1. Introduction

For the stability analysis and design of feedback controllers, frequency response analysis is very useful.

When a linear system is subjected to a sinusoidal input, its ultimate response (after a long time) is also a sustained sinusoidal wave (will be proved...). This constitutes the basis of frequency response analysis.

With frequency response analysis we are interested primarily in determining how the features of the output sinusoidal wave (amplitude, phase shift) change with the frequency of the input sinusoid.