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Class 12 Physics Communication Systems Notes

Introduction

We live in the world of information. Information needs to be communicated from one entity to another entity. This act of sending and receiving message from one place to another place, successfully, is called communication.

The word successful in the above definition, implies many things like

• Common understanding by the sender and the receiver in interpreting the information
• Quality in communication, which implies no addition, deletion or modification of the actual information

The growing needs of human beings in the field of communication imposed demands on

• Complexity of information
• Speed of transmission

Evolution in communication

The table below shows us how physical messengers who travelled from one place to another changed to the current day situation where information comes to your doorstep anytime with easy access.

Time period	Event	Remarks
When Kings ruled	·         Announcement to common people·         Peace and war message from one country to another	·         Messengers travelled from one place to another·         Drum beaters announced Kings decisions
1835	Invention of Telegraph by F.B.Morse and Sir Charles Wheatstone	Messengers physically going from one place to another reduced
1876	Invention of Telephone by Alexander Graham Bell and Antonio Meucci	Even now this communication is very useful
1895	Wireless Telegraphy by Jagadis Chandra Bose and G Marconi	Leap in communication history from using wires to wireless
1936	Television broadcast by John Logi Baird	Being used even today
1955	Radio FAX by Alexander Bain	Being used even today
1968	ARPANET by JCR LIcklider	First internet where file transfer from one computer to another computer was possible
1975	Fiber Optics by Bell Laborataries	More economical means of communication
1989-91	World Wide Web by Tim Berners-Lee	Information access made so easy in modern world

Communication System

The general form of communication system is depicted below:

As we see here, the basic elements of communication includes transmitter, Channel and the receiver.  The transmitter and the receiver may be located geographically at different places. The Channel connects the transmitter and the receiver.

Information Source – The source produces signal of the information which needs to be communicated.

Signal – Information in electrical form suitable for transmission is called signal.

Transmitter – Converts the source signal into suitable form for transmission through the channel.

Channel – The channel connecting the transmitter and the receiver is a physical medium. The channel can be in the form of wires, cables or wireless.

Noise – When the transmitted signal propagates along the channel, it may get distorted due to channel imperfection.

Thus, noise refers to unwanted signals that tend to disturb the process of communication from the transmitter to the receiver.

Receiver – Due to noise and other factors, the corrupted version of signal arrives at the receiver. The receiver has to reconstruct the signal into recognisable form of the original message for delivering it to the user. The signal at the receiver forms the output.

Modes of communication

Point to point communication – There is a single link between the transmitter and the receiver. Communication takes place between single transmitter and receiver

Example – Telephone

Broadcast mode – There are large number of receivers though information is sent by a single transmitter.

Example – Television and Radio

Communication – Terminology

Transducer – Any device which converts energy in one form to another form is called transducer.

Electrical transducer: A device that converts some physical variable like pressure, displacement, force, temperature, into corresponding variations in electrical signal. Hence, the output of this would be an electrical signal.

Signal Types – Information in electrical form suitable for transmission called signal, is of two types

Analog signal –

• Continuous variations of voltage and current. Hence, single valued functions of time.
• Sine wave is a fundamental analog signal
• Example – Sound and picture signals in television

Digital signal –

• Digital step value based
• Binary system where 0 represents low level and 1 represents high level is used
• Universal digital coding methods like BCD – Binary Coded Decimal and ASCII – American Standard Code of Information Interchange is used in common

Amplitude –

The maximum extent of vibration or oscillation from the position of equilibrium

Frequency –

The frequency is the number of waves which pass a fixed place in a given amount of time.

Phase –

The two waves depicted below have a phase difference indicated by the phase shift which is the fraction of the wave cycle which has elapsed relative to the origin.

Signal propagation – Terminology

Attenuation – The loss of strength of the signal while propagating through a medium is known as attenuation.

Amplification – The process of increasing the amplitude of the signal by using an electronic circuit is called amplification. This also increases the strength of the signal. Hence, it compensates the attenuation of the signal.

Range – It is the largest distance between the source and the destination upto which the signal is received with sufficient strength.

Bandwidth – It refers to the frequency range for which the equipment operates.

Modulation – If the information signal is of low frequency, it cannot be transmitted to long distances. Hence, at transmission point, it is super imposed on high frequency wave. This high frequency wave acts as a carrier of information. This is modulation

Sinusoidal wave modulation:

There are 3 types of modulation, namely 1. Amplitude modulation 2.Frequency modulation and 3. Phase modulation

Amplitude modulation –

• The amplitude of the carrier wave is varied in accordance with the information signal

Frequency modulation –

• The frequency of the carrier wave is varied in accordance with the information signal

Phase modulation –

• The phase of the carrier wave is varied in accordance with the information signal

Pulse wave modulation:

There are 3 types of pulse wave modulation – namely (a) Pulse amplitude modulation (b) Pulse width modulation (c) Pulse position modulation

Demodulation – The process of retrieval of information from the carrier wave at the receiver is termed as demodulation. This is a reverse of modulation.

