# JEE Main

Explore popular questions from Communication System for JEE Main. This collection covers Communication System previous year JEE Main questions hand picked by experienced teachers.

# Mathematics

Communication System

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Q 1. Consider an optical communication system operating at {tex} \lambda - 800 \mathrm { nm } {/tex}. Suppose, only {tex} 1 \% {/tex} of the optical source frequency is the available channel bandwidth for optical communication. How many channels can be accommodated for transmitting audio signals requiring a bandwidth of {tex} 8 \mathrm { kHz } {/tex}

4.8 x 10

B

48

C

6.2 x 10

D

{tex} 4.8 \times 10 {/tex}

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Q 2. For sky wave propagation of a {tex} 10 \mathrm\ { MHz } {/tex} signal, what should be the minimum electron density in ionosphere

~{tex} 1.2 \times 10^{12} \mathrm{ \,m^{-3}} {/tex}

B

~{tex} 10^{6} \mathrm{ \,m^{-3}} {/tex}

C

~{tex} 10^{4} \mathrm{ \,m^{-3}} {/tex}

D

~{tex} 10^{22} \mathrm{ \,m^{-3}} {/tex}

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Q 3. In frequency modulation

A

The amplitude of modulated wave varies as frequency of carrier wave

The frequency of modulated wave varies as amplitude of modulating wave

C

The amplitude of modulated wave varies as amplitude of carrier wave

D

The frequency of modulated wave varies as frequency of modulating wave

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Q 4. A wave is represented as $e = 10 \sin \left( 10 ^ { 8 } t + 6 \sin 1250 t \right)$ then the modulating index is

A

{tex} 10 {/tex}

B

{tex} 1250 {/tex}

C

{tex} 10 ^ {8} {/tex}

{tex} 6 {/tex}

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Q 5. A carrier is simultaneously modulated by two sine waves with modulation indices of 0.4 and {tex} 0.3 . {/tex} The resultant modulation index will be

A

1

B

0.7

0.5

D

0.35

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A

Ground wave

B

lonospheric wave

Direct wave

D

Sky wave

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Q 7. A step index fibre has a relative refractive index of {tex} 0.88 \% . {/tex} What is the critical angle at the corecladding interface

A

{tex} 60 ^ { \circ } {/tex}

B

{tex} 75 ^ { \circ } {/tex}

C

{tex} 45 ^ { \circ } {/tex}

None of these

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Q 8. A sky wave with a frequency {tex} 55 \mathrm { MHz } {/tex} is incident on {tex} D {/tex} -region of earth's atmosphere at {tex} 45 . {/tex} The angle of refraction is (electron density for {tex} D {/tex} region is 400 electron/cm)

A

{tex} 60 ^ { \circ } {/tex}

{tex} 45 ^ { \circ } {/tex}

C

{tex} 30 ^ { \circ } {/tex}

D

{tex} 15 ^ { \circ } {/tex}

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Q 9. Mean optical power launched into an 8 km fibre is {tex} 120 \mu W {/tex} and mean output power is {tex} 4 \mu W , {/tex} then the overall attenuation is (Given {tex} \log 30 = 1.477 ) {/tex}

{tex} 14.77 d B {/tex}

B

{tex} 16.77 d B {/tex}

C

{tex} 3.01 d B {/tex}

D

None of these

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Q 10. Maximum useable frequency (MUF) in F region layer is {tex} x , {/tex} when the critical frequency is {tex} 60 \mathrm { MHz } {/tex} and the angle of incidence is {tex} 70 ^ { \circ } . {/tex} Then {tex} x {/tex} is

A

{tex} 150 \mathrm { MHz } {/tex}

B

{tex} 170 \mathrm{ MHz} {/tex}

{tex} 175 \mathrm { MHz } {/tex}

D

{tex} 190\mathrm { MHz } {/tex}

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Q 11. An amplitude modulated wave is modulated to {tex} 50 \% {/tex}. What is the saving in power if carrier as well as one of the side bands are suppressed

A

{tex} 70 \% {/tex}

B

{tex} 65.4 \% {/tex}

{tex} 94.4 \% {/tex}

D

{tex} 25.5 \% {/tex}

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Q 12. In an FM system a 7 kHz signal modulates 108 MHz carrier so that frequency deviation is 50 kHz. The carrier swing is

7.143

B

8

C

0.71

D

350

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Q 13. Through which mode of propagation, the radio waves can be sent
from one place to another

