TABLE OF CONTENTS Certain applications such as satellite communications require very severe filtering functions. Low passband loss may be combined with extreme selectivity and group delay linearity requirements. In such cases, conventional all-pole transfer functions may not be suitable and filters with generalised Chebyshev characteristics are required.
Classical cascade synthesis procedures enable transmission zeros to be independently realised by cascades of two-port networks such as the Brune section. However, when the transmission zeros are on the real axis or in the complex plane it is usually more convenient to synthesise cross-coupled ladder network prototypes as described in Chapter 3. Certain transfer functions are then realisable by the generalised direct-coupled cavity waveguide filter described in this section [7].
The symmetrical generalised direct-coupled cavity waveguide filter is shown in Figure 6.19. It consists of two identical shunt inductive-iris coupled waveguide structures where adjacent cavities in the two halves are cross-coupled through apertures in the common narrow wall. The equivalent circuit for the filter will be derived and equated to the cross-coupled lowpass prototype filter at midband. Formulae for the iris susceptances and the electrical lengths of the cavities, shown in Figure 6.20, then follow directly.
The cross-coupled lowpass prototype network is shown in Figure 6.21. Note that for an N th-degree filter n = N/2 in this prototype. The even-mode equivalent circuit of...
