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(A) : In case of frequency modulation, t...

(A) : In case of frequency modulation, the frequency of the RF carrier is changed by the AF signal and the change is proportional to the amplitude of the AF signal at any instant.
(R) : When the AF signal is positive, the carrier frequency increases but it decreases when the AF signal is negative.

A

Both 'A' and 'R' are true and 'R' is the correct explanation of 'A'.

B

Both 'A' and 'R' are true and 'R' is not the correct explanation of 'A'.

C

A' is true abd 'R' false

D

A' is false and 'R' is false

Text Solution

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The correct Answer is:
A
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The efficient transmission of signals is achieved by superimposing electrical audio signals on a high frequency carrier wave (the process is known as modulation). When the amplitude of high frequency carrier wave is changed in accordance with the intensity of modulating signal, it is called amplitude modulation. The extent to which the amplitude of carrier wave is changed by the signal is described by modulation factor. It is given as m="Amplitude change of carrier wave"/"Amplitude of unmodulated carrier wave" Let a carrier wave is represented by V_c=V_c cos omega_ct Let the modulation factor be m, the maximum change in amplitude of carrier wave is mV_c So, modulating signal can be represented as v_m=mV_c cosomega_mt So, the amplitude of modulated wave is =V_c+mV_c cosomega_m t Using this value, the instantaneous voltage of modulated wave is E=V_c cos omega_c t+ (mV_c)/2 cos (omega_c+omega_m)t + (mV_c)/2 cos (omega_c-omega_m ) t The above wave contains three frequencies namely, f_c, f_c+f_m and f_c-f_m . The frequencies f_c+f_m and f_c-f_m are called side band frequencies , USB and LSB respectively. The fraction of total power carried by side band frequencies is

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The efficient transmission of signals is achieved by superimposing electrical audio signals on a high frequency carrier wave (the process is known as modulation). When the amplitude of high frequency carrier wave is changed in accordance with the intensity of modulating signal, it is called amplitude modulation. The extent to which the amplitude of carrier wave is changed by the signal is described by modulation factor. It is given as m="Amplitude change of carrier wave"/"Amplitude of unmodulated carrier wave" Let a carrier wave is represented by V_c=V_c cos omega_ct Let the modulation factor be m, the maximum change in amplitude of carrier wave is mV_c So, modulating signal can be represented as v_m=mV_c cosomega_mt So, the amplitude of modulated wave is =V_c+mV_c cosomega_m t Using this value, the instantaneous voltage of modulated wave is E=V_c cos omega_c t+ (mV_c)/2 cos (omega_c+omega_m)t + (mV_c)/2 cos (omega_c-omega_m ) t The above wave contains three frequencies namely, f_c, f_c+f_m and f_c-f_m . The frequencies f_c+f_m and f_c-f_m are called side band frequencies , USB and LSB respectively. If modulation factor is 100% , the amplitude change of carrier wave is

AAKASH SERIES-COMMUNICATION SYSTEMS -Exercise - IB (Assertion (A) & Reason (R) Type Questions)
  1. (A) : The sky waves are not used in the transmission of television sig...

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  2. A long solenoid having n = 200 turns per metre has a circular cross-se...

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  3. This question has Statement-1 and Statement-2. Of the four choices giv...

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  4. Assertion(A): Na^(o+) ions are discharged in preference to H^(o+) ions...

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  5. (A) : Optical communication is preferred to microwave communication ...

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  6. Statement-I : The surface wave propagation is used for medium wave ban...

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  7. Statement-I : The television broadcasting becomes weaker with increasi...

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  8. (A) : For long distance transmission of radio waves, short wave bands ...

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  9. (A) : Propagation of electromagnetic waves as microwaves is better tha...

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  10. (A) : The lectromagnetic waves of shorter wavelengths can travel long...

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  11. (A) : In case of frequency modulation, the frequency of the RF carrier...

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  12. (A) : Digital signals are preferred to analog signals for transmission...

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  13. (A) : A portable AM radio set must be kept horizontal to receive the s...

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  14. Assertion: Telephony is an example of point-to-point communication mod...

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  15. (A) : Range is the largest distance between a source and a destination...

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  16. (A) : For transmitting a signal, the antenna should have a size compa...

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  17. (A) : At frequencies above 40 MHz, communication is essentially limite...

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  18. (A) : Sky wave is reflected by ionosphere (R) : The reflected wave ...

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  19. (A) : Communication in UHF/VHF regions can be established by space wa...

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  20. (A) : In a communication system based on amplitude modulation the modu...

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