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Frequency Modulation (FM)
is a
method of modulation wherein information is transmitted as
variations in the instantaneous frequency of the carrier signal.
In analog FM, the carrier frequency is changed in direct proportion
to variations in the input signal's amplitude.
The
modulation percentage for FM is defined simply as the ratio of
actual frequency variation to the maximum frequency deviation
expressed in %.
Thus,
100% modulation means that the carrier's frequency variation covers
the entire allowable amount, i.e., its maximum frequency deviation.
This maximum deviation frequency is chosen arbitrarily according to
the application of the FM transmitter.
For example, if the
modulating signal's frequency band is from 80 kHz below the carrier
frequency to 80 kHz above it, then 50% modulation means that the
carrier's frequency is being deviated by only 40 kHz above and below
the resting frequency of the carrier signal.
The
following equations apply to FM.
Percent Modulation
M = [∆f /
D] x 100%
where
M = %
Modulation
∆f =
change in frequency
D =
maximum frequency deviation, i.e., frequency deviation for 100%
modulation
Modulation Index
Mi = fd /
fa
where
Mi =
modulation index
fd =
deviation frequency, kHz
fa =
modulating audio frequency, kHz
See Also:
Amplitude Modulation Equations; More
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