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# Communication Systems

#### Multiple Choice Questions

1.

What maximum frequency can be reflectedfrom ionosphere?

• 5 MHz

• 6 GHz

• 5 KHz

• 500 MHz

A.

5 MHz

The ionosphere extends from a height of 80 kmto 300 km. The refractive index of ionosphere isless than its free space value. That is, it behavesas a rarer medium.As we go deep into theionosphere, the refractive index keeps ondecreasing. The bending of beam (away fromthe normal) will continue till it reaches criticalangle after which it will be reflected back. Thedifferent points on earth receive signalsreflected from different depths of theionosphere. There is a critical frequency f, (S to100 MHz) beyond which the waves cross theionosphere and do not return back to earth.

2.

Assertion: Microwave communication ispeferred over optical communication.

Reason:Microwaves provide large number ofchannels and bandwidth compared to opticalsignals.

• if both assertion and reason are true but reason is the correct explanation of assertion

• if both assertion and reason are true and the reason is not the correct explanation of assertion

• if assertion is true and reason is false

• if both assertion and reason are false statements

D.

if both assertion and reason are false statements

The quality of a communication system isdetermined by its information carrying capacity, Theinformation carrying capacity is directly proportionalto its bandwidth. The wider the bandwidth, the greaterits information carrying capacity.

As the light frequencies used in optical fibre systems is much larger than the highest frequency microwave transmissions, so the bandwidth in case of optical communication systems is much larger than that inmicrowave communications. The bandwidth of theoptical fibre systems is quite suitable for the presentday communication needs and also for the future.That's why optical communication is preferred overmicrowave communication.

3.

Given below is the circuit diagram of an AMdemodulator. For good demodulation of AMsignal of carrier frequency f, the value of RCshould be

• RC = 1/f

• RC < 1/f

• RC ≥ 1/f

• RC >> 1/f

D.

RC >> 1/f

From the modulated signal, we can demodulate or detect the modulating or message signal by using a diode and a suitable capacitor filter. TheAMsignal given to the input of the circuit appears at the output of the diode as a rectified wave. For good demodulation ofAMsignal the value of RC (which is a lime-constant) is chosen such that

$\frac{1}{\mathrm{f}}$<< RC

⇒ RC>>$\frac{1}{\mathrm{f}}$

where f is the frequency of the carrier signal.

4.

For skywave propagation of a 10 MHz signal,what should be the minimum electron density in ionosphere?

• ∼ 1.2 × 1012 m-3

• ∼ 106 m-3

• ∼ 1014 m-3

• ∼ 1022 m-3

A.

∼ 1.2 × 1012 m-3

The critical frequency of a wave for reflection from a layer of atmosphere is given by

fc = 9 ( Nmax )1/2

where N is the density of electron/m3

10× 106 = 9 ( Nmax )1/2

Nmax =${\left(\frac{10×{10}^{6}}{9}\right)}^{2}$

Nmax≅ 1.2× 1012 m-3

5.

Assertion:Television signals are received through sky-wave propagation.

Reason:The ionosphere reflects electromagneticwaves of frequencies greater than a certain criticalfrequency

• If both assertion and reason are true and reason is the correct explanation of assertion

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

• If assertion is true but reason is false

• If both assertion and reason are false

D.

If both assertion and reason are false

In skywave propagation, the radio waveshaving frequency range 2 MHz to 30 MHz are reflectedback by the ionosphere. Radio waves havingfrequency nearly greater than 30 MHz penetratesthe ionosphere and is not reflected back by theionosphere. The TV signal having frequency greaterthan 30 MHz therefore cannot be propagated throughsky wave propagation.

In case of skywave propagation, critical frequency isdefined as the highest frequency that is returned tothe earth by the considered layer of the ionosphereafter having sent straight to it. Above this frequency,a wave will penetrate the ionosphere and is notreflected by it.

6.

The binary number 10111 is equivalent to the decimal number

• 19

• 31

• 23

• 22

C.

23

Hence, (10111)B = 1× 24 + 0× 23 + 1× 22 + 1× 21 + 1× 20

= 16 + 0 + 4 + 2 + 1 = 23

7.

If thehighest modulating frequency of the waveis 5 kHz, the number of stations that can beaccomodated in a 150 kHz bandwidth ?

• 15

• 10

• 5

• None of these

A.

15

Number of stations =$\frac{\mathrm{Band}\mathrm{width}}{\mathrm{Band}\mathrm{width}}{\mathrm{stations}}}$

andwidth per station = 2× 5 kHz

total bandwidth = 150 kHz

Number of stations =$\frac{150}{10}$

Number of stations = 15

8.

A sound absorber attenuates the sound level by20 dB. The intensity decreases by a factor of

• 1000

• 10000

• 10

• 100

D.

100

Let intensity of sound be I and I'.Loudness of sound initially

${\mathrm{\beta }}_{1}=10\mathrm{log}\left(\frac{\mathrm{I}}{{\mathrm{I}}_{\mathrm{o}}}\right)\phantom{\rule{0ex}{0ex}}{\mathrm{\beta }}_{2}=10\mathrm{log}\left(\frac{\mathrm{I}\text{'}}{{\mathrm{I}}_{\mathrm{o}}}\right)$

β2 - β1 = 20

$\therefore 20=20\mathrm{log}\left(\frac{\mathrm{I}\text{'}}{\mathrm{I}}\right)$

∴ I' = 100 I

9.

Assertion: In high latitudes one sees colourfulcurtains of light hanging down from high altitudes.

Reason:The high energy charged particles fromthe are deflected to polar regions by themagnetic field of the earth.

• if both assertion and reason are true and the reason is the correct explanation of the assertion

• if both assertion and reason are true and the reason is not the correct explanation of the assertion

• if assertion is true but reason is false

• if both assertion and reason are false statements

A.

if both assertion and reason are true and the reason is the correct explanation of the assertion

Earth is surrounded by its magnetic field in space called magnetosphere, which forms a barrier to Solar wind. The solar wind pressure strongly compresses the magnetosphere on the dayside and draws it out into an extremely long tail on the night side of the earth. Since, the charged particles of the solar wind cannot cross the earths magnetic field lines they flow around the magnetosphere. This forms a standing shock wave in space upstream of the earth. The magnetosphere and the solar wind form an enormous electrical dynamo where large and complicated electrical currents flow. These electrons descend down to altitudes of 100 - 300 km they may collide with the atmospheric gas causing it to glow. This is the basic cause of aurora.

Aurora is a natural electrical phenomenon characterized by the appearance of streamers of reddish or greenish in the sky, especially near the northern or southern magnetic pole.

Note that the plasma sheet is only connected to magnetic field lines from the earths Polar Regions on the night sight, which explains why the aurora is mostly seen only at high latitudes at night.

10.

Assertion: Electromagnetic waves withfrequencies smaller than the critical frequencyof ionosphere are used for communication usingsky wave propagation.

Reason: The refractive index of the ionospherebecomes very high for frequencies higher thanthe critical frequency.

• If both assertion and reason are true and reason is the correct explanation of the assertion

• If both assertion and reason are true but reason is not the correct explanation ofthe assertion

• If assertion is true, but reason is false

• Both assertion and reason are false statements

B.

If both assertion and reason are true but reason is not the correct explanation ofthe assertion

If the frequencies of the waves are less thanthat of the critical frequency. They will be reflectedback as the ionosphere has refractive index lessthan free space. This is the principle of sky wavepropagation.

The statement given in the reason that therefractive index becomes high is correct. But thereason given is not true for the assertion.