Page 283 - Complete Wireless Design

Filter Design

282 Chapter Six

The fundamental building block in low-pass filter design is the half-section,
as shown in Fig. 6.16. This half-section is composed of one series inductor and
one shunt capacitor. By cascading an increasing amount of half-sections, we
can obtain a filter with any filter skirt steepness desired. This is referred to as
increasing the number of poles of a filter; where four poles equal one section
in a low-pass filter. The half-sections may be joined either as pi sections (Fig.
6.17) or as T sections (Fig. 6.18). These sections will then combine with the
next adjoining reactive element, creating a single inductor for the pi of Fig.
6.19. This is because there would be absolutely no reason to use two inductors
in series with each other, or two capacitors in shunt with each other, when
their values can simply be added to obtain the proper value (Fig. 6.20).
Combining half-sections with either the pi or the T technique is equally valid,
but the design that results in fewer inductors is usually preferred from a cost,
Q, and size standpoint. A filter with either an even or an odd number of cas-
caded sections is also perfectly sound.

Figure 6.16 Series inductor and
shunt capacitor LPFs, or half-
sections.

Figure 6.17 Half-sections joined
as pi sections.

Figure 6.18 Half-sections joined
as T sections.

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