# CBSE Important Questions Class 12 Physics Chapter 15

## Important Questions for CBSE Class 12 Physics Chapter 15 – Communication Systems

These important questions in class 12 physics chapter 15 will teach students about communication systems. After reviewing these CBSE revision notes, students will understand how communication systems work. Students would also be better prepared for their exams by studying these chapter 15 class 12 physics important questions and could solve several CBSE sample papers after solving these physics class 12 chapter 15 important questions.

These notes also come with important formulas and CBSE extra questions that can be used to assess your comprehension.

### Study Important Questions for Class 12 Physics Chapter 15-Communication Systems

1. What communication system uses discrete, binary-coded signals to transmit messages or information?

Ans: A digital communication system is one in which the signal is discrete, and the message or information is encoded in binary.

1. What does it mean to modulate a signal during transmission?

Ans: Modulation extends the transmission range of low-frequency signals because they cannot be carried over large distances.

1. What are the conditions for sending microwaves from one location on the earth to another?

Ans: The transmitting and receiving antennas must be pointed in the same direction.

1. A TV tower is 300 metres tall. What is the farthest distance over which a TV signal may be received?

Ans: The answer to the question states that the tower is 300 metres tall. The maximum transmission distance must be determined.

By using the formula: d = 2Rh

Where d is the transmission distance, R is the Earth’s radius, which is 6400 km, and h is the tower’s height.

h= 300, R= 6400 X 1000 in the formula d= 2Rh

d= 2X6400X1000X300

= 61.96 X 1000

62 km

Up to 62 kilometres is the highest distance at which TV transmissions can be received.

1. Why are high frequencies not suitable for ground wave propagation?

Ans: Signals with frequencies higher than 1500 kHz cannot be transmitted because they are primarily absorbed by the Earth’s surface. This makes higher frequencies unsuitable for ground wave propagation.

1. What kind of modulation is applied to speech broadcasts for commercial purposes?

Ans: Amplitude modulation is used to broadcast voice signals for commercial purposes.

1. An immediate level jump occurs in a signal. How often will it occur?

Ans: If the signal abruptly switches between levels, its frequency is limitless.

1. The sky is infinite, yet the spread of sky waves has a limit. Tell us why.

Ans: Sky wave propagation is limited because radio signals are reflected by the ionosphere, while high-frequency waves are absorbed by the ionosphere rather than reflected.

1. The height of a transmitting antenna is 50 meters. If the earth’s radius is assumed to be 6250 km, find the area that it covers.

Ans: The answer to the question states that the antenna is 50 metres high. It is asked to determine the region that it covers.

Using the formula, d = 2Rh

Where d is the transmission distance, R is the Earth’s radius, which is 6250 km, and h is the antenna’s height.

Put, h = 50 and R = 6250X1000 in the formula, d= 2Rh

d= 2rh

d= 2X6250X50X103

d= 2.5 X 104 m

Put the value of d in the formula of area of the circle, d2.

Area covered = d2

= 3.14 X (2.5 X 104)2

= 1963 km2

1. What function does layer play in communication?

Ans: F2 refers to the top layer of the ionosphere. It has a height of up to 400 kilometres and acts as a reflecting layer for high-frequency radio waves.

1. How should an entire communication system be put up?

Ans: A whole communication system consists of an information source, a transmitter, a link, and a receiver. The transmitter is often located in one location in a basic communication system, the receiver is placed in a separate location to distinguish them from the transmitter, and the channel is the specific physical medium that establishes the connections between them.

1. Differentiate between digital and analogue communication.

Ans:

 Analog Communication Digital Communication In analogue communication, an analog signal continuously varies with time. A digital signal with only two levels, high and low, is used in digital communication.
1. Which frequencies will be appropriate for communication utilizing sky waves beyond the horizon?

(a) 10 kHz

(b) 10 MHz

(c) 1 GHz

(d) 1000 GHz

Ans: As of 10 MHz.

Beyond the horizon, the signal waves must travel a great distance to communicate. 10 kHz transmissions cannot be adequately disseminated due to the antenna size. High-energy signal waves enter the ionosphere (1GHz – 1000GHz). The ionosphere readily reflects frequencies up to 10 MHz. Therefore, the use of signal waves at such frequencies for communication beyond the horizon is allowed.

