Compact balanced tri-band superconducting band-pass filter using double square ring loaded resonators

This article presents a balanced tri-band superconducting band-pass filter (BPF) with controllable center frequencies (CFs) and fractional bandwidths (FBWs) using novel double square ring loaded resonