Question 1
A TV tower has a height of 300m. What is the maximum distance upto which this TV transmission can be received?
(a) 62 km
(b) 64 km
(c) 65 km
(d) 60 km Solution
Question 2
Three waves A, B and C of frequencies 70 MHz, 1400 kHz and 5 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 are transmitted via sky wave.
(b) A is transmitted via ground wave, B via sky wave and C via space wave.
(c) A is transmitted via space wave, B via ground wave and C via sky wave.
(d) B is transmitted via ground wave while A and C are transmitted via space wave. Solution
(c)
Question 3
How does the effective power (P) radiated by an antenna vary with wavelenght ($\lambda$)?
(a) $P \; \alpha \; (\frac {1}{\lambda})^{-2}$
(b) $P \; \alpha \; (\frac {1}{\lambda})^{2}$
(c) $P \; \alpha \; (\frac {1}{\lambda})^{-1}$
(d) $P \; \alpha \; (\frac {1}{\lambda})$ Solution
(b)
Question 4
What should be the lenght of the diple antenna for a carrier wave of frequency $1.5 \times 10^8$ Hz?
(a) 1 m
(b) .6 m
(c) .5 m
(d) .2 m Solution
Length of dipole antenna is given by
$l= \frac {\lambda}{2} = \frac {c}{2\nu} = \frac {3 \times 10^8}{2 \times 1.5 \times 10^8}= 1$ m
Hence (a) is the correct option
Question 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 Solution
(b)
Question 6
A transmitting antenna at the top of a tower has a height 32 m and the height of the receiving antenna is 50 m. What is the maximum distance between them for satisfactory communication in LOS mode? Given radius of earth 6400 km
(a) 64.5 km
(b) 51 km
(c) 40 km
(d) 45.5 km Solution
Question 7
An audio signal of 15kHz 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. Solution
(a) ,(b) and (d)
Question 8
(a) a -> s, b ->r, c ->p, d-> q
(b) a -> r, b ->s, c ->p, d-> q
(c) a -> s, b ->r, c ->q, d-> p
(d) a -> p, b ->r, c ->s, d-> q Solution
(a)
Question 9
(a)a -> p, b ->q, c ->p, d-> q
(b)a -> q, b ->p, c ->q, d-> p
(c)a -> p, b ->p, c ->q, d-> p
(d) a -> q, b ->q, c ->q, d-> p Solution
(b)
Question 10
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 false?
(a) (i)
(b) (ii) and (iii)
(c) (i), (ii) and (iii)
(d) (iv) Solution
(d)
Question 11
The maximum amplitude of an A.M. wave is found to be 15 V while its minimum amplitude is found to be 3 V. What is the modulation index?
(a) 1/3
(b) 1/2
(c) 2/3
(d) 3/4 Solution
Question 12
A.T.V. tower has a height of 200m. By how much the height of tower be increased to triple its coverage range?
(a)1600 m
(b)88 m
(c)1800 m
(d)600 m Solution
$d=\sqrt {2R \times 200}$
$d_1 = 3d = \sqrt {2R \times h}$
From above
$\sqrt {2R \times h}=3 \sqrt {2R \times 200}$
squaring both sides, we get
$2hR=9 \times 2R \times 200$
$h= 1800$m
Increase in height of tower =1800m-200m
=1600m
Hence (a) is correct
Question 13
The super imposing of audio waves with carrier wave is called
(a) Transmission
(b) Reception
(c) Modulation
(d) Defection. Solution
(c) Modulation
Question 14
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 6kHz.
(c) The bandwidth required for amplitude modulation is 3 MHz.
(d) The side band frequencies are 1503 kHz and 1497 kHz. Solution
(b) and (d)
Question 15
Identify the mathematical expression for amplitude modulated wave
(a) $A_c \sin [\left \{ \omega _c + k_1v_m(t) \right \}t + \phi ]$.
(b) $A_c sin [\omega _c t + \phi + k_2 v_m(t)]$.
(c) $(A_c+ k_2 v_m(t)) sin (\omega _ct + \phi )$
(d) $A_c v_m(t) sin (\omega _c t + \phi )$ Solution
(c)
Text Based Questions(long)
Question 1
Write the function of a transducer and Repeater in the context of communication systems
Question 2
(i)What is modulation and Write few factors justifying the need of modulation for transmission of a signal?
(ii)Write two advantage of frequency modulation over amplitude modulation
Question 3
(i)How can a rectangular wave be generated from superimposition of the sinusoidal waves?
(ii) What is Ionosphere ? Explain its importance in Communication
Question 4
Explain and draw the schemeatic diagram for the following propagation. Give two examples of each of them also
(i) Ground wave
(ii) Sky wave
(iii) Space Wave
Why is the sky wave mode propagatio restricted to frequencies upto 40mHz.
Question 5
Draw a block diagram of a detector for AM signal and show using necessary processes and the waveforms. How the original message signal is detected from the Input AM wave?
