Question
What is the modulation index of an FM phone signal having a maximum carrier deviation of plus or minus 6 kHz if the highest modulating frequency is 2 kHz?
Answer Options
- A) 0.3
- B) 3
- C) 0.6
- D) 6
Correct Answer: B
Explanation
The modulation index (\beta) for an FM signal is defined as the ratio of the carrier’s maximum frequency deviation (\Delta f) to the highest modulating frequency (f_m) present in the audio signal. The units must be consistent (e.g., both in kHz or Hz) for the ratio to be correct.
Given the maximum carrier deviation (\Delta f) is 6 kHz and the highest modulating frequency (f_m) is 2 kHz, the modulation index is \beta = \Delta f / f_m = 6 kHz / 2 kHz = \mathbf{3}. A modulation index of 3 indicates that the signal bandwidth is significantly wider than the sum of the carrier deviation and the modulating frequency.
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