Practical Filter Design Part 6

111 PRACTICAL FILTER DESIGN PART 6 by H. Baggott In this sixth part in the series we start our discourse of the tables and characteristics of filters and as first we deal with those pertaining to the Butterworth type because that is the best known and probably also the most often used kind of filter. The Butterworth filter owes its popu- larity to a combination of flat amplitude response in the pass band and reasonable roll-off. A drawback is its non-linear phase characteristic. The roll-off is fairly precisely 6n dB per octave, where n is the order of the fil- ter. The Butterworth filter may be consid- ered a compromise between the Bessel network (moderate roll-off but linear phase response) and the Chebishev filter (steep roll-off, poor phase response and ripple in the pass band). For applications that require a flat pass band and steep roll- off, the Butterworth filter is undoubtedly the best choice. Table 1 gives the pole locations of Butterworth filters of the second to the t...