Compact Dual-Band HTS Bandpass Filter Using Spirally Asymmetric Stepped-Impedance Resonators

In this paper, a compact dual-band high-temperature superconducting (HTS) bandpass filter (BPF) is constructed by spirally asymmetric stepped-impedance resonators (SIRs) and a pair of parallel-coupled feed lines. The resonant characteristics of spirally asymmetric SIRs with one step discontinuity are investigated in the dual-band BPF design. Owing to its intrinsic characteristics and the multipath propagation effect, multiple transmission zeros are created to improve the selectivity of the passband and the rejection level of stopband. For demonstration purposes, a compact HTS filter operating at 2.4/5.8 GHz for wireless local area networks is fabricated. Measured insertion losses at each passband are 0.12 and 0.18 dB, respectively. A good agreement between the measurements and the simulations is observed, which verifies the circuit design methodology. Finally, the temperature dependence and the input-power dependence of microwave nonlinear properties of the dual-band HTS filter at each passband are also measured and analyzed. Measured results have a good agreement with simulations.