Introduction to the PLL

Though the idea of the phase-locked loop (PLL) surfaced as early as 1932, it remained an esoteric and expensive concept until the arrival of the integrated circuit. The PLL has the unique ability to capture a signal without requiring precision components, a welcome feature in a world of large variations. The basic block diagram of a PLL is shown in Figure 14-1.

Block diagram of a simple phase-locked loop.

Figure 14-1. Block diagram of a simple phase-locked loop.

The phase-locked loop is primarily an analog concept, and only when we treat it as such can we fathom its powerful capabilities. And here is where simulation fails us. Anyone who has ever sat down at a bench and observed a phase-locked loop grabbing a minute signal seemingly buried in noise will agree that a simulation cannot give you the same sensation.

In a real circuit, a phase-locked loop almost seems to be alive, capturing and hanging on to a signal as the frequency is changed. A simulation of the same circuit is a cold experience, giving you no intuition (and taking up an enormous amount of time).