Repeater – A repeater is a combination of receiver and a transmitter.

A repeater picks up the signal from the transmitter, amplifies and retransmits it to the receiver. Thus repeaters are used to extend the range of communication system

Example – Communication satellite is a repeater station in space.

Propagation of electromagnetic waves

While communication using radio waves, the transmitter antenna radiates electromagnetic waves. These waves travel through the space and reach the receiving antenna at the other end. We have considered below some of the wave propagation methods in brief.

Ground or Surface wave propagation:

• In this mode of wave propagation, ground has a strong influence on propagation of signal waves from the transmitting antenna to the receiving antenna. The signal wave glides over the surface of the earth
• While propagating on the surface of the earth, the ground wave induces current in the ground. It also bends around the corner of the objects on the earth
• Due to this, the energy of the ground wave is gradually absorbed by the earth and the power of the ground wave decreases
• The power of the ground wave decreases with the increase in the distance from the transmitting station. This phenomenon of loss of power of the ground wave is called attenuation
• The attenuation of ground waves increases very rapidly with the increase in its frequency
• Thus, ground wave communication is not suited for high frequency signal wave and for very long range communication
• To radiate signals with high efficiency, the antennas should have a size comparable to the wavelength of the signal

Sky waves:

• The ionosphere plays a major role in sky wave propagation. We know that the earth’s atmosphere is divided into various regions like – Troposphere, Stratosphere, Mesosphere and Ionosphere.
• The ionosphere is also called as thermosphere as temperature increases rapidly here and it is the outermost part of the earth’s atmosphere
• Above troposphere, we have various layers like D (part of stratosphere), E (part of stratosphere), F₁ (part of mesosphere), F₂ (part of ionosphere)
• The ionosphere is called so because of the presence of large number of ions or charged particles. Ionisation occurs due to the absorption of the ultraviolet and other high energy radiation coming from the sun, by the air molecues
• The phenomenon of bending of electromagnetic waves in this layer so that they are diverted towards the earth is helpful in skywavepropogation. This is similar to total internal reflection in optics
• The radiowaves of frequency range from 1710 kHz to 40 MHz are propagated in sky wave propagation

Space waves:

• The space waves travel in straight line from the transmitting antenna to the receiving antenna.
• Hence, space waves are used for line of sight communication such as television broadcast, microwave link and satellite communication
• The line of sight communication is limited by (a) the line of sight distance (b) the curvature of the earth
• At some point by the curvature of the earth, the line of sight propagation gets blocked.
• The line of sight distance is the distance between transmitting antenna and receiving antenna at which they can see each other. It is also called range of communication d_M
• The range of space wave communication can be increased by increasing the heights of the transmitting antenna and receiving antenna.
• The maximum line of sight distance (range of communication) d_M between two transmitting anternna of height h_T and the receiving antenna of height h_R above the earth is given by

d_M= (2Rh_T)^1/2 + (2Rh_R)^1/2

Problem – A transmitting antenna at the top of a tower has a height of 32m and the height of the receiving antenna is 50m. What is the maximum distance between them for satisfactory communication in LOS mode? Given radius of earth = 6.4 * 10⁶ m

Solution –Given h_{T =} 32m ,h_r= 50m, R = 6.4 * 10⁶ m

d_M= (2Rh_T)^1/2 + (2Rh_R)^1/2

= (2 * 6.4 * 10⁶ * 32 )^1/2  + (2 * 6.4 * 10⁶ * 50 )^1/2

     = 45.5 Km

Modulation and its necessity

Any message signal, in general, is not a single frequency sinusoidal. But it spreads over a range of frequencies called the signal bandwidth.

Suppose we wish to transmit an electronic signal, in the audio frequency range, say 20 Hz to 20kHz range, over a long distance we need to consider factors like

• Size of the antenna
• Effective power radiated by the antenna
• Avoiding mixing of signals from different transmitters

Size of antenna:

• Antenna is needed for both transmission and reception
• Antenna should have a size comparable to the wavelength of the signal, atleastλ/4 where λ is the wavelength of the signal
• In the above audio frequency range, if we consider frequency ט = 15,000 Hz. Then λ = c / ט = 3 * 10⁸ / 15,000 = 20,000 m
• Hence, antenna length = λ/4 = 20,000 / 4 = 5000 m.
• It is practically impossible to design an antenna of height 5000m
• So the transmission frequency should be raised in such a way that the length of the antenna is within 100m which is feasible for practical purpose
• This shows that there is a need for converting low frequency signal to high frequency before transmission

Effective power radiated by the antenna:

• Effective power rated by the antenna = P = E/t
• Also, E = hט = hc/λ
• Hence, P = E/t = hc/λ * c/λ
• Studies reveal that if l is the linear length of the antenna, then P is proporational to (l/λ)²
• Hence, for good transmission, high power and hence small wavelength and high frequency waves are required
• High frequency waves becomes inevitable in this case also

Avoiding mixing of signals from different transmitters:

• When many transmitters are transmitting baseband information signals simultaneously, they all gets mixed up
• There is no way to distinguish between them
• Possible solution is communication at high frequencies and allotting a band of frequencies for each transmitter so that there is no mixing
• This is what is being dene for different radio and TV broadcast stations

Hence, we understand the necessity of modulation.

