IISER Physics - Principles of Communication
Exam Duration: 45 Mins Total Questions : 30
A modulator
- (a)
generates constant frequency radiowaves
- (b)
generates variable frequency radiowaves
- (c)
improves signal-to-noise ratio
- (d)
improves thermal stablity
In AM, total modulation index must not exceed unity or else
- (a)
the system will fail
- (b)
distortion will result
- (c)
amplifier wil be damaged
- (d)
resonant waves will be generated
For a 100% modulated AM wave, if the carrier wave is suppressed, the amount of power saved will be
- (a)
33%
- (b)
50%
- (c)
67%
- (d)
75%
In amplitude modulation :
A. amplitude of a carrier wave is varied by signal voltage
B. frequency of the carrier wave remains unchanged
C. the wave shape of the modulated signal is different from that of the modulating signal
D. amplitude of the modulated wave varies at the modulating frequency.
- (a)
if A, B, C are correct
- (b)
if B, C, D are correct
- (c)
if C, D, A are correct
- (d)
All the correct
With 100% modulation in an AM wave, the tatio of sideband power to the total power is
- (a)
2/3
- (b)
1/3
- (c)
1/2
- (d)
1/4
For a 100% modulated A. M. wave, if the total power is 1200 W, power transmitted at each sideband is
- (a)
400 W
- (b)
800 W
- (c)
200 W
- (d)
300 W
Choose the CORRECT statement :
- (a)
Modulation index and modulation factor indicate the same quantity
- (b)
In frequancy modulation, the frequency of the carrier wave varies according to the amplitude of modulating signal
- (c)
Frequency modulation is superior to amplitude modulation because of noise elimination.
- (d)
Only (a) and (c) are correct
The FM broadcast band is from
- (a)
550 to 1600 kHz
- (b)
88 to 108 kHz
- (c)
88 to 108 MHz
- (d)
1600 kHz to 5975 MHz
Which of the following statements is not true for modulation?
- (a)
It allows the use of practicable antennas
- (b)
It ensures transmissions over long distances
- (c)
It reduces the bandwitdth
- (d)
It separates different transmissions
If VHF and higher frequency radiation is to be focussed into a beam by an antenna, the antenna diameter required for a given beam width
- (a)
increases as the square of frequency
- (b)
increases directly with frequency
- (c)
decreases with frequency
- (d)
is independent of frequency
Which layer in ionospheric layer is the most important reflecting medium for high frequency radio waves?
- (a)
D-layer
- (b)
E-layer
- (c)
F1-layer
- (d)
F2-layer
The effective refractive index of the ionosphere for radiowaves is
- (a)
more then unity
- (b)
less than unity
- (c)
equal to unity
- (d)
either more or less than unity depending on the electron concentraction
Which of the following fibres gives highest modal dispersion?
- (a)
Graded-index single-mode
- (b)
Graded-index multimode
- (c)
Step-index single-mode
- (d)
Step-index multimode
Operating wavelength of GaA1As LEDs and laser diodes is about
- (a)
850 nm
- (b)
665 nm
- (c)
1300 nm
- (d)
1550 nm
Amongst different types of fibres, sigle-mode step-index fibre has
- (a)
highest data rate
- (b)
lowest attenuation
- (c)
highest thickness
- (d)
both (a) and (b)
An all-pllastic fibre is strong mechanically as compared to an all-glass fibre but has
A. higher loses
B. lower bandwidth
C. lower tensile strength
Mark
- (a)
If A is correct only
- (b)
If B is correct only
- (c)
If A, B is correct only
- (d)
If A, C is correct only
The range of light wavelength in which the high-quality glass fibres have least losses is
- (a)
820 nm to 880 nm
- (b)
1200 nm to 1320 nm
- (c)
1550 nm to 1610 nm
- (d)
1620 nm to 1710 nm
The numerical aperture of an optical fibre depends on
A. core refractive index
B. cladding refractive index
C. critical angle
Mark :
- (a)
if A, B are correct only
- (b)
if B, C are correct only
- (c)
if A, C are correct only
- (d)
if A, B, C all are correct
Splices are generally
- (a)
temporary fibre joints
- (b)
butt fibre joints
- (c)
permanent fibre joints
- (d)
welded fibre joints
The performance of an optical fibre is measured in terms of
- (a)
its attenuation
- (b)
its pulse spreading
- (c)
its numerical aperture
- (d)
All of the above
Digital signals
- (a)
do not provide a continuous set of values
- (b)
utilizer binary systems
- (c)
ensures error and noise free communication
- (d)
All of the above
Range of frequencies allotted for commercial FM radio broadcast is
- (a)
88 to 108 MHz
- (b)
88 to 108 kHz
- (c)
8 to 88 MHz
- (d)
88 to 108 GHz
In earth's atmosphere, for F1- layer; the virtual height and critical frequency are
- (a)
150 km and 3 MHz
- (b)
160 km and 3.5 MHz
- (c)
170 km and 4.5 MHz
- (d)
180 km and 5 MHz
The process of changing some characteristic of a carrier wave in accordance with the intensity of signal to be transmitted is called
- (a)
amplification
- (b)
rectification
- (c)
modulation
- (d)
None of the above
In an amplitude modulated wave for audio frequency of 500 cycle/s, the appropriated carrier frequency will be
- (a)
50 cycle/s
- (b)
100 cycle/s
- (c)
500 cycle/s
- (d)
50000 cycle/s
An amplitude modulated wave is modulated to 50%. What is the saving in power, if carrier as well as one of the sidebands are suppressed?
- (a)
70%
- (b)
65.4%
- (c)
94.4%
- (d)
25.5%
Identify the mathematical expression for amplitude modulated wave.
- (a)
\(\quad { A }_{ c }sin\left[ \left\{ { \omega }_{ c }+{ k }_{ 1 }\left( { v }_{ m } \right) \right\} t+\phi \right] \)
- (b)
\(\quad { A }_{ c }sin{ \{ \omega }_{ c }t+\phi +{ k }_{ 2 }{ v }_{ m }\left( t \right) \)
- (c)
\(\{ { A }_{ c }+{ k }_{ 2 }{ v }_{ m }(t)\} sin({ \omega }_{ c }t+\phi )\)
- (d)
\({ A }_{ c }({ v }_{ m })(t)sin({ \omega }_{ c }t+\phi )\)
Three waves A,B and C of frequencies 1600 kHz, 5 MHz and 60 MHz, respectively are 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)
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
Match the communication services given in Column I with their frequency range given in Column II and select the correct option from the choices given below
Column I | Column II | ||
A. | FM broadcast | 1. | 76-88 MHz |
B. | Television | 2. | 3.7-6.4 GHz |
C. | Satellite communication | 3. | 88-108 MHz |
- (a)
A B C 1 2 3 - (b)
A B C 3 1 2 - (c)
A B C 3 2 1 - (d)
A B C 1 3 2
Which of the following four alternatives is not correct?
- (a)
to increase the selectivity
- (b)
to reduce the time lag between transmission and reception of the information signal
- (c)
to reduce the size of antenna
- (d)
to reduce the fractional bandwidth, that is the ratio of the signal bandwidth to the centre frequency.