Abstract
This letter presents a new multimode ring resonator ultrawideband (UWB) bandpass filter by loading a microstrip line on the vertical symmetrical plane of a ring resonator and a pair of stepped-impedance open stubs at the center of this microstrip line. Five modes are allocated within UWB band, and a pair of transmission zeros are introduced to improve the passband selectivity. By adopting the parallel-coupled lines, not only suitable external coupling is achieved, but also the performance of upper stopband is improved. This quintuple-mode high-temperature superconducting UWB filter is designed on a 0.5-mm-thick MgO substrate with double-sided YBCO films. The measured results show that 3-dB bandwidth covers 1.57–6.51 GHz, the return loss is better than 10.70 dB, the out-of-band rejection is greater than 24.75 dB in the lower stopband, and the upper stopband with 20-dB attenuation level is extended to about 9 GHz.
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