ABSTRACT
At the end of this chapter you should be able to: • appreciate the purpose of a filter network • understand basic types of filter sections, i.e. lowpass, high-pass, band-pass and band-stop filters
• understand characteristic impedance and attenuation of filter sections
• understand low and high pass ladder networks
• design a low and high pass filter section • calculate propagation coefficient and time delay in filter sections
• understand and design ‘m-derived’ filter sections • understand and design practical composite filters
42.1 Introduction
A filter is a network designed to pass signals having frequencies within certain bands (called passbands) with little attenuation, but greatly attenuates signals within other bands (called attenuation bands or stopbands). As explained in the previous chapter, an attenuator net-
work pad is composed of resistances only, the attenuation resulting being constant and independant of frequency. However, a filter is frequency sensitive and is thus composed of reactive elements. Since certain frequencies are to be passed with minimal loss, ideally the inductors and capacitors need to be pure components since the presence of resistance results in some attenuation at all frequencies.
