Communication Systems

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Questions:

Multiple Choice Question

5 x 1 = 5

An audio signal of 15kHz frequency cannot be transmitted over long distances without modulation because

(a)

the size of the required antenna would be at least 5km which is not convenient.

(b)

the audio signal can not be transmitted through sky waves.

(c)

the size of the required antenna would be at least 20km, which is not convenient

(d)

effective power transmitted would be very low, if the size of the antenna is less than 5 km.

A male voice after modulation transmission sounds like that of a female to the receiver.The problem is due to

(a)

poor selection of modulation index (selected 0

(b)

poor bandwidth selection of amplifiers

(c)

poor selection of carrier frequency

(d)

loss of energy in transmission

A message signal of frequency \({ \omega }_{ m }\) is superposed on a carrier wave of frequency \({ \omega }_{ c }\) to get an amplitude modulated wave (AM). The frequency of the AM wave will be

(a)

\({ \omega }_{ m }\)

(b)

\({ \omega }_{ c }\)

(c)

\(\frac { { \omega }_{ c }+{ \omega }_{ m } }{ 2 } \)

(d)

\(\frac { { \omega }_{ c }-{ \omega }_{ m } }{ 2 } \)

Frequencies in the UHF range normally propagate by means of:

(a)

Ground waves

(b)

sky waves

(c)

surface waves

(d)

space waves

Which of the following frequencies will be suitable for beyond the horizon communication using sky waves.

(a)

10 kHz

(b)

10 MHz

(c)

1 GHz

(d)

1000 GHz

1 Marks

5 x 1 = 5

Name the device which can represent digital data by analog signals and vice versa.

Which of the two _________AM or FM is preferred for high fidelity reception?

Name three different compression techniques used in fax.

What is modulation index of an FM wave?

10.

What is varied in an FM wave?

Generic 2

3 x 2 = 6

11.

A message signal of frequency 10 kHz and peak voltage 10 V is used to modulate a carrier wave of frequency 1 MHz and peak voltage 20 V. Determine
(i) The modulation index
(ii) the side bands produced.

12.

Define modulation index. Why is it kept low? What is the role of a bandpass filter?

13.

AM and FM transmission are said to be 'real time systems'.Why?

Generic 3

5 x 3 = 15

14.

Explain with reason
(i) In amplitude modulation, the modulation index μ is kept less than or equal to 1.
(ii) The maximum amplitude of an amplitude modulated wave is found to be 15V while its minimum amplitude is found to be 3 V. What is the modulation index?
(iii) Why amplitude modulated signal be noisier than a frequency modulated signal upon transmission through a channel?

15.

Distinguish between sky wave and space wave propagation. Explain in brief with the help of suitable diagram, how space waves are used for line of sight communication?

16.

A radio set can be turned to any station in the frequency range 6 MHz to 12 MHz band.What is the corresponding wavelength band of radio set?

17.

Explain the following terms in relation to the use of internet?

18.

A 1000 kHZ carrier is modulated with 800 Hz audio signals.What are the frequencies of first pair of side bands?

Generic 5

3 x 5 = 15

19.

One day Samir though if Moon is a natural satellite then why there is need of other artificial satellites. finally, he came to the conclusion that Moon cannot be used as a communication satellite. So, he requested his teacher for its answer.The teacher explained him the the reasons behind it properly, after which he shared his knowledge with his classmates.
(a) What are the value shown by Samir?
(b) Why can Moon be not as a communication satellite?

20.

Raj was cycling from his village to a neighbouring village.On the way, he met an old man who was walking in the direction of the same neighbouring village. Raj invited the old man to sit on his cycle and took him to the neighbouring village.The old man got down from the cycle, thanked Raj and gave him blessing.
Read the above passage and answer the following questions:
(i) What are the values displayed by Raj?
(ii) Can you relate the given paragraph with the process of modulation?

21.

In the famous conversation, Rakesh Sharma, the first Indian Astronaut in space, was asked by the Prime Minister Indira Gandhi as to how India looked from space.To which he replied,'Sare Jahan Se Achcha' (better than the whole world).
Read the above passage and answer the following questions:
(i) Which scientific mode of communication enabled the Prime Minister to speak to the Astronaut?
(ii) Name the scientific values displayed in this anecdote.
(iii) Which values are being reflected in the reply given by the astronaut?
(iv) Give one more example of this scientific mode of communication in everyday life situations.

