Triple-Mode Band-Stop and Low-Pass Cavity Filters Using Parallel Coupled Resonators

Triple-mode band-stop and low-pass cavity filters utilizing parallel-coupled resonators are presented. In the proposed approach, a transmission line suspended within an air cavity couples to the three fundamental cavity modes in parallel, avoiding any inter-resonator cavity coupling perturbations. By varying only a few structural parameters, the coupling strengths into each mode can be controlled, and highly flexible filtering functions can be generated, including band-stop and low-pass filtering. A cubic 60 mm air cavity, four-pole, three-zero, 50 MHz bandwidth band-stop filter with sharp selectivity, centred at 3500 MHz, is fabricated and measured. Without silver plating, it achieves 0.3 dB of transmission loss 10 MHz away from each stop-band edge and 0.3 dB of reflection loss inside the stop-band.