Text Book Back Questions and Answers - Chapter 10 Communication Systems 12th Science Guide Samacheer Kalvi Solutions - SaraNextGen [2024-2025]

Updated On May 15, 2024

By SaraNextGen

Communication Systems
Multiple Choice Questions
Question 1.
The output transducer of the communication system converts the radio signal into
(a) Sound
(b) Mechanical energy
(c) Kinetic energy
(d) None of the above
Answer:
(a) Sound.
Question 2.
The signal is affected by noise in a communication system
(a) At the transmitter
(b) At the modulator
(c) In the channel
(d) At the receiver
Answer:
(c) In the channel.
Question 3.
The variation of frequency of carrier wave with respect to the amplitude of the modulating signal is called
(a) Amplitude modulation
(b) Frequency modulation
(c) Phase modulation
(d) Pulse width modulation
Answer:
(b) Frequency modulation.
Question 4.
The internationally accepted frequency deviation for the purpose of FM broadcasts
(a) $75 \mathrm{kHz}$
(h) $68 \mathrm{kHz}$
(c) $80 \mathrm{kHz}$
(d) $70 \mathrm{kHz}$
Answer:
(a) $75 \mathrm{kHz}$.

Question 5.
The frequency range of $3 \mathrm{MHz}$ to $30 \mathrm{MHz}$ is used for
(a) Ground wave propagation
(b) Space wave propagation
(c) Sky wave propagation
(d) Satellite communication
Answer:
(c) Sky wave propagation.
Short Answer Questions
Question 1.
Give the factors that are responsible for transmission impairments.
Answer:
- Attenuation
- Distortion (Harmonic)
- Noise
Question 2.
Distinguish between wireline and wireless communication? Specify the range of electromagnetic waves in which it is used.
Answer:

Question 3.
Explain centre frequency or resting frequency in frequency modulation.
Answer:
When the frequency of the baseband signal is zero (no input signal), there is no change in the frequency of the carrier wave. It is at its normal frequency and is called as centre frequency or resting frequency.
Question 4.
What does RADAR stand for?
Answer:
Radar basically stands for Radio Detection and Ranging System.
Question 5.
What do you mean by Internet of Things?
Answer:
Internet of Things (IoT), it is made possible to control various devices from a single device. Example: home automation using a mobile phone.
Long Answer Questions
Question 1.
What is modulation? Explain the types of modulation with necessary diagrams.
Answer:
Modulation:
For long distance transmission, the low frequency baseband signal (input signal) is superimposed onto a high frequency radio signal by a process called modulation. There are 3 types of modulation based on which parameter is modified. They are (i) Amplitude modulation, (ii) Frequency modulation, and (iii) Phase modulation.
(i) Amplitude Modulation (AM):
If the amplitude of the carrier signal is modified according to the instantaneous amplitude of the baseband signal, then it is called amplitude modulation. Here the frequency and the phase of the carrier signal remain constant. Amplitude modulation is used in radio and TV broadcasting.

The signal shown in figure (a) is the message signal or baseband signal that carries information, figure (b) shows the high frequency carrier signal and figure (c) gives the amplitude modulated signal. We can see clearly that the carrier wave is modified in proportion to the amplitude of the baseband signal

(ii) Frequency Modulation (FM):
The frequency of the carrier signal is modified according to the instantaneous amplitude of the baseband signal in frequency modulation. Here the amplitude and the phase of the carrier signal remain constant. Increase in the amplitude of the baseband signal increases the frequency of the carrier signal and vice versa. This leads to compressions and rarefactions in the frequency spectrum of the modulated wave. Louder signal leads to compressions and relatively weaker signals to rarefactions.

When the amplitude of the baseband signal is zero in Figure (a), the frequency of the modulated signal is the same as the carrier signal. The frequency of the modulated wave increases when the amplitude of the baseband signal increases in the positive direction (A,C). The increase in amplitude in the negative half cycle (B,D) reduces the frequency of the modulated wave (Figure (c)).
(iii) Phase Modulation (PM)
The instantaneous amplitude of the baseband signal modifies the phase of the carrier signal keeping the amplitude and frequency constant is called phase modulation. This modulation is used to generate frequency modulated signals. It is similar to frequency modulation except that the phase of the carrier is varied instead of varying frequency.

