Modulation
Name of Student: __________________________________________________________
Roll No.: ______________________________Group& Section: ____________________
Date of Experiment
: ________________________________________________
Report Submitted on
: ________________________________________________
Marks Obtained
:
Remarks if any
: _________________________________________________
Signature
: _________________________________________________
Amplitude Modulation:
An Amplitude Modulated signal is composed of both low frequency and high frequency
components. The amplitude of the high frequency (carrier) of the signal is controlled by the low
frequency (modulating) signal. The envelope of the signal is created by the low frequency signal.
If the modulating signal is sinusoidal, then the envelope of the modulated Radio Frequency (RF)
signal will also be sinusoidal. This would be the case in a common AM radio. The low frequency
signal would be an audio signal and the high frequency would be the transmitting frequency of the
AM radio station. Shown in Fig. 1 is an example of an AM signal in the time domain.
Fig. 1 Amplitude Modulated Signal
The mathematical representation for this waveform is as follows:
f(t)= A[1+m cos (2fmt)] cos (2fct)
Where,
A=Amplitude of carrier
m = modulation index
fm = modulating frequency
fc = carrier frequency
Figure 2 shows the physical interpretation of the mathematical equation given above. In this
diagram, the quantities A and mp are indicated.
The circuit for generating an AM modulated waveform must produce the product of the carrier
and the modulating signal. After receiving an AM signal, it can be demodulated to recover the low
frequency signal. One of the simplest types of AM demodulating circuits is the envelope detector.
Fig. 2 AM Modulated Signal Showing Values
Modulation Index:
It is the ratio of the amplitudes of modulating signal and carrier. A complementary figure to
modulation index is also used for amplitude modulation signals known as the modulation depth,
it is typically the modulation index expressed as a percentage
Where,
M=Amplitude of message signal
A=Amplitude of carrier
Typically the modulation index of a signal will vary as the modulating signal intensity varies.
However some static values (given below) will enable the various levels to visualize more easily.
Fig. 3a Under-modulation
Fig. 3b Exact-modulation
Fig. 3c Over-modulation



Under-modulation is achieved when the modulation index, m is 0<m<1
Exact-modulation is achieved when the modulation index, m=1
Over-modulation is achieved when the modulation, m>1
The amplitude modulation index is key parameter for any AM transmission as it is necessary to
keep the index or depth within limits to reduce distortion and interference
Objective: 1
To observe the AM wave with respect to the variation in modulation index on MATLAB
MATLAB is used as a tool to observe the change in shape of AM modulated signal by varying
modulation index
Type in the following code in an m-file
MATLAB Code
clear all
close all
clc
fm=1000;
fc=10*1000;
Ac=2;
T=1/fm;
t=0:T/1000:6*T;
%Frequency of message/modulating signal
%Frequency of carrier signal
%Amplitude of carrier signal
%Time period of message signal
m=input('Enter the value of modulation index: ');
Am=Ac*m;
%Amplitude of message signal
Vm=Am*cos(2*pi*fm*t);
Vc=Ac*cos(2*pi*fc*t);
Mod=Ac*(1 + m*cos(2*pi*fm*t)).*cos(2*pi*fc*t);
figure,
subplot(311)
%Message signal
%Carrier signal
%Modulated signal
plot(t,Vm)
grid on
xlabel('Time'); ylabel('Amplitude');
title('Modulating Signal')
subplot(312)
plot(t,Vc)
grid on
xlabel('Time'); ylabel('Amplitude');
title('Carrier Signal')
subplot(313)
plot(t,Mod)
grid on
xlabel('Time'); ylabel('Amplitude');
title('Modulated Signal')
Procedure:
1. Open MATLAB editor and type in the code
2. Run the m-file and enter the modulation index values at 0.5, 1 and 1.5. Sketch the
modulated waveforms below and write your comments
3. Observe and write what will happen if the modulation index exceeds 1
Objective: 2
To observe the AM wave by varying modulating and carrier signal voltage levels
Procedure:
1. Provide power supply to the trainer containing AM block.
2. Apply the modulating and carrier voltages with frequencies 500 Hz and 5 KHz at J5
and J4 respectively and observe the waveform at J6
3. Fill the following observation column:
S.No.
Modulating
Signal
Voltage (Vp-p)
Carrier Signal
Voltage
(Vp-p)
1
6
2
2
6
4
3
6
6
4
6
8
5
6
12
Modulation
Index
Waveform
Result:
……………………………………………………………………………………
……………………………………………………………………………………
……………………………………………………………………………………
…………………………………………………………………………………