Triband High-Temperature Superconducting Bandpass Filters Using Multimode Resonators

In this paper, a simple multimode resonator (MMR) is proposed to design a compact triband high-temperature superconducting (HTS) bandpass filter (BPF). The MMR is constructed with three transmission line sections with different characteristic impedance, named asymmetric trisection stepped-impedance resonator (TSIR). Asymmetric TSIR can produce three controllable resonant modes, and its resonant characteristics are analyzed based on the transmission line theory. Meanwhile, some graphs for the relationship of size parameters and ratio of resonances are built for design conveniently. For demonstration, a second-order triband HTS BPF that centered at 1.5/3.5/5.8 GHz is well designed. The filters are fabricated on a 0.5-mm-thick MgO wafer with double-sided YBCO thin films and measured at the temperature of 77 K. Measured results agree well with the simulated ones and show that the insertion losses in all passbands are less than 0.16 dB, which exhibit superiority in midband insertion loss.