E8B06

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 7.5 kHz if the highest modulation frequency is 3.5 kHz?

A2.14
B0.214
C0.47
D47

Why Deviation ratio is defined as the maximum carrier frequency swing divided by the highest modulating frequency. Here that is 7.5 kHz divided by 3.5 kHz, which gives about 2.14. Note the difference from modulation index, which uses the actual modulating frequency in use at the moment; deviation ratio uses the maximum values the system allows.

Watch out The value near 0.47 comes from dividing the numbers the wrong way (3.5 / 7.5), and the choices differing only by a factor of ten are decimal-point traps.

Deviation ratio = swing over audio: big number on top. 7.5/3.5 = 2.14.

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

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