Two novel microstrip bandpass filters with improved selectivity based on dual-mode ring resonators are proposed. Tow and four transmission zeros close to the passband are realized to improve the selectivity for the passband. Sixth-order passband is realized with two open/shorted stubs and a dual-mode ring resonator. The transmission zeros near the passband can be adjusted conveniently by changing the characteristic impedances of the coupled lines. For demonstration, two planar bandpass filters (3-dB bandwidths 20.9%, 20.2%) are designed and fabricated.
"Bandpass Filters with Improved Selectivity Based on Dual-Mode Ring Resonators," Progress In Electromagnetics Research Letters,
Vol. 56, 1-7, 2015. doi:10.2528/PIERL15072609
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