Fundamentals of Radio Modulation Techniques

AM Linearity

Amplitude Modulation (AM) is linear because it involves the propagation of information between a signal and a carrier. The carrier's amplitude is varied linearly according to the variations in the level of the modulating signal.

Synchronous Detection

In synchronous detection, the input signal is first multiplied with a locally generated sinusoid and then passed through a low-pass filter. The filter's bandwidth is typically the same as the message bandwidth or slightly greater. It is assumed that the local oscillator is synchronized with the carrier in both phase and frequency.

Phase and Frequency Modulation

Frequency Modulation (FM) conveys information through variations in the carrier frequency (ωc).

Phase Modulation (PM) conveys information through variations in the carrier phase.

Furthermore, the modulation index in PM is directly proportional to the amplitude of the modulating signal and independent of its frequency. In FM, the modulation index is directly proportional to the modulating signal amplitude and inversely proportional to its frequency.

Single Sideband and Vestigial Sideband

Single Sideband (SSB) modulation eliminates one of the sidebands of a DSB-SC signal using a band-pass filter. In a Vestigial Sideband (VSB) system, instead of completely eliminating one sideband (like USB), it allows a gradual cutting of a portion of one sideband.

Carrier Power in AM

The modulation index (B) has no effect on the total transmitted power in AM because the transmission power primarily depends on the amplitude of the carrier signal.

Maximum FM Efficiency

You can obtain a transmission efficiency of nearly 100% in FM. This is achieved by choosing an appropriate value for the modulation index (B). For example, a value around 2.4 concentrates significant power in the sideband components.

FM Bandwidth vs. Carrier Power

The carrier power (Pc) does not affect the FM bandwidth. The bandwidth depends on the modulation index (B) and the maximum modulating frequency (Fm).

FM Bandwidth vs. Modulation Index

Increasing the modulation index (B) increases the number of significant sidebands, and thus also increases the bandwidth of the FM signal.

Narrowband vs. Broadband FM

Narrowband FM transmits significantly less information compared to Broadband FM.

A Narrowband FM signal occupies a bandwidth similar to AM, approximately BW = 2ωm. Broadband FM occupies a much wider bandwidth.

AM Demodulation Advantage

Amplitude Modulation offers the advantage of simple demodulation. The ability to use an envelope detector in the receiver simplifies the recovery of the original modulating signal f(t).