DC Pandey Solutions: Communication Systems | DC Pandey Solutions for JEE Physics PDF Download

 Page 1


Exercises 
  Single Correct Option 
Q 1.  Three waves A, B and C of frequencies 1600 kHz, 5 MHz and 60 MHz, respectively are to be 
transmitted from one place to another. Which of the following is the most appropriate mode of 
communication? 
  (a) A is transmitted via space wave while B and C transmitted via sky wave. 
  (b) A is transmitted via ground wave, B via sky wave and C via space wave. 
  (c) B and C are transmitted via ground wave while A is transmitted via sky wave. 
  (d) B is transmitted via ground wave while A and C are transmitted via space wave. 
Q 2.  A 100 m long antenna is mounted on a 500 m tall building. The complex can become a 
transmission tower for waves with ?.  
  (a) ~ 400m   (b) ~ 25m   (c) ~ 150 m   (d) ~ 2400 m 
Q 3.  A speech signal of 3 kHz is used to modulate a carrier signal of frequency 1 MHz, using amplitude 
modulation. The frequencies of the side bands will be 
  (a) 1.003 MHz and 0.997 MHz   (b) 3001 kHz and 2997 kHz 
  (c) 1003 kHz and 1000 kHz    (d) 1 MHz and 0.997 MHz 
Q 4.  A message signal of frequency ? m
 
is superposed on a carrier wave of frequency ? c
 
to get an 
amplitude modulated wave (AM). The frequency of the AM wave will be 
 (a) ? m   (b) ? c   (c) 
cm
2
? ? ?
   (c) 
cm
2
? ? ?
 
Q 5.  A basic communication system consists of 
  (A) transmitter (B) information source (C) user of information (D) channel (E) receiver 
  Choose the correct sequence in which these are arranged in a basic communication system.  
  (a)ABCDE   (b) BADEC   (c) BDACE   (d) BEADC  
Q 6.  Which of the following frequencies will be suitable for beyond the horizon communication using 
sky waves? 
  (a) 10 kHz   (b) 10 MHz   (c) 1 GHz   (d) 1000 GHz 
Q 7.  Frequencies in the UHF range normally propagate by means of 
  (a) ground waves  (b) sky waves   (c) surface waves  (d) space waves 
Q 8.  Digital signals 
  (i) do not provide a continuous set of values 
  (ii) represent values as discrete steps 
  (iii) can utilize binary system and 
  (iv) can utilize decimal as well as binary systems 
  Which of the above statements are true? 
  (a) (i) and (ii) only     (b) (ii) and (iii) only 
  (c) (i), (ii) and (iii) but not (iv)   (d) All of (i), (ii), (iii) and (iv) 
  More than One Correct Options 
Q 9.  A TV transmission tower has a height of 240 m. Signals broadcast from this tower will be 
received by LOS communication at a distance of (assume the radius of earth to be 6.4 × 10
6
m)  
  (a) 100 km   (b) 24 km   (c) 55 km   (d) 50 km 
Page 2


Exercises 
  Single Correct Option 
Q 1.  Three waves A, B and C of frequencies 1600 kHz, 5 MHz and 60 MHz, respectively are to be 
transmitted from one place to another. Which of the following is the most appropriate mode of 
communication? 
  (a) A is transmitted via space wave while B and C transmitted via sky wave. 
  (b) A is transmitted via ground wave, B via sky wave and C via space wave. 
  (c) B and C are transmitted via ground wave while A is transmitted via sky wave. 
  (d) B is transmitted via ground wave while A and C are transmitted via space wave. 
Q 2.  A 100 m long antenna is mounted on a 500 m tall building. The complex can become a 
transmission tower for waves with ?.  
  (a) ~ 400m   (b) ~ 25m   (c) ~ 150 m   (d) ~ 2400 m 
Q 3.  A speech signal of 3 kHz is used to modulate a carrier signal of frequency 1 MHz, using amplitude 
modulation. The frequencies of the side bands will be 
  (a) 1.003 MHz and 0.997 MHz   (b) 3001 kHz and 2997 kHz 
  (c) 1003 kHz and 1000 kHz    (d) 1 MHz and 0.997 MHz 
Q 4.  A message signal of frequency ? m
 
is superposed on a carrier wave of frequency ? c
 
to get an 
amplitude modulated wave (AM). The frequency of the AM wave will be 
 (a) ? m   (b) ? c   (c) 
cm
2
? ? ?
   (c) 
cm
2
? ? ?
 
