Communication Systems - 1 File Download

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Level - II

SECTION - A

Objective Type Questions

(Elements of a Communication System)

1. A device which converts one type of energy into other is known as

(1) Attenuator (2) Transmitter (3) Receiver (4) Transducer

Sol. Answer (4)

By definition.

2. Modem is a device used for

(1) Modulation (2) Demodulation

(3) Converting physical variables into electrical form (4) Both (1) & (2)

Sol. Answer (4)

By definition/Fact.

3. The ‘Facsimile Transmission (FAX)’ involves

(1) Speech communication

(2) Telegraphy

(3) Exact reproduction of a document or static picture

(4) Taking photograph

Sol. Answer (3)

Fact.

4. Modem is a device used for

(1) Converting digital to analog signals (2) Converting analog signal to digital signals

(3) Either of these (4) None of these

Sol. Answer (3)

By definition.

Solutions

Chapter 15

Communication Systems

144 Communication Systems Solutions of Assignment (Level-II)


(Propagation of Electromagnetic Waves)

5. The intensity of a light pulse travelling along an optical fibre decreases exponentially with distance accordingto the relation I = I

0e–0.0693x

where x is in km and I

0 is intensity of incident pulse. The intensity of pulse reduces

to 4

1 after travelling a distance

(1) 1 km (2) 10 km

(3) 20 km (4) 40 km

Sol. Answer (3)

I = I0e–0.0693x

0.06930

04

xI

I e−

=

0.06931

4

xe−

=

1log 0.0632

4e

x

loge 1 – log

e 4 = – 0.06932 x

0 – 2.303 × 2 log10

2 = – 0.06932 x

x = 20 km

6. Numerical aperture of an optical fibre (w.r.t. air) having core and cladding refractive indices n1 and n

2 respectively

is

(1) 2

2

2

1 nn (2) 2

2

2

1

1sin nn (3) 2

2

2

1

1cos nn (4) 2

2

2

1

1tan nn

Sol. Answer (1)

n1 = of core

n2 = of cladding '

i

= 90 –

n1 sin

c = n

2 sin90º

n1 sin = n

2

sin = 2

1

n

n

Numerical Perture = n1 sin = n

1 sin(90 – ) = n

1 cos

c

Numerical aperture = 2

11 sin

cn − θ = 2 2

1 2n n−

7. On a particular day, the maximum frequency reflected from the ionosphere is 10 MHz. The maximum electrondensity in ionosphere is

(1) 106/m2 (2) 1012/m3 (3) 1.23 × 1012/m3 (4) 1/9 × 106/m3

Sol. Answer (3)

fc

= max)9 (N

145Solutions of Assignment (Level-II) Communication Systems


8. An optical fibre having core of refractive index 3 and cladding of refractive index 1.5 is kept in air. The

maximum angle of acceptance is

(1) 60° (2) 45°

(3) 30° (4) 15°

Sol. Answer (1)2 2

1 1 2

0

sinC

n n

n

−

⎛ ⎞−θ = ⎜ ⎟⎝ ⎠

( )12sin 3 1.5

−

=−

( )1sin 0.75

−

=

= 60°

9. A ground receiver station is receiving a signal at 100 MHz transmitted from a ground transmitter at a heightof 300 m located at a distance of 1000 km. It is coming via

(1) Ground wave (2) Space wave (L.O.S)

(3) Sky wave (4) Satellite communication

Sol. Answer (4)

Fact.

10. If 1 and

2 are the refractive indices of the material of core and cladding respectively of an optical fiber, then

(1) 1 >

2(2)

1 <

2

(3) 1 =

2(4)

1

2

Sol. Answer (1)

Refractive index of core is always greater than cladding in an optical fibre.

11. A transmitting antenna at the top of tower has a height 32 m and height of receiving antenna is 50 m. Minimumdistance between them for satisfactory LOS mode of communication is

(1) 40 km (2) 45 km

(3) 60 km (4) 100 km

Sol. Answer (2)

2 2 45536 m 45.536 kmT R

d RH RH ∵

(Modulation and Its Necessity)

12. Broadcasting antennas are generally

(1) Omnidirectional type (2) Vertical type

(3) Horizontal type (4) None of these

Sol. Answer (2)

Fact.



