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Lab 4

Objective

• Understand the Amplitude Modulation

• Analysis of Amplitude modulation waveform using different frequencies

• Analysis of Amplitude modulation waveform using different modulating index

Introduction

In order that a steady radio signal or "radio carrier" can carry information it must be changed or

modulated in one way so that the information can be conveyed from one place to another. There are a

number of ways in which a carrier can be modulated to carry a signal often an audio signal and the most

obvious way is to vary its amplitude.

Amplitude modulation applications

•  Broadcast transmissions

•  Air band radio

• Quadrature amplitude modulation

Amplitude modulation

AM, the information signal varies the amplitude of the carrier sine wave. The

instantaneous value of the carrier amplitude changes in accordance with the

amplitude and frequency variations of the modulating signal.

 The carrier frequency remains constant during the modulation process, but its

amplitude varies in accordance with the modulating signal. An increase in the

amplitude of the modulating signal causes the amplitude of the carrier to increase.

Both the positive and the negative peaks of the carrier wave vary with the

modulating signal. An increase or a decrease in the amplitude of the modulating

signal causes a corresponding increase or decrease in both the positive and the

negative peaks of the carrier amplitude.Using trigonometric functions, we can

epress the sine wave carrier with the simple epression

v m !"m sin #fmt

$here v m !instantaneous value of information signal

"m !peak amplitude of information signal

fm !frequency of modulating signal

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Modulating signal

%arrier signal

Modulation &nde

'or undistorted AM to occur, the modulating signal voltage must be less than the

carrier voltage Therefore the relationship between the amplitude of the modulating

signal and the amplitude of the carrier signal is important. This relationship, known

as the modulation inde m (also called the modulating factor or coe)cient, or the

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degree of modulation*, is the ratio these are the peak values of the signals, and the

carrier voltage is the un+modulated value. Multiplying the modulation inde by --

gives the percentage of modulation.

'or eample, if the carrier voltage is " and the modulating signal voltage is /.0 ",

the modulation factor is -.1222 and the percentage of modulation is 3vermodulation and 4istortion The modulation inde should be a number between - and

. &f the amplitude of the modulating voltage is higher than the carrier voltage, m

will be greater than , causing -.122 5 -- !12.22.

3ver modulation and 4istortion

 The modulation inde should be a number between - and . &f the amplitude of the

modulating voltage is higher than the carrier voltage, m will be greater than ,causing distortion of the modulated waveform.

 The ideal condition for AM is when or, which gives -- percent modulation. This

results in the greatest output power at the transmitter and the greatest output

voltage at the receiver, with no distortion

3ver modulated signal

Matlab %ode

clc!

clear all!close all!t#$#.##%$%!

set&#'(defaultlinelinewidth(')*!

A+!,Amplitude of signal

fminput&(Message frequency(*!,Accepting input valuefcinput&(-arrier frequency(*!,Accepting input value &f)f%*

miinput&(Modulation Index(*!,Modulation Index

/mA0sin&)0pi0fm0t*!,Message /ignal

subplot&1'%'%*!,2lotting frame divided in to 1 rows and this fig appear at %st

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 plot&t'/m*!

xlabel&(Time(*!

ylabel&(Amplitude(*!

title&(Message /ignal(*!grid on!

/cA0sin&)0pi0fc0t*!,-arrier /ignal

subplot&1'%')*!

 plot&t'/c*!xlabel&(Time(*!

ylabel&(Amplitude(*!

title&(-arrier /ignal(*!

grid on!/fm&A3mi0/m*.0sin&)0pi0fc0t*!,AM /ignal' Amplitude of -arrier changes to &A3Message*

subplot&1'%'1*!

 plot&t'/fm*!

xlabel&(Time(*!ylabel&(Amplitude(*!

title&(AM /ignal(*!

grid on!

Lab Task -hoose different frequencies and show output results

6ercise

78 $hat would happen if the modulating inde were bigger than .

7#8 Use modulating inde # and 9 show results in your report and discuss its

output.

728 %hoose modulating voltage much greater than the carrier voltage show resultsand discuss its output.