Chapter03.ppt

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Principles of ElectronicPrinciples of Electronic

Communication SystemsCommunication Systems

Third Edition

Louis E. Frenzel, Jr.

2008 The McGraw-Hill o!"anies


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Chapter 3Chapter 3

#!"litude Modulation Funda!entals


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2008 The McGraw-Hill o! anies

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Topics Covered in Chapter 3Topics Covered in Chapter 3

$-%& #M once"ts

$-2& Modulation 'nde( and )ercenta*e o+ Modulation

$-$& ideands and the Freuenc/ o!ain

$-1& #M )ower

$-& in*le-ideand Modulation

$-3& lassi+ication o+ 4adio E!issions


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3-1: AM Concepts3-1: AM Concepts

'n the !odulation "rocess, the 5oice, 5ideo, or di*italsi*nal !odi+ies another si*nal called the carrier.

'n amplitude modulation(AM)the in+or!ation si*nal

5aries the a!"litude o+ the carrier sine wa5e. The instantaneous 5alue o+ the carrier a!"litude

chan*es in accordance with the a!"litude and+reuenc/ 5ariations o+ the !odulatin* si*nal.

#n i!a*inar/ line called the envelope connects the"ositi5e and ne*ati5e "ea6s o+ the carrier wa5e+or!.


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Fi*ure $-%& #!"litude !odulation. 7a The !odulatin* or in+or!ation si*nal.


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Fi*ure $-%& #!"litude !odulation. 7b The !odulated carrier.


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'n #M, it is "articularl/ i!"ortant that the "ea6 5alue o+

the !odulatin* si*nal e less than the "ea6 5alue o+

the carrier.

Vm< Vc

Distortionoccurs when the a!"litude o+ the

!odulatin* si*nal is *reater than the a!"litude o+ the

carrier.

# !odulator is a circuit used to "roduce #M.

#!"litude !odulators co!"ute the "roduct o+ the

carrier and !odulatin* si*nals.



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Fi*ure $-$& #!"litude !odulator showin* in"ut and out"ut si*nals.



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3-2: Modulation nde! and3-2: Modulation nde! and

Percenta"e of ModulationPercenta"e of Modulation

The modulation index (m)is a 5alue that descries

the relationshi" etween the a!"litude o+ the

!odulatin* si*nal and the a!"litude o+ the carrier

si*nal.m =Vm/ Vc

This inde( is also 6nown as the modulating factoror

coefficient, or the degree of modulation.

Multi"l/in* the !odulation inde( / %00 *i5es the

"ercenta*e o+ !odulation.



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95er!odulation and istortion

The !odulation inde( should e a nu!er etween 0

and %.

'+ the a!"litude o+ the !odulatin* 5olta*e is hi*her thanthe carrier 5olta*e, mwill e *reater than %, causin*

distortion.

'+ the distortion is *reat enou*h, the intelli*ence si*nal

eco!es unintelli*ile.



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95er!odulation and istortion

istortion o+ 5oice trans!issions "roduces *arled,

harsh, or unnatural sounds in the s"ea6er.

istortion o+ 5ideo si*nals "roduces a scra!led andinaccurate "icture on a T: screen.



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Fi*ure $-1& istortion o+ the en5elo"e caused / o5er!odulation where the !odulatin*

si*nal a!"litude Vmis *reater than the carrier si*nal Vc.



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)ercenta*e o+ Modulation

The !odulation inde( is co!!onl/ co!"uted +ro!

!easure!ents ta6en on the co!"osite !odulated

wa5e+or!. ;sin* oscillosco"e 5olta*e 5alues&

Vm

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Chapter03.ppt