AM Modulation & Demodulation

Modulation Definition: Modulation: The process by which some characteristics of a carrier are varied in accordance with the modulating signal (message signal). The purpose: - To transmit messages over long distances. - To transmit signals from various sources simultaneously over a common channel.

AM Modulation Definition: Amplitude Modulation: The process in which the amplitude of the carrier varies linearly with message signal.

Generation of AM signal - Analog Multiplier is considered one of the basic methods that is used in amplitude modulation. - The analog multiplier is a component with 2 inputs and one output as shown below:

The general formula of the AM modulated signal S(T):

AM Demodulation Two types of AM demodulators are explained: 1- Diode detector (asynchronous detector). 2- Product detector (synchronous detector).

Diode Detector After the diode: After the low pass filter ( proper choice of R and C):

In order to retrieve the original message using the diode detector, the following three conditions must be satisfied: 1. fc<<10 fm 2. µ=[0,1] 3. Tc>>RC>>Tm; where Tc=1/fc, Tm=1/fm, and RC is the time constant of the RC low pass filter.

Product detector The General Block diagram of the product detector is as shown below:

Advantages of AM Modulation - There is symmetry in the upper and the lower side of the modulated signal. - It can be demodulated using simple circuit in case of using the envelope detector provided that μ is less than one.

Disadvantages of AM Modulation - Large bandwidth: to transmit an AM signal, twice of the message signal bandwidth is needed. - Low power efficiency η : most of the total transmitted power is wasted in the carrier. Note: In case of a sinusoidal signal, given that µ is the modulation index, the efficiency is given as: η=µ²/ µ²+2