Vestigial side band (vsb)

Explains about Vestigial side band (vsb) modulation

1. VESTIGIALSIDEBAND(VSB)
MODULATION
BY
G.GNANAPRIYA
AP/ECE
RAMCOINSTITUTEOF
TECHNOLOGY

2. INTRODUCTION
Drawbacks of SSB(Single Side Band)
Generation is difficult
ToproduceSSBsignalfromDSBsignalidealfilters
shouldbeused.
Selectivefilteringistobeusedtoreconstructthe
originalsignal
Phase shift should be exactly 90degree
Inordertoovercomethesedrawbacks,VSB
modulationisused.ItisacompromisebetweenSSBand
DSBSCmodulations.
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3. INTRODUCTION
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ToproduceSSBsignalfromDSBsignalidealfilters
shouldbeused.
InVSBsystemonesidebandandavestigeofother
sidebandaretransmittedtogether.
Theresultingsignalhasabandwidth>thebandwidth
ofthemodulating(baseband)signalbut<theDSB
signalbandwidth.
InVSB,Transmissionbandwidth,BWv=B+v,where‘v’
isthevestigialoffrequencyband

4. SPECTRUMOFDSB SC, SSB ANDVSB
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5. Sideband
shaping
Filter H(f)
Carrier
Product
Modulator
m(t) s(t)
Filter Method
Ac Cos(2πfct)
Baseband signal
DSB SC
VSB WAVE
–GenerateaDSB-SCAMsignal
–PasstheDSB-SCAMsignalthroughabandpassfilterwith
frequencyresponseH(f)
•The exact design of the filter depends on the desired
spectrum of the VSB modulated wave.
•The VSB filter required to have the vestigial symmetry
around fc.
GENERATIONOFVSB

6. TIMEDOMAINREPRESENTATION OFVSB
WAVE
Time domain representation of VSB wave is
HereSI(t)isthetheinphasecomponentandSq(t)is
thequadraturecomponentofs(t).
Theinphasecomponentmaybederivedfroms(t)
byfirstmultiplyings(t)bycos2πfctandthenpassing
theproductthroughaLPF.
Similarly,thequadraturenphasecomponentmay
bederivedfroms(t)bymultiplyings(t)bysin2πfct
andthenpassingtheproductthroughaLPF.
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7. Spectrum of VSB signal is
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8. 8
Thus the VSB modulated wave in time domain is

9. COHERENT DETECTION OF VSB
VSB wave s(t)
Where M(f) is the fourier transform of the base band signal.
The output of the product modulator is
Taking the fourier transform on both sides of equation (2)
)
Substituting for S(f) from first equation we get

10. The spectrum of the signal so(t) at the output of the LPF is
For a distortionless reproduction of the original baseband signal at the
output of detector, it is necessary that the transfer function of the filter
must satisfy the following condition

11. MAGNITUDE RESPONSE OF VSB
FILTER
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12. EXAMPLE OF VSB SIGNAL -TV SIGNAL
TV signal has a nominal bandwidth of 4.5MHz
If DSB modulation is used it requires 9MHz for each TV
channel
But if VSB modulation is used, the whole TV signal is confined
to about 6MHz
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13. ADVANTAGESOFVSB
OffersacompromisebetweenSSBandDSB-SC
VSBisstandardfortransmissionofTVandsimilar
signals
Bandwidthsavingcanbesignificantifmodulating
signalsareoflargebandwidthasinTVandwide
banddatasignals.
ForexamplewithTVthebandwidthofthe
modulatingsignalcanextendupto5.5MHz;with
fullAMthebandwidthrequiredis11MHz
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14. THANK YOU
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