introduction to communication Cha two ppt

Introduction to communication system

1. Chapter-2 : Amplitude (linear) modulation

2. Outlinesofthechapter
2.1.introductiontomodulation
2.2.linearmodulation
 AM,
 DSB,
 DSB-SC,
 SSBandVSB
2.3.modulationindex,powerspectrumandbandwidthofDSB,SSBand
VSBAMsystems

3. 2.1. Introduction to modulation
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3
What is Modulation?
Modulationis the process of encoding information from a message
source in a manner suitable for transmission
•isusedtocontroltheparametersofacarriersignal.
•Encodinginformationinasuitableformfortransmission.
Translatebasebandsourcesignaltobandpasssignal
Bandpasssignal:“modulatedsignal”
•Howsignalmodulate?-Varyamplitude,phaseorfrequencyofacarrier
WhatisDemodulation?
•Decodingorextractbasebandmessagefromcarrier
•isthereverseprocessofmodulationtorecoverthemessagesignal
m(t)ord(t)atthereceiver

4. Con’t
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4

5. 2.2. Linear modulations and demodulations
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2.2.1. Classification of Modulation
1.Analog Modulation
•Modulating (m(t)):
✓The input signal is continuous signal
2.Digital Modulation
•Modulating (d(t)):
✓The input is time sequence of symbols or pulses
d(t)
d(t)
d(t)
•ASK
•FSK
•PSK
•QAM

6. con’t
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6
•Type of analog modulation
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase modulation (PM)
•Amplitude Modulation (AM)
linear dependence on quality & power of rcvd signal
The out signal is continuous and increased in strength
The modulating signal m(t) is 'impressed' on to the amplitude of the carrier
m(t)
m(t)
m(t)
•AM
•DSB
•DSB-SC
•SSB
•VSB

7. Con’t
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7
•
Amplitude Modulation
•The amplitude of high-carrier signal is varied according to the
instantaneous amplitude of the modulating message signal m(t).)2cos()](1[)(
)(
)2cos(
:Signal AM The
:Signal Message Modulating
:SignalCarrier
tftmAts
tm
tfA
ccAM
cc


+=
AM Modulator s
AM(t)m(t)

8. Con’t
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8
NB:
USB-upper side band
LSB-lower side band
•Nowletm(t)=V
mcos
mt,i.e.a'test'signal
•Usingthetrig-identity
•wehave() () ()()tωtωV+tωV=tv
cmmcCs coscoscos ( ) ( ) BA+B+A=BA −coscos
2
1
coscos () () ( )( ) ( )( )tωω
V
+tω+ω
V
+tωV=tv
mc
m
mc
m
cCs −cos
2
cos
2
cos
Comments Carrier USB LSB
Amplitude V
C V
m/2 V
m/2
Frequency f
c f
c+ f
m f
c-f
m
Angular
frequency

c 
c+ 
m
c–
m
•This equation represents Double Side Band Amplitude Modulation
DSBAM
The modulationindex.c
m
A
A
m=

9. Amplitude modulation
Con’t
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9
•

10. Con’t
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10
•
The total bandwidth required for AM
can be determined
from the bandwidth of the Message
signal: B
AM= 2B. Where, B is the bandwidth
of the message signal m(t)

11. 1Thecarriersignalisccccc
ftAts  2 where)cos()( ==
2In the same way, a modulating signal(information
signal) can also be expressed as tAts
mmm
cos)(=
Derive the Frequency Spectrum for Full-AM
Modulation (DSB-LC)
1/17/2021 11by Getachew

12. 3The amplitude-modulated wave can be expressed as  )cos()()( ttsAts
cmc
+=   )cos()cos()( ttAAts
cmmc
+=
4 By substitution of the message signalc
m
A
A
m=
5 The modulation index.
1/17/2021 12

13. 6 Therefore The full AM signal may be
written as)cos())cos(1()( ttmAts
cmc
+= )]cos()[cos(2/1coscos BABABA −++= t
mA
t
mA
tAts
mc
c
mc
c
cc )cos(
2
)cos(
2
)(cos)(  −+++=
1/17/2021 13

