E8B05

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 deviation ratio of an FM phone signal having a maximum frequency swing of plus or minus 5 kHz if the highest modulation frequency is 3 kHz?

A6
B0.167
C0.6
D1.67

Why Deviation ratio is defined as the maximum carrier frequency swing divided by the highest modulating (audio) frequency. Here that is 5 kHz divided by 3 kHz, which gives about 1.67. Note this is a fixed design figure for the system, using the peak allowed deviation and the top audio frequency, unlike modulation index which changes with the instantaneous audio frequency.

Watch out The 0.6 value comes from dividing the two numbers the wrong way (3 divided by 5); deviation always goes on top.

Deviation ratio = swing over highest audio. Deviation first, divided by audio: 5/3 = 1.67.

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

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