Overview

In Chapter 4, we discussed dividers and oscillators. In this chapter we discuss detectors and other circuits that support PLLs. Detailed designs of phase detectors, lock detectors, and acquisition aids are studied.

Phase detectors are studied because understanding how phase detectors work is one of the major keys to understanding how PLLs work. Many systems use the lock detector to reset the system. This is a disastrous change to the operation of most systems. In a PLL, a reset can start the loop operating at a very low frequency (or with no output), which will then acquire lock at the normally much higher output operating frequency. Consequently, a small phase shift in the PLL that would marginally affect the system can cause a huge disruption in the operation of the system if a reset occurs. The key to lock detection is to alarm on behavior that shows that the PLL is broken. Quadrature phase detection, time-window edge comparison, tune-voltage window comparator, and cycle-slip detection are the lock-detection methods that are covered.

Open-loop sweep, closed-loop sweep, and discriminator-aided acquisitions are the methods that are covered. The phase/frequency detector uses discriminator-aided acquisition and is the most popular choice. Clock-recovery circuits cannot use phase/frequency detectors. Consequently, these circuits require an acquisition aid. Understanding the design details and trade-offs in these components is critical in designing monolithic PLLs.

A charge pump and operational amplifier provide a method to convert the differential up and down outputs of the phase/frequency detector to a single-ended positive...