Balanced Dual-Band HTS BPF With Controllable Frequency Ratio and Transmission Zeros Using Mixed Cross-Coupling

Abstract

In this brief, a fourth-order balanced dual-band high temperature superconducting (HTS) bandpass filter (BPF) is presented based on modified stub-loaded resonators (SLRs). Firstly, the differential mode (DM) and common mode (CM) resonant characteristics of the modified SLRs are analyzed. Under DM operation, two types of mixed cross-coupling topologies are constructed by placing the spiral modified SLRs and their DM equivalent structures in an appropriate way. Then, a dual-band response with controllable center frequencies, bandwidths, and transmission zeros (TZs) is achieved. Under CM operation, an effective CM suppression circuit is proposed for improving the in-band CM suppression level. Finally, for verification, a fourth-order balanced dual-band HTS BPF with center frequencies of 3.494/4.797 GHz and insertion loss of 0.26/0.21 dB is designed and fabricated on MgO substrate with double-sided YBCO thin films. The measured results agree well with the simulated ones, and the BPF shows the attractive features of low insertion loss and good out-of-band rejection.