# Communication System Class 12 Notes Physics Chapter 15

## Introduction

The act of transmission and reception of information is known as communication. Every living creature in the world experiences the need to impart or receive information almost continuously from others in the surrounding world.

The aim of this chapter is to introduce the concepts of communication, namely the mode of communication, the need for modulation, production, and deduction of amplitude modulation.

## Elements of a Communication System

Irrespective of its nature, every communication system has three essential elements- transmitter, medium (also known as a channel), and receiver.

In a communication system, the transmitter is located at one place, the receiver is located at some other place (far or near) separated from the transmitter and the channel is the physical medium that connects them. Depending upon the type of communication system, the channel may be in the form of wires or cables connecting the transmitter and the receiver or it may be wireless.

The purpose of the transmitter is to convert the message signal, produced by the source of information, into a form, suitable for transmission through the channel. If the output of the information source is a non-electrical signal like a voice signal, a transducer converts it to an electrical form before giving it as an input to the transmitter.

When a transmitted signal propagates along the channel it may get distorted due to channel imperfection. Moreover, noise adds to the transmitted signal and the receiver receives a corrupted version of the transmitted signal. The receiver has the task of operating on the received signal. It reconstructs a recognizable form of the original message signal for delivering it to the user of information.

There are two basic modes of communication: point-to-point and broadcast.

• In point-to-point communication mode, communication takes place over a link between a single transmitter and a receiver. Telephony is an example of such a mode of communication.

• In the broadcast mode, there are a large number of receivers corresponding to a single transmitter. Radio and television are examples of the broadcast mode of communication.

## Basic Terminology of Communication Systems

To understand the principles of communication, we must know some basic terms which will be used frequently.

• (i) Transducer: It is a device that converts one form of energy into another. In electronic communication systems, we usually come across devices that have either their inputs or outputs in the electrical form. An electrical transducer may be defined as a device that converts some physical variable (pressure, displacement, force, temperature, etc.) into corresponding variations in the electrical signal at its output. e.g. microphones, tape heads, photocells, LED etc.

• (ii) Signal: It is the Information converted into electrical form and suitable for transmission. Signals can be either analog or digital. Analog signals are continuous variations of voltage or current. They are essentially single-valued functions of time. The sine wave is a fundamental analog signal. All other analog signals can be fully understood in terms of their sine wave components. Sound and picture signals in TV are analog in nature. Digital signals are those which can take only discrete stepwise values. A binary system that is extensively used in digital electronics employs just two levels of a signal. ‘0’ corresponds to a low level and ‘1’ corresponds to a high level of voltage/current.

• (iii) Noise: The unwanted signals that tend to disturb the transmission and processing of message signals in a communication system are known as noise. The source generating the noise may be located inside or outside the system.

• (iv) Transmitter: It processes the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception.

• (v) Receiver: It extracts the desired message signals from the received signals at the channel output.

• (vi) Attenuation: It is the loss of strength of a signal while propagating through a medium.

• (vii) Amplification: Amplification refers to the process of increasing the amplitude of a signal using an electronic circuit called the amplifier. Amplification is necessary to compensate for the attenuation of the signal in communication systems. Amplification is done at a place between the source and the destination wherever signal strength becomes weaker than the required strength.

• (viii) Range: It is the largest distance between a source and a destination up to which the signal is received with sufficient strength.

• (ix) Bandwidth: It is the frequency range over which equipment operates or the portion of the spectrum occupied by the signal.

• (x) Modulation: The original low-frequency message/information signal cannot be transmitted long distances. Therefore, at the transmitter, the information contained in the low-frequency message signal is superimposed on a high-frequency wave, which acts as a carrier of the information. This process is known as modulation. There are several types of modulation, abbreviated as AM, FM, and PM.

• (xi) Demodulation: It is the process of extraction of information from the carrier wave at the receiver. It is the reverse process of modulation.

• (xii) Repeater: It is a combination of a receiver and a transmitter. A repeater picks up the signal from the transmitter, amplifies, and retransmits it to the receiver sometimes with a change in carrier frequency. Repeaters are used to extend the range of a communication system. A communication satellite is essentially a repeater station in space.

## Bandwidth of Signals

Bandwidth is the difference between the upper and lower frequencies in a continuous band of frequencies. It is typically measured in hertz (Hz).

The message signal can be voice, music, picture, or computer data. In a communication system, each of these signals has different ranges of frequencies. The types of communication system needed for a given signal depends on the range of frequencies which is considered essential for the communication process.

## The bandwidth of Transmission Medium

Different types of transmission media offer different bandwidths similar to massage signals. The commonly used transmission media are wire, free space and fiber optic cable. Coaxial cable is a widely used wire medium, which offers a bandwidth of approximately 750 MHz.

Broadly, transmission media have been divided into two types:

Guided transmission medium: It is that communication medium or channel which is used in point to point communication between a single transmitter and a receiver. For example, Optical fibers, parallel wire lines, twisted pairs, and co-axial cables are guided transmission media. Thus, a guided transmission medium is used in line communication.

Unguided transmission medium: It is that communication medium that is used, where there is no point to point contact between the transmitter and receiver. Free space is an example of an unguided transmission medium. It is used in space communication and satellite communication, such as in radio and television.

## Signals

We may define a message signal as a single-valued function of time that conveys the information. Such a function has a unique value at any instant in time. The message signals have to be converted into electrical signals to make them suitable for transmission through the channel.

