E8B

FM index, deviation ratio and multiplexing

How to calculate modulation index and deviation ratio, and what OFDM and frequency or time multiplexing do.

11 pool questions1 on the exam

Syllabus: Modulation and demodulation: modulation methods; modulation index and deviation ratio; frequency- and time-division multiplexing; orthogonal frequency-division multiplexing (OFDM)

Modulation index = deviation ÷ modulating frequency

Frequency deviation is how far an FM signal swings either side of the carrier. The modulation index compares that swing with the modulating frequency (the tone doing the swinging).

Bigger swing or slower modulating tone = bigger index.

Frequency deviation (either side)±3 kHzModulating frequency1 kHz

Modulation index3

Deviation	Modulating frequency	Index
±3000 Hz	1000 Hz	3000 ÷ 1000 = 3
±6 kHz	2 kHz	6 ÷ 2 = 3

The carrier frequency is not in the formula. For phase modulation the index is independent of the RF carrier frequency.

RememberIndex = deviation ÷ modulating frequency. Swing on top, tone below.

Deviation ratio = maximum deviation ÷ highest modulating frequency

Deviation ratio is the same division at the system's limits: the maximum carrier deviation over the highest audio modulating frequency.

Bigger swing or slower modulating tone = bigger index.

Maximum deviation (either side)±5 kHzHighest modulating frequency3 kHz

Deviation ratio1.67

Max swing	Highest audio	Ratio
±5 kHz	3 kHz	5 ÷ 3 = 1.67
±7.5 kHz	3.5 kHz	7.5 ÷ 3.5 = 2.14

Memory hookDeviation on top, frequency on the bottom. If you get 0.6 instead of 1.67, you flipped it.

OFDM splits data over many orthogonal subcarriers

Orthogonal frequency-division multiplexing is a digital modulation technique used in amateur digital modes. It sends one data stream as many slow subcarriers, with frequencies chosen so each peak lands where the others are zero. That avoids intersymbol interference.

Overlapping subcarriers, spaced so they stay out of each other's way.

RememberOFDM = many closely spaced digital subcarriers that do not interfere.

FDM splits the band. TDM splits the time.

Multiplexing puts several data streams on one transmission. FDM divides the signal into separate frequency bands, each carrying a different stream. Digital TDM has the signals share discrete time slots of one data transmission.

FDM splits the band. TDM splits the time.

RememberFDM: different frequencies at once. TDM: same channel, taking turns.

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