1. In the UHF band, frequencies often spread using the:

(a) Ground waves

(b) Skywaves

(c) Surface waves

(d) Space waves

Ans: (d) Space waves

Due to their extremely high frequency, ultra-high frequency (UHF) waves are incapable of being reflected by the ionosphere or following the earth’s curvature. Line-of-sight communication, which simply means that the waves move in a straight line, is how UHF signals are transmitted.

1. Digital signals

(i) avoid supplying a continuous range of numbers;

(ii) convey values in the form of discrete steps,

(iii) can use the binary system;

(iv) can work with binary and decimal systems.

Which of the claims given above are true?

(a): Only (i) and (ii);

(b): Only (ii) and (iii);

(c): Only (i), (ii), and (iii);

(d): (i) (ii), (iii), and (iv).

Ans: (c) A digital signal sends messages using the binary (0 and 1) system. The decimal system cannot be trusted with this type of system (which correlates to analogue signals). This is because digital signals are made up of discrete values.

1. Describe briefly why modulating low-frequency signals into high-frequency signals is necessary.

Ans: Modulation is required

(i) to send a low-frequency signal to a far-off location;

(ii) to minimize the height of the antenna;

(iii) to prevent the signals from various stations from becoming muddled.

1. Describe the roles of the

(i) repeater,

Ans: (i) A repeater combines a transmitter and a receiver. It is employed to extend the range of signal communication. A repeater picks up the signal, amplifies it, and then sends it back to the receiver.

(ii) The receiver separates the required message signals from the received signals at the channel output. It consists of a receiving antenna, an amplifier, a demodulator, an intermediate frequency converter, and an amplifier for the audio signal.

1. Differentiate between space and sky wave communication methods. What is the space wave mode’s biggest drawback? When using this style of communication, calculate the expression for the ideal distance between the sending and receiving antennas.

Ans: (i) In the sky waveform of communication, waves move from the transmitting antenna to the receiving antenna via ionosphere reflections. In the space wave style of communication, waves travel directly from the transmitter to the receiver or via satellites.

(ii) Direct waves eventually become obstructed because of the globe’s curvature.

(iii) The ideal separation between the transmitting and receiving antennas is 2hTR + 2hRR

1. Create a block diagram for a system of general communications. Each of the following should have a function listed:

The transmitter, the channel, and the receiver.

Ans: Generalized communication system block diagram:

Functions:

(a) Transmitter: A transmitter transforms an incoming message signal into a form that may be transmitted across a channel and then received.

(b) Channel: It transports the message’s signal from a transmitter to a receiver.

(c) Receiver: At the channel output, a receiver separates the desired message signals from the received signals.

1. Describe the concept of signal amplitude modulation. The highest and minimum amplitudes of an amplitude-modulated wave are respectively 10 V and 2 V. Make a modulation index calculation.

Ans: The process of superimposing a message signal over a carrier wave in which the carrier wave’s amplitude is altered in accordance with the message/information signal is known as amplitude modulation.

Given, (A) maximum = 10V

(A) minimum = 2V, = ?

ac + am = 10

amac = 2

Hence, 2ac = 12 and ac = 6V

am = 4V

= amac = 46 = 23

Therefore, Modulation Index () = 23

1. What does “modulation,” a term used in communication systems, mean? First, identify the two modulation types in this example. Then, give two reasons why any one of these is better than the other.

Ans: (i) Modulation is a procedure in which one of a high-frequency carrier wave’s characteristics (amplitude, frequency, or phase) is changed in response to a specific low-frequency message signal.

(ii) (a) Modulated wave 1 : Frequency Modulation (FM)

(b) Modulated wave 2 : Amplitude Modulation (AM)

FM has two benefits over AM:

1. Better power efficiency and less noise.
2. Increased operational range.
3. Improved reception quality.

AM has two benefits over FM:

1. We need simple circuits.
2. Lower transmission frequency range.
3. “Detection of a modulated signal” – what does that mean? First, draw a block diagram of an AM wave detector, then quickly describe how the original message signal is obtained while displaying the waveforms.

Ans: Recognizing a modulated signal recovery of the modulated signal from the modulated carrier wave is the detection process. The following straightforward approach is provided in the form of a block diagram to obtain the original message signal m (+) of angular frequency com. The modulated carrier wave has the frequencies of and eoc+ com.

1. Describe the need for high-frequency carrier waves to efficiently transmit signals. A carrier wave with a frequency of 12 MHz and a peak voltage of 30 V is modulated using a message signal with a peak voltage of 20 V at 12 kHz. Determine the:

(i) Modulation index

(ii) Side-band frequencies

Ans: (a) With high-frequency carrier waves, transmission is possible with a tolerable antenna length because power radiation grows proportionately to 1/2.

