Analog Communication - Practical 3

Aim

To study the DSB-SC modulation using a balance modulator.

Theory

Double Sideband Suppressed Carrier (DSB-SC) modulation is a technique where the carrier is suppressed, and only the sidebands (containing the information) are transmitted. This improves power efficiency compared to conventional AM.

A balanced modulator is an electronic device that generates the product of two signals, commonly used to produce a DSB-SC signal. It eliminates the carrier component by maintaining symmetry in the circuit, ensuring that the output contains only the product terms.

Mathematically, for a carrier signal c(t) = Ac cos(2πf_ct) and a message signal m(t), the modulated signal is given by:

v(t) = Ac m(t) cos(2πf_ct)

The spectrum of a DSB-SC signal contains two sidebands, each symmetric about the carrier frequency, without a carrier component.

MATLAB Code

% MATLAB Code for DSB-SC Modulation
t =0:0.000001:.001;
Vm= 1;
Vc= 1;
fm = 2000;
fc= 50000;
m_t = Vm*sin(2*pi*fm*t);
subplot(4,1,1);
plot(t,m_t);
c_t = Vc*sin(2*pi*fc*t);
subplot(4,1,2);
plot(t,c_t);
subplot(4,1,3);
s_t = m_t.*c_t;
hold on;
plot(t,s_t);
plot(t,m_t,'r:');
plot(t,-m_t,'r:');
hold off;
r = s_t.*c_t;
[b a] = butter(1,0.01);
mr= filter(b,a,r);
subplot(4,1,4);
plot(t,mr)

Expected Output

The MATLAB code generates three plots demonstrating the DSB-SC modulation process:

Message Signal: A sinusoidal wave representing the information signal.
Carrier Signal: A high-frequency sinusoidal wave.
DSB-SC Modulated Signal: The product of the message and carrier signals, containing two sidebands without a carrier component.

The modulated signal clearly shows how the amplitude varies in accordance with the message signal, while the carrier frequency remains the same.