OVERVIEW

The fundamental use of filters in electrical engineering is to shape signal spectrum, which is especially crucial in reducing input signal noise in receivers and spurious emissions in transmitters. It should be noted that electrical filters are not limited to shaping the magnitude response of the signal spectrum; they can be used to change the phase response as well. The fundamentals of filter theory are based on the works of the great French mathematician Jean Baptiste Joseph Fourier who showed that arbitrary functions can be represented by trigonometric series, called the spectrum of the signal. Broadly speaking, a filter is a continuous, translation- invariant, linear system. In electrical engineering, filtering can be intentional, as in the input stage of a receiver, or unintentional, as in the transmission path of a microwave signal. It is important to realize that almost every physical system has some sort of filtering action built in whenever a signal, an input, an output, and a transmission path can be defined in that object. The signals can be either electrical or mechanical vibrations, although many analogies exist between the two.

Microwave filters have traditionally been built using waveguide and coaxial lines. Following the enormous expansion in printed circuit technology and modeling techniques, many of them are now built using printed circuits, with few exceptions, which we will introduce in a moment. Printed circuit filters have advantages over rectangular or coaxial waveguide filters in terms of low cost, repeatability, high accuracy, and compact size. Repeatability and high...