TABLE OF CONTENTS Steve Winder
| Note | Bandstop (notch) filters are tricky because they usually involve subtracting large signals. Thus any imperfections can cause a relatively large error. Trimming will probably be necessary to get a crisp notch with correct rolloff and a deep notch at the desired center frequency. Here Steve Winder shows us several good ways to make a notch./rap |
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 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 could occur that would lead to an incorrect overall frequency response. The denormalization and scaling process for lowpass and highpass filters has already been described (in Chapters 6 and 7).
This chapter describes how to design narrowband active and passive bandstop filters to almost any specification. Narrowband filters have upper and lower frequencies that are less than about three octaves apart. The design of these filters uses the normalized lowpass filter pole and zero or component values as a starting point. In this chapter I use information from previous chapters and give examples where they help in understanding. I...