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Answers Key:

Multiple Choice Question

5 x 1 = 5

(d)
effective power transmitted would be very low, if the size of the antenna is less than 5 km.

(b)
poor bandwidth selection of amplifiers

(d)
space waves

(b)
10 MHz

1 Marks

5 x 1 = 5

()
Modem.

()
Frequency modulation is preferred for high fidelity reception.

()
(i) Modified Huffman(MH)
(ii) Modified Read (MR)
(iii) Modified Modified Read (MMR).

()
Modulation Index of FM wave
\(=\frac { maximum \ frequency \ deviation }{ modulation \ frequency }\)
\(=m_{ f }=\frac { { \delta }_{ max } }{ { v }_{ m } } \)

10.

()
In frequency modulation,the frequency of carrier wave is varied in accordance with the instantaneous value of the modulating voltage.

Generic 2

3 x 2 = 6

11.

(a) Modulation index =10 / 20 = 0.5
(b) The side bands are at (1000+10 kHz)=1010 kHz and (1000 –10 kHz) = 990 kHz.

12.

Modulation index is the ratio of the amplitude of modulating signal to that of carrier wave
Alternatively \(\mu =\frac { { A }_{ m } }{ { A }_{ c } } \)
Reasons:
To avoid distortion
Role:
A bandpass filter rejects low and high frequencies and allows a band of frequencies to pass through.

13.

It implies that the AM and FM signals can be received only at the time they are transmitted.

Generic 3

5 x 3 = 15

14.

Modulation is a process in which one of the characteristics (amplitude, frequency, phase) of a high frequency carrier wave is made to change in accordance with a give low frequency message signal.
Modulated wave 1 : Frequency Modulation
Modulated wave 2 : Amplitude Modulation
Two advantages of FM over AM :
(i) Lower noise, better power efficiency
(ii) Higher operating range
(iii) Higher fidelity reception.
[Alternatively, Two advantages of AMover FM]
(i) Simple circuits are required.
(ii) Lower frequency space for transmission

15.

Propagation of space wave Space wave communication involves the wave transmission from transmitter and travelling along a straight line in space, reaches to receiving antenna. The range of space wave propagation is limited by line of sight distance between transmission and receiver/repeater antenna.
Range of transmission antenna Let. d be range of transmission antenna of height h.

In \(\Delta OQT, OQ^{2}=QT^{2}+OT^{2} \Rightarrow (R_e+h)^{2}=d^{2}R_{e}^{2}\)
\(\Rightarrow d^{2}=h^{2}+2hR_e \quad [\because h<\)
\(\Rightarrow h may be neglected, d^{2}=2hR_e\)
\(\Rightarrow d=\sqrt{2hR_e}\)

16.

For v = 6MHz ,\(\lambda =\frac { c }{ v } =\frac { 3\times{ 10 }^{ 8 } }{ 6\times{ 10 }^{ 6 } } =50\ m\)
for v = 12 MHz,\(\lambda =\frac { c }{ v } =\frac { 3\times{ 10 }^{ 8 } }{ 12\times{ 10 }^{ 6 } } =25\ m\)
Thus the corresponding wavelength band is 25 m to 50 m band.

17.

(i) Internet working
(ii) social networking
(iii) E-mail.

18.

\({ v }_{ SB }=v_{ c }\pm { v }_{ m }=1000+0.8\)
\(=1000.8\ kHz\ and\ 999.2\ kHz\)

Generic 5

3 x 5 = 15

19.

(a) The value shown by Samir are:
(i) Curiosity
(ii) Creativity
(iii) Sharing Knowledge.
(b) Moon cannot be used as a communication satellite, because:
(i) The surface of the Moon is so rough and uneven that most of the signal scatters back into space instead of bouncing back to the earth.
(ii) The moon can only reflect signals while modern communication satellites actively rebroadcast the signal back to the earth.
(iii) The moon is only available for half of the day and it keeps moving.

20.

(i) Raj showed concern for the stranger and respect for the old age of the person.
(ii) Just as the old man sat on the cycle, the message signal is superposed on the carrier wave.The modulated carrier wave travels some distance.The message signal is then recovered from the modulated wave in the same way as the old man got down from the cycle.

21.

(i) Radio wave communication system.
(ii) Use of scientific and technological advancement in service to mankind.
(iii) Patriotism and love for the country, presence of mind.
(iv) Television communication system.