Important Questions for CBSE Class 12 Physics Chapter 15 - Communication Systems

CBSE Class 12 Physics Chapter-15 Important Questions – Free PDF Download

Free PDF download of Important Questions with Answers for CBSE Class 12 Physics Chapter 15 – Communication Systems prepared by expert Physics teachers from latest edition of CBSE(NCERT) books only by CoolGyan to score more marks in CBSE board examination.
You can also Download Physics Revision Notes Class 12 to help you to revise complete Syllabus and score more marks in your examinations.

CBSE Class 12 Physics Important Questions
15 – Communication Systems

1.Communication Communication is the act of transmission and reception of information.
2.Communication System A system comprises of transmitter, communication channel and receiver.
A block diagram of a generalised communication system is shown as below

3.Transmitter It consists of transducer/signal generators, modulators and transmitting antenna.
4.Receiver Its main function is to decode the original signals. The main function involves picking up the signals, demodulating and displace the original message signal
5.Communication Channel The physical path between the transmitter and receiver is known as communication channel. They are of two types namely (i) Guided (point-to-point) (ii) Unguided
6.Bandwidth of Communication Channel The range of frequencies used to pass through channel is known as bandwidth.


8.There are two basic modes of communication given as below:

• Point-to-point In this type of communication mode, communication takes place over a link between a signal transmitter and a receiver, e.g. telephony.
• Broadcast In the broadcast mode, there are a large number of receivers corresponding to a signal transmitter, e.g. radio and TV.

9.Basic Terminology used in Electronic Communication Systems

• Signal Information converted into electrical form and suitable for transmission is called a signal.
• Transducer Any device/arrangement that converts one form of energy into another is called a transducer, e.g. microphone.
• Noise It refers to the unwanted signals that tends to disturb the transmission and processing of message signals in communication system.
• Attenuation It refers to the loss of strength of a signal during its propagation through the communication channel.
• Amplification It is the process by which amplitude of a signal is increased using an electronic circuit called the amplifier.
• Range It is the largest distance between a source and a destination up to which the signal is received with sufficient strength.
• Baseband Band of frequencies representing the original signal is called baseband.
•  Repeater Repeaters are erected at suitable distances between the transmitter and
  receiver. Repeaters are used to extend the range of a communication system

10.Message Signals A time varying electrical signal generated by a transducer out of original signal is termed as message signal.
The electrical signals are of two types such as below:

• Analog signal A continuous signal value which at any instant lies within the range of a maximum and a minimum value.

Graphical representation of analog signal can be represented as given below:

• Digital Signal (Pulse Signal) Digital signals are those which can take only discrete stepwise values e.g. output of a computer, fax, etc.

11.Coding schemes used for digital communication are given as below:

• Binary Coded Decimal (BCD) In this, a digit is represented by two binary numbers 0 or 1.
• American Standard Code for Information Interchange (ASCII) It is a universally popular digital code to represent numbers, letters and certain characters.

12. Bandwidth of Signals Bandwidth of signal is defined as the difference between the upper and lower frequencies of signal. In a communication system, the message signal can be voice, music, picture or computer data. This has been shown in the table given as below

13.Bandwidth of Transmission Medium The commonly used transmission media are wire, free space, fibre optic cable (750 MHz ) and optical fibre (100 GHz.)
This range is sub-divided further and allocated for various services as indicated in the table given as below:

14.Antenna Antenna is a device which acts as an emitter of electromagnetic waves and it also acts as a first receiver of energy. It is generally a metallic object often a wire or collection of wires.

15.Propagation of Electromagnetic Waves In communication using radio waves, an antenna at the transmitter radiates the EM waves, which travel through the space and reach the receiver at the other end.
16.Depending upon frequency and ways of propagation, electromagnetic waves categorised as follows
(i) Ground Wave Propagation (f< 2MHz) In ground wave propagation, the radio waves (AM) travel along the surface of the earth. These waves are guided along the earths surface and they follow the curvature of the earth.

(ii) Sky Wave Propagation (2 MHz < f < 30 MHz) Long distance communication can be achieved by ionospheric reflection of radio waves back towards earth. This mode of propagation is called sky wave propagation and is used by short wave broadcast services. The ionosphere is so called because of the presence of a large number of ions. It extends from height of 65 km to about 400 km above the earth’s surface.
The details are in the table as below:

The density of atmosphere decreases with height.
The ionospheric layer acts as a reflector for a certain range of frequencies. These phenomena are shown as below


17.Satellite Communication In this communication, frequency band 5.9 GHz to 6.4 GHz is used for uplinking and 3.7 GHz to 2 GHz is used for downlinking.