14. f
C f
c+f
mf
c-f
m
2f
m
Draw the Frequency Spectrum of the above AM
signal and calculate the Bandwidth
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15. ThefrequencyspectrumofAMwaveformcontains
threeparts:
1.Acomponentatthecarrierfrequencyf
c
2.Anuppersideband(USB),whosehighest
frequencycomponentisatf
c+f
m
3.Alowersideband(LSB),whosehighestfrequency
componentisatf
c-f
m
Thebandwidthofthemodulatedwaveformistwicethe
informationsignalbandwidth.
Frequency Spectrum of an AM signal
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16. •Becauseofthetwosidebandsinthefrequency
spectrumitsoftencalledDoubleSidebandwithLarge
Carrier.(DSB-LC)
•Theinformationinthebaseband(information)signal
isduplicatedintheLSBandUSBandthecarrier
conveysnoinformation.
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17. Con’t 17
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•AmplitudeModulationisalinearprocess,hencetheprincipleofsuperposition
applies.Theoutputspectrummaybefoundbyconsideringeachcomponent
cosinewaveinm(t)separatelyandsummingattheoutput.
•Note:
FrequencyinversionoftheLSBthemodulationprocesshaseffectively
shiftedorfrequencytranslatedthebasebandm(t)messagesignaltoUSBand
LSBsignalscentredonthecarrierfrequencyf
c.
USBisafrequencyshiftedreplicaofm(t)andLSBisafrequency
inverted/shiftedreplicaofm(t)
bothsidebandseachcontainthesamemessageinformation,henceeitherthe
LSBorUSBcouldberemoved(becausetheybothcontainthesame
information).

18. Con’t 18
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•ThebandwidthoftheDSBsignalis2BHz,i.e.twicethehighestfrequencyin
thebasebandsignal,m(t)
•Theprocessofmultiplying(ormixing)togivefrequencytranslation(orup-
conversion)formsthebasisofradiotransmittersandfrequencydivision
multiplexingwhichwillbediscussedlater.
Exercise
•AstandardAMbroadcaststationisallowedtotransmittedmodulating
frequencyupto5kHZ.IftheAMstationistransmittingonafrequencyof
980KHZ.ComputethemaximumandminimumupperandlowerSBandthe
totalbandwidthoccupiedbytheAMstation.

19. Con’t 19
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PowerinDSBAM
•NormalisedAveragePower=(V
RMS)
2
/R=
•wemaytabulateforAMcomponentsasfollows:
•Fromthiswemaywritetwoequivalentequationsforthetotalpower
P
T,inaDSBAMsignal
•ThecarrierpowerR
V
pk
/
2
2







 () () ( )( ) ( )( )tωω
V
+tω+ω
V
+tωV=tv
mc
m
mc
m
cCs −cos
2
cos
2
cos 4R2R8R8R2R
22222
mCmmC
T
V
+
V
=
V
+
V
+
V
=P
and8R8R2R
22222
CCC
T
Vm
+
Vm
+
V
=P 2R
2
C
c
V
=P
i.e.44
22
m
P+
m
P+P=P
cccT
or







2
1
2
m
+P=P
cT

20. Con’t 20
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Component Carrier USB LSB
Amplitude pk
V
C
Power
Power2
mV 2
mV 2R
2
CV 8R22R
22
mm V
=
V





 8R
22
CVm
Total Power P
T= Carrier Power P
c+ P
USB+ P
LSB8R22R
22
mm V
=
V





 2R
2
C
V 8R
22
C
Vm

21. Con’t 21
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•Eitheroftheseformsmaybeuseful.SincebothUSBandLSBcontainthe
sameinformationausefulratiowhichshowstheproportionof'useful'power
tototalpoweris
•ForDSBAM(m1),allowingform(t)withadynamicrange,theaverage
valueofmmaybeassumedtobem=0.3
•Hence,onaverageonlyabout2.15%ofthetotalpowertransmittedmayberegarded
as'useful'power.(95.7%ofthetotalpowerisinthecarrier!)
•Evenforamaximummodulationdepthofm=1forDSBAMtheratio2
2
2
2
24
2
1
4
m+
m
=
m
+P
m
P
=
P
P
c
c
T
USB








Hence, ()
()
0.0215
0.324
0.3
24
2
2
2
2
=
+
=
m+
m 6
1
24
2
2
=
m+
m
i.e.only1/6thofthetotalpoweris'useful'power(with2/3of
thetotalpowerinthecarrier).