Problem – Find the minimum length of the antenna required to transmit a radio signal of frequency 10 MHz.

Solution – We know that  λ = c / ט = 3 * 10⁸ / 10 * 10⁶ = 30 m

Minimum length of the antenna = λ / 4 = 30 / 4 = 7.5 m

Band width

Bandwidth is also defined as the amount of data that can be transmitted in a fixed amount of time

Signals – Bandwidth:

• The message signal can be voice, music, picture or computer data
• Each of the above have different frequency ranges
• The speech signals frequency range from 300Hz to 3100Hz. Hence, bandwidth = 3100 -300 = 2800 Hz
• Any music requires bandwidth of 20kHz because of high frequencies produced by musical instruments
• Video signals for transmission of picture requires 4.2 MHz of bandwidth
• The Television signal which contains both voice and picture is usually allocated a bandwidth of 6MHz bandwidth for transmission

Transmission Medium – Bandwidth:

• Different types of transmission media offers different bandwidth
• Coaxial cables, widely used wire medium offers bandwidth of approximately 750 MHz
• Communication through free space using radio waves offers wide range from hundreds of kHz to few GHz
• Optical fibres are used in the frequency range of 1THz to 1000 THz (THz – Tera Hertz; 1THz = 10¹²Hz)
• As mentioned earlier, to avoid mixing of signals, allotting a band of frequencies to a specific transmitter is in practise
• The International Telecommunication Union administers this frequency allocation
• Services like FM Broadcast, Television, Cellular Mobile Radio and Satellite communication operate under fixed frequency bands

Let us now consider amplitude modulation in detail.

Amplitude modulation

1. As we know, in amplitude modulation, the amplitude of the carrier wave is varied in accordance with the amplitude of the information signal or modulating signal.
2. For sinusoidal modulating wave,

m(t) = A_msinω_mt  ——————————–(1)

• A_m – Amplitude of modulating signal
• ω_m– 2πט_m – Angular frequency of modulating signal

3. For carrier wave,

C_m(t) = A_csinω_ct  ——————————–(2)

• A_c – Amplitude of carrier wave
• ω_m– 2πט_c– Angular frequency of carrier wave

4. For carrier wave, the amplitude is changed by adding the amplitude of the modulating signal which is A_{c plus} A_msinω_mt

C_m(t) = (A_c+ A_msinω_mt) sin ω_ct  ——————————–(3)

5. Multiply and Divide equation (3) RHS by A_c

C_m(t) = A_c (A_c/A_c+A_m/A_c sin ω_mt) sin ω_ct  —————————(4)

Replace A_m / A_c = µ

µ is called Amplitude modulation index and is always less than or equal to 1 to avoid distortion.

C_m(t) = A_csin ω_ct ₊ µA_c sin ω_ct sin ω_mt   —————————(5)

6. We know that sin A sin B = ½ [ cos (A-B) – cos (A + B)]

Hence, sin ω_ctsin ω_mt = [cos (ω_c–ω_m)t – cos(ω_c+ω_m)t]

C_m(t) = A_csin ω_ct ₊ µA_c/2[cos (ω_c-ω_m)t – cos(ω_c+ω_m)t]

C_m(t) = A_csin ω_ct ₊ µA_c/2cos (ω_c–ω_m)t – µA_c/2cos(ω_c+ω_m)t——–(6)

7. Equation (6) shows that the amplitude modulated signal consists of
   1. Carrier wave of frequency ω_c
   2. Sinusoidal wave of frequency (ω_c–ω_m)
   3. Sinusoidal wave of frequency (ω_c+ω_m)
8. The two additional waves are called side bands. The frequency of these bands are called side band frequencies
   1. Frequency of lower side band = (ω_c–ω_m)
   2. Frequency of upper side band = (ω_c+ω_m)

9. The band width of the AM wave is Frequency of lower side band minus Frequency of upper side band

(ω_c+ω_m) – (ω_c–ω_m) = 2ω_m (Twice the frequency of modulating signal)

