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# Communication Systems

#### Multiple Choice Questions

1.

A telephonic communication service is working at carrier frequency of 10 GHz. Only 10% of it is utilized for transmission. How many telephonic channels can be transmitted simultaneously if each channel requires a bandwidth of 5 kHz?

• 2 x 106

• 2 x 105

• 2 x 104

• 2 x 105

D.

2 x 105

If n = no. of channels

10% of 10 GHz = n x 5 KHz

or$\frac{10}{1000}x10x{10}^{9}\phantom{\rule{0ex}{0ex}}=nx5x{10}^{3}\phantom{\rule{0ex}{0ex}}⇒n=2x{10}^{5}$

2.

A diode detector is used to detect an amplitude modulated wave of 60% modulation by using a condenser of capacity 250 pico farad in parallel with a load resistance 100 kilo ohm. Find the maximum modulated frequency which could be detected by it.

• 10.62 MHz

• 10.62 kHz

• 5.31 mHz

• 5.31 mHz

B.

10.62 kHz

The frequency is given as

μ =0.6
R = 100 k = 100 x1000 Ω
c = 250 pico farad = 250 x 10-12 F
So,

313 Views

3.

A train is moving on a straight track with speed 20 ms-1 .It is blowing its whistle at the frequency of 1000 Hz. The percentage change in the frequency heard by a person standing near the track as the train passes him is (speed of sound = 320 ms-1 ) close to :

• 6%

• 12%

• 18%

• 18%

B.

12%

Apparent frequency heard by the person before crossing the train.

Similarly, apparent frequency heard, after crossing the trains

472 Views

4.

In a full wave rectifier circuit operating from 50 Hz mains frequency, the fundamental frequency in the ripple would be

• 50 Hz

• 25 Hz

• 100 Hz

• 100 Hz

C.

100 Hz

frequency = 2 (frequency of input signal).

285 Views

5.

This question has Statement 1 and Statement 2. Of the four choices given after the statements, choose the one that best describes the two statements.
Statement 1 - Sky wave signals are used for long distance radio communication. These signals are in general, less stable than ground wave signals.
Statement 2- The state of ionosphere varies from hour to hour, day to day and season to season.

•  Statement 1 is true, statement 2 is false.

• Statement 1 is true, Statement 2 is true, Statement 2 is the correct explanation of Statement 1

•  Statement 1 is true, Statement 2 is true, Statement 2 is not the correct explanation of Statement 1

•  Statement 1 is true, Statement 2 is true, Statement 2 is not the correct explanation of Statement 1

D.

Statement 1 is true, Statement 2 is true, Statement 2 is not the correct explanation of Statement 1

233 Views

6.

A sound absorber attenuates the sound level by 20 dB. The intensity decreases by a factor of

• 1000

• 10000

• 10

• 10

D.

10

491 Views

7.

Choose the correct statement:

• In amplitude modulation, the amplitude of the high-frequency carrier wave is made to vary in proportion to the amplitude of the audio signal.

• In amplitude modulation, the frequency of the high-frequency carrier wave is made to vary in proportion to the amplitude of the audio signal.

• In frequency modulation, the amplitude of the high-frequency carrier wave is made to vary in proportion to the amplitude of the audio signal.

• In frequency modulation, the amplitude of the high-frequency carrier wave is made to vary in proportion to the amplitude of the audio signal.

B.

In amplitude modulation, the frequency of the high-frequency carrier wave is made to vary in proportion to the amplitude of the audio signal.

As, we know, an amplitude modulated wave, the bandwidth is twice the frequency of modulating the signal. Therefore, amplitude modulation (AM), the frequency of the high-frequency carrier wave is made to vary in proportion to the amplitude of the audio signal.

214 Views

8.

A radar has a power of 1 Kw and is operating at a frequency of 10 GHz. It is located on a mountain top of height 500 m. The maximum distance upto which it can detect object located on the surface of the earth (Radius of earth = 6.4 x 106 m) is

• 80 km

• 16 km

• 40km

• 40km

A.

80 km

Maximum distance on earth where object can be detected is d, then

(h + R)2 = d2 + R2

⇒ d2 = h2 + 2Rh
Since, h<<R,
⇒d2 =2hR

321 Views

9.

A signal of 5 kHz frequency is amplitude modulated on a carrier wave of frequency 2 MHz. The frequencies of the resultant signal is/are:

• 2005 kHz, and 1995 kHz

• 2005 kHz, 2000 kHz and 1995 kHz

• 2000 kHz and 1995 kHz

• 2000 kHz and 1995 kHz

C.

2000 kHz and 1995 kHz

Frequency associated with AM are
fc - fm, f, fc + fm
according to the question
fc = 2 MHz = 2000 kHz

fm = 5 kHz
Thus, frequency of the resultant signal is are carrier frequency fc = 2000 kHz, LSB frequency  fc-fm = 2000 kHz-5kHz = 1995 kHz and USB frequency fc+fm = 2005 kHz
397 Views

10.

Arrange the following electromagnetic radiations per quantum in the order of increasing energy:

A: Blue light
B: Yellow light
C: X-ray

• D, B, A, C

• A, B, D, C

• C, A, B, D

• C, A, B, D

A.

D, B, A, C

As, we know energy liberated, E = hc/λ

i.e
So, lesser the wavelength greater will be energy liberated by electromagnetic radiations per quantum.

As, order of wavelength is given by