The electrical signals are of two types:

(i) Analog signals: An analog signal is that in which the current or voltage value varies continuously with time. In the simplest form of an analog signal, the amplitude of the signal varies sinusoidally with time, as shown in the figure. It is represented by the equation E = E0 sin(ωt+Φ) where E0 is the maximum value of voltage, called the amplitude, t is the time period and ω=\frac{2π}{T} is the angular frequency of the signal and Φ represents the phase angle.

(ii) Digital signals: A digital signal is a discontinuous function of time, in contrast to an analog signal, wherein current or voltage value varies continuously with time. Digital signal is usually in the form of pulses. Each pulse has two levels of current or voltage, represented by 0 and 1.

## Propagation of Electromagnetic Waves

An antenna at the transmitter in communication using radio waves radiates the electromagnetic waves which travel through space and reach the receiving antenna at the other end. Several factors affect the propagation of EM waves and the path, they follow.

(i) Ground Wave: The antennas should have a size comparable to the wavelength λ of the signal (at least ~ λ/4) to radiate signals with high efficiency. At longer wavelengths (i.e., at lower frequencies), the antennas have a large physical size and they are located on or very near to the ground. In standard AM broadcast, ground-based vertical towers are generally used as transmitting antennas. For such antennas, the ground has a strong influence on the propagation of the signal. The mode of propagation is called surface wave propagation and the wave glides over the surface of the earth.

A wave induces a current in the ground over which it passes and it is attenuated as a result of the absorption of energy by the earth. The attenuation of surface waves increases very rapidly with an increase in frequency. The maximum range of coverage depends on the transmitted power and frequency (less than a few MHz). more

(ii) Sky waves: Long-distance communication can be achieved by the ionospheric reflection of radio waves back towards the earth. Sky wave propagation is used by short wave broadcast services. The ionosphere is so called because of the presence of a large number of ions or charged particles. It extends from a height of ~ 65 km to about 400 km above the earth’s surface. Ionization occurs due to the absorption of the ultraviolet and other high-energy radiation coming from the sun by air molecules. The degree of ionization varies with the height. The density of the atmosphere decreases with height. At great heights, the solar radiation is intense but there are few molecules to be ionized. Close to the earth, even though the molecular concentration is very high, the radiation intensity is low so that the ionization is again low.

However, at some intermediate heights, there occurs a peak of ionisation density. The ionospheric layer acts as a reflector for a certain range of frequencies (3 to 30 MHz). Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and escape. These phenomena are shown in the figure. The phenomenon of bending em waves so that they are diverted towards the earth is similar to total internal reflection in optics. more

(iii) Space wave: A space wave travels in a straight line from the transmitting antenna to the receiving antenna. Space waves are used for line-of-sight (LOS) communication as well as satellite communication. At frequencies above 40 MHz, communication is essentially limited to line-of-sight paths. At these frequencies, the antennas are relatively smaller and can be placed at heights of many wavelengths above the ground.

Because of the line-of-sight nature of propagation, direct waves get blocked at some point by the curvature of the earth as illustrated in Figure. If the signal is to be received beyond the horizon then the receiving antenna must be high enough to intercept the line-of-sight waves. more

## Modulation and Its Necessity

Modulation is the phenomenon of superimposing the low audio frequency baseband message or information signals (called the modulating signals) on a high-frequency wave (called, the carrier wave). The resultant wave is called the modulated wave, which is transmitted. more

## Amplitude Modulation

The amplitude of the carrier wave is varied in accordance with the amplitude of the audio frequency modulating signal. However, the frequency of the amplitude modulated wave remains the same as that of the carrier wave. To explain amplitude modulation, let us take a sinusoidal modulating signal, represented by

m(t) = Am sin ωmt .....(1)

Let the sinusoidal wave be represented as

c(t) = Ac sin ωct .....(2)

we represent amplitude modulated wave as

cm(t) = (Ac + Am sin ωct) sin ωct .....(3)

c_{m}(t)=A_{c}[1+\frac{A_m}{A_c}sin ω_{m}t] sin ω_{c}t

\frac{A_m}{A_c}=μ

where, μ is called amplitude modulation index.

Am = amplitude of modulating signal

ωmangular modulating wave frequency

Ac = amplitude of carrier wave

ωcangular carrier wave frequency

## Internet

The first use of the word "Computer" was recorded in 1613. It referred to a person who carried out calculations, often as employment. From the end of the 19th century, the word began to take on its more familiar meaning, a machine that carries out computations. It is a general-purpose device that can be programmed to carry out a set of arithmetic or logical operations automatically. Embedded computers found in many devices from MP3 players to fighter aircraft and from toys to industrial robots are the most numerous.

This was not enough and before the end of the twentieth century, we succeeded in creating a global network of computers that provides ways to exchange information and communicate among all computers connected to the network. "Internet"- a global system of interconnected computer networks that links several billion devices worldwide.

## Summary

• Communication is the act of transmission and reception of information.

• The transmitter, transmission channel, and receiver are three basic units of a communication system.

• Two important forms of the communication system are Analog and Digital.

• Two basic modes of communication are point-to-point and broadcast.

• Noise refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system.

• Attenuation is the loss of strength of a signal while propagating through a medium.

• Amplification is the process of increasing the amplitude (and consequently the strength) of a signal using an electronic circuit caller amplifier.

• The range is the largest distance between a source and destination up to which the signal is received with sufficient strength.

• Band-width refers to the frequency range over which equipment operates or the portion of spectrum occupied by the signal.

• Modulation: At the transmitter, the information contained in the low-frequency message, is superimposed on a high-frequency wave. This process is known as modulation.

• Three types of modulation are amplitude modulation (AM), frequency modulation (FM), and pulse modulation (PM).

• De-modulation is the reverse process of modulation. This is the process of extraction of information from the carrier wave.