(Amplitude Modulation)

13. An AM broadcasting station has a vertical telescopic transmitting antenna and a receiver has a vertical telescopicantenna. The receiver will respond to

(1) Electric component of the electromagnetic wave produced by antenna

(2) Magnetic component of the electromagnetic wave produced by antenna

(3) Both electric and magnetic components of electromagnetic wave produced by antenna

(4) 50% of both components of electromagnetic wave produced by antenna

Sol. Answer (1)

14. In an AM wave for audio frequency of 3400 cycle/s, the appropriate carrier frequency will be

(1) 1000 Hz (2) 34000 MHz

(3) 60000 Hz (4) 800,000 Hz

Sol. Answer (4)

Fact.

15. In frequency modulation, the amount of frequency deviation depends on the

(1) Frequency of audio signal (2) Amplitude of audio signal

(3) Both the frequency and amplitude of audio signal (4) None of these

Sol. Answer (2)

( )2

m

c f

Af f t f k

16. Radio wave of constant amplitude can be generated with

(1) Filter (2) Rectifier

(3) FET (4) Oscillator

Sol. Answer (4)

Fact.

17. Which of the following is an advantage of FM over AM?

(1) Less noise (2) Larger bandwidth

(3) More circuit complexity (4) Can be transmitted to longer distance

Sol. Answer (1)

Fact.

(Production of Amplitude Modulated Wave)

18. A demodulator circuit contains

(1) A diode (2) A transistor

(3) A solar cell (4) Logic gate

Sol. Answer (1)

Fact.



19. An audio signal of amplitude one half the carrier amplitude is used in an amplitude modulation. The modulationindex is

(1) 2 (2) 0.25

(3) 0.50 (4) 0.125

Sol. Answer (3)

0.5m

c

A

Aµ = =

20. Which of the following modulation index produces noise?

(1) 0.5 (2) 0.8

(3) 1.2 (4) 0.7

Sol. Answer (3)

Medulation index greater than one produces noise. We keep modulation index less than 1.

21. A 400 W carrier wave is modulated to a depth of 75%. Calculate the total power in the modulated wave.

(1) 512.5 W (2) 400 W

(3) 112.5 W (4) 200 W

Sol. Answer (1)

2

12

cP P

22. Which of the following is not used in analog communication?

(1) PAM (2) AM

(3) FM (4) ASK

Sol. Answer (4)

SECTION - B

Previous Years Questions

1. The radiowaves of frequency 300 MHz to 3000 MHz belong to

(1) High frequency band (2) Very high frequency band

(3) Ultra high frequency band (4) Super high frequency band

Sol. Answer (3)

Fact.

2. Range of frequencies alotted for commerical FM radio broad cast is

(1) 88 to 108 MHz (2) 88 to 108 kHz

(3) 8 to 88 MHz (4) 88 to 108 GHz

Sol. Answer (1)

Fact.



3. The length of a half wave dipole antenna at 30 MHz is

(1) 10 m (2) 50 m

(3) 5 m (4) 100 m

Sol. Answer (3)

8

7

3 1010 m

3 10

c

f

×λ = = =

×

Length of half wave antenna = 2

λ = 5 m

4. In a single reflection from the ionosphere, the sky waves cover a distance on ground not more than

(1) 400 m (2) 4000 m

(3) 400 km (4) 4000 km

Sol. Answer (4)

Fact.

5. The maximum range of ground or surface wave propagation depends on

(1) The frequency of radiowaves only

(2) Power of the transmitter only

(3) Both on frequency as well as power

(4) Wavelength of radiowaves

Sol. Answer (3)

Fact.

6. Refractive index of ionosphere is

(1) Zero (2) More than one

(3) Less than one (4) One

Sol. Answer (3)

Refractive index of ionosphere is less than 1.

7. For sky wave propagation of a 10 MHz signal, what should be the minimum electron density in ionosphere?

(1) 1.2 × 108 m–3 (2) 1.2 × 1012 m–3

(3) 1.2 × 1014 m–3 (4) 1.2 × 1016 m–3

Sol. Answer (2)

max

9cf N

8. The TV tower has a height of 100 m. What is the maximum distance upto which the T.V. transmission canbe received ?

(1) 34.77 km (2) 32.70 km

(3) 35.2 km (4) 40.70 km



Sol. Answer (3)

Maximum distance is given by 2d Rh=

R = 6.4 × 106m, h = 100 m

9. If both the length of the antenna and the wavelength of the signal to the transmitted are doubled, the power

radiated by the antenna

(1) Remains constant (2) Is doubled

(3) Is halved (4) Increases 16 times

Sol. Answer (1)

2L

P

∵

10. Arrange the following communication frequency bands in the increasing order of frequencies

a. AM broadcast

b. Cellular mobile radio

c. FM broadcast

d. Television UHF

e. Satellite communication

(1) a, b, c, d, e (2) a, c, b, d, e

(3) e, b, d, c, a (4) a, c, d, b, e

Sol. Answer (4)

Fact.

