Question

In FM, if we decreases modulating frequency then the modulation index ________

(a) will increase, if the modulating voltage amplitude increases

(b) will decrease, if the modulating voltage amplitude increases

(c) will increase, if the modulating voltage amplitude remains constant

(d) will decrease, if the modulating voltage amplitude remains constant

This question was addressed to me during an interview.

Question is taken from Frequency Domain Representation in section Double Sideband Suppressed Carrier Modulation of Analog Communications

Answer 1

Correct answer is (c) will increase, if the modulating voltage amplitude remains constant

The explanation is: In frequency modulation, the modulation index is the ratio of frequency deviation to modulating frequency. Therefore, modulation index can be made inversely proportional to the modulating frequency by keeping amplitude constant. Thus, if we decrease modulating frequency then the modulation index will increase, with amplitude remaining constant.