The carrier phase changes according to increase or decrease in the amplitude of the baseband signal. When the modulating signal goes positive, the amount of phase lead increases with the amplitude of the modulating signal. Due to this, the carrier signal is compressed or its frequency is increased.
On the other hand, the negative half cycle of the baseband signal produces a phase lag in the carrier signal. This appears to have stretched the frequency of the carrier wave. Hence similar to frequency modulated wave, phase modulated wave also comprises of compressions and rarefactions. When the signal voltage is zero ( $\mathrm{A}, \mathrm{C}$ and $\mathrm{E}$ ) the carrier frequency is unchanged.
Question 2.
Elaborate on the basic elements of communication system with the necessary block diagram.

Answer:
Elements of an electronic communication system:
1. Information (Baseband or input signal):
Information can be in the form of a sound signal like speech, music, pictures, or computer data
Information can be in the form of a sound signal like speech, music, pictures, or computer data which is
given as input to the input transducer.

2. Input transducer:
A transducer is a device that converts variations in a physical quantity (pressure, temperature, sound) into an equivalent electrical signal or vice versa. In communication system, the transducer converts the information which is in the form of sound, music, pictures or computer data into corresponding electrical signals. The electrical equivalent of the original information is called the baseband signal. The best example for the transducer is the microphone that converts sound energy into electrical energy.
3. Transmitter:
It feeds the electrical signal from the transducer to the communication channel. It consists of circuits such as amplifier, oscillator, modulator, and power amplifier. The transmitter is located at the broadcasting station.
4. Amplifier:
The transducer output is very weak and is amplified by the amplifier.
5. Oscillator:
It generates high-frequency carrier wave (a sinusoidal wave) for long distance transmission into space. As the energy of a wave is proportional to its frequency, the carrier wave has very high energy.
6. Modulator:
It superimposes the baseband signal onto the carrier signal and generates the modulated signal.
7. Power amplifier:
It increases the power level of the electrical signal in order to cover a large distance.
8. Transmitting antenna:
It radiates the radio signal into space in all directions. It travels in the form of electromagnetic waves with the velocity of light $\left(3 \times 10^8 \mathrm{~ms}^{-1}\right)$.
9. Communication channel:
Communication channel is used to carry the electrical signal from transmitter to receiver with less noise or distortion. The communication medium is basically of two types: wireline communication and wireless communication.
10. Noise:
It is the undesirable electrical signal that interfaces with the transmitted signal. Noise attenuates or reduces the quality of the transmitted signal. It may be man-made (automobiles, welding machines, electric motors etc .) or natural (lightening, radiation from sun and stars and environmental effects). Noise cannot be completely eliminated. However, it can be reduced using various techniques.

11. Receiver:
The signals that are transmitted through the communication medium are received with the help of a receiving antenna and are fed into the receiver. The receiver consists of electronic circuits like demodulator, amplifier, detector etc. The demodulator extracts the baseband signal from the carrier signal. Then the baseband signal is detected and amplified using amplifiers. Finally, it is fed to the output transducer.
12. Repeaters:
Repeaters are used to increase the range or distance through which the signals are sent. It is a combination of transmitter and receiver. The signals are received, amplified, and retransmitted with a carrier signal of different frequency to the destination. The best example is the communication satellite in space.
13. Output transducer:
It converts the electrical signal back to its original form such as sound, music, pictures or data. Examples of output transducers are loudspeakers, picture tubes, computer monitor, etc.
14. Attenuation:
The loss of strength of a signal while propagating through a medium is known as attenuation. Range It is the maximum distance between the source and the destination up to which the signal is received with sufficient strength.
Question 3.
Explain the three modes of propagation of electromagnetic waves through space. Propagation of electromagnetic waves:
Answer:
The electromagnetic wave transmitted by the transmitter travels in three different modes to reach the receiver according to its frequency range:
1. Ground wave propagation (or) surface wave propagation (nearly $2 \mathrm{kHz}$ to $2 \mathrm{MHz}$ )
2. Sky wave propagation (or) ionospheric propagation (nearly $3 \mathrm{MHz}$ to $30 \mathrm{MHz}$ )
3. Space wave propagation (nearly $30 \mathrm{MHz}$ to $400 \mathrm{GHz}$ )
1. Ground wave propagation:
If the electromagnetic waves transmitted by the transmitter glide over the surface of the earth to reach the receiver, then the propagation is called ground wave propagation. The corresponding waves are called ground waves or surface waves.

