Communication Systems

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Questions:

Generic 5

9 x 5 = 45

Due to regular constant headaches, Anuj's mother in a medical check-up was diagnosed with tumour. With this, Anuj realized that there was a telecommunication tower, which is every close to their house. After enquiring from the doctor about the cause of tumour because of the antenna radiation, Anuj lodged a complaint with the police and finally succeeded in getting the tower removed from the nearby place to distant place away from his residential location.
Answer the following
(i) What values were displayed by Anuj?
(ii) Anuj made a rough estimate about the height of the antenna to be about 20m from the ground. Calculate the maximum distance up to which radiations from the tower are likely to reach. Use the value of radius of the Earth =6400 x 10³ m

Group discussion was arranged in class XII on the topic of atmosphere. Three groups were made. Teacher asked the question "why can Moonbe not used as a communication satellite?" Answers were given by all the three groups. Each, group can give only one reason. Teacher told them that reason given by each group is correct. The groups collected all the three, reasons and came to correct conclusion,"
(i) Give the correct reason for the above question.
(ii) What values were showed by all the three groups?

(a) Distinguish between sinusoidal and pulseshaped signals.
(b) Explain, showing graphically, how a sinusoidal carrier wave is superimposed on a modulating signal to obtain the resultant amplitude modulated (AM) wave.

Seema wants admission in a professional course in a village. It was last date of admission and she left a concerning certificate at home. She became upset as college was very far from the home. She called her brother for it and he faxed the certificate. She got admission and thanked her brother for timely help.
(a) What are the value displayed by Seema and her brother?
(b) What do you mean by Fax?
(c) sky waves are not used in transmitting TV signals.Why?

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?

Sweta had no proper knowledge about point-to-point communication and broadcast. So, she took help from her friend Rakhi explained him that in point-to-point communication mode, the communication takes place over a link between a single transmitter and a receiver. The example of this mode of communication is telephony. But in broadcast mode, there are a number of receivers corresponding to a single transmitter. The examples of this mode of communication are radio and television. Then Sweta was convinced with answer of Rakhi,
(a) What are the values displayed by Rakhi?b
(b) A message signal of frequency 10 kHz and peak voltage of 10 V is used to modulate a frequency of 1 MHz and peak voltage of 20 V. Determine
(i) modulation index
(ii) frequency of the side band produced.

Anuj's mother was having constant headaches. After a medical check-up, she was diagnosed with tumour. Anuj realized there was a telecommunication tower very close to their house. He enquired from the doctor if the radiation from the tower could have caused the tumour. As the doctor supported his anxiety, he lodged a complaint with the police and ultimately succeeded in getting the tower removed to a distant place away the from residential colony?
Answer the following :
(i) What values were displayed by Anuj?
(ii) Anuj made a rough estimate about the height of the antenna to be 20 m from the ground. Calculate the maximum distance upto which radiations from the tower are likely to reach. Use the value of radius of the Earth = 6400 x 10³ m.

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?

Mohan went to the market to purchase a TV set. He got confused with so many features and functions of electronic appliances. He took the help of his friend Sohan, a science student.Sohan explained him about the communication system, digital and analog signals. This knowledge proved of great help to Mohan in purchasing a colour TV.
Read the above passage and the answer the following questions:
(i) What do you mean by the term communication?
(ii) Which type of signals are better?
(iii) What type of nature Sohan has?
(iv)What is the minimum number of communication satellites required for global communication coverage?

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Answers Key:

Generic 5

9 x 5 = 45

(i) (a) Concern
(b) Scientific temperament
(c) Keen observer
(d) Alertness
(ii) d= \(\sqrt { 2hR } \)
= \(\sqrt { 2\times 20\times 6.4\times { 10 }^{ 6 } } \) m
= 2 x 8 x 10³ m
= 16 km

(i) Moon is a natural satellite of the earth but it is not an active satellite as it carries no electronic equipments for receiving, amplifying and transmitting the signal back to the earth. Moon cannot be used as a communication satellite because
(a) the distance between the earth and the moon is not proper.
(b) the period of revolution of the moon is not 24 hr.
(c) the moon is not revolving in the equitorial plane of the earth.
(ii) The three groups showed the values of eagerness to engage in a debate, promptness in taking decision and ability to work in a group in a constructive manner.

