In previous chapters, we discussed stability and developed various tools to determine
stability and to assess relative stability. We continue that discussion in this chapter by
showing how frequency response methods can be used to investigate stability. The
important concepts of gain margin, phase margin, and bandwidth are developed in the
context of Bode plots and Nyquist diagrams. A frequency response stability result—
known as the Nyquist stability criterion—is presented and its use illustrated through
several interesting examples. The implications of having pure time delays in the sys-tem
on both stability and performance are discussed. We will see that the phase lag
introduced by the time delay can destabilize an otherwise stable system.The chapter
concludes with a frequency response analysis of the Sequential Design Example:
Disk Drive Read System.