Both transmitting and receiving antennas must be close to the earth. The size of the antenna plays a major role in deciding the efficiency of the radiation of signals. During transmission, the electrical 

signals are attenuated over a distance. Some reasons for attenuation are as follows:

1. Increasing distance:
The attenuation of the signal depends on (i) power of the transmitter (ii) frequency of the transmitter, and (iii) condition of the earth surface.
2. Absorption of energy by the Earth: When the transmitted signal in the form of EM wave is in contact with the Earth, it induces charges in the Earth and constitutes a current. Due to this, the earth behaves like a leaky capacitor which leads to the attenuation of the wave.
3. Tilting of the wave:
As the wave progresses, the wavefront starts gradually tilting according to the Skywave curvature of the Earth. This increase in the tilt decreases the electric field strength of the wave. Finally, at some distance, the surface wave dies out due to energy loss.

The frequency of the ground waves is mostly less than $2 \mathrm{MHz}$ as high frequency waves undergo more absorption of energy at the earth's atmosphere. The medium wave signals received during the day time use surface wave propagation. It is mainly used in local broadcasting, radio navigation, for ship-to-ship, ship-to-shore communication and mobile communication.
2. Sky Wave Propagation:
The mode of propagation in which the electromagnetic waves radiated from an antenna, directed upwards at large angles gets reflected by the ionosphere back to earth is called sky wave propagation or ionospheric propagation. The corresponding waves are called sky waves.

The frequency range of EM waves in this mode of propagation is 3 to $30 \mathrm{MHz}$. EM waves of frequency more than $30 \mathrm{MHz}$ can easily penetrate through the ionosphere and does not undergo reflection. It is
used for short wave broadcast services. Medium and high frequencies are for long-distance radio communication.

Extremely long distance communication is also possible as the radio waves can undergo multiple reflections between the earth and the ionosphere. A single reflection helps the radio waves to travel a distance of approximately $4000 \mathrm{~km}$.

Ionosphere acts as a reflecting surface. It is at a distance of approximately $50 \mathrm{~km}$ and spreads up to 400 $\mathrm{km}$ above the Earth surface. Due to the absorption of ultraviolet rays, cosmic ray, and other high energy radiations like a, p rays from sun, the air molecules in the ionosphere get ionized.

This produces charged ions and these ions provide a reflecting medium for the reflection of radio waves or communication waves back to earth within the permitted frequency range. The phenomenon of bending the radio waves back to earth is nothing but the total internal reflection.
3. Space wave propagation:
The process of sending and receiving information signal through space is called space wave communication. The electromagnetic waves of very high frequencies above $30 \mathrm{MHz}$ are called as space waves. These waves travel in a straight line from the transmitter to the receiver. Hence, it is used for a line of sight communication (LOS).

