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Published on: 19/08/2019
Communication Systems
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Questions + Answers key
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1.
The fraction of world's human population using the services of internet is
1/2
1/3
1/4
1/5
2.
The correct relation between plasma frequency v of a radio wave and electron density N of the ionosphere layer is
v = 109 N
v = 9 N
\(v = 9 \sqrt{N}\)
\(N = 9 \sqrt{v}\)
3.
The frequency band of FM broadcast is
88 MHz to 108 MHz
88 Hz to 88 kHz
800 kHz to 8000 MHz
none of the above
4.
Out of the following, which is not an essential element of a communication system?
Transmitter
Transducer
Receiver
Communication Channel
5.
A 100m long antenna is mounted on a 500 m tall building. The complex can become a transmission tower for waves with \(\lambda \).
\(\sim \) 400 m
\(\sim \) 25 m
\(\sim \) 150 m
\(\sim \) 2400 m
6.
what is heterodyning?
7.
What is meant by attenuation and distortion?
8.
What is the total modulation index when a carrier wave is modulated by a number of sine waves?
9.
What is changed in amplitude modulation?
10.
A 100 MHz carrier is modulated by a 12 kHz sine wave so as to cause a frequency swing of +50kHz. Find the modulation index
11.
Block diagram of a receiver is shown in the figure:
(a) Identify 'X' and 'Y'.
(b) Write their functions.
12.
What is the frequencies used in satellite communication? What is common between these waves and light waves?
13.
What are the limitations of amplitude modulation?
14.
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?
15.
By what percentage will the transmission range of a T.V. tower be affected when the height of the tower is increased by 21%?
16.
The distance between transmitting antenna and receiving antenna at which they can see each other is called -------------
17.
Estimate the fastest bit rate capable of being carried by light of wavelength 1.3 \(\mu m\). How many phone calls could be carried at this bit rate? Bandwidth of optical fibre = 2 GHz.
1.
(b)
1/3
2.
(c)
\(v = 9 \sqrt{N}\)
3.
(a)
88 MHz to 108 MHz
4.
(b)
Transducer
5.
(a)
\(\sim \) 400 m
6.
( )
Mixing two frequencies across nonlinear impedance
7.
( )
Attenuation is the phenomenon of decrease in signal strength due to energy loses Distortion is the name given to the wave from perturbation.
8.
( )
\(\mu _{ A }=\sqrt { { \mu }_{ 1 }^{ 2 }+{ \mu }_{ 2 }^{ 2 }+{ \mu }_{ 3 }^{ 2 }+..... } \)
9.
( )
Amplitude of high-frequency carrier wave is made proportional to the instantaneous amplitude of audio frequency modulating voltage.
10.
Modulation index ,mf = \(\frac { Maximum\ frequency\ deviation }{ minimum\ signal\ freqency } \)
11.
X: IF stage
Y: Amplifier
The carrier frequency is changed to a lower frequency by intermediate frequency (IF) stage preceding the detection.
It increases the strength of the detected signal.
12.
The range of frequencies used in satellite communication is uplink: 5.925 to 6.425 GHz and downlink: 3.7 to 4.2 GHz. Both the waves used in satellite communication and light waves are electromagnetic waves and can travel through a vacuum at the same speed.
13.
(i) Efficiency of AM transmission is low
(ii) Amplitude modulation suffers from noise
(iii) Quality of audio signal is often poor.
14.
(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.
15.
Original transmission range,
\(d=\sqrt { 2hR } \)
New height, \({ h }^{ \prime }=h+\frac { 21h }{ 100 } =1.21h\)
New transmission range,
\({ d }^{ \prime }=\sqrt { 2{ h }^{ \prime }R } =\sqrt { 2\times 1.21hR } =1.1d\)
% increase in range
\(=\frac { { d }^{ \prime }-d }{ d } \times 100=\frac { \left( 1.1d \right) -d }{ d } \times 100\)= 10%
16.
( )
range of communication,
17.
Here, \(\lambda =1.3\times { 10 }^{ -6 }m\) \(v=\frac { c }{ \lambda } =\frac { 3\times { 10 }^{ 8 } }{ 1.3\times { 10 }^{ -6 } } =2.3\times { 10 }^{ 14 }Hz\)
Max bit rate = 2 v = 2 x 2.3 x 1014 = 4.6 x 1014 Hz
For optical fibre, bandwidth = 2 GHz = 2 x 109 Hz
No of phone calls = \(\frac { 4.6\times { 10 }^{ 14 } }{ 2\times { 10 }^{ 9 } } =2.3\times { 10 }^{ 5 }\)
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