E8B04

SIGNALS AND EMISSIONS · Modulation and demodulation: modulation methods; modulation index and deviation ratio; frequency- and time-division multiplexing; orthogonal frequency-division multiplexing (OFDM)

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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?

A0.3
B3
C0.6
D6

Why For FM, the modulation index is the peak carrier deviation divided by the frequency of the modulating tone at that instant. Here 6 kHz of deviation divided by a 2 kHz modulating frequency gives an index of 3. Note this is a ratio of two frequencies, so it has no units, and it changes as the modulating frequency changes.

Watch out The value 0.3 comes from dividing in the wrong order (2 divided by 6, then misplacing the decimal), and 6 is just the deviation in kHz with no division done at all.

Index = deviation / modulating frequency. Big deviation over small tone = big index. 6/2 = 3.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

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