Previous Years Examination Questions

1 Mark Questions

1.The figure given below shows the block diagram of a generalised communication system. Identify the element labelled X and write its function.      [Delhi 2014 C]

Ans. Labelled element X represents the channel. Its function is to transmit information from one place to another.
2.What is the meaning of the term attenuation used in communication system?  [All India 2014C]
Ans.It refers to the loss of strength of a signal during its propagation through the communication channel output.
3.Give the one example of point-to-point communication mode. [All India 2014C]
Ans. Telephone is the example of point-to-point communication mode.
4.What is the function of a transducer used in a communication system? [Delhi 2012]
Ans. Transducer used as a sensor or detector in communication system. It converts the physical signal into electrical signal.
5.What does the term attenuation used in communication system mean? [Delhi 2012, 2008C]
Ans. It refers to the loss of strength of a signal during its propagation through the communication channel output.
6.What is the function of a repeater in a communication system? [Foreign 2011; Delhi 2010]
Ans. Repeater It picks up the signals from the transmitter, amplifies it and transmit it to the receiver. Thus, repeater comprises up of receiver, transmitter and amplifier. Its function is to extend the range of communication
7.What is the function of a transmitter in a communication system? [Foreign 2011]
Ans. Transmitter It comprises of message signal source, modulator and transmitting antenna. Transmitter make signals compatible for communication channel via modulator and antenna.
8.How are microwaves produced?  [Foreign 2011]
Ans. A type of electromagnetic wave is microwave whose wavelength ranging from as long as meter to as short as millimeter and having the frequency range 3000 MHz to 300 GHz. This also includes UHF, EHF and various sources with different boundaries.
9.What is the sky wave propagation?  [Delhi 2009]
Ans.  Sky wave propagation When radio wave propagates from one place of earth to other after reflection by ionosphere, the range of frequencies from few MHz to 30 MHz gets reflected back by ionosphere. This range also reflected as short wave band. This mode of propagation is used by short wave broadcast service.
10.What is ground wave propagation?  [Delhi 2009]
Ans.Ground wave propagation The radio waves whose frequencies ranged up to 1500 kHz, propagates from one place of earth to other following its transmission along the surface of earth. These waves get attenuated and hence cannot travel over long distances. This range of frequencies also referred as amplitude modulated band (AM band).
11.What is space wave propagation?  [Delhi 2009]
Ans. Space wave propagation It is also known as Line of Sight propagation (LOS). The radio wave transmitted by antenna directly reaches the  receiving antenna travelling along a straight line.
TV waves (80 MHz-200 MHz) propagate through space wave propagation
12.What does the term transducer mean in an electronic communication system? [Delhi 2009c]
Ans. Transducer Any device which converts one form of energy into other, e.g. electric transducer converts pressure, temperature, etc., into varying electrical signals, i.e. transducer converts physical signals into electrical signals.
13.Distinguish between sinusoidal and pulse shaped signals. [All India 2009C]
Ans. A signal in which current or voltage change continuously with time sinusoidally is known as sinusoidal signal.

A signal in which current or voltage can take only two discrete values for it is called pulse shaped signals.

14.What are the three basic units of a communication system? [Delhi 2008C]
Ans.  Three basic units of communication system are given below:
(i) Transmitter (ii) Communication channel (iii) Receiver
Basically, the transmitter is located at one place, the receiver is located at some other place (near or far) and the channel is the physical medium that connects the transmitter and receiver.
15.Name the mode of propagation of radio waves which travels in a straight line, from the transmitting antenna to the receiving antenna. [All India 2008C]
Ans. Space wave propagation is that mode of wave propagationjn which the radio waves emitted from the transmitter antenna reaches the receiving antenna through the space.

2 Marks Questions

16. Write the function of the following in communication systems. [All India 2014]
(i) Transducer (ii) Repeater
Ans.  (i)Transducer – Refer to ans. 12.
(ii) Repeater – Refer to ans. 6.
17.Write the function of the following in communication systems. [All India 2014]
(i) Transmitter (ii) Modulator
Ans. (i) Transmitter – Refer to ans.7
(ii) Modulator – A modulator is a device that performs modulation.
18. Write the function of the following in communication system [All India 2014]
(i) Receiver (ii) Demodulator
Ans. (i) Receiver – A receiver extracts the desired message signal from the received signals at the channel output.
(ii) Demodulator – A demodulator is a device that performs demodulation, i.e. inverse of modulation.

