# MP Board Class 12th Physics Important Questions Chapter 15 Communication Systems

## MP Board Class 12th Physics Important Questions Chapter 15 Communication Systems

### Communication Systems Important Questions

Communication Systems Objective Type Questions

Question 1.
Choose the correct answer of the following:

Question 1.
The elements of communication system are :
(a) One transmitter
(c) Only communication channel
(d) All of the above.
(d) All of the above.

Question 2.
Sound wave are not directly transmitted after converting them into electric signals because:
(a) They propagates with speed of sound
(b) Their frequency does not remain constant
(c) For their transmission very high antenna is needed
(d) Their energy is very high.
(c) For their transmission very high antenna is needed

Question 3.
The super imposing of audio waves with carrier wave is called :
(a) Transmission
(b) Reception
(c) Modulation
(d) Defection.
(c) Modulation

Question 4.
The frequency range used for T.V. transmission is :
(a) 30 – 300 MHz
(b) 30 – 300 GHz
(c) 30 – 300 KHz
(d) 30 – 300 Hz.
(a) 30 – 300 MHz

Question 5.
The periodic time of communication satellite is :
(a) 1 year
(b) 1 day
(c) 12 hours
(d) 12 minutes.
(b) 1 day

Question 6.
T. V. signals are reflacted by :
(a) Mesosphere
(b) Ionosphere
(c) Troposphere
(d) None of these.
(d) None of these.

Question 7.
The short wave band of radio waves are transmitted through :
(a) Sky wave propagation
(b) Ground wave propagation
(c) Artificial satellite
(d) Direct sending from transmitter to receiver.
(a) Sky wave propagation

Question 8.
T.V. network uses :
(a) Microwaves
(b) Radio waves of very high frequency
(c) Gamma rays
(d) X – ray.
(b) Radio waves of very high frequency

Question 9.
The waves used in telecommunication are :
(a) Infrared rays
(b) UV – rays
(c) Microwaves
(d) Cosmic rays.
(c) Microwaves

Question 2.
Fill in the blanks :

1. …………….. is that signal which varies periodically with time.
2. Digital signal is represented by two binary number …………….. and……………..
3. The range of audio signals is ……………..
4. The frequency of carrier waves is of order of ……………..
5. The order of frequency of radio waves which can be transmitted by total internal reflection through ionosphere is ……………..
6. The band width of AM waves is ……………..
7. The T.V. transmission was invented by……………..
8. WWW means ……………..
9. …………….. is a device with the help of which the location of any place can be obtained.

1. Analog signals
2. 0 and 1
3. 20 Hz to 20000 Hz
4. MHz
5. 30 – 300 MHz
6. Frequency of modulating wave
7. J. L. Baird
8. World Wide Web
9. GPS.

Question 3.
Match the columns:
I.

1. (e)
2. (d)
3. (b)
4. (c)
5. (a)

II.

1. (e)
2. (c)
3. (d)
4. (b)
5. (a)

Question 4.
Write the answer in one word/sentence:

1. Write any two light sources which are used in optical communication?
2. On what principle does the optical fibre work?
3. Who invented the T, V. transmission?
4. What is FAX?
5. What is meant by channel width? Write channel width of AM and FM radio station.
6. What is demodulation or detection?

1. Light emitting diode (LED) and Diode LASER
2. Total internal reflection
3. J. L. Baird
4. The electronic reproduction of a document at a distant place is known as facsimile telegraphy or FAX
5. Channel width is that frequency range in which signals can be transmitted from a station. The channel width of AM is 10
6. kHz and of FM is 150 kHz
7. The process of extracting the audio signal from the modulated wave is known as demodulation or detection.

Communication Systems Very Short Answer Type Questions

Question 1.
What is principle of semiconductor LASER?
In these LASER Gallium Arsenide (Ga – As) is used. This LASER the ability to produce LASER rays in the range 0.75 to 0.9 um.

Question 2.
Why is a parallel wire line not suitable for microwave transmission?
In microwave transmission, half the operating wavelength approaches the separation between the two parallel wire lines. Therefore energy loss in the form of radiation becomes maximum.

Question 3.
What do you understand by channel and channel noise?
The frequency range prescribed for a given transmission is called channel. The unwanted signals present by known or unknown reason in the transmitted signal is called noise.

Question 4.
What is importance of modulation index?
The modulation index determines the strength and quality of the transmitted signal. If the modulation index is small the amount of variation in the carrier amplitude will be small.

Question 5.
Write full form of LED and LASER.
Full form of LED is “Light Emitting Diode”. Full form of LASER is “Light Amplification by Stimulated Emission of Radiations”.

