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. low-pass, 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
• understand and
A filter is a network quencies within certain little attenuation, but other bands (called
As explained in the previous chapter, an attenuator network pad is composed of resistances only, the attenuation resulting being constant and independent 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
some attenuation at all frequencies. Between the passband of a filter, where ideally the
attenuation is zero, and the attenuation band, where ideally the attenuation is infinite, is the cut-off frequency, this being the frequency at which the attenuation changes from zero to some finite value. A filter network containing no source of power is termed passive, and one containing one or more power sources is known as an active filter network. The filters considered in this chapter are symmetrical unbalanced T and π sections, the reactances used being considered as ideal. Filters are used for a variety of purposes in nearly every type of electronic communications and control equipment. The bandwidths of filters used in communications systems vary from a fraction of a hertz to many megahertz, depending on the application.
