ECE 110: Communications 1 – Principles of Communication Systems
Laboratory Report 1
Amplitude Modulation Using SCILAB
Software
March 5, 2025
2nd Sem AY 2024-2025
Submitted by:
Allen Chreed Ebasco
Submitted to:
Engr. Lovely Mae D. Balamad
1.0 OBJECTIVES
To analyze Amplitude Modulation (AM) signals with different
modulation indices and other types of AM signals.
2.0 EQUIPMENT / APPARATUS
SCILAB Software
3.0 THEORY
Amplitude Modulation (AM) is a technique used in electronic
communication, most commonly for transmitting information via a radio
carrier wave. AM works by varying the amplitude of the carrier signal in
proportion to the amplitude of the message signal (modulating signal).
The equation for an AM wave is:
s ( t )=( A c + A m cos ( 2 π f m t ) )cos( 2 π f c t ¿ )¿where:
Ac = Amplitude of the carrier signal
f c = Carrier frequency
Am = Amplitude of the modulating signal
f m = Modulating frequency
The modulation index (m) is given by:
Am
m=
Ac
which determines how much the carrier amplitude varies. If m>1, the
signal is over-modulated, leading to distortion. If m=1 , it is 100%
modulated, and if m<1, it is under-modulated.
4.0 LABORATORY PROCEDURE
1. Open SCILAB and create a new script.
2. Define the parameters:
Carrier frequency = 5 kHz
Amplitude of carrier = 9 V
Modulating frequency = 500 Hz
Amplitude of modulating signal = 4.5 V
Sampling time = 100 ms
3. Use the SCILAB code provided below to generate and plot the AM
signal.
4. Run the script and observe the generated graphs for the carrier
signal, modulating signal, and amplitude-modulated signal.
5.0 SCILAB CODE
// Define parameters
fc = 5000; // Carrier frequency in Hz
Ac = 9; // Amplitude of carrier signal in V
fm = 500; // Modulating frequency in Hz
Am = 4.5; // Amplitude of modulating signal in V
fs = 10*fc; // Sampling frequency (10 times carrier frequency)
t = 0:1/fs:0.1; // Time vector for 100 ms
// Generate signals
carrier = Ac * cos(2 * %pi * fc * t);
modulating = Am * cos(2 * %pi * fm * t);
modulated = (Ac + modulating) .* cos(2 * %pi * fc * t);
// Plot Carrier Signal
subplot(3,1,1);
plot(t, carrier);
title("Carrier Signal");
xlabel("Time (s)");
ylabel("Amplitude (V)");
grid();
// Plot Modulating Signal
subplot(3,1,2);
plot(t, modulating);
title("Modulating Signal");
xlabel("Time (s)");
ylabel("Amplitude (V)");
grid();
// Plot AM Signal
subplot(3,1,3);
plot(t, modulated);
title("Amplitude Modulated Signal");
xlabel("Time (s)");
ylabel("Amplitude (V)");
grid();
6.0 RESULTS AND DISCUSSION
The results of the experiment are presented in three graphical
representations. The first graph (Figure 1) illustrates the carrier signal,
which is a high-frequency cosine wave with a constant amplitude. The
second graph (Figure 2) represents the modulating signal, which is a
lower-frequency cosine wave that dictates the variations in the carrier
wave. Finally, the third graph (Figure 3) shows the amplitude-modulated
(AM) signal, where the carrier wave’s amplitude is modulated according to
the amplitude variations of the modulating signal.
From the analysis of the generated AM waveform, it can be
observed that the amplitude of the carrier varies in direct proportion to
the modulating signal. The modulation index is calculated as: which
indicates a 50% modulation depth. This means that the carrier amplitude
fluctuates between 4.5V and 13.5V, following the variations in the
modulating wave. The expected AM characteristics are clearly observed,
demonstrating the fundamental principles of amplitude modulation.
Figure 1. Carrier Signal
Figure 2. Modulating Signal
Figure 3. Amplitude Modulated Signal
7.0 CONCLUSION
This experiment demonstrated the generation of an Amplitude
Modulated (AM) signal using SCILAB. The results confirm that AM varies
the carrier amplitude according to the modulating signal. The modulation
index was calculated, and its effect on the waveform was observed.
Future work can include testing different modulation indices and analyzing
frequency spectra.
