E8B: Modulation Techniques

Modulation is the process of encoding information onto a carrier signal. The specific numbers that describe how much modulation is applied — and how multiple signals share a channel — are the focus of this group.

This lesson covers FM modulation index and deviation ratio, how phase modulation index behaves differently from FM, worked calculations for both, orthogonal frequency-division multiplexing (OFDM), frequency-division multiplexing (FDM), and time-division multiplexing (TDM).

FM Modulation Index

The modulation index of an FM signal describes how much the carrier frequency deviates relative to the modulating signal frequency. It is defined as:

Modulation index = frequency deviation / modulating signal frequency

A higher modulation index means more deviation — a wider signal that occupies more bandwidth but offers better signal-to-noise performance. A lower modulation index means less deviation and a narrower signal.

Deviation Ratio

The deviation ratio is a related but distinct measurement used to characterize an FM system's design limits. It is the ratio of the maximum carrier frequency deviation to the highest audio modulating frequency the system is designed to handle:

Deviation ratio = maximum carrier deviation / highest modulating frequency

While modulation index is a measurement of a specific signal at a given moment, deviation ratio is a system specification that describes the worst-case (maximum deviation, highest frequency) scenario.

Phase Modulation Index

For a phase-modulated (PM) signal, the modulation index behaves differently than for FM. The PM modulation index does not depend on the RF carrier frequency — it is determined only by the phase deviation applied by the modulating signal. This is in contrast to FM where the frequency deviation is the key parameter, and in contrast to AM where carrier frequency also plays no role in the depth of modulation.

Worked Calculations

OFDM

Orthogonal frequency-division multiplexing (OFDM) is a digital modulation technique that transmits data on many subcarriers simultaneously. The subcarriers are spaced at frequencies specifically chosen so that each one is at a zero-crossing of all the others — making them orthogonal and eliminating intersymbol interference between subcarriers.

In amateur radio, OFDM is used for digital modes. Its ability to pack many subcarriers into a band while maintaining orthogonality makes it highly bandwidth-efficient. OFDM also handles multipath propagation well, which is why it is used in modern wireless communications systems worldwide.

Frequency-Division Multiplexing (FDM)

Frequency-division multiplexing (FDM) is a technique that divides the available transmission bandwidth into separate sub-bands, each carrying a different data stream. Each channel occupies its own frequency slot and does not overlap with adjacent channels. Traditional landline telephone systems used FDM extensively; in amateur radio, systems with multiple audio subcarriers use a form of FDM.

Time-Division Multiplexing (TDM)

Digital time-division multiplexing (TDM) is a technique where two or more signals are arranged to share discrete time slots of a single data transmission channel. Each signal is given a repeating time slot in sequence, and the signals are interleaved in time rather than separated in frequency. TDM is the basis for many digital telephony and networking systems.

E8B Practice Questions