For high frequencies, the transmission towers must be high enough so that the transmitted and received signals (direct waves) will not encounter the curvature of the earth and hence travel with less attenuation and loss of signal strength. Certain waves reach the receiver after getting reflected from the ground.
Question 4
What do you know about GPS? Write a few applications of GPS.
Answer:
GPS stands for Global Positioning System. It is a global navigation satellite system that offers geolocation and time information to a GPS receiver anywhere on or near the Earth. GPS system works with the assistance of a satellite network. Each of these satellites broadcasts a precise signal like an ordinary radio signal.
These signals that convey the location data are received by a low-cost aerial which is then translated by the GPS software. The software is able to recognize the satellite, its location, and the time taken by the signals to travel from each satellite. The software then processes the data it accepts from each satellite to estimate the location of the receiver.
Applications:
Global positioning system is highly useful many fields such as fleet vehicle management (for tracking cars, trucks and buses), wildlife management (for counting of wild animals) and engineering (for making tunnels, bridges etc).
Question 5.
Give the applications of ICT in mining and agriculture sectors.
Answer:
(i) Agriculture:
The implementation of information and communication technology (ICT) in agriculture sector enhances the productivity, improves the living standards of farmers and overcomes the challenges and risk factors.
(a) ICT is widely used in increasing food productivity and farm management.
(b) It helps to optimize the use of water, seeds and fertilizers etc.
(c) Sophisticated technologies that include robots, temperature and moisture sensors, aerial images, and GPS technology can be used.
(d) Geographic information systems are extensively used in farming to decide the suitable place for the species to be planted.

(ii) Mining:
(a) ICT in mining improves operational efficiency, remote monitoring and disaster locating system.
(b) Information and communication technology provides audio-visual warning to the trapped underground miners.
(c) It helps to connect remote sites.
Question 6.
Modulation helps to reduce the antenna size in wireless communication - Explain. Antenna size:
Answer:
Antenna is used at both transmitter and receiver end. Antenna height is an important parameter to be discussed. The height of the antenna must be a multiple of $\frac{\lambda}{4}$, $\mathrm{h}=\frac{\lambda}{4} \ldots(1)$
where $\lambda$ is wavelength $\left(\lambda=\frac{c}{v}\right), \mathrm{c}$ is the velocity of light and $\mathrm{v}$ is the frequency of the signal to be transmitted.
An example:
Let us consider two baseband signals. One signal is modulated and the other is not modulated. The frequency of the original baseband signal is taken as $\mathrm{v}=10 \mathrm{kHz}$ while the modulated signal is $\mathrm{v}=1$ $\mathrm{MHz}$. The height of the antenna required to transmit the original baseband signal of frequency $\mathrm{v}=10$ $\mathrm{kHz}$ is
$
\mathrm{h}_1=\frac{\lambda}{4}=\frac{c}{4 v}=\frac{3 \times 10^8}{4 \times 10 \times 10^3}=7.5 \mathrm{~km} \text {.... (2) }
$
The height of the antenna required to transmit the modulated signal of frequency $v=10 \mathrm{kHz}$ is $\mathrm{h}_1=\frac{\lambda}{4}=\frac{c}{4 v}=\frac{3 \times 10^8}{4 \times 10 \times 10^6}=75 \mathrm{~m}$
Comparing equations (2) and (3), we can infer that it is practically feasible to construct an antenna of height $75 \mathrm{~m}$ while the one with $7.5 \mathrm{~km}$ is not possible. It clearly manifests that modulated signals reduce the antenna height and are required for long distance transmission.
Question 7.
Fiber optic communication is gaining popularity among the various transmission media -justify.
Answer:
The method of transmitting information from one place to another in terms of light pulses through an optical fiber is called fiber optic communication. It is in the process of replacing wire transmission in communication systems. Light has very high frequency $(400 \mathrm{THz}-790 \mathrm{THz})$ than microwave radio systems. The fibers are made up of silica glass or silicon dioxide which is highly abundant on Earth.

Now it has been replaced with materials such as chalcogenide glasses, fluoroaluminate crystalline materials because they provide larger infrared wavelength and better transmission capability. As fibers are not electrically conductive, it is preferred in places where multiple channels are to be laid and isolation is required from electrical and electromagnetic interference.
Applications:
Optical fiber system has a number of applications namely, international communication, inter-city communication, data links, plant and traffic control and defense applications.
Merits:
1. Fiber cables are very thin and weight lesser than copper cables.
2. This system has much larger bandwidth. This means that its information canying capacity is larger.
3. Fiber optic system is immune to electrical interferences.
4. Fiber optic cables are cheaper than copper cables.
Demerits:
1. Fiber optic cables are more fragile when compared to copper wires.
2. It is an expensive technology.