Dielectric Resonator Filters
It is sometimes desirable to include couplings between non-adjacent resonators to obtain more optimum filter transfer functions. An example of a fourth order dielectric resonator design incorporating a single capacitive cross coupling is shown in Fig. 1.
This filter employs a short, single-section evanescent mode filter to couple between the first resonator and the last resonator. The couplings between the other resonators are inductive irises, tuned but not resonated. The evanescent filter coupling between resonators (1) and (4) uses a re-entrant section of coaxial line to provide the capacitance required to resonate the equivalent shunt inductance of the below cutoff (evanescent mode) section of waveguide which is what the iris really represents.
The single cross-coupling in this even order circuit (4th order) provides a pair of transmission zeros which emphasize the attenuation close to the passband. Inclusion of both capacitive and inductive couplings are possible, with respective effects on stopband attenuation or on passband group delay and amplitude flatness. The technique can also be applied to dual mode filters, bandstop filters or most kinds of TEM, quasi-TEM or evanescent mode filters.