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MCQs

Communication Systems(12th Grade -Physics)

Total Questions : 167 | Page 1 of 17 pages
Question 1. 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 6.4 ×106m.
  1.    40 km
  2.    44 km
  3.    45.5 km
  4.    48.5 km
  5.    The frequency of modulated wave varies as frequency of carrier wave
 Discuss Question
Answer: Option C. -> 45.5 km
:
C
2×64×105×32+2×64×105×50 = 64 × 10^2 × 10 + 8 × 10^3 × 10m = 144 × 10^2 × 10= 45.5 km
Question 2. What is the modulation index of an over modulated wave
  1.    1
  2.    >1
  3.    Zero
 Discuss Question
Answer: Option B. -> >1
:
B
When m > 1 then carrier is said to be over modulated.
Question 3. The maximum distance upto which TV transmission from a TV tower of height h can be received is proportional to
  1.    h12
  2.    h
  3.    h32
  4.    h2
 Discuss Question
Answer: Option A. -> h12
:
A
d = 2hRdαh12
Question 4. Laser beams are used to measure long distances because
  1.    They are monochromatic
  2.    They are highly polarised
  3.    They are coherent
  4.    They have high degree of parallelism
 Discuss Question
Answer: Option D. -> They have high degree of parallelism
:
D
Laser beams are perfectly parallel, so that they are very narrow and can travel a long distance without spreading. This is the feature of laser while monochromatic and coherent are characteristics only.
Question 5. Consider telecommunication through optical fibres. Which of the following statements is not true.
  1.    Optical fibres may have homogeneous core with a suitable cladding.
  2.    Optical fibres are subject to electromagnetic interference from outside.
  3.    Optical fibres can be of graded refractive index
  4.    Optical fibres have extremely low transmission loss
 Discuss Question
Answer: Option B. -> Optical fibres are subject to electromagnetic interference from outside.
:
B
Optical fibres are not subjected to electromagnetic interference from outside.
Question 6. Sinusoidal carrier voltage of frequency 1.5 MHz and amplitude 50 V is amplitude modulated by sinusoidal voltage of frequency 10 kHz producing 50% modulation. The lower and upper side-band frequencies in kHz are
  1.    1490, 1510
  2.    1510, 1490
  3.    11490, 11510
  4.    11510, 11490
  5.    If assertion is false but reason is true.
 Discuss Question
Answer: Option A. -> 1490, 1510
:
A
Here, fc = 1.5 MHz = 1500 kHz ,fm = = 10 kHz
Low side band frequency
= fc - fm = 1500 kHz - 10 kHz = 1490 kHz
Upper side band frequency
= fc + fm = 1500 kHz + 10 kHz = 1510 kHz
Question 7. Read the assertion and reason carefully to mark the correct option out of the options given below:
Assertion : The electrical conductivity of earth's atmosphere decreases with altitude.
Reason : The high energy particles (i.e. g-rays and cosmic rays) coming from outer space and entering our earth's atmosphere cause ionisation of the atoms of the gases present there and the pressure of gases decreases with increase in altitude.
(a)If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b)If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c)If assertion is true but reason is false.
(d)If the assertion and reason both are false.
(e)If assertion is false but reason is true.
  1.    a
  2.    b
  3.    c
  4.    d
  5.    e
 Discuss Question
Answer: Option E. -> e
:
E
The electrical conductivity of earth's atmosphere increases with height so assertion is false.
When high energy particles enters in earth's atmosphere. They ionises the gases present in atmosphere. Also as we go up, the air thins out gradually and air pressure decreases.
Question 8. The attenuation in optical fiber is mainly due to
  1.    Absorption
  2.    Scattering
  3.    Neither absorption nor scattering
  4.    Both (a) and (b)
 Discuss Question
Answer: Option D. -> Both (a) and (b)
:
D
A very small part of light energy is lost from an optical fiber due to absorption or due to light leaving the fiber as a result of scattering of light sideways by impurities in the glass fiber.
Question 9. A photodetector is made from a semiconductor In 0.53Ga0.47As with Eg = 0.73 eV. What is the maximum wavelength, which it can detect
  1.    1000 nm
  2.    1703 nm
  3.    500 nm
  4.    173 nm
  5.    The frequency of modulated wave varies as frequency of carrier wave
 Discuss Question
Answer: Option B. -> 1703 nm
:
B
Limiting value of hv is Eg, such that hv = hcλ = Eg
or λ=hcEg=6.63×1034Js×3×108ms10.73×1.6×1019J
= 1703 nm
Question 10. In frequency modulation
  1.    The amplitude of modulated wave varies as frequency of carrier wave
  2.    The frequency of modulated wave varies as amplitude of modulating wave
  3.    The amplitude of modulated wave varies as amplitude of carrier wave
  4.    The frequency of modulated wave varies as frequency of modulating wave
  5.    The frequency of modulated wave varies as frequency of carrier wave
 Discuss Question
Answer: Option B. -> The frequency of modulated wave varies as amplitude of modulating wave
:
B
The process of changing the frequency of a carrier wave (modulated wave) in accordance with the audio frequency signal (modulating wave) is known as frequency modulation (FM).

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