11. Modulation is the process of superposing

(1) Low frequency radio signal on low frequency audiowaves

(2) Low frequency audio signal on high frequency waves

(3) High frequency radio signal on low frequency audio signal

(4) High frequency audio signal on low frequency radiowaves

Sol. Answer (2)

By definition/Fact.

12. The waves used by artificial satellites for communication purposes are

(1) Microwaves (2) AM radiowaves

(3) X-rays (4) FM radiowaves

Sol. Answer (1)

Fact.



13. In communication with help of antenna, if height is doubled, then the range covered which was initially ‘r’ wouldbecome

(1) 2r (2) 3r

(3) 4r (4) 5r

Sol. Answer (1)

Range(r) = 2Rh

22 2r R h=

22r r=

14. Which range of frequencies can be transmitted suitably by sky wave propagation?

(1) 5 kHz to 500 kHz (2) 1 MHz to 2 MHz

(3) 2 MHz to 20 MHz (4) Above 50 MHz

Sol. Answer (3)

Fact.

15. In an optical fibre, the light signal is transmitted by

(1) Refraction at the core-cladding interface

(2) Interference between the incident and inflected waves

(3) Repeated total internal reflections at the core and cladding interface

(4) Polarisation of optical signals

Sol. Answer (3)

Fact.

16. Fraction of total power carried by the side bands (PS/P

T) is given by

(1) m2 (2)2

2

2 m

m

(3)2 m

m

(4)

2

22

m

m

Sol. Answer (4)

Fact.

17. The band width of amplitude modulation is

(1) Equal to frequency of aduio signal

(2) Double the frequency of audio signal

(3) Half the frequency of carrier wave

(4) Double the frequency of carrier wave

Sol. Answer (2)

Band width = 2m



SECTION - C

Assertion-Reason Type Questions

1. A : Modulator is an essential component of a transmitter.

R : Modulator superimposes a low frequency message signal on a high frequency carrier wave.

Sol. Answer (2)

2. A : Short wave bands are used for transmission of radio waves to a large distance.

R : Short waves are reflected by ionosphere.

Sol. Answer (1)

3. A : If the amplitude of a message signal is half of the amplitude of the carrier wave, then the index of

modulation is 50%.

R : In amplitude modulation the amplitude of the carrier wave varies in accordance with the signal voltage of

the message signal.

Sol. Answer (2)

m

c

A

Aµ = . Hence assertion is true.

4. A : The bandwidth of a modulated signal is 2fm

in amplitude modulation, where fm

is the frequency of the

modulating signal.

R : In case of amplitude modulation the frequency of a modulated signal is equal to the frequency of the carrier

wave.

Sol. Answer (2)

5. A : The process of demodulation is carried out to retrieve the message signal.

R : The range of the line-of-sight propagation is limited mainly due to earth’s curvature.

Sol. Answer (2)

6. A : Transducer in communication system converts only electrical signal into a physical quantity.

R : For information signal to be transmitted directly to long distances, modulation is not a necessary process.

Sol. Answer (4)

7. A : Microwave propagation is better than the skywave propagation.

R : Microwaves have frequencies 100 to 300 GHz which have very good directional properties.

Sol. Answer (1)

8. A : In satellite communication, generally we keep different uplink and downlink frequencies.

R : In case of failures, the detection of faulty link becomes easier if frequencies are kept different.

Sol. Answer (1)



9. A : Remote sensing satellites can send continuous pictures of the earth even when clouds are present or itis dark night.

R : These satellites use infrared as well as microwaves apart from visible light photography.

Sol. Answer (1)

10. A : Optical communication system is more economical than other systems of communications.

R : The information carrying capacity of a communication system is directly proportional to its band width.

Sol. Answer (1)

11. A : Long distance radio broadcasts use short-wave bands.

R : Ionosphere reflectes waves in these bands.

Sol. Answer (1)

12. A : It is necessary to use satellites for long distance TV transmission.

R : Television signals are not properly reflected by the ionosphere therefore, reflection is effected by satellites.

Sol. Answer (1)

13. A : The electrical conductivity of the earth's atmosphere does not change with altitude.

R : Cosmic rays from outer space entering the earth's atmosphere do not affect it.

Sol. Answer (4)

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