(a) In the process of modulation, some specific characteristics of the carrier wave is varied in accordance with the information or message signal.
The carrier wave may be :
(i) Continuous (sinusoidal) wave, or
(ii) Pulse, which is discontinuous.
A continuous sinusoidal carrier wave can be expressed as
\(E={ E }_{ 0 }sin\left( \omega t+\phi \right)\)
Three distinct characteristics of such a wave are amplitude (E₀ ), angular frequency \(\left( \omega \right) \) and phase angle \(\left( \phi \right) \) . Anyone of these three characteristics can be varied in accordance with the modulating baseband (AF) signal, giving rise to the respective amplitude modulation; frequency modulation and phase modulation.
Again, the significant characteristics of a pulse are pulse amplitude, pulse duration or pulse width and pulse position (representing the time of rise or fall of the pulse amplitude). Anyone of these characteristics can be varied in accordance with the modulating baseband (AF)signal, giving rise to the respective, pulse amplitude modulation (PAM), pulse duration modulation (PDM) or pulse width modulation (PWM) and pulse position modulation (PPM).
(b) Amplitude Modulation: When a modulating AF wave is superimposed on a high frequency carrier wave in a manner that the frequency of modulated wave is same as that of the carrier wave, but its amplitude is made proportional to the instantaneous amplitude of the audio frequency modulating voltage, the process is called amplitude modulation (AM).
Let the instantaneous carrier voltage (e_c ) and modulating voltage (em) be represented by
\({ e }_{ c }={ E }_{ c }sin{ \omega }_{ c }t ...(i)\)
\(\\ { e }_{ m }={ E }_{ m }sin{ \omega }_{ m } \quad ...(ii)\)
Thus, in amplitude modulation, amplitude A of modulated wave is made proportional to the instantaneous modulating voltage e_m
\(i.e., A={ E }_{ c }+{ ke }_{ m }\quad ...(iii)\)
where k is a constant of proportionality.
In amplitude modulation, the proportionality constant k is made equal to unity. Therefore, maximum positive amplitude of AM wave is given by
\(A={ E }_{ c }+{ e }_{ m }={ E }_{ c }+{ E }_{ m }\)
\(sin \ { \omega }_{ m }t \ \ ...(iv)\)
It is called top envelope.
The maximum negative amplitude of AM wave is given by
\(-A=-{ E }_{ c }-{ e }_{ m }\)
\(=-\left( { E }_{ c }+{ E }_{ m }sin\ { \omega }_{ m }t \right) ...(v)\)

(a) The values displayed by Seema and her brother are :
(i) Awareness
(ii) Understanding
(iii) Sharing knowledge
(b) FAX means transmission as a whole. It scans the contents of a document to create electronic signals. The signals are sent to the destination in an orderly manner using telephone lines. The signals are reconverted into a replica of the original document at the destination.
(c) Skywaves cannot be used in transmitting TV signals having frequencies 30 MHz or more because these waves are not reflected back by the ionosphere and penetrate into outer space.

(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.

(a) The values displayed by Rakhi are :
(i) High degree of general awareness.
(ii) The proper knowledge of subject matter.
(iii) Helping and caring nature.
(b) Given, V_m = 10 kHz, V_c = 1 MHz,
A_m = 10V and A_c = 20 V
(i) Thus, modulation index,
\(\mu =\frac { { A }_{ m } }{ { A }_{ c } } =\frac { 10 }{ 20 } =0.5\)
(ii) Frequencies of side bands
\(={ V }_{ c }\overset { + }{ - } { V }_{ m }\)
\(= 1MHz\overset { + }{ - } 10kHz\)
\(=990kHz\ and\ 1010kHz\)

(i) (a) Concern
(b) Scientific temperament
(c) Keen observer
(d) Alertness
(or any other two correct values)
(ii) \({ d }=\sqrt { { 2hR } } \)
\(=\sqrt { 2\times 20\times { 6.4\times 10 }^{ 6 } \ } m\)
\(= \ 2\times 8\times { 10 }^{ 3 }\)
= 16 km

(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.

(i) As set up that transfer information implicitly from one point to another is called communication system. Analog signal is better than a digital signal.
(ii) Sohan is an intelligent and knowledgeable person.
(iii) There are three communication satellites.
(iv) The three groups display the values if eagerness to engage in adebate, promptness in taking decision and ability to work in a constructive manner.