Parallel Multimode Cavity Filters With Generalized Frequency Response

A generalized frequency response for cavity filters is demonstrated from six modes coupled in parallel. The six modes belong to two parallel-coupled triple-mode spheres and are coupled in parallel via two half-wavelength transmission-line resonators. The theory of phase and amplitude control of parallel couplings into spherical cavities is introduced by way of coupled resonator models, and the structure of two combined spheres is detailed. The theory and simulations prove that multiple transmission zeros can be arbitrarily placed, including inside the passband to create multiple bands and beyond infinity (imaginary frequencies) for flattened group delay. Several design examples of two spheres in parallel are presented to demonstrate a high degree of flexibility of transmission zero placement, and one example is fabricated and successfully tested.