Question 6.
Which device is used to transmit the T.V. signals up to long distances?
Communication satellites are used to transmit the T.V. signals up to long distances.

Question 7.
What is a communication system?
Communication system is that system through which the signals are transmitted from one place and required at the other place.

Question 8.
What are message signals?
A message signal is a single valued function of time that conveys the information. The electrical analog of information or basic message is called message signal. These are obtained by suitable transducers.

Question 9.
What are analog signals?
An analog signal is continuous single value which at any instant lies within the range of a maximum and minimum value.

Question 10.
What is a digital signal?
Digital signal is a discontinuous signal value which appears in steps in predetermined levels rather than having the continuous change.

Question 11.
What is noise?
Noise refers to the unwanted signals that tends to disturb the transmission and processing of message signals in a communication system.

Question 12.
What is modulation?
Modulation is a process in which the signal of low frequency (audio signals) are superimposed over a signal of high frequency (carrier signal).

Question 13.
What is FAX?
Fax is a type of communication in which an electronic copy of a document is sent to a distance place.

Question 14.
Which type of signal is used in communication through computer?
Digital Signal.

Question 15.
Write basic difference between the light emitted by LED and LASER.
The light emitted by LASER is completely coherent and monochromatic while the light emitted by the LED is not coherent.

Question 16.
What is bandwidth? What is bandwidth of audio signals?
The frequency range of a signal is called its bandwidth. The bandwidth audio signals is 20 Hz to 20 KHz.

Communication Systems Short Answer Type Questions

Question 1.
Write name of two system used for pulse modulation.

1. PAM : Pulse Amplitude Modulation.
2. PCM : Pulse Code Modulation.

Question 2.
Write name of two systems which are used to convert the digital data into analog data.

2. Phase Shift Keying (PSK).

Question 3.
FM signals are less sensitive as compared to AM signals. Why?
In FM transmission the information (message) in the carrier waves is in the form ofvariation (change) in frequency. In the AM the noise get amplitude modulated. Hence the amplitude carrier wave varies. That is why the FM signals are less sensitive as compared to AM signals.

Question 4.
Write limitations of frequency modulation.

1. Area of reception for FM is much smaller.
2. About 10 times wider channel is required by FM.
3. FM receivers and transmitters are very complex and costly.

Question 5.
Write the meaning of LASER and two uses.
Meaning of LASER:
Light Amplification by stimulated Emission of Radiations.

Uses:

1. Communication and
2. The medical field to determine vary long distances.

Question 6.
Write any two uses of optical communication.

1. It is free from noise.
2. Its bandwidth is large, so number of channels can be transmitted simultaneously.

Question 7.
What is Light Emitting Diode?
It is forward biased P – N junction which emits the light. It converts electrical energy into light energy.

Question 8.
What is optical fibre? On what principle does it work?
An optical fibre is a device with the help of which the optical signals can be transmitted over through a zig-zag path without loss of energy. It is based on principle of total internal reflection.

Question 9.
What is meant by population inversion and optical pumping?
Population inversion:
The process by which the number of atoms are increased . in the excited state as compared to ground state by stimulated absorption, is called population inversion.

Optical pumping:
The process by which population inversion is obtained, is called optical pumping.

Question 10.
Give two characteristics of LASER rays.

1. LASER rays are monochromatic.
2. These are highly coherent.

Question 11.
What are the elements of communication system? Explain with the help of block diagram and also describe the different kinds of communication system.
Or
What is communication system? What are its main parts? Explain with block diagram.
Communication system is that system through which the signals are transmitted from one place and received at the other place.

Elements of communication system : Following are the elements of a communication system:

1. Transmitter : Its function is to transmit the information after modifying it, to a form suitable for transmission.
Block diagram of transmitter :

2. Communication channel : The free space through which the electromagnetic waves sent by the transmitter, reach the receiver is called communication channel. It may be a coupled wire, coaxial cable or radio wave.

Question 12.
What is noise? Why digital communication is popular? What are A/D and D/A converter?
Noise : Noise is unwanted disturbance or foreign element interfering with the desired information or signal. Noise can come up in any part of the communnication system but it has its worst effect when the signal is weakest.

The digital communication is popular nowaday because of the following characteristics :

1. Its quality is good.
2. Digital signals are in the form of pulses, they can produce easily by using logic gates.
3. These signals do not get distorted by noise.
4. These signals can be stored as digital data.
5. In this transmission many information can be transmitted using one channel.

A/D and D/A converter:
The electric circuit which can convert the analog signal to digital signal is called A/D converter. The electric circuit which can convert the digital signal into analog signal is called D/A converter.

