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ANALOG COMMUNICATION SYSTEMSCHAPTER-1

1.1 Introduction:

Attention, the universe! By kingdoms, right wheel! , this prophetic phrase is the first

telegraphic message on record. Samuel F.B. Morse sent it over a 16km line in 1838. Thuswas born a new era, the era of electrical communication. Over the next century and half,

communication engineering has advanced to the point that earthbound viewers can watch

astronauts working on space.

A communication system is the one used to transfer the information from one place(Source) to another place (Destination). Since there are so many systems existing and its

hard to cover each and every one of them so we approach the subject from a more general

viewpoint. Recognizing that all the communication systems have the basic function of

information transfer, well seek out and isolate the principles and problems of conveyinginformation in electrical form.

The term communication refers to sending and receiving and processing the information

by electrical means. Radio communication was made possible by the invention of triode

tube, and was stimulated by the work done during world war-II (sometimes war plays andimportant role in technological development, e.g.-ARPANET) .A modern communication

system is concerned with sorting processing and storing of information.

1.2 Block Diagram of Communication System

TX- Transmitter

RX- ReceiverFig: Block Diagram of Communication system

The information source is the one that generates the information to be transmitted. As we

are dealing with electrical communication systems, the information should be in electrical

form so the first task is to convert any information into electrical forms using transducers.

Example: the microphone in the mouthpiece of a telephone set converts the audio signals

into electrical signals.

Unless the message that comes from the information source is electrical in nature, it is

unsuitable for immediate transmission. Even then, a lot of work must be done to makesuch message suitable for transmission . Example: restriction of audio frequency i.e.

filtering of the signal, compression of amplitude range. Eventually in the transmitter, the

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Informationsource

TX Destination.

RXChannel

Noise

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information modulates the high frequency carrier i.e. the information is impressed on

high frequency carrier wave and this is transmitted from the transmitter via the antenna in

radio communication and via wires in base band communication systems.

The acoustic channel (shouting) is not used in long distance communication neither wasthe visual channel until the advent of laser. Later came the channels like radio, wire, fiber

optic etc. The medium through which the transmitter transmits the information towards

the destination is known as channel. The channel might be a wired media (cables), wave-guides, or free space (radio communication). It is inevitable that the signal will be

deteriorated during the process of transmission and reception. This deterioration of the

signal is caused by the presence of Noise, Interference and Distortion. As thedeterioration of the signal cause by interference and distortion can be reduced with the

help of filters and equalizers the channel noise is of greatest concern for any

communication system designers.

Noise is unwanted energy, usually of random nature and is present in transmission

systems due to any cause. It obviously places a limitation on transmission systems. Whenthe noise is severe it may mask a given signal so much that the signal becomes

unintelligible and therefore useless. Noise may interfere at any point in the system but itwill have its greatest effect when the signal is weakest, this means that noise at the

channel or at the input point of the receiver is most noticeable so the channel noise is of

greatest concern to any communication system designer.

There are great varieties of receivers available in the market. The type of receiver ischosen according to the requirements, modulation and demodulation technique used and

the frequency range. The most common is the superheterodyne/supersonic heterodyne

(means mixing of frequencies higher than the audio frequency.). The task ofdemodulation and decoding is done at the receiver. The output of the receiver is fed to the

loudspeakers to produce audio signals in the case of voice communication.

1.3 Why Analog Communication Systems?

We can say that todays world is a digital world. The digital communication systems

have advantages like increased immunity to channel noise and external interference(main and the important point in any communication system), common format for

transmission of data and voice signals, improved security using encryption techniques

etc. However all the above-mentioned advantages are achieved at the cost of increasedchannel bandwidth and increased system complexity. As most of the present broadcasting

systems and a large part of the telephony in use today are based on analog

communication systems principle and moreover they will remain on service in future

also.

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1.4 Noise, Interference And Distortion

The transmitted signal becomes weak due to attenuation present in long distance

communication. This attenuation is undesirable as it reduces signal strength at the

receiver and the contamination of signal occurs. Although such contamination may occurat any point on the receiver but as the signals strength is least at the input point of the

receiver and the contamination is maximum so the standard convention is to blame thechannel entirely treating the transmitter and receiver as being ideal.

Distortion is waveform perturbation caused by imperfect response of the system to thedesired signal itself. Unlike Noise and interference, distortion disappears when the signal

is turned off. If the channel has linear but distorting response, then distortion may be

corrected or at least reduced with the help of special filters called equalizers.(Sub-signalsand systems- Distortionless transmission line-linear and non linear distortion-Phase and

Amplitude distortion.)

Interference is contamination by extraneous signals from human sources, other

transmitters, power lines, machineries, switching circuits etc. Interference occurs most

often in radio communication system whose receiving antenna usually intercepts severalsignals at the same time. Radio frequency interference (RFI) also appears in cable system

if the transmission wires or receiver circuitries pick up signals radiated from nearbysources. The interference can be removed or reduced by filtering.

