A comparison of five methods of low-pass passive filter design

The phase and magnitude characteristics of five network functions, all three-pole approximations to an ideal low-pass filter characteristic, are compared. The functions considered are the Butterworth, Chebyshev, Papoulis Class L, `maximally flat delay¿ and `in-line pole¿; all of these can be realized with a simple passive circuit. The equations for the pole locations are quoted. Plots of the pole locations and the magnitude and phase responses are given for each of the functions normalized for unit resistance and a pass band from ¿ = 0 to ¿ = 1. The suitability of the different functions for use in audio circuits and when the output waveform is to be observed on a c.r.o. is also discussed. A plot of the pole locations for the five functions is given and the effect of changes in the pole positions is noted. Finally, experimental results are given showing the magnitude, phase and pulse responses of practical filters designed to realize the network functions discussed.