10. Graphical representation

Problem – The frequencies of two side bands in an AM wave are 640 kHz and 660 kHz respectively. Find the frequencies of carrier and modulating signal. What is the bandwidth required for amplitude modulation ?(CBSE 2017 Set 1)

Solution –  Given(ω_c+ω_m) = 660 kHz ;  (ω_c–ω_m) = 640 kHz

• (ω_c+ω_m) – (ω_c–ω_m) = 2 ω_m= 660 – 640 = 20 kHz

Hence, frequency of modulating signal ω_{m =} 10 kHz

• Now, (ω_c+ω_m) = 660 kHz, (ω_c+10) = 660 kHz, Hence, frequency of carrier waveω_{c =}660 – 10 = 650 kHz
• The band width of the AM wave is Frequency of lower side band minus Frequency of upper side band (ω_c+ω_m) – (ω_c–ω_m) = 2ω_m

Hence, 2 ω_m= 20 kHz

Problem – An audio signal of amplitude 0.1 V is used in amplitude modulation of a carrier wave of amplitude 0.2 V. Calculate the modulation index.

Solution –

We know that A_m / A_c = µ , given A_m = 0.1 V ,  A_c = 0.2 V

µ = 0.1 / 0.2 = 0.5

Production of amplitude modulated wave

We know that modulating signal is represented by

m(t) = A_msinω_mt  ——————————–(1)

• A_m – Amplitude of modulating signal
• ω_m– 2πט_m – Angular frequency of modulating signal
• Similarly, carrier wave is represented by

C_m(t) = A_csinω_ct  ——————————–(2)

• A_c – Amplitude of carrier wave
• ω_m– 2πט_c– Angular frequency of carrier wave
• Modulating signal is added to the carrier wave, Hence, the representation is

x(t) =A_msinω_mt   + A_csinω_ct

The above signal is passed to a square law device (non-linear device)

y(t) =B x(t) + c [x(t)]²

            B,C – Arbitrary constants

Substitute for x(t) in y(t) and use formula (A + B)² = A² + B² + 2AB

y(t) =B[A_msinω_mt   + A_csinω_ct] + c[A_msinω_mt   + A_csinω_ct]²

 =B[A_msinω_mt   + A_csinω_ct] + c[A_m²sin²ω_mt   + A_c ²sin ²ω_ct

 + 2 A_m A_csin ω_mtsin ω_ct]

We know sin A sin B = ½ [cos (A-B) – cos (A+B)]

               Hence,

                                sinω_ctsin ω_mt = ½ [ cos (ω_c – ω_m)t – cos (ω_c+ ω_m)t ]

Also, sin²A = ( 1- cos 2A) /2

Hence,

                sin ²ω_ct = (1 – cos 2ω_ct) / 2

                sin ²ω_mt = (1 – cos 2ω_mt) / 2

Therefore y(t) can be re-written as

Y(t) =B[A_msinω_mt   + A_csinω_ct] + cA_m²/ 2( 1 – cos2ω_mt)

 + cA_c²/ 2( 1 – cos2ω_ct)

+ 2 A_m A_c (c/2)[ cos (ω_c – ω_m)t – cos (ω_c+ ω_m)t ] ]

Y(t) =BA_msinω_mt   + BA_csinω_ct + c/ 2[A_m ² + A_c²]

 – cA_m ²/ 2 cos2ω_mt – c A_c²/ 2 cos2ω_ct

+ cA_m A_ccos (ω_c – ω_m)t – c A_m A_ccos (ω_c+ ω_m)tIn the above equation, there is a d.c. term ½ c [A_m ² + A_c²] and sinusoidal waves of frequencyω_c, ω_m, 2ω_m,(ω_c – ω_m) and (ω_c+ ω_m)

The signal is passed through band pass filter centered atω_c

This rejects the low and high frequencies. In the above case, the filter rejects d.c, ω_c,ω_m, 2ω_m,(ω_c – ω_m). The frequenciesω_c, (ω_c – ω_m) and (ω_c+ ω_m) are passed. This is amplitude modulated wave.

This wave cannot be passed as such. It needs to be amplified and then fed to an antenna of appropriate size for radiation.

Chapter-15-Communication-Systems

CBSE Class 12 Physics Important Questions Chapter 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

Types of signal	Frequency range	Bandwidth
Speech signal	300-3100 Hz	2800 Hz
Music signal	20-20000 Hz	20 kHz
Video signal		4.2 MHz
TV signal		6 MHz

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 ( Communication Systems )

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. Labelledelement 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( Communication Systems )

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

Characteristic	Light wave	Radio wave (100 -200 MHz) TV waves
Speed	Travel with speed c = 3 x 108 m/s	Also travel with speed c = 3 x 108 m/s
Reflection	Occurs and get affected by ground terrain, atmosphere and other objects.	It occurs and also get affected by ground terrain, atmosphere and other objects.

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.

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