22. Con’t 22
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Example 2
•IfthecarrierofanAMtransmitteris1000wanditismodulated100%,
a)findthetotalpower
b)PowerinDSBandpowerinoneSB

23. •ThecarriercomponentinfullAMorDSB-LCdoesnotconveyany
information.Henceitmayberemovedorsuppressedduringthe
modulationprocesstoattainhigherpowerefficiency.
•ThetradeoffofachievingahigherpowerefficiencyusingDSB-SC
isattheexpenseofrequiringacomplexandexpensivereceiverdue
totheabsenceofcarrierinordertomaintaintransmitter/receiver
synchronization.
Double Side Band Suppressed Carrier
(DSB-SC) Modulation
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24. 1Considerthecarrierccccc
ftAts  2 where)cos()( ==
2modulated by a single sinusoidal signalmmmm
ftAts  2 wherecos)(
m
==
3Themodulatedsignalissimplytheproductofthesetwo( )

LSB
mc
cm
USB
mc
cm
mcmc
mmcc
t
AA
t
AA
BABABA
ttAA
tAtAts
)cos(
2
)cos(
2

)cos()cos(
2
1
coscos since
)cos()cos(
)cos()cos()(



−++=
−++=
=
=
Derive the Frequency Spectrum for Double
Sideband Suppressed Carrier Modulation (DSB-
SC)
1/17/2021 24by Getachew

25. tAts
mmm
cos)(= tAts
ccc
cos)(= )cos()cos()( tAtAts
mmcc
= X
f
c
f
c-f
m f
c+f
m
Frequency Spectrum of a DSB-SC AM Signal
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26. Generation and Detectionof DSB-SC
•ThesimplestmethodofgeneratingaDSB-SC
signalismerelytofilteroutthecarrierportionof
afullAM(orDSB-LC)waveform.
•Givencarrierreference,modulationand
demodulation(detection)canbeimplemented
usingproductdevicesorbalancedmodulators.
1/17/2021 26by Getachew

27. Block diagram of Balanced modulator
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28. Balanced Cont’d..
1/17/2021 28by Getachew

29. Single-Sideband Modulation
How to generate SSB signal?
•Generate DSB-SC signal
•Band-pass filter to pass only one of the sideband
and suppress the other.
For the generation of an SSB modulated signal
to be possible, the message spectrum must have
an energy gap centered at the origin.
Single Side Band Modulation (SSB)
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30. Con’t 30
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2. Single Sideband AM (SSB)

31. Con’t 31
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2. Single Sideband AM (SSB)

32. Con’t 32
•NotethatthebandwidthoftheSSB
signalBHzishalfoftheDSBsignal
bandwidth.
•NotealsothatanidealSSBfilter
responseisshown.Inpracticethefilter
willnotbeidealasillustrated.
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33. Con’t 33
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Power in SSB
•Frompreviousdiscussion,thetotalpowerintheDSBsignalis
•Hence,ifP
candmareknown,thecarrierpowerandpowerinonesideband
maybedetermined.Alternatively,sinceSSBsignal
•thenthepowerinSSBsignal(NormalisedAveragePower)is
•PowerinSSBsignal=







2
1
2
m
+P=P
cT 44
22
m
P+
m
P+P=P
cccT=
for DSBAM.() () ( )( )tω+ω
V
+tωV=tv
mc
m
cCs cos
2
cos 8R2R22R2R
2222
mCmC
SSB
V
+
V
=
V
+
V
=P 




 )
4
1(
8R2R
222
m
P
V
+
V
c
mC
+=

34. Vestigial Side Band (VSB) Modulation
1/17/2021 by Getachew 34

35. Vestigial Side Band (VSB) Modulation
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36. Con’t
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36
Percentage of Modulation
•modulatingsignalvoltageVmmustbelessthanthecarriervoltageVc.
•relationshipbetweentheamplitudeofthemodulatingsignalandthe
amplitudeofthecarriersignal,knownasthemodulationindexm(also
calledthemodulatingfactororcoefficient,orthedegreeofmodulation),is
theratio
•Thesearethepeakvaluesofthesignals,andthecarriervoltageistheun
modulatedvalue.
•Multiplyingthemodulationindexby100givesthepercentageof
modulation.
•Forexample,ifthecarriervoltageis9Vandthemodulatingsignalvoltage
is7.5V,themodulationfactoris0.8333andthepercentageofmodulationis
0.8333*100=83.33%.c
m
V
V
=m

37. Comparison of Amplitude Modulation methods
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38. Con’t
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38
•Goals of Modulation Techniques
HighBitRate
HighSpectralEfficiency(maxBps/Hz)
HighPowerEfficiency(minpowertoachieveatargetBER)
Low-Cost/Low-PowerImplementation
RobustnesstoImpairments
HowAMandFMWorks-YouTube.mkv