Amplitude Demodulation Simulation: Theory, Working & Results

Instructions for Amplitude Modulation (AM)

• Step 1: Click on 'Generate Message' button to generate input message signal
• Step 2: Then click on 'Generate Carrier' button to generate carrier signal. The carrier frequency has to be more than the message frequency and You can change frequencies using sliders
• Step 3: Click on 'Generate Amplitude Modulated Signal' button to generate Amplitude Modulated Signal
• Step 4: Click the 'Show Frequency Spectrums' button to view the AM spectra.
• Here, the modulation index is defined as the ratio of the message signal amplitude to the carrier signal amplitude. You can adjust both values.

50Hz

Step 1:

500Hz

Step 2:

Message Signal Amplitude

1

Carrier Signal Amplitude

1

Step 3:

Step 4:

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Amplitude Demodulation

Synchronous (coherent) detection recovers the original message by multiplying the received signal with a locally generated carrier synchronized with the transmitter.

Multiplier-Based Detection (Product Modulator)

The received signal \( s(t) \) is multiplied by the local carrier \( c(t) = \cos(2\pi f_c t) \):

\( v(t) = s(t)c(t) = A_c[1 + K_a m(t)]\cos^2(2\pi f_c t) \)

Using \( \cos^2\theta = \frac{1}{2}(1+\cos2\theta) \):

\( v(t) = \frac{A_c}{2}[1+K_a m(t)] + \frac{A_c}{2}[1+K_a m(t)]\cos(4\pi f_c t) \)

This produces a baseband component and a high-frequency component at \( 2f_c \).

Low-Pass Filtering (LPF) and Baseband Recovery

A low-pass filter (LPF) removes the high-frequency component and retains the baseband signal:

\( v_{LPF}(t) = \frac{A_c}{2} + \frac{A_cK_a}{2}m(t) \)

After removing the DC offset and applying signal conditioning, the original message signal \( m(t) \) is successfully recovered.

Message Frequency (Hz)

Carrier Frequency (Hz)

Cut-off Frequency (Hz)