Filters: Theory & Practice 2

15 16 Z1 EE September 1987 FILTERS: THEORY & PRACTICE 2 Fundamental sections from which more complex ones are built are called prototype filters. The majority of filters used are high-pass, low-pass, band-pass, or band-stop. All of these are usually made up of low-loss reactances connected in T, n, or L form as shown in Fig. 1. Practical filters are nor- mally designed as a combi- nation of these configurations. In the following, the reactance, i.e., the imaginary part of a com- plex impedance, is designated by X, which is equal to l/wC or wL or a combination of these. Low-pass filters In a correctly terminated low- pass T filter, see Fig. 15: ZOT = VZ1Z2 + Zi/4 = jX2 - XI/4 which can also be expressed as ZOT = j Xi (X1/4 + X2) [20] If X, and (X1/4+X2) are of op- posite sign, ZOT is resistive and the filter draws power: it is operating in its pass-band-see Fig. 17. If XI and (Xi /4 +X2) are of the same sign, ZOT is reactive and the filter reflects power: it is operating in its...