Question 6
Draw the block diagram of the generalized communication system. Write the function of the each of the following
(i)Transmitter
(ii)Channel
(iii)Receiver
Question 7
What is pulse modulation? Describe the various type of pulse modulation with the help of suitable diagram
Question 8
Define the terms carrier swing,modulation index, and bandwidth in connection with F.M?
Question 9
Define the terms modulation index for an AM wave. What would be the modulation index for an AM wave for which the maximum amplitude is 'a' and the minimum amplitude is
'b'?
Short Answer type
Question 1
Write the functions of the following in communication systems:
(i) Transmitter
(ii) Modulator
Question 2
Why are high frequency carrier waves used for transmission?
Question 3
Distinguish between 'Analog and Digital signals'.
Question 4
Deduce an expression for the distance upto which the T.V signals can directly be received from a T.V tower of height H?
Question 5
Distinguish between a Email and FAX
Question 6
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? Question 7
A carrier wave of peak voltage 15V is used to transmit a message signal. find the peak voltage of the modulating signal in order to have a modulation index of 60%? Solution
Question 8
How do we make the choice of the communication channel?. A message signal has a bandwidth of 5 Mhz. Suggest a possible communication channel for its transmission? Question 9
Write the Full form for following
a. LASER
b. LED
c. WWW
d. FTP
e. MTSO
f. BCD
g. ASCII
h. PAM
i. PDM Solution
a. Light Amplification by Stimulated Emission of Radiations
b. Light Emitting Diode
c. World Wide web
d.File Transfer Programme
e.Mobile Telephone Switching Office
f.Binary coded decimal
g.American Standard Code for Information Interchange
h. pulse amplitude modulation
i. pulse duration modulation
Numerical Questions
Question 1
Consider an optical communication system operating at $\lambda$ =80nm.Suppose only 1% of the optical source frequency is the available channel bandwidth for optical communication. How many channels can be accommoodated for transmitting
(a) Audio-signals requiring a bandwidth of 8 kHz
(b) Video TV signals requiring an approximate bandwidth of 4.5 MHz Solution
Optical Wavelength used= 800 nm= $8 \times 10^{-7}$ m
Optical source frequency is given by
$f= \frac {c}{\lambda} = \frac { 3 \times 10^8}{8 \times 10^{-7}}= 3.75 \times 10^{14}$Hz
Total bandwidth of channel available=
=1% of $3.75 \times 10^{14}$Hz= $3.75 \times 10^{12}$Hz
Number of channel is defined as
$\text{Number of channel} = \frac {\text{Total bandwidth of channel}}{\text{Bandwidth needed per channel}}$
(a) Number of channels for audio signal
$= \frac {3.75 \times 10^{12}}{8 \times 10^3}= 4.7 \times 10^8$
(b) Number of channels for Video TV signal
$= \frac {3.75 \times 10^{12}}{4.5 \times 10^6}= 8.3 \times 10^5$
Question 2
A message signal of 2KHz and peak voltage 20V is used to modulate a carrier wave of frequency 12 MHz and peak voltage 30V.Calculate
(a) Modulation index
(b) Side band frequencies Solution
Question 3
The TV transmission tower at a particular station has a height of 160 m
(a) What is the coverage range
(b) How much population is covered by transmission if the average density around tower is 1200 per sq km
(c) by how much should the height be increased to double its coverage range
Given Radius of Earth=6400 Km Solution
Coverage range is given by
$D= \sqrt {2RH} = \sqrt {2 \times 6400 \times .16} = 45 $ km
Population Covered is given by
$=\text {population density} \times \text {Area covered}= 1200 \times \pi D^2= 76.37$ Lakh
Let $h_1$ be the height to double the coverage,then
$2d= \sqrt {2RHh_1}$
$2 \sqrt{2R \times .16}= \sqrt {2RHh_1}$
$h_1= .640$km
Hence height to be increased = 640 -160=480 m
Question 4
A message signal of frequency 10 kHz and peak voltage of 10 volts is used to modulate a carrier of frequency 1 MHz and peak voltage of 20 volts.
Determine (a) modulation index, (b) the side bands produced. Solution
(a) Modulation index =10/20 = 0.5
(b) The side bands are at (1000+10 kHz)=1010 kHz and
(1000 -10 kHz) = 990 kHz.
Question 5
In a diode AM-detector , the output circuit consists of R=1000 $\Omega$ and C=10pF.A carrier signal of 100 KHz is to be detected. Is it good? If yes then explain why? If not what value would you suggest? Solution
For demodulation
$ \frac {1}{f_c} << RC$
Here
$\frac {1}{f_c} = \frac {1}{100 \times 10^3}= 10^{-5} $s
$RC= 10^3 \times 10 \times 10^{-12} = 10^{-8}$s
Here we can clear see that
The above relation is not true. Therefore it is not good.
Let take an capacitor of 1 $\mu F$, then
$RC= 10^3 \times 1 \times 10^{-6} = 10^{-3}$s
This value is much larger than $ \frac {1}{f_c} $, hence a capacitor of 1 $\mu F$ provides a satisfactort circuit for detector
