Communication Systems, Laser Practice Questions - DPP for NEET

 Page 1


1. (a) By using 
( )
1/2
max
9 2.84
cc
f N f MHz Þ= ;
2. (d) Carrier frequency > audio frequency
3. (a) A maximum frequency deviation of 75 kHz is permitted
for commercial FM broadcast stations in the 88 to 108
MHz VHF band.
4. (c) Carrier + signal ® modulation.
5. (c) 270 W
6. (a)
12
m >m
7. (c)
60
175
cos
cos 70
= ==
q
o
c
f
MUF MHz
8. (d) coordinated waves of a particular wavelength
9. (d) Surgery needs sharply focused beam of light and laser
can be sharply focused.
10. (d) Laser beams are perfectly parallel. So that they are very
narrow and can travel a long distance without
spreading. This is the feature of laser while they are
monochromatic  and coherent, these are characteristics
only.
11. (b) The formula for modulating index is given by
3
f
3
m
Frequency var iation 10 10
m5
v Modulating frequency
2 10
d´
= = ==
´
12. (b)
af
ff <
13. (c) An antenna is a metallic structure used to radiate or
receive EM waves.
14. (a) V arying the local oscillator frequency
15. (b) In the channel or in the transmission line
16. (a) Carrier swing 
Frequency deviation 50
7.143
Modulating frequency 7
= ==
17. (c) In optical fibre, light travels inside it, due to total
internal reflection.
18. (b) The process of changing the frequency of a carrier wave
(modulated wave) in accordance with the audio
frequency signal (modulating wave) is known as
frequency modulation (FM).
19. (d) Following are the problems which are faced while
transmitting audio signals directly .
(i) These signals are relatively of short range.
(ii) If every body started transmitting these low frequency
signals directly , mutual interference will render all of
them ineffective.
(iii) Size of antenna required for their efficient radiation
would be larger i.e. about 75 km.
20. (a) The critical frequency of a sky wave for relection from
a layer of atmosphere is given by 
1/2
max
9( )1 =
c
fN
6 1/2
max
10 10 9( )1 Þ ´=N
2
6
123
max
10 10
1.2 10
9
Nm
-
æö
´
Þ=´ ç÷
ç÷
èø
;
21. (b) Core of acceptance angle 
1 22
12
sin nn
-
q=-
22 (a) Remote sensing is the technique to collect information
about an object in respect of its size, colour, nature,
location, temperature etc without physically touching
it. There are some areas or location which are
inaccessible. So to explore sensing is used. Remote
sensing is done through a satellite.
23. (b)
24. (a) Laser beams are perfectly parallel. They are
monochromatic and coherent. These are characteristics
only.
25. (d)
10
cm
f 9 N 9 9 10 = = ´´
                      
6
2.7 10 Hz 2.7 MHz = ´=
26. (b)
( )
( )
2
23
6
c
22
3
D 250 10
f f 1 2.7 10 1
4h
4 150 10
´
=+= ´ ´ +
´´
       = 3.17 × 10
6
Hz
27. (c)
ci
f f sec = =f
6
i
6
c
f 3.17× 10
Sec 1.174
f
2.7 10
f===
´
–1
i
sec (1.174) 31.6 f= =° .
28. (d) TV signals (frequency greater than 30 MHz) cannot
be  propagated through sky wave propagation.
Above critical frequency, an electromagnetic wave
penetrates the ionosphere and is not reflected by it.
29. (d) The electromagnetic waves of shorter wavelength do
not suffer much diffraction from the obstacles of earth’s
atmosphere so they can travel long distance.
Also, shorter the wavelength, shorter is the velocity of
wave propagation.
30. (b) A dish antenna is a directional antenna because it can
transmit or signals in all direction.