(b) (i) Modulation index, = AmAc = 2030 = 0.67

(ii) U.S.B. = c + m

= (12 X 106 + 12 X 103) Hz

Hence, U.S.B. = 1.21 X 107 Hz

L.S.B. = cm

= (12 X 106 – 12 X 103) Hz

Hence, L.S.B. = 1.19 X 107 Hz

(i) Plot the amplitude variation against an amplitude-modulated wave. (ii) Define modulation index. (iii) Mention how crucial it is for efficient amplitude modulation.

Ans: (i) Plot of ‘amplitude’ versus ‘ω’ for an amplitude-modulated signal

(ii) The modulation index measures the relationship between the message or modulating signal’s amplitude and the carrier wave’s amplitude.

= AmAc

(iii) Importance:

• The modulation index determines the strength and calibre of the transmitted signal.
• By maintaining p<1, distortions are prevented.
1. A carrier with a frequency of 1 MHz and a peak voltage of 20 V is modulated using a message signal with a peak voltage of 10 V and a frequency of 10 kHz. Determine

(i) The modulation index

(ii) The created sidebands

Ans:  m = 10 KHz

Em= 10 V

c = 1 MHz

Ec = 20 V

(i) Modulation Index = EmEc = 10 V20 V = 12 = 0.50

(ii) Side Bands = c +m

Side Bands = 106 Hz + 10 Hz

Side Bands = 1000 Hz + 10 Hz

Hence, Upper sidebands = 1010 Hz

Lower sidebands = 990 Hz

1. Find the boxes marked X and Y in the provided block diagram of a receiver and describe what they do.

Ans: X- Intermediate frequency

Y- Power amplifier/amplifier

The IF stage’s purpose is to represent the intermediate frequency step before detection. Therefore, the IF step often modifies the carrier frequency to a lower frequency. The purpose of an amplifier is to amplify or raise the strength of an input signal.

1. List two reasons why modulating a signal is necessary. First, an information signal is transmitted using a carrier wave with a peak voltage of 12 V. What should the modulating signal’s peak voltage be to achieve a modulation index of 75%?

Ans: (a) Modulation is necessary for a signal because:

(i) A reasonable-sized antenna can transmit a modulated signal with a high frequency.

(ii) A modulated signal can be transmitted with more power because the power radiated is proportional to (12)

(b) We know, = AmAc

Hence, Am = Ac = 75100X12 = 9V

1. Write down the purpose of

(i) the transducer

(ii) In the context of a communication system, a repeater

Ans: (i) Transducer: A transducer is a tool that transforms one type of energy into another.

(ii) Repeater: A repeater is a receiver and transmitter combo that increases the communication system’s range.

1. Differentiate between frequency and amplitude modulation.

Ans: In contrast to frequency modulation, which offers higher transmission quality, amplitude modulation offers a broader coverage area.

1. Give the name of the wave type employed in line-of-sight (LOS) communication. What frequency range do they have? The receiving antenna is on the ground, while the sending antenna is 45 metres high at the top of the tower. Determine the farthest distance to allow them to communicate effectively in LOS mode. (Earth’s radius is 6.4 106 m)

Ans: Space waves

Frequency range above 40 MHz

Max. distance, d = 2hTR

d = 2 X 45 X 6.4 X 106

d = 24 X 103 m

= 24 km

To help students with exam preparation, Extramarks provides Class 12 Physics Chapter 15 important questions. Extramarks also provides the best quality CBSE previous year question papers based on NCERT books and according to the latest CBSE syllabus. The questions are well-structured and cover all the important concepts. They can be of great help in last-minute revisions.

## 1. What is the propagation of sky waves?

Sky wave propagation sends signals at frequencies below 40 MHz by utilizing the ionosphere’s reflective properties.

## 2. What purpose does a repeater serve in a communications system?

In a communication system, a repeater’s job is to increase the range of communication.

## 3. Give a specific illustration of point-to-point communication.

Point-to-point communication mode illustration: Telephone

## 4. Why do incoming and outgoing signals on a mobile phone have different frequencies?

To prevent signal overlap.

## 5. What does the term "attenuation," used in communication systems, mean?

“Attenuation” refers to a signal’s loss of strength as it travels through a medium.