A

Ground wave propagation

B

Sky wave propagation

C

Space wave propagation

All of them

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Q 14. In {tex} \mathrm { AM } {/tex}, the centpercent modulation is achieved when

Carrier amplitude {tex} = {/tex} signal amplitude

B

Carrier amplitude {tex} \neq {/tex} signal amplitude

C

Carrier frequency {tex} = {/tex} signal frequency

D

Carrier frequency {tex} \neq {/tex} signal frequency

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Q 15. Range of frequencies allotted for commercial FM radio broadcast is

{tex} \mathrm {88 \,\,to\,\, 108 }\,\,\ MHz {/tex}

B

{tex} \mathrm {88 \,\,to\,\, 108 }\,\,\ KHz {/tex}

C

{tex} \mathrm {8 \,\,to\,\, 88 }\,\,\ MHz {/tex}

D

{tex} \mathrm {88 \,\,to\,\, 108 }\,\,\ GHz {/tex}

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Q 16. An AM wave has 1800 watt of total power content, For {tex} 100 \% {/tex} modulation the carrier should have power content equal to

A

1000 watt

1200 watt

C

1500 watt

D

1600 watt

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Q 17. Advantage of optical fibre

A

High bandwidth and EM interference

B

Low bandwidth and EM interference

C

High band width, low transmission capacity and no EM interference

High bandwidth, high data transmission capacity and no EM interference

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Q 18. The velocity factor of a transmission line {tex} x {/tex}. If dielectric constant of the medium is {tex} 2.6 , {/tex} the value of {tex} x {/tex} is

A

0.26

0.62

C

2.6

D

6.2

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Q 19. The electron density of {tex} E , F , F {/tex} layers of ionosphere is {tex} 2 \times 10 ^{11},5 \times 10 ^ { n } {/tex}. and {tex} 8 \times 10 ^ { n} {/tex}. {tex} m ^{3} {/tex} respectively. What is the ratio of critical frequency for reflection of radiowaves

A

2:4:3

B

4:3:2

2:3:4

D

3:2:4

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Q 20. In a diode AM-detector, the output circuit consist of {tex} R = 1k \Omega {/tex} and {tex} C = 10 {/tex} {tex} p F . {/tex} A carrier signal of {tex} 100 \mathrm { kHz } {/tex} is to be detected. Is it good

A

Yes

No

C

Information is not sufficient

D

None of these

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Q 21. The antenna current of an AM transmitter is {tex} 8 A {/tex} when only carrier is sent but increases to {tex} 8.96 A {/tex} when the carrier is sinusoidally modulated. The percentage modulation is

A

{tex} 50 \% {/tex}

B

{tex} 60 \% {/tex}

C

{tex} 65 \% {/tex}

{tex} 71 \% {/tex}

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Q 22. The total power content of an AM wave is 900 {tex} W . {/tex} For {tex} 100 \% {/tex} modulation, the power transmitted by each side band is

A

{tex} 50 W {/tex}

B

{tex} 100\, W {/tex}

{tex} 150 W {/tex}

D

{tex} 200 \mathrm { W } {/tex}

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Q 23. If {tex} \mu {/tex} and {tex} \mu_{1}{/tex} are the refractive indices of the materials of core and cladding of an optical fibre, then the loss of light due to its leakage
can be minimised by having

{tex} \mu > \mu_{1} {/tex}

B

{tex} \mu < \mu_{1} {/tex}

C

{tex} \mu = \mu_{1} {/tex}

D

None of these

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Q 24. In a radio receiver, the short wave and medium wave stations are tuned by using the same capacitor but coils of different inductance {tex} L_{s}{/tex} and {tex} L_{m}{/tex} respectively then

A

{tex} L_{s}{/tex}> {tex} L_{m}{/tex}

{tex} L_{s}{/tex} < {tex} L_{m}{/tex}

C

{tex} L_{s}{/tex}= {tex} L_{m}{/tex}

D

None of these

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Q 25. The audio signal used to modulate {tex} 60 \sin \left( 2 \pi \times 10 \mathrm { t } \right) {/tex} is {tex} 15 \sin 300 \pi \mathrm { t } {/tex}. The depth of modulation is

A

{tex} 50 \% {/tex}

B

{tex} 40 \% {/tex}

{tex} 25 \% {/tex}

D

{tex} 15 \% {/tex}

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