Q 5.  A basic communication system consists of 
  (A) transmitter (B) information source (C) user of information (D) channel (E) receiver 
  Choose the correct sequence in which these are arranged in a basic communication system.  
  (a)ABCDE   (b) BADEC   (c) BDACE   (d) BEADC  
Q 6.  Which of the following frequencies will be suitable for beyond the horizon communication using 
sky waves? 
  (a) 10 kHz   (b) 10 MHz   (c) 1 GHz   (d) 1000 GHz 
Q 7.  Frequencies in the UHF range normally propagate by means of 
  (a) ground waves  (b) sky waves   (c) surface waves  (d) space waves 
Q 8.  Digital signals 
  (i) do not provide a continuous set of values 
  (ii) represent values as discrete steps 
  (iii) can utilize binary system and 
  (iv) can utilize decimal as well as binary systems 
  Which of the above statements are true? 
  (a) (i) and (ii) only     (b) (ii) and (iii) only 
  (c) (i), (ii) and (iii) but not (iv)   (d) All of (i), (ii), (iii) and (iv) 
  More than One Correct Options 
Q 9.  A TV transmission tower has a height of 240 m. Signals broadcast from this tower will be 
received by LOS communication at a distance of (assume the radius of earth to be 6.4 × 10
6
m)  
  (a) 100 km   (b) 24 km   (c) 55 km   (d) 50 km 
Q 10.  An audio signal of 15 kHz frequency cannot be transmitted over long distances without 
modulation because 
  (a) the size of the required antenna would be at least 5 km which is not convenient 
  (b) the audio signal can not be transmitted through sky waves 
  (c) the size of the required antenna would be at least 20 km, which is not convenient 
  (d) effective power transmitted would be very low, if the size of the antenna is less than 5 km 
Q 11.  Audio sine waves of 3 kHz frequency are used to amplitude modulate a carrier signal of 1.5 MHz. 
Which of the following statements are true? 
  (a) The side band frequencies are 1506 kHz and 1494 kHz 
  (b) The bandwidth required for amplitude modulation is 6 kHz 
  (c) The bandwidth required for amplitude modulation is 3 MHz 
  (d) The side band frequencies are 1503 kHz and 1497 kHz 
Q 12.  In amplitude modulation, the modulation index (X, is kept less than or equal to 1 because 
(a) ? > 1 will result in interference between carrier frequency and message frequency, resulting 
into distortion. 
  (b) ? > 1 will result in overlapping of both side bands resulting into loss of information. 
  (c) ? > 1 will result in change in phase between carrier signal and message signal. 
(d) ? > 1 indicates amplitude of message signal greater than amplitude of carrier signal resulting 
into distortion. 
Subjective Questions 
Q 13.  Compute the LC product of a tuned amplifier circuit required to generate a carrier wave of 1 MHz 
for amplitude modulation. 
Q 14.  A carrier wave of peak voltage 12 V is used to transmit a message signal. What should be the peak 
voltage of the modulating signal in order to have a modulation index of 75%? 
Q 15.  Which of the following would produce analog signals and which would produce digital signals?  
  (i) A vibrating tuning fork  (ii) Musical sound due to a vibrating sitar string 
  (iii) Light pulse   (iv) Output of NAND gate 
Q 16.  Two waves A and B of frequencies 2 MHz and 3 MHz, respectively are beamed in the same 
direction for communication via sky wave. Which one of these is likely to travel longer distance in 
the ionosphere before suffering total internal reflection? 
Q 17.  The maximum amplitude of an AM wave is found to be 15 V while its minimum amplitude is 
found to be 3V. What is the modulation index? 
Q 18.  Why is a AM signal likely to be more noisy than a FM signal upon transmission through a 
channel? 
Q 19.  Is it necessary for a transmitting antenna to be at the same height as that of the receiving antenna 
for line of sight communication? A TV transmitting antenna is 81 m tall. How much service area 
can it cover, if the receiving antenna is at the ground level? 
Q 20.  A TV transmission tower antenna is at a height of 20 m. How much service area can it cover if the 
receiving antenna is (i) at ground level, (ii) at a height of 25 m? Calculate the percentage increase 
in area covered in case (ii) relative to case (i). 
Page 3


