chapter03.ppt
TRANSCRIPT
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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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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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3-1: AM Concepts3-1: AM Concepts
Fi*ure $-%& #!"litude !odulation. 7a The !odulatin* or in+or!ation si*nal.
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3-1: AM Concepts3-1: AM Concepts
Fi*ure $-%& #!"litude !odulation. 7b The !odulated carrier.
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3-1: AM Concepts3-1: AM Concepts
'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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3-1: AM Concepts3-1: AM Concepts
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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3-2: Modulation nde! and3-2: Modulation nde! and
Percenta"e of ModulationPercenta"e of Modulation
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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3-2: Modulation nde! and3-2: Modulation nde! and
Percenta"e of ModulationPercenta"e of Modulation
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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3-2: Modulation nde! and3-2: Modulation nde! and
Percenta"e of ModulationPercenta"e of Modulation
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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3-2: Modulation nde! and3-2: Modulation nde! and
Percenta"e of ModulationPercenta"e of Modulation
)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