Compact Triple-Mode Bandstop Ceramic Cavity Filters Using Parallel Coupled Resonators

Triple-mode bandstop ceramic cavity filters utilizing parallel-coupled resonators are presented. A transmission line, etched onto the face of a silver-plated ceramic cube, couples to three modes in parallel, avoiding any interresonator coupling perturbations. By varying only a few structural parameters, the coupling strengths into each mode can be controlled, and highly flexible bandstop filtering functions are demonstrated. A 30-MHz bandstop filter at 1800 MHz, from a roughly 27 mm³ ceramic block mounted to a copper base, is fabricated and measured. It achieves 26 dB of attenuation, with less than 0.3 dB of insertion loss 20 MHz away from the stopband, and less than 0.3 dB of reflection loss in the stopband, closely matching the simulation.