A phase-locked loop (PLL) helps keep the electronic world orderly. In a television set, one PLL keeps the head at the top of the screen and the feet at the bottom of the screen. Another PLL makes the color green remain green and the color red remain red.

This chapter presents background information to understand how a loop operates and gives reference material for further study of loops. Key signals of interest within a PLL and the major components are discussed so that we can immediately identify performance by monitoring these signals or major components on the test bench or in simulations. Key design requirements are presented so that we will be able to identify the important requirements from among the many trivial ones that can be found in statements of work or specifications. A history of PLL is presented to show the progression from the first implementations with large racks of equipment to today's tiny monolithic versions.

Applications of PLLs are presented to show the many applications in modern systems. We will need to understand the differences in specification requirements for these applications because the design differences in the PLL literature are driven by these application differences. Good design engineers must understand how their circuits will be used in order to provide the optimum circuit for the customer. Not everything the customer desires is in the specification, and the ranking of the importance of specifications must be determined.