Amplitude Modulation Technique

8/18/2019 Amplitude Modulation Technique

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Amplitude modulation (AM) radio is a commonplace

technology today, and is standard in any type of

commercial stereo device. Because of the low cost of the

parts necessary to implement AM transmission and the

simplicity of the underlying technology, using amplitude

modulations is a cheap and effective way to perform many

tasks that require wireless communication.

The most wellknown application of an AM transmitter is in

radio. Am radio receivers are availa!le in numerous

devices, from automo!ile stereos to clock radios.

"owever, the usage of AM transmitters is not restricted to

professional radio stations


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Transmit information !earing (message) or !ase!and signal

(voice music) through a communication channel

Base!and # is a range of frequency signal to be transmitted.

eg: Audio (0 - 4 kHz), ideo (0 - ! "Hz).

$ommunication channel% #ransmission $it%out frequency s%ifting.

#ransmission t%roug% t$isted &air cable, coa'ial cable and fibero&tic cable.

ignificant &o$er for $%ole range of frequencies. ot suitable for radio*micro$a+e and satellite communication.

$arrier communication se tec%nique of modulation to s%ift t%e frequency. %ange t%e carrier signal c%aracteristics (am&litude,

frequency and &%ase) follo$ing t%e modulatingsignal am&litude.

uitable for radio*micro$a+e and satellitecommunication.


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the instantaneous amplitude of a carrier wave is varied in

accordance with the instantaneous amplitude of the

modulating signal. Main advantages of AM are small

!andwidth and simple transmitter and receiver designs.

Amplitude modulation is implemented !y mi&ing the

carrier wave in a nonlinear device with the modulating

signal. This produces upper and lower side!ands, which

are the sum and difference frequencies of the carrier

wave and modulating signal. The carrier signal is represented !y

c(t) # A cos(wct)

The modulating signal is represented !y m(t) # B sin(wmt)

Then the final modulated signal is

' m(t)* c(t) # A ' m(t)* cos(wct)

# A ' B sin(wmt)* cos(wct)

# A cos(wct) A m+ (cos((wcwm)t)) A m+ (cos((wcwm)t))


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--B-B -/01A

--B2-B -/01A


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--B signal frequency spectrum

--B-B signal

--B2-B signal


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"artley modulator A direct approach for creating a single side!and AM signal (--BAM)

is to remove either the upper or lower -B !y filtering the 3-B-$AM signal (frequency discriminator method)

--B modulator using the frequency discrimination approach

Magnitude spectra % (a) !ase!and 4 (!) 3-B-$AM4 (c) upper --B4 (d) lower --B


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5uadrature modulator can !e used to create a --BAM

signal !y selecting the quadrature signal to coherently

cancel either the upper +lower -B from the inphase

channel --BAM signal given%

2sing the minus sign in equation () results in upper--B,whereas selection of the plus sign yields lower --B

The hil!ert transform is a wide!and 678 phase shifter

()

()


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"artley modulator for --B4 sign gives upper --B, signgives lower --B

Amplitude Modulation Technique

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