Compact Tri-Band Bandpass Filter Based on Ring Multi-Mode Resonator

A novel tri-band bandpass filter (BPF) based on ring multi-mode resonator (R-MMR) is proposed in this paper. The filter consists of an R-MMR and a pair of parallel coupled lines. Because the R-MMR is symmetrical, its resonant characteristics can be analyzed by using odd- and even-mode analysis method. When a couple of stubs are loaded on the ring resonator, six transmission poles and four transmission zeros are created, and a tri-band BPF is realized by employing a couple of parallel coupled lines. To miniaturize the size, the R-MMR is folded. A prototype is designed, fabricated and measured. Good agreement is obtained between the simulated and measured results.