Filters: Theory 8C Practice 3

EE 28 October 1987 FILTERS: THEORY 8c PRACTICE 3 36 Istop band approximation transition width f~~ approximation Brick wall (ideal) Butterworth Very flat pass band. Attenuation continues to increase in stop band. Poor pass/slop band transition. The underlying principle of the network theory is to synthesize the filter from a knowledge of its voltage transfer function (voltage vs frequency charac- teristic). This is an essentially mathematical approach using the ratio of certain types of polynomials. Known as approxi- mation theory, it is a very powerful technique. Using these methods, three of the ap- proximations yield familiar transfer functions. Together with the ideal brick wall response, they are shown in Fig. 36. The development of the digital computer greatly aided in the evolution of the polynomial coefficients. This enables filter design tables to be produced from suitable computer pro- grams. The use of such tables, coupled with a step-by-step design pro- cedure, enables ...