Chapter 7 - Bandstop filters

Abstract

This chapter describes how to design narrowband active and passive bandstop filters to almost any specification. There are two categories of bandstop filters: wideband and narrowband. Filters are classified as wideband if their upper and lower passband cutoff frequencies are several octaves apart. This is when the upper frequency is many times than that of the lower frequency. Wideband filters are ideally constructed from odd-order lowpass and highpass filters connected in parallel. Odd-order filters are necessary because, outside their passband, these have both high input impedance and high output impedance. High impedance in the stopband prevents loading of the parallel connected filter, otherwise impedance mismatches can occur that would lead to an incorrect overall frequency response. Narrowband filters have upper and lower frequencies that are less than about three octaves apart. The design of these uses the normalized lowpass filter pole and zero or component values as a starting point. Examples and formulae for passive designs in the denormalization and scaling of normalized component values are provided and the method of denormalizing pole and zero information is described in the chapter.