High-Temperature Superconducting Bandpass Filter Using Asymmetric Stepped-Impedance Resonators With Wide-Stopband Performance

This paper presents a wide-stopband high-temperature superconducting (HTS) bandpass filter (BPF). The filter is implemented by the asymmetric stepped-impedance resonators (SIRs) with only one step discontinuity, which leads to low loss and compact size. With varied characteristic impedance ratios and length ratios, the diversity of asymmetric SIR has been studied, and the spurious resonant-mode responses are discussed. Adopting different asymmetric SIR units operated at the same fundamental frequency f
₀
(2.4 GHz), its wide upper stopband can be realized by dispersing the higher order spurious resonant modes of these resonators. Furthermore, a modified input/output (I/O) feedline structure has been utilized to enhance the stopband performance. Finally, a fourth-order HTS BPF for wireless application is designed and fabricated on a MgO wafer with double-sided YBCO films. The filter measurement shows excellent performance with 0.05-dB minimum insertion loss and better than -20-dB rejection level in the stopband up to 10.46 GHz (about 4.4f
₀
). In addition, the size of the HTS filter is 12.45 mm × 23 mm (i.e., 0.256 λ
_g
by 0.472 λ
_g
, where λ
_g
is the guided wavelength at the center frequency).