Exercises 
  Single Correct Option 
Q 1.  Three waves A, B and C of frequencies 1600 kHz, 5 MHz and 60 MHz, respectively are to be 
transmitted from one place to another. Which of the following is the most appropriate mode of 
communication? 
  (a) A is transmitted via space wave while B and C transmitted via sky wave. 
  (b) A is transmitted via ground wave, B via sky wave and C via space wave. 
  (c) B and C are transmitted via ground wave while A is transmitted via sky wave. 
  (d) B is transmitted via ground wave while A and C are transmitted via space wave. 
Q 2.  A 100 m long antenna is mounted on a 500 m tall building. The complex can become a 
transmission tower for waves with ?.  
  (a) ~ 400m   (b) ~ 25m   (c) ~ 150 m   (d) ~ 2400 m 
Q 3.  A speech signal of 3 kHz is used to modulate a carrier signal of frequency 1 MHz, using amplitude 
modulation. The frequencies of the side bands will be 
  (a) 1.003 MHz and 0.997 MHz   (b) 3001 kHz and 2997 kHz 
  (c) 1003 kHz and 1000 kHz    (d) 1 MHz and 0.997 MHz 
Q 4.  A message signal of frequency ? m
 
is superposed on a carrier wave of frequency ? c
 
to get an 
amplitude modulated wave (AM). The frequency of the AM wave will be 
 (a) ? m   (b) ? c   (c) 
cm
2
? ? ?
   (c) 
cm
2
? ? ?
 
