Bandpass Filter Loaded With Open Stubs Using Dual-mode Ring Resonator

A high selectivity bandpass filter loaded with open stubs using dual-mode ring resonator is presented in this letter. Sixth-order passband with six transmission zeros close to the passband is realized with two open stubs and open/shorted coupled lines. The transmission zeros near the passband can be adjusted conveniently by only changing the characteristic impedances of the coupled lines. For demonstration, a planar bandpass filter (3 dB bandwidth 20.6%) is designed and fabricated. Good filtering performance and high selectivity for the filter are realized and experimentally verified.