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Chapter Eight
Direct-Coupled Filters
Direct-coupled filters are the most common narrow-band networks in radio
frequency engineering. They may be found in very low frequency through
microwave applications, often in such different forms as to appear totally
unrelated. A large amount of design information exists, the formal basis
usually being attributed to Dishal (1949) and Cohn (1957). The design method
presented here evolved over the last three decades and is based on the
"loaded-Q" concept discussed in Section 6.1.3. It is unique to the extent that
loaded Q's of internal resonators are treated as design parameters. It precisely
accounts for midband impedance matching and dissipative loss. For passband
widths of less than 20%, it provides an accurate estimate of stopband selectivity
and an approximation of ideal passband response shapes. Unlike synthesis
methods, the loaded-Q design technique enables adjustment of surplus parameters,
so that bounds on practical component values may be accommodated.
Practical, direct-coupled filters evolve from an ideal prototype network,
shown in Figure 8.1. It is a lumped-element representation, although several
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Figure 8.1. The direct-coupled-filter prototype network,
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