As, signals of these frequency ranges are reflected back by ionosphere up to receiver end after travel ling a large distance.
19.Which mode of wave propagation is suitable for television broadcast and satellite communication and why? Draw a suitable diagram depicting this mode of propagation Of wave.      [Foreign 2012]
Ans.  Sky wave
Television —> 1710 kHz to 40 MHz
20.Distinguish between Analog and Digital signals.                      [Delhi 2012]
Ans. Continuously with time At any time, the value of signal is represented by its amplitude.
Digital signal In digital signal, the amplitudes are not continuous with time. Amplitude of a signal has only its two levels (i.e. low or high).
21.Mention the function of any two of the following used in communication system. (i) Transducer (ii) Repeater (iii) Transmitter (iv) Bandpass filter  [Delhi 2012]
Ans. (i) Transducer Refer to ans. 12.
(ii) Repeater Refer to ans. 6.
(iii) Transmitter Refer to ans. 7.
(iv) Bandpass filter A device which passes the signals with certain frequency range only.
22.What is sky wave communication? Why is this mode of propagation restricted to the frequencies only up to few MHz?       [All India 2011]
Ans. Sky wave propagation Refer to ans. 9. Reason behind restriction up to Few MHz. The radio wave of frequencies up to 30 MHz cannot penetrate the ionosphere and they get reflected back to earth whereas higher frequencies (> 40 MHz) bends slightly but not reflected back to the earth. Because, frequencies up to few MHz (< 30 MHz) gets reflected back to earth. Hence, this frequency range is used for sky wave communication
23.What is space wave communication? Write the range of frequencies suitable for space wave communication. [All India 2011]
Ans. Space wave propagation Refer to ans. 11.
24.Draw a block diagram showing the important component in a communication system. What is the function of a transducer? [Foreign 2011]
Ans.  Range of frequency suitable for space wave propagation is 100 MHz to 220 MHz.

25.What is the range of frequencies used for TV transmission? What is common between these waves and light waves?       [Delhi 2010]
Ans.The range of frequencies used for TV transmission is 100 MHz to 220 MHz

26.What is the range of frequencies used in satellite communication? What is common between these waves and light waves?      [Delhi 2010]
Ans. The range of frequencies used in satellite communication is 3.7 GHz to 6.4 GHz.
Common between these waves and light waves refer to frequency range for light wave which is of GHz order.
27.In standard AM broadcast, what mode of propagation is used for transmitting a signal? Why is this mode of propagation limited to frequencies up to a few MHz?  [Foreign 2010]
Ans. In standard AM broadcast, surface wave propagation is used for transmitting the signals.
Attenuation of surface wave increases very ripidly with increase in frequency that is why it is limited to frequencies up to a few MHz. In AM broadcast, range of frequencies are limited to 30 MHz
28.Name any two types of transmission media that are commonly used for transmission of signals. Write the range of frequencies of signals for which these transmission media are used.           [All India 2010c]
Ans.  For the transmission of signals, following two types of transmission media are used.
(i) Sky wave propagation or short wave propagation.
(ii) Space wave communication or line of sight communication.
Range of frequencies
(i) Sky wave propagation          30 MHz > f > 1500 kHz
(ii) Space wave communication 1 MHz > f > 100 MHz
29.(i) What is line of sight communication?
(ii) Why is it not possible to use sky wave propagation for transmission of TV signals? [Foreign 2009]
Ans.  (i) For line of sight communication
Refer to Ans. 11.
(ii) The frequency     of   waves     used  for transmission of TV signals are of range 100 MHz-220 MHz. But ionosphere may be able to reflect waves back on earth of frequency   upto   30  MHz.     Therefore, ionosphere is unable to reflect TV waves (space waves) back on the earth
30.A communication satellite is essentially a repeater station in space. Justify this statement by analyzing the function of a repeater. [All India 2009C]
Ans. A communication satellite
(i) pick up the signal transmitted by transmitter
(ii) amplifies it
(iii) retransmit it towards information users. These all are also a function of repeater to receive, amplify and retransmission of signal.
31.Write the function of

• transducer and
• repeater in the context of communication system.  [All India 2009]

Ans. Transducer Any device which converts one form of energy into other, e.g. electric transducer converts pressure, temperature, etc., into varying electrical signals, i.e. transducer converts physical signals into electrical signals.
Repeater It picks up the signals from the transmitter, amplifies it and transmit it to the receiver. Thus, repeater comprises up of receiver, transmitter and amplifier. Its function is to extend the range of communication