TABLE OF CONTENTS This chapter is included for the novice. It provides a brief historical perspective and a review of very basic analog, high-frequency, electronic filter terminology.
As the wireless era began, selectivity was provided by a single series or shunt resonator. Modern filters date back to 1915 when Wagner in Germany and Campbell in the United States working independently proposed the filter [1]. In 1923, Zobel [2] at Bell Laboratories published a method for filter design using simple mathematics. His approximate "image parameter" technique was the only practical filter design method used for decades.
Around 1940, Foster's earlier theories were extended by Darlington and Cauer to exactly synthesize networks to prescribed transfer functions. Due to a heavy computational burden, these methods remained primarily of academic interest until digital computers were used to synthesize lowpass prototypes, from which other filter structures were easily derived. These lowpass prototypes have been tabulated for many specific and useful transfer approximations named after the mathematicians credited with the development of the polynomials, such as Butterworth, Chebyshev, Bessel, Gaussian and others. Although in practical use since the 1950s, this method is referred to as modern filter theory because it is the most recent of this triad of techniques.
At this point, two schools of interest developed. One pursued the extension and refinement of filter mathematics. For example, even digital computer precision is generally unsuitable when direct synthesis of bandpass instead of lowpass filters is attempted [3]. The final result of this pursuit is the ability...
