Questions and answers for exam Preparation

MCQs

Communication Systems(12th Grade -Physics)

Total Questions : 167 | Page 3 of 17 pages

Question 21. In an FM system a 7 kHz signal modulates 108 MHz carrier so that frequency deviation is 50 kHz. The carrier swing is

7.143
8
0.71
350
If assertion is false but reason is true.

Discuss Question

Answer: Option A. -> 7.143
:
A
Carrier swing =

\frac{F r e q u e n c y d e v 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{50}{7}

= 7.143

Question 22. The electromagnetic waves of frequency 2 MHz to 30 MHz are

In ground wave propagation
In sky wave propagation
In microwave propagation
In satellite communication
If assertion is false but reason is true.

Discuss Question

Answer: Option B. -> In sky wave propagation
:
B
The EM waves of frequency from 2MHz to 30 MHz are reflected from the ionized upper layers of earth's atmosphere and hence the sky wave propagation

Question 23. The waves used in telecommunication are?

IR
UV
Microwave
Cosmic rays
If assertion is false but reason is true.

Discuss Question

Answer: Option C. -> Microwave
:
C
In telecommunication, microwaves are used.

Question 24. The frequency of a FM transmitter without signal input is called

Lower side band frequency
Upper side band frequency
Resting frequency
None of these
If assertion is false but reason is true.

Discuss Question

Answer: Option C. -> Resting frequency
:
C
The frequency of a FM transmitter without signal input is calledResting frequency

Question 25. If

f_{a}

and

f_{f}

represent the carrier wave frequencies for amplitude and frequency modulations respectively, then

fa > ff
fa
fa ≈ ff
fa ≥ ff
If assertion is false but reason is true.

Discuss Question

Answer: Option B. -> fa
:
B

f_{a}

f_{f}

Question 26. An amplitude modulated wave is modulated to 50%. What is the saving in power if carrier as well as one of the side bands are suppressed

70%
65.4%
94.4%
25.5%
e

Discuss Question

Answer: Option C. -> 94.4%
:
C

P_{s b}

P_{c} (\frac{m_{a}}{2})^{2}

P_{c} \frac{({0.5}^{2})}{4} = 0.0625 P_{c}

Also P =

P_{c} (1 + \frac{{m_{a}}^{2}}{2}) = P_{c} (1 + \frac{{(0.5)}^{2}}{2}) = 1.125 P_{c}

∴

% saving =

\frac{(1.125 P_{c} - 0.0625 P_{c})}{1.125 P_{c}} \times 100

= 94.4%

Question 27. Read the assertion and reason carefully to mark the correct option out of the options given below:
Assertion : Television signals are received through sky-wave propagation.
Reason : The ionosphere reflects electromagnetic waves of frequencies greater than a certain critical frequency.

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

Discuss Question

Answer: Option D. -> If the assertion and reason both are false.
:
D
TV signals (frequency greater than 30 MHz) cannot be propagated through sky wave propagation.
Above critical frequency, an electromagnetic wave penetratates the ionosphere and is not reflected by it.

Question 28. Read the assertion and reason carefully to mark the correct option out of the options given below:
Assertion: Short wave bands are used for transmission of radio waves to a large distance.
Reason : Short waves are reflected by ionosphere
(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.

Discuss Question

Answer: Option A. -> a
:
A
Having the range of wavelength from 30kmto 30cmare known as short wave. These waves are used for radio transmission and for general communication purpose to a longer distance from ionosphere. Ionosphere is the outermost region of atmosphere extending from height of 80kmto 400kmapproximately, above the surface of earth. Therefore, both the assertion and reason are true and reason is the correct explanation of assertion.

Question 29. Indicate which one of the following system is digital?