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

Medicaps Institute (2)

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