ABSTRACT
This chapter describes the behavior of linear time-invariant systems in the frequency domain. It is important to stress that the control engineer should be able to understand the behavior of a system both in the time and in the frequency domain. The analytical correlation between the frequency domain and the time domain response for higher-order systems is, in general, very difficult. The Nyquist stability criterion refers to the stability of closed-loop systems. In cases where a mathematical model is not available and the system transfer function is described graphically, then the use of the Nyquist stability criterion comes in very handy. Since the Nyquist diagram of the open-loop transfer function constitutes the basis for the application of the Nyquist criterion, it follows that one must be familiar with how to construct such diagrams.
