In amplitude modulation, the modulation index m is kept less than or equal to 1 because:
m > 1 will result in interference between carrier frequency and message frequency, resulting in distortion.
m < 1 will result in overlapping of both sidebands resulting in loss of information.
m > 1 will result in change in phase between carrier signal and message signal.
m > 1 indicates amplitude of message signal greater than amplitude of carrier signal, resulting in distortion.
Answer and explanation
The correct answer is D.
Concept:
The modulation index in AM is always kept less than 1 to avoid distortion or clipping of the message signal. When m > 1, the carrier undergoes a phase reversal of 180° and the envelope of the modulated signal no longer contains the actual message signal, resulting in a distorted output from the envelope detector.
Explanation:
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When m > 1, the amplitude of the message signal exceeds the amplitude of the carrier signal.
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This causes over-modulation, leading to envelope distortion where the envelope detector cannot faithfully recover the original message.
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Additionally, m > 1 results in overlapping of both sidebands, causing loss of information.
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Therefore, option D is the correct answer.