Experiment-01

 1.1  Title: Design and Implementation of a DSB-SC Modulator and Demodulator

1.2  Aim of the experiment:

To design and implement a Double Sideband Suppressed Carrier (DSB-SC) modulator and demodulator and demonstrate its principle of operation.

1.3   Theoretical background for the experiment: DSB-SC (Double Sideband Suppressed Carrier) modulation is a form of amplitude modulation where the carrier signal is suppressed, leaving only the sidebands to carry information. This technique improves power efficiency compared to standard AM, as no power is wasted on the carrier. The transmitted signal can be expressed as:

where:

·         is the message signal (modulating signal).

·         is the angular frequency of the carrier.

·         Demodulation is performed using a coherent detector, where the received signal is multiplied by a synchronized carrier signal followed by low-pass filtering to recover the message signal.  

1.4   Design: Modulator:

·         Multiplier circuit or software implementation for the product .

·         Demodulator: A multiplier to mix the received signal with and a low-pass filter to extract.

1.5  Step by step procedure to carry out the experiment:

• • Generate a sinusoidal message signal using a function generator or software.

  • Generate a sinusoidal carrier signal of higher frequency than .

  •   Multiply and to produce the modulated signal .

  •  Observe the spectrum of the modulated signal to confirm suppression of the carrier.

For demodulation:

        Multiply with a synchronized carrier signal .

         Pass the resulting signal through a low-pass filter to recover .

•  Observe and compare the demodulated signal with the original message signal.

1.6  Code:

fc = 1000;   

fm = 100;    

Ac = 1;      

Am = 0.5;    

fs = 10000;  

t = 0:1/fs:0.05; 

carrier = Ac * cos(2 * pi * fc * t);     

message = Am * sin(2 * pi * fm * t);     

modulated_signal = message .* carrier;

demodulated_signal=modulated_signal.*carrier;

[b, a] = butter(5, 2 * fm / fs);               % 5th order Butterworth low-pass filter

recovered_message = filter(b, a, demodulated_signal);

figure;

subplot(6,1,1);

plot(t, message, 'r');

title('Message Signal');

xlabel('Time (s)');

ylabel('Amplitude');

subplot(6,1,2);

plot(t, carrier, 'b');

title('Carrier Signal');

xlabel('Time (s)');

ylabel('Amplitude');

subplot(6,1,3);

plot(t, modulated_signal, 'g');

title('DSB-SC Modulated Signal');

xlabel('Time (s)');

ylabel('Amplitude');

subplot(6,1,4);

plot(t, demodulated_signal, 'b');

title('DSB-SC Modulated Signal');

xlabel('Time (s)');

ylabel('Amplitude');

subplot(6,1,5);

plot(t, recovered_message, 'r');

title('DSB-SC Modulated Signal');

xlabel('Time (s)');

ylabel('Amplitude');

subplot(6, 1, 6);

plot(t, demodulated_signal, 'b', 'DisplayName', 'Demodulated Signal');

hold on;

plot(t, recovered message, 'r', 'DisplayName', 'Recovered Message');

title ('Demodulated and Recovered Signals');

xlabel('Time (s)');

ylabel('Amplitude');

legend;

hold off;

1.7   Observations and results:

·         The modulated signal contains the upper and lower sidebands without the carrier component.

·         The demodulated signal closely matches the original message signal after low-pass filtering.

1.8  Plotting of the graph: