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2016 Extra Class study guide: E8B - modulation and demodulation

Posted: 19 Feb 2016 11:50 AM PST
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E8B Modulation and demodulation: modulation methods; modulation index and
deviation ratio; frequency and time division multiplexing; Orthogonal
Frequency Division Multiplexing

In FM modulation, the two primary parameters of interest are deviation
ratio and modulation index. Deviation ratio is the ratio of the maximum
carrier frequency deviation to the highest audio modulating frequency.
(E8B09) The deviation ratio of an FM-phone signal having a maximum
frequency swing of plus-or-minus 5 kHz when the maximum modulation
frequency is 3 kHz is 1.67. (E8B05)The deviation ratio of an FM-phone
signal having a maximum frequency swing of plus or minus 7.5 kHz when the
maximum modulation frequency is 3.5 kHz is 2.14. (E8B06)

The term for the ratio between the frequency deviation of an RF carrier
wave, and the modulating frequency of its corresponding FM-phone signal is
modulation index. (E8B01) The modulation index is equal to the ratio of the
frequency deviation to the modulating frequency. The modulation index of a
phase-modulated emission does not depend on the RF carrier frequency.
(E8B02)

The modulation index of an FM-phone signal having a maximum frequency
deviation of 3000 Hz either side of the carrier frequency, when the
modulating frequency is 1000 Hz is 3. (E8B03) The modulation index of an
FM-phone signal having a maximum carrier deviation of plus or minus 6 kHz
when modulated with a 2-kHz modulating frequency is 3. (E8B04)

Some communications systems use multiplexing techniques to to combine
several separate analog information streams into a single analog radio
frequency signal. When a system uses frequency division multiplexing, two
or more information streams are merged into a “baseband,” which then
modulates the transmitter. (E8B10). When a system uses digital time
division multiplexing, two or more signals are arranged to share discrete
time slots of a data transmission. (E8B11)

Orthogonal Frequency Division Multiplexing is a digital modulation
technique using subcarriers at frequencies chosen to avoid intersymbol
interference. (E8B08) Orthogonal Frequency Division Multiplexing is a
technique used for high speed digital modes. (E8B07)

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