Q 5.  A basic communication system consists of 
  (A) transmitter (B) information source (C) user of information (D) channel (E) receiver 
  Choose the correct sequence in which these are arranged in a basic communication system.  
  (a)ABCDE   (b) BADEC   (c) BDACE   (d) BEADC  
Q 6.  Which of the following frequencies will be suitable for beyond the horizon communication using 
sky waves? 
  (a) 10 kHz   (b) 10 MHz   (c) 1 GHz   (d) 1000 GHz 
Q 7.  Frequencies in the UHF range normally propagate by means of 
  (a) ground waves  (b) sky waves   (c) surface waves  (d) space waves 
Q 8.  Digital signals 
  (i) do not provide a continuous set of values 
  (ii) represent values as discrete steps 
  (iii) can utilize binary system and 
  (iv) can utilize decimal as well as binary systems 
  Which of the above statements are true? 
  (a) (i) and (ii) only     (b) (ii) and (iii) only 
  (c) (i), (ii) and (iii) but not (iv)   (d) All of (i), (ii), (iii) and (iv) 
  More than One Correct Options 
Q 9.  A TV transmission tower has a height of 240 m. Signals broadcast from this tower will be 
received by LOS communication at a distance of (assume the radius of earth to be 6.4 × 10
6
m)  
  (a) 100 km   (b) 24 km   (c) 55 km   (d) 50 km 
Q 10.  An audio signal of 15 kHz frequency cannot be transmitted over long distances without 
modulation because 
  (a) the size of the required antenna would be at least 5 km which is not convenient 
  (b) the audio signal can not be transmitted through sky waves 
  (c) the size of the required antenna would be at least 20 km, which is not convenient 
  (d) effective power transmitted would be very low, if the size of the antenna is less than 5 km 
Q 11.  Audio sine waves of 3 kHz frequency are used to amplitude modulate a carrier signal of 1.5 MHz. 
Which of the following statements are true? 
  (a) The side band frequencies are 1506 kHz and 1494 kHz 
  (b) The bandwidth required for amplitude modulation is 6 kHz 
  (c) The bandwidth required for amplitude modulation is 3 MHz 
  (d) The side band frequencies are 1503 kHz and 1497 kHz 
Q 12.  In amplitude modulation, the modulation index (X, is kept less than or equal to 1 because 
(a) ? > 1 will result in interference between carrier frequency and message frequency, resulting 
into distortion. 
  (b) ? > 1 will result in overlapping of both side bands resulting into loss of information. 
  (c) ? > 1 will result in change in phase between carrier signal and message signal. 
(d) ? > 1 indicates amplitude of message signal greater than amplitude of carrier signal resulting 
into distortion. 
Subjective Questions 
Q 13.  Compute the LC product of a tuned amplifier circuit required to generate a carrier wave of 1 MHz 
for amplitude modulation. 
Q 14.  A carrier wave of peak voltage 12 V is used to transmit a message signal. What should be the peak 
voltage of the modulating signal in order to have a modulation index of 75%? 
Q 15.  Which of the following would produce analog signals and which would produce digital signals?  
  (i) A vibrating tuning fork  (ii) Musical sound due to a vibrating sitar string 
  (iii) Light pulse   (iv) Output of NAND gate 
Q 16.  Two waves A and B of frequencies 2 MHz and 3 MHz, respectively are beamed in the same 
direction for communication via sky wave. Which one of these is likely to travel longer distance in 
the ionosphere before suffering total internal reflection? 
Q 17.  The maximum amplitude of an AM wave is found to be 15 V while its minimum amplitude is 
found to be 3V. What is the modulation index? 
Q 18.  Why is a AM signal likely to be more noisy than a FM signal upon transmission through a 
channel? 
Q 19.  Is it necessary for a transmitting antenna to be at the same height as that of the receiving antenna 
for line of sight communication? A TV transmitting antenna is 81 m tall. How much service area 
can it cover, if the receiving antenna is at the ground level? 
Q 20.  A TV transmission tower antenna is at a height of 20 m. How much service area can it cover if the 
receiving antenna is (i) at ground level, (ii) at a height of 25 m? Calculate the percentage increase 
in area covered in case (ii) relative to case (i). 
Q 21.  If the whole earth is to be connected by LOS communication using space waves (no restriction of 
antenna size or tower height), what is the minimum number of antennas required? Calculate the 
tower height of these antennas in terms of earth's radius? 
Q 22.  The maximum frequency for reflection of sky waves from a certain layer of the ionosphere is 
found to be fmax = 9(Nmax)
1/2
, where Nmax is the maximum electron density at that layer of the 
ionosphere. On a certain day it is observed that signals of frequencies higher than 5 MHz are not 
received by reflection from the F1 layer of the ionosphere while signals of frequencies higher than 
8 MHz are not received by reflection from the F2 layer of the ionosphere. Estimate the maximum 
electron densities of the F1 and F2 layers on that day. 
Q 23.  A 50 MHz sky wave takes 4.04 ms to reach a receiver via re-transmission from a satellite 600 km 
above earth's surface. Assuming re-transmission time by satellite negligible, find the distance 
between source and receiver. If communication between the two was to be done by Line of Sight 
(LOS) method, what should be the size of transmitting antenna? 
Answers 
Exercises 
  1. (b) 2. (a) 3. (a) 4. (b) 5. (b) 6. (b) 7. (d) 8. (d) 9. (b,c,d) 10. (a,b,d) 11. (b,d) 12. (b,d)  
13. 2.53 × 10
-14
 s
2
  14.9 V  15. (i) analog (ii) analog (iii) digital (iv) digital 
16. 3 MHz  17.2/3   18. See the hints 
19. No, 3258 km
2
  20. (i) 803.84 km
2
 (ii) 3608.52 km
2
, 348.9%  21. Six, h = radius of earth  
22. 3.086 × 10
11
 m
-3
,7.9 × 10
11
 m
-3   
23. 170 km, 565 m