COMMUNICATION SYSTEM:The science of communication involving long distance is called Tele communication. Technically speaking the earliest communication system namely line telegraphy invented in eighteenth century.Modern communication consist of following three points:SpreadProcessingStorage of information

A TYPICAL ELECTRONIC COMMUNICATION SYSTEM

SOURCE

RECEIVER

NOISE

I|P TRANSDUCER

TRANSM-

ITTER

O\P TRANSDUCER

DESTIN-

ATION

CHANNEL

TRANSMITTER:The purpose of a transmitter is to modify the message signal in a form suitable for transmission medium orSimply the channel. This can easily be done through a process known as modulation.

MODULATION: It is performed in a transmitter by a circuit calleda modulator. A Carrier that has been acted on by an informationsignal is called a modulated wave .

DEMODULATION :is the reverse process of modulation and converts the modulated carrier back to the original information and performed in a receiver.

NEED OF MODULATION:Frequency multiplexing

Practicability of antenna

Narrow banding

Common processing

MASTER

OSCILLATIONS

RF VOLPWRAMP

RF O|PPWR AMP

MODPROCE

S

RF BUFFE

R AMP

MOD PWR AMP

MOD FEQ VOLAMP

Diagram of high level broadcast transmitter:TRANSMITTING ANTENNA

CHANNEL: This is basically a medium ,which connects the transmitter to receiver. It may be a coxial wires,Optical fiber and may be a atmosphere.

Noise is a major limiting factor in the performance of a Communication system.Noise may be of following categories:Thermal noise Intermodulation processCross talkImpulse noise

RECEIVER: The main function of this element is to reproduce the original message from the distorted signal. The recovery of the message accomplishedby a process known as demodulation which is basicallythe process ,reverse of the modulation.

RF AMP

AUDIO

FREQAMP

LOCAL

OSC

FREQMIXE

R

IFAMP

DETECTOR

LOUDSPEAKER

RECEIVING ANTENNA IF=455KHZ

DIAGRAM:

MIXING:The mixing process may be defined as a processOf combining two or more signals. In essence there are two ways in which signals can be combined or mixed:Linear mixingNon Linear mixing

LINEAR MIXING: It occurs when two or more signalscombine in a linear device, such as passive networkor a small signal amplifier.Linear mixing can be explained with the help of two different cases:

SIGNAL INPUT FREQUENCY: Fig(a) shows the amplification of a single input frequency by alinear amplifier. Fig(b) shows the output signal in thetime domain. Fig© shows the frequency domain.

LIN

EA

RA

MP

Vin=Va Vout=AVa

FIG(a)Linear amplificationVout=AVinVin=Va sinwatVout=AVa sinwat

Vin=Va

Vout=AVa

Fig(B)time domain

Fig©frequency domain

VaAVa

fa fb

MULTIPLE INPUT FREQUENCY: Fig(a) shows two inputFrequencies combining in a small signal amplifier. Each Input signal is amplified by the gain and mathematicallyExpressed as :Vout=AvinVin=Va sinwat+Vb sinwbt

Vout=A(Va sinwat+Vb sinwbt)Vout=AVa sinwat+AVb sinwbt

Vin=Va

Vin=Vb

Vout=Ava+AVb

FIG(a)Linear amplification

fa

fbfa

Vin=Va

Vout=Vb

Fig(B)time domain

Fig©frequency domain

VaAVa

fa fb

AVb

Vout=AVa+AVb

ttt

tt

fa fb

NON LINEAR MIXING:It occurs when two or more Signals are combined in a non linear device such asDiode or large signal amplifier.It is also explained withDifferent forms:

SIGNAL INPUT FREQUENCY: Fig(a) shows the amplification of a single input frequency by a nonlinear amplifier. Fig(b) shows the output signal in thetime domain. Fig© shows the frequency domain.

LIN

EA

RA

MP

Vin=Va Vout=AVa

FIG(a)non-Linear amplificationfa fb

Vin=Va

Vout=AVb

Fig(B)time domain

tt

Fig©frequency domain

VaAVa

fa fa

Vout=Avin+Bvin+CVinVout=A(Va sinwat)+B(Va sinwat)+C(Va sinwat)

2

2

3

3

MULTIPLE-INPUT FREQUENCY:Fig(a) shows two inputFrequencies combining in a small signal amplifier. Each Input signal is amplified by the gain and mathematicallyExpressed as :

Vout=A(V1+V2)+B(V1+V2)+- - - - -Vin=A[V1 sinw1t+V2 sinw2t]+b[V1 sinw1t+V2sinW2t]+- - - - - - - -

2

PUTTING THE VALUE OF * I EQUATION I

Vo=A[V1 sinw1t+V2 sinw2t]+B[V1 sinw1t+V2sinW2t]+- - - - - - - -

23

Vin=Va

Vin=Vb

FIG(a)Linear amplification

fa

fbfa

Vout=A(V1+V2)+B(V1+V2)+- - - - -2

Vin=Va

Vin=Vb

Fig(B)time domain

Vout=A(Va+Vb)+B(Va+Vb)+- - -

ttt

2

Fig©frequency domain

VaAVa

fa fb

AVb

tt

fa fb