Question 13.
Write difference between Analog and Digital signals.
Difference between Analog and Digital signals :

Analog signal:

• In this signal value of current or voltage changes continuously with time.
• It is a continuous function of time.
• A simple analog signal is represented by a sine wave.
• The signal obtained by converting a speech, music or intensity of reflected light are analog signals.

Digital signal:

• In these signals the current or voltage has only two descrete levels 0 and 1.
• It is discontinuous function of time.
• Digital signal is represented in the form of pulses.
• The letters of a book, output of digital computers or the information obtained from a FAX are digital signals.

Communication Systems Long Answer Type Questions

Question 1.
Explain need of modulation and define the term modulation.
Or
What is modulation? Why it is needed for the transmission of signals?
Modulation:
Modulation is a process in which the signal of low frequency (audio signals) superimposed over a signal of high frequency (carrier signal). Such that same properties of carrier wave like amplitude, frequency or phase varies in accordance with the instantaneous value of audio signals.

Need of Modulation:
Modulation is necessary for a low frequency signal when it is to be sent to a distant place so that the information may not die out in the way itself as well as for the proper identification of a signal and to keep the height of antenna small also.

For the transmission of electromagnetic waves the length of antenna should be of order of wavelength of the transmitted waves. Since, λ= $$\frac {c}{ ν}$$ therefore the required length of antenna for the audible range should be equal to $$\frac { 3\times { 10 }^{ 8 } }{ 20,000 }$$ = 1.5 x 104m to $$\frac { 3\times { 10 }^{ 8 } }{ 20 }$$ = 1.5 x 107

This length of antenna is not possible in practical. Now if the waves of 300kHz or more than it are to be transmitted then the required length of antenna will be $$\frac { 3\times { 10 }^{ 8 } }{ 3,00,000 }$$ = 100 m or less than it. The antenna of this size can be constructed easily. Hence to transmit the audio signals they are superimposed with the radio waves of frequency of order of Mega Hertz. These waves are called carrier waves and this process is called modulation.

Question 2.
How many types of modulation are there? Explain each types of modula-tion.
The equation of carrier wave is given as :
ec = Ec. cos (ωct + θ)
Where ec is instantaneous value of carrier signal. Ec is amplitude of carrier wave, ωc is angular frequency and θ is phase angle. Thus, there are three types of modulation corresponding to Ec, mc and θ.

1. Amplitude modulation:
When an audio frequency (modulating) signal is superimposed that the amplitude of modulated wave is linear function of instantaneous value of modulating signal, then this type of modulation is called as amplitude modulation.

2. Frequency modulation:
Frequency modulation is that modulation in which the frequency of carrier wave varies in accordance with the instantaneous value of modulating signal. In this modulation the amplitude and phase of modulating signal are equal to that of carrier wave.

3. Phase Modulation:
In the phase modulation, the modulating waves are super imposed with carrier waves such that the phase of the modulated wave is the linear function of the amplitude of the modulating wave but the frequency and amplitude of the modulated signal remains same as the frequency and amplitude of the carrier wave.

Question 3.
What are limitations of amplitude modulation?
The limitations (disadvantages) of amplitude modulation are as follows :

1. Efficiency of Amplitude modulation is smaller:
In AM modulation the message signals are contained in side bands, but not contained in the carrier wave. It is found that in amplitude modulation only one third power is contained inside bands, remaining power is contained in carrier wave. Hence efficiency decreases.

2. Amplitude modulation is more likely to suffer from noise.

3. In Amplitude modulation the fidelity of reception is less:
The range of audio signal is 20 Hz to 20 kHz. Hence the bandwidth must be 40 kHz. But the disturbance created by the nearby radio station should be taken into account and hence the bandwidth is kept only of about 20 kHz.

4. Its transmission range is low. Due to less power it is not possible to transmit the signals up to long distance. Inspite of these limitations the AM is mostly used for the transmission of audio signals.

Question 4.

1. Frequency modulation is inherently and practically free from the effects of noise.
2. In frequency modulation, noise can further be decreased by increasing the deviation 8.
3. FM receiver can further be improved with the help of limiters to remove amplitude changes, if any which controls the noise level.
4. In FM it is possible to operate many independent transmitters on same frequency without interference.

1. A bout 10 times wider channel is required by FM.
2. Area of reception for FM is much smaller.
3. FM receivers and transmitters are very complicated and costly.

Question 5.
Explain the data transmission and retreival with the help of block diagram.
The important use of data transmission and retrieval is in the microprocessor and computers. All the informations and signals which are to be transmitted are converted into digital signals by coding, which are then modulated with the carrier signals and are sent to far distant places. These signals are received by receivers. These received signals are amplified and demodulated in their original form. The block diagram of this process is shown in fig.

Question 6.
What is FAX machine? Draw its block diagram and explain its working.

