AMPLITUDE MODULATION CIRCUIT

SOFTWARE: THE MATHWORKS™ SIMULINK®

Circuit Diagram:

A.M. circuit

Components:

Signal Generators, Constant, Adder, Multiplier, Oscilloscopes.

Working:

First of all we take the signal generator and add a constant 2 to its output sinusoidal signal using an ADDER. The constant here acts as a Bias to distinguish between to important types of amplitude modulation called DSB-SC and DSB-with large carrier. Then we will multiply the output of the adder circuit with the carrier. The output of the multiplier will be our final Amplitude Modulated Wave ready to be transmitted. As shown in the figure we have placed 3 Scopes at three different points. There output are shown the following figures using Simulink®.

Outputs:

Output of Scope2 i.e. actual Message

Output of Scope1 i.e. Carrier

Output of the multiplier Amplitude Modulated Wave

The output we have just seen is the type of A.M. that contains a double side band with Large Carrier because a constant is used here.

Now if we remove the constant then the circuit will become

Then the output of the multiplier will be like this:

Amplitude Modulated Wave with Suppressed Carrier (DSB-SC).

Simulation in MATLAB™

Source Code:

t=0:0.001:1;

vd=8*cos(2*pi*5*t);

vc=0.1*cos(2*pi*15*t);

ft=vc.*vd;

am=ft+vc;

figure(1)

plot(t,vd);

figure(2)

plot(t,vc);

figure(3)

plot(t,am);

Outputs:

Message Signal.

Carrier Signal.

Amplitude Modulated Signal.

Notes:

To recover the original message found in DSB-SC AM will require some sophisticated circuitry like phased-locked loops PLL). PLL is used to implement the concept of coherent detection since it requires precise knowledge of how the transmitted carrier behaves.

On the other hand, to recover DSB-LC AM, only a simple envelope detector is needed. Envelope detection requires a simple diode, a resistor and capacitor. It does not require the phase information of the transmitted carrier. Envelope detection is also known as non-coherent detection.