Introduction

The purpose of any communication system is to carry information — a message signal — from a source to a distant receiver. Before that message can travel through a communication channel, it is processed by a key technique called modulation. Modulation is the process of superimposing the information contained in a low-frequency signal onto a high-frequency signal. Without it, practical electronic communication over any useful distance would not exist.

More formally, modulation is the process by which some characteristic of a high-frequency signal called the carrier is varied in step with the instantaneous value of another signal called the modulating signal. The low-frequency message is also called the intelligence signal or baseband signal. The higher-frequency carrier, being high enough in frequency to be radiated through free space as a radio wave, is termed a radio-frequency (RF) signal. The signal that comes out of the modulator is the modulated signal (Figure 8.1.1).

A sinusoidal carrier wave can be written in general as

There are three parameters associated with this carrier: its amplitude $V_C$, its frequency $f_c$ and its phase $\theta$. Depending on which one of these is varied in accordance with the amplitude of the modulating signal, the process is named:

• Amplitude modulation (AM) — the amplitude of the carrier is varied in proportion to the modulating signal, keeping frequency and phase constant.
• Frequency modulation (FM) — the frequency of the carrier is varied in proportion to the modulating signal, keeping amplitude and phase constant.
• Phase modulation (PM) — the phase of the carrier is varied in proportion to the modulating signal, keeping amplitude and frequency constant.

Frequency modulation and phase modulation both effectively vary the total angle $\phi = (\omega_c t + \theta)$ of the carrier, so together they are called angle modulation.

Need for modulation

Modulation is used so that message signals can be transmitted effectively over long distances. The main reasons are:

• Length of antenna — For efficient radiation or reception, the minimum antenna length must be about one-fourth the wavelength of the signal, i.e. $\lambda/4$. A low audio frequency demands an impractically huge antenna. For example, a 20 kHz audio signal has $\lambda = C/f = (3\times10^{8})/(20\times10^{3}) = 15000$ m, needing an antenna of roughly 3750 m. If instead the audio is used to modulate a 10 MHz carrier, the required antenna length $\lambda/4 = C/4f = (3\times10^{8})/(4\times10\times10^{6}) = 7.5$ m — perfectly practical.
• Operating range — A higher-frequency wave carries greater energy, so it travels much farther. Low audio frequencies radiated directly cannot cover any useful distance. …