Questions and answers for exam Preparation

MCQs

Communication Systems(12th Grade -Physics)

Total Questions : 167 | Page 2 of 17 pages

Question 11. A 500 Hz modulating voltage fed into an FM generator produces a frequency deviation of 2.25 kHz. If amplitude of the voltage is kept constant but frequency is raised to 6 kHz then the new deviation will be

4.5 kHz
54 kHz
27 kHz
15 kHz
The frequency of modulated wave varies as frequency of carrier wave

Discuss Question

Answer: Option B. -> 54 kHz
:
B

m_{f}

\frac{δ}{f_{m}} = \frac{2250}{500}

= 4.5

∴

New deviation = 2(

m_{f} f_{m}

) = 2

\times

4.5

\times

6 = 54 kHz.

Question 12. Broadcasting antennas are generally

Omni directional type
Vertical type
Horizontal type
None of these

Discuss Question

Answer: Option B. -> Vertical type
:
B
Broadcasting antennas are generally vertical type.

Question 13. In AM, the centpercent modulation is achieved when

Carrier amplitude = signal amplitude
Carrier amplitude−1 = signal amplitude
Carrier frequency = signal frequency
Carrier frequency−1 = signal frequency
The frequency of modulated wave varies as frequency of carrier wave

Discuss Question

Answer: Option A. -> Carrier amplitude = signal amplitude
:
A
When signal amplitude is qual to the carrier amplitude, the amplitude of carrier ware varies between 2A and zero.

In AM, The Centpercent Modulation Is Achieved When

m_{a} = \frac{A m p l i t u d e c h a r g e o f c a r r i e r}{A m p l i t u d e o f n o r m a l c a r r i e r} = \frac{2 A - A}{A} \times 100

= 100%

Question 14. For television broadcasting, the frequency employed is normally

30-300 MHz
30-300 GHz
30-300 KHz
30-300 Hz

Discuss Question

Answer: Option A. -> 30-300 MHz
:
A
VHF (Very High Frequency) band having frequency range 30MHzto 300MHzis typically used for TV and radar transmission.

Question 15. An oscillator is producing FM waves of frequency 2 kHz with a variation of 10 kHz. What is the modulating index

0.20
5.0
0.67
1.5

Discuss Question

Answer: Option B. -> 5.0
:
B
The formula for modulation index is given by

m_{f} = \frac{δ}{v_{m}} = \frac{F r e q u e n c y v a r i a t i o n}{M o d u l a t i n g f r e q u e n c y} = \frac{10 \times 10^{3}}{2 \times 10^{3}} = 5

Question 16. A transmitter supplies 9 kW to the aerial when unmodulated. The power radiated when modulated to 40% is

5 kW
9.72 kW
10 kW
12 kW
The frequency of modulated wave varies as frequency of carrier wave

Discuss Question

Answer: Option B. -> 9.72 kW
:
B

P_{t}

P_{c} [1 + \frac{m^{2}}{2}]

= 9 [1 +

\frac{{(0.4)}^{2}}{2}

]
= 9 [1 +

\frac{0.16}{2}

] (

∵

m = 40 % = 0.4)
= 9 (1.08) = 9.72 kW

Question 17. The audio signal used to modulate 60 sin 2

π 10^{6}

t is 15sin 300

π

t. The depth of modulation is

50%
40%
25%
15%
The frequency of modulated wave varies as frequency of carrier wave

Discuss Question

Answer: Option C. -> 25%
:
C

m_{a} = \frac{E_{m}}{E_{c}} = \frac{15}{60} \times 100

= 25 %

Question 18. Read the assertion and reason carefully to mark the correct option out of the options given below:
Assertion :The electromagnetic waves of shorter wavelength can travel longer distances on earth's surface than those of longer wavelengths.
Reason : Shorter the wavelength, the larger is the velocity of wave propagation.

Both assertion and reason are true and the reason is the correct explanation of the assertion.
Both assertion and reason are true but reason is not the correct explanation of the assertion.
Assertion is true but reason is false.
The assertion and reason both are false.
If assertion is false but reason is true.

Discuss Question

Answer: Option C. -> Assertion is true but reason is false.
:
C
The electromagnetic waves of shorter wavelength do not suffer much diffraction from the obstacles of earth's atmosphere so they can travel long distance.
Also, shorter the wavelength, shorter is the velocity of wave propagation.

Question 19. In which of the following remote sensing technique is not used

Forest density
Pollution
Wetland mapping
Medical treatment

Discuss Question

Answer: Option D. -> Medical treatment
:
D
Remote sensing is the technique to collect information about an object in respect of its size, colour, nature, location, temperatureetc.without physically touching it. There are some areas or location which are inaccessible. So to explore these areas or locations, a technique known as remote sensing is used. Remote sensing is done through a satellite.

Question 20. The bit rate for a signal, which has a sampling rate of 8 kHz and where 16 quantisation levels have been used is