Jan 17, 2016
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