(To understand Noise please refer to article no 1.6)

1.5 Communication system types:

1. Simplex- one-way communication system. For e.g. radio broadcasting

systems.

2. Half Duplex- Two way but one at a time. For e.g.- Walky-talky, Telegraph.

3. Duplex- simultaneous two way communication. For e.g. Telephony.

1.6 Fundamental Limitations of Communication systems

Engineers have to face two general kinds of constraints while designing a communication

system. On the one hand he/she has to deal with the technological problems including

such diverse considerations like hardware availability, economic factors. Federalregulations etc. These are the problems of feasibility which can be solved with the help of

theoretical results even though the result may not be perfect.. On the other hand are the

two fundamental limitations and are noise and bandwidth.As noise is unavoidable, The answer for its inevitability comes from kinetic theory.

Before going into kinetic theory of thermodynamics, let us classify Noise. As noise is

unwanted energy of random nature and is present anywhere in the system we classify

noise as:

External noise- the various kind of noise present outside the receiver is known as externalnoise. For e.g. atmospheric noise, extraterrestrial noise (solar noise, cosmic noise),

industrial noise,

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Internal noise: - internal noise is the noise produced by various active and passive

devices present in the system .For e.g. Thermal noise, Shot noise, Transit time noise etc.

Now let us see how the kinetic theory throws light on the point why noise is inevitable.

The kinetic theory shows that the temperature of a particle is a way of expressing itsinternal kinetic energy. At any temperature above absolute zero, thermal energy causes

microscopic particle to exhibit random motion. Let us take the case of a resistor at atemperature of 17degree centigrade not connected to a voltage source., we may think thatthere is no voltage to measure across it. This is true if the measuring instrument is a dc

voltmeter, but is false if a very sensitive electronic voltmeter is connected across it. This

voltage is the result of random motion of electrons. Thus a resistor acts as a Noise source.Now the point to be remembered here is that we cannot make thermal noise zero, as

absolute zero is not realizable till date.

Now let us come to the point how bandwidth places a limitation for the communication

system designer. If the bandwidth of the channel is insufficient to hold the spectrum ofthe signal then this results in distortion. Suppose the signal to be transmitted has a

bandwidth of 10 KHz but if the channel has a bandwidth of 5 KHz only then the channel

will pass the signals frequency component up to 5KHz and after this it will startattenuating the signal thus resulting in unintelligible signal at the receiver output.

Taking both of these limitations into consideration Hartley-Shannon in 1948 stated that

the rate of information transfer could not exceed the channel capacity.

C= B log2 (1+S/N)

Where, C= channel capacity

B= Bandwidth

S/N= Signal to Noise ratio.

The channel capacity of a channel is defined as a maximum rate of information transferwithout error through the channel.

1.7 Need Of Modulation:

The baseband signal is not suitable for transmission. So these signals are further

modified to facilitate transmission. This conversion process is called modulation where

the baseband signal is used to modify some parameters (Amplitude, frequency, phase) ofa high frequency carrier wave resulting in AM, FM, PM.

(i) Modulation for efficient transmission and reception.

For line of sight propagation the antenna used has a physical dimension of at least 1/10th

of signals wavelength. So unmodulated signal results in nonviable size of receiver and

transmitter antenna. For e.g. let us consider a not modulated signal of 3KHz. and let uscalculate the antenna dimension for this case. Now again let us consider the signal to be

modulated and the frequency is 3MHz. now on comparing the result we will find that the

antenna size for the first case is impractical while as the antenna dimension for the laterseems feasible.

Antenna size= 1/10th of wavelength ()

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(ii) Modulation to overcome hardware limitations:

(iii) Modulation to reduce noise:

A brute force method for combating noise and interference is to increase the signalpower until it overwhelms the contaminations. But increasing the signal power is costly

and may damage the equipment.(The transatlantic cable was damaged by high signal

power). Fortunately the modulation techniques like FM and other digital carrier systemshave the valuable property of noise reduction.

(iv) Modulation for frequency assignment:

When we tune a radio or a television to a particular station we are selecting one of the

many signals present at that time. This is possible because each station uses different

frequencies and the filtering process can select the desired frequency. This frequency

assignment was possible with the help of modulation. Modulation helps to place thesignal to be transmitted at different frequencies by using different carrier frequencies by

different station. If modulation was not there then only one station can transmit orbroadcast at one particular area.

(v) Modulation for multiplexing:

Multiplexing is the process of combining several signals for simultaneous transmission

through a single channel. Frequency division multiplexing uses continuous (AM, FM,

PM) to put each signal in different frequency slot and a bank of filters separates the signalat the receiving end. Similarly time division multiplexing uses pulse modulation to put

different signals in non-overlapping time slots.

End of Chapter: 1

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