TABLE OF CONTENTS In this chapter we will discuss applications of the PLL in the domain of data communications. There are many different kinds of communications, however; hence we will first look at the most important variants. First of all, analog or digital data can be transmitted over a data link. Historically, the PLL was developed to be used in the analog domain: the inventor Henri de Bellescize22 designed a vacuum-tube-based synchronous demodulator for an AM receiver. The first important application of the PLL was in the recovery of the color sub-carrier in television receivers around 1950.2
In recent years digital communications have become increasingly important, even in the classical analog domains such as telephone, radio, and television. Because synchronization is an extremely important task whenever digital data are transmitted, the PLL and related circuits find widespread applications.
Analog and digital signals can be transmitted either as baseband signals or by modulation of a carrier; in the latter case we speak about bandpass modulation. If analog signals are communicated in the baseband, there is no need for synchronization; hence this is not an issue for the PLL. PLLs and related circuits come into play, however, when analog signals are modulating a high-frequency carrier. For analog signals, the classical modulation schemes still are amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM).
Historically, AM is the oldest modulation scheme. To modulate an analog signal (e.g., voice, music, and measurements of temperature...
