Communication Systems

Assertion: In a communication system based on amplitude modulation the modulation index is kept <1. 

Reason: It ensures minimum distortion of signal.

• 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.

Voltage of modulating wave of 5 V with 10 MHz frequency was superimposed on carrier wave of frequency 20 MHz and voltage 20 V then the modulation index is

• 0.25

• 1.25

• 2.43

• 64.0

The area covered by a transmitting antenna of height 50 m is

• 320$\mathrm{\pi }$ km²

• 1440$\mathrm{\pi }$ km²

• 640$\mathrm{\pi }$ km²

• 120$\mathrm{\pi }$ km²

Assertion:  In the transmission of long distance radio signals, short wave band is used. 

Reason: In shorter wavelength, attenuation is very less.

• If both assertion and reason are true and reason is correct the 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.

Transmission lines transmit a voltage of V volt to our houses from power stations, then the power P supplied by them is proportional to

• $\frac{1}{\mathrm{V}}$

• V

• V²

• $\frac{1}{{\mathrm{V}}^2}$

Assertion:  Communication in UHF/VHF regions can be established by space wave or tropospheric wave. 

Reason:  Communication in UHF/VHF regions is limited to line of sight distance

• 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.

If the highest modulating frequency of the wave is 5 kHz, the number of stations that can be accomodated in a 150 kHz bandwidth ?

• 15

• 10

• 5

• None of these

Given below is the circuit diagram of an AM demodulator. For good demodulation of AM signal of carrier frequency f, the value of RC should be

• RC = 1/f

• RC < 1/f

• RC ≥ 1/f

• RC >> 1/f

Assertion:  Electromagnetic waves with frequencies smaller than the critical frequency of ionosphere are used for communication using sky wave propagation. 

Reason:  The refractive index of the ionosphere becomes very high for frequencies higher than the 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

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