1. FAX : The electronic reproduction of a document at a distant place is known as facsimile telegraphy or Fax. To send the document through FAX following functions are performed.

• Optical scanning
• Conversion of data for transmission and reception.
• Printing a copy of images of the original document at the receiving end.

The block diagram of working of FAX is shown in fig.

Question 7.
Write notes on ‘MODEM’.
Or
What is MODEM? Write its working and uses.
The block diagram of a modem is shown ahead : A modem is basically a modulator and a demodulator circuit.

Working:
At the transmitting end i.e., at section A, a modem change the digital data (obtained from source) into analogue form in audio frequency range.

So, it is easily modulated and transmitted by communication network through telephone lines and transmitter and sent to receiver for section B. At the receiving end i.e., at sermon B, the modem converts the analogue signal to digital signal.

Uses : It is used for short distance communication by connecting one computer to another through telephone lines.

Kinds of MODEM : MODEM are of two types.

• Internal MODEM : These are internally attached in the computer.
• External MODEM : These are connected externally with the computer.

Question 8.
Describe the following in short:

1. E – mail
2. Internet
3. World Wide Web
4. Celular phone
5. Pager.

1. E – mail:
Its full form is electronic mail the message produced by using word processing, programs are transmitted over the world wide network called internet and they get stored in a computer called mail server. Any person connected to the internet can contact mail server. Any person connected to the internet can contact the mail server to check whether it is holding mail for him.

2. Internet:
Internet is a world wide network which connects the millions of computers which are linked together for data transmission. It is a global system of inter connected computer networks.

3. World Wide Web:
Its abreviation is WWW. It was invented by Tim Berners Lee in 1989 – 91 which is highly enlarged encylopedia which is accessible to every one for knowledge.

4. Celular Phone:
It is basically mini wireless phones for exchanging messages as well as for mobile telephonic conversation.

5. Pager:
It is a wireless device which records the messages in writing.

Question 9.
What is line communication? Explain.
In a communication system the communication channel is a medium which provides the physicals path between transmitter and receiver. There are two types of communication medium:

1. Guided medium:
This medium connects transmitter and receiver in the case of point to point communication, double line cable, coaxial cable and optical fibre are the examples of guided medium.

2. Unguided medium:
This medium connects one transmitter and many receivers. Sky wave communication is an example of this medium. Its band frequency rays from few thousand kHz to few GHZ.

In the sky wave communication there is no point to point connection between transmitter and antenna. But in some communication applications the point to point connection is needed for example in telephone and telegraph the transmitter and receiver are connected by wire lines. It is called as line communication.

Question 10.
What is Global Positioning System (G.P.S.)? Write its applications.
It is a space based satellite navigation system that provides users with accurate information on position time wherein the world even through the local streets and in all weather conditions.

To use GP.S. system of a satellite the user must have a GP.S. device fitted with transmitter and receiver for sending and receiving radio wave signal.

Applications of G.P.S

1. Navigation on land, water and air.
2. Keeping standard time all over the world.
3. Used in unmanned automatic vehicles movement and mobile telephony.
4. In desiging map of a location.

Communication Systems Numericals Questions

Question 1.
When there is 50% modulation the transmitter emits the 20 kW power calculate the power of carrier wave.
Solution:
Given : ma = 50% = $$\frac {50}{100}$$ = Pt = 20 kW = 20,000 W
Formula:

Question 2.
When a modulating of 500 Hz is given to a frequency modulation generator then it produces a frequency deviation of 2.25 kHz. Calculate modulation depth.
Solution:
Given : δmax = 2.25 kHz = 2250 Hz, fm = 500 Hz

Question 3.
The maximum and minimum amplitude of a amplitude modulated wave are 16 mV and 4 mV. What is the depth of modulation?
Solution:
Given : Emax = 16mV, Emin = 4mV
Formula:

Question 4.
A 20 kHz modulating signal is modulated with a carrier wave of 4 MHz. What will be the upper side band and lower side band? What will be the channel width?
Solution:
Given: fm = 20 kHz, fc = 4 MHz =4,000 kHz
fUSB = fc + fm = 4000 + 20 = 4020 kHz.
fLSB = fc – fm= 4000 – 20 = 3980 kHz.
Channel width = fUSB – fLSB
= 4020 – 3980 = 40 kHz.

Question 5.
Find out the modulation index of a frequency modulated wave whose modulating frequency is 2 kHz and maximum frequency deviation is 10 kHz.
Solution:
Given : fm = 2 kHz and δmax = 10 kHz
Formula : mf = $$\frac { { \delta }_{ max } }{ { f }_{ m } }$$
mf = $$\frac {10}{2}$$ = 5