Novel wideband bandpass filter using open stub and short stub-loaded multi-mode ring resonator

A novel wideband bandpass filter (BPF) using open-stub and short-stub loaded multi-mode ring resonator (MMRR) is proposed in this paper. The characteristics of the multi-mode resonator are investigated using even-odd mode analysis. The odd-mode resonance frequency is mainly determined by the length of ring resonator, while the even-mode frequencies can be tuned effectively by changing the lengths of the stubs. A prototype of wideband bandpass filter for ultra-wideband (UWB) application is designed and fabricated for demonstration. Good agreement is observed between the measured and the theoretical expectation, indicating fractional bandwidth of 100.8%, a small group delay variation and insertion loss less than 0.5dB.