AM Modulation &

Demodulation

ModulationDefinition: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 ModulationDefinition: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):

unit. with voltsignalcarrier theof Amplitude:

signal.carrier theofFrequency :

signal. message theofFrequency :

unit. with voltsignal message theof Amplitude :

. toequal ish index whic Modulation :

)2cos()2cos(1)(

c

m

m

c

m

mc

mcAM

A

fc

f

A

A

A

fcttfA

AAtS

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 detectorThe 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