medicaps institute (2)
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Medicaps
institute [ mitm ]
ALI
PRESENTS..
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COMMUNICATIONENGINEERINGPOWER POINT PRESENTATION
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TOPICS TO BE COVERED
INTRODUCTION
DEFINATION
CLASSIFICATION AM MODULATION
DOUBLE SIDE BAND (DSB-SC)
SINGLE SIDE BAND (SSC-SC)
VESTIGAL SIDE BAND (VSB)
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INTRODUCTION
The purpose of communication system is to transmita message signal from an information source, located
at one point, to a destination or user located at
another point some distance away .
The signal produced by information source may not
be electrical in nature.
For transmission purpose we need an electrical
signal, because all the signal processing devices orequipments are electrical form and then transmitted.
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DEFINATIONS
MODULATION:- Modulation is a process by whichsome characteristics of a signal known as carrier wave is
varied according to the instantaneous value of another
signal known as modulating signal.
MODULATING SIGNAL:- The signal containingintelligence or some information which is to be
transmitted are called modulating signal. These signals
are also called as base band signals
c > m
where,
c is called carrier frequency
m is called modulating frequency.
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CLASSIFICATION1) CONTINUOS WAVE MODULATION:- If the carrier
waveform is continuos in nature then its called as continuos
wave modulation.
i. AMPLITUDE MODULATION:- The amplitude of a sinosuidal
carrier wave is varied accordance with base band signal.ii. ANGLE MODULATION:- The angle of the sinosuidal carrier
wave is varied according with base band signal.
1) PULSE MODULATION:- If the carrier waveform is pulse
type waveform, then its called as pulse modulation.
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MODULATION
IN
SHORT
Amplitude modulation is defined as a system in
which the maximum amplitude is made
proportional to the instantaneous value of the
modulating or base band signal
In this process amplitude modulation, the
frequency and phase remains constant..whereasthe maximum amplitude varies according to the
instanteneous value of the information signal.
The am wave has a time varying amplitude
called as the ENVELOPE of the am wave.
S(t) = [A+X(t)] Coswc(t) =
E(t)Coswc(t) E(t)= A+X(t)
E(t) is envelope of the am wave
This envelope consist of base band signalx(t). We generally keep Wc > Wm which ensures
that the two side bands do not overlap each
other.
At this point , it may be noted that the
negative frequencies appeared in the spectrumanalysis due to the exponential representation
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OUBLE SIDE BAND
SUPPRESSED CARRIER
IN SHORTq The modulated signal which contains no
carrier but two side bands is called double
side band.
q
It is also clear that the impulses at the Wcare missing which means that the carrier
term is suppressed and only two side bands
USB and LSB are left.
q Considering only positive side USB frequency
is (Wc + Wm) and LSB frequency is (Wc
-Wm) the difference of these two bands isequal to the transmission bandwidth.
q DISADVANTAGE:- BANDWIDTH IS2wm
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v As shown before AM modulation and DSB-SC
is wasteful because of bandwidth. Since they
both need a transmission bandwidth equal to
twice of the message signal.
v
The message signal is present in both USB andLSB. This means that if we transmit only one
sideband than no information is lost.
v Modulation of this type width provides a single
side band with suppressed carrier is known as
SSB-SC.v AS SHOWN IN FIGURE THE BANDWIDTH IS ONLY wm
CO MPATIBLE S SB-SC : - An SSBsignal may be generated in which the
carrier is suppressed then it can be detected
SINGLE SIDE BAND-UPPRESSEDARRIER
INHORT
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JOURNEY
TOWARDSVSB
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VESTIGIAL SIDE BAND
v A VSB modulation system is actually acompromise b/w DSB and SSB modulationsystem.
v The generation of VSB mpodulation signalis easier than other modulated
signals,such as AM, DSB-SC, SSB.
v
The bandwidth of VSB is slightly higherthan SSB signal but considerably less thanDSB-SC signal.
v When signals containing frequency
components at extremely low frequencies
(as in telegraph signals) the USB and LSBof the translated signal tend to meet at the
carrier frequency so, this becomes difficultto isolate one side band from the other.
Hence SSB signal schemes becomes
unsuitable for handling such types ofsignals, now this problem overcome in thisscheme known as VESTIGIAL SIDE BAND.
Few points about VSB
In VSB modulation instead of rejecting one side band completely, a gradual cut off one side
band is allowed. This gradual cut-off is a part of other side band. This part of other side bandis called as VESTIGE.
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Spectrum of VSBsignal
BANDWIDTH of VSB
= (wv + wc) (wc - wm)= wv + wc wc + wm
= wv + wm
Where,
wv= width of VSB
wm = bandwidth of the message signal
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GENERATION OF VSB SIGNAL
To generate a VSB signal, first we have to generate a DSB-SC signal and
then pass through a filter. This filter will pass the wanted side-band as
it is along with a part of unwanted side-band
FREQUENCY DOMAIN DESCRIPTION
SCHEME FOR GENERATION OF VSB SIGNAL
F R E Q U E N C Y S P E C T R U M : -In the frequency spectrum it is assumed thatthe VSB is transmitted as it is and the lower side-band is modified into
Vestigial side band.
T R A N S M I S S I O N B A N D W I D T H :-The transmission of the VSB modulated wave
= +
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ADVANTAGES OF VSB
VSB modulation is used in TELEVISION signalsbecause the video signal needs a large transmission
bandwidth.
A. The main disadvantage of VSB modulation is the reduction
in bandwidth. It is almost as efficient as the SSB.
A. It possesses good phase characteristics and makes thetransmission of the low frequency components possible.
APPLICATIONS
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THANKS
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REFRENCED
BY
MR. AJYAY BOYAT