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NCERT Class 12 Physics Textbook Chapter 15 With Answer PDF Free Download
Chapter 15: Communication Systems
15.1 Introduction
Communication is the act of transmission of information. Every living creature in the world experiences the need to impart or receive information almost continuously from others in the surrounding world.
For communication to be successful, it is essential that the sender and the receiver understand a common language.
Man has constantly made endeavors to improve the quality of communication with other human
beings.
Languages and methods used in communication have kept evolving from prehistoric to modern times, to meet the growing demands in terms of speed and complexity of information.
It would be worthwhile to look at the major milestones in events that promoted developments in
communications, as presented in Table 15.1.
Modern communication has its roots in the 19th and 20th centuries in the work of scientists like J.C. Bose, F.B. Morse, G. Marconi, and Alexander Graham Bell.
The pace of development seems to have increased dramatically after the first half of the 20th century. We can hope to see many more accomplishments in the coming decades.
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.
15.3 Basic Terminology Used In Electronic Communication Systems
By now, we have become familiar with some terms like information source, transmitter, receiver, channel, noise, etc.
It would be easy to understand the principles underlying any communication if we get ourselves acquainted with the following basic terminology.
(i) Transducer: Any device that converts one form of energy into another can be termed a transducer.
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 variables (pressure, displacement, force, temperature, etc) into corresponding variations in the electrical signal at its output.
(ii) Signal: Information converted into electrical form and suitable for transmission is called a signal.
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.
The 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. There are several coding schemes useful for digital communication.
| Author | NCERT |
| Language | English |
| No. of Pages | 19 |
| PDF Size | 1657 KB |
| Category | Physics |
| Source/Credits | ncert.nic.in |
NCERT Solutions Class 12 Physics Chapter 15 Communication Systems
Q.1: Which of the following frequencies will be suitable for beyond-the horizon communication using sky waves?
(1) 10 kHz
(2) 10 MHz
(3) 1 GHz
(4) 1000 GHz
Soln:
(2) 10 MHz
The signal waves need to travel a large distance for beyond–horizon communication.
Due to the antenna size, the 10 kHz signals cannot be radiated efficiently.
The 1 GHz – 1000 GHz (high energy) signal waves penetrate the ionosphere.
The 10 MHz frequencies get reflected easily from the ionosphere. Therefore, for beyond–horizon communication signal waves of 10 MHz frequencies are suitable.
Q.2: Frequencies in the UHF range normally propagate by means of :
(1) Ground Waves
(2) Sky Waves
(3) Surface Waves
(4) Space Waves
Soln:
(4) Space Waves
Due to its high frequency, an ultra-high frequency (UHF) wave cannot travel along the trajectory of the ground also it cannot get reflected by the ionosphere. The ultrahigh-frequency signals are propagated through line–of–sight communication, which is actually space wave propagation.
Q.3: Digital signals
(i) Do not provide a continuous set of values
(ii) Represent value as discrete steps
(iii) Can utilize the binary system
(iv) Can utilize decimal as well as binary systems
State which statement(s) are true?
(a) (1), (2), and (3)
(b) (1) and (2) only
(c) All statements are true
(d) (2) and (3) only
Soln:
(a) (1), (2), and (3) For transferring message signals the digital signals use the binary (0 and 1) system. Such a system cannot utilize the decimal system. Discontinuous values are represented in digital signals.
Q.4: Is it necessary for a transmitting antenna to be at the same height as that of the receiving antenna for line-of-sight communication? A TV transmitting antenna is 81 m tall. How much service area can it cover if the receiving antenna is at the ground level?
Soln: In line–of–sight communication, between the transmitter and the receiver there is no physical obstruction. So, there is no need for the transmitting and receiving antenna to be at the same height.
Height of the antenna, h = 81 m
Radius of earth, R = 6.4 x 106m
d = √2Rh, for range
The service area of the antenna is given by the relation :
A = πd2 = π(2Rh)
= 3.14 x 2 x 6.4 x 106 x 81
= 3255.55 x 106 m2 = 3255.55 = 3256 km2
Q.5: A carrier wave of peak voltage 12 V is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75%?
Soln:
Given:
The amplitude of carrier wave, Ac = 12 V
Modulation index, m = 75% = 0.75
The amplitude of the modulating wave = Am
Modulation index is given by the relation :
m = \frac{A_{m}}{A_{c}}AcAm
Therefore, Am = m.Ac
= 0.75 x 12 V= 9 V
NCERT Class 12 Physics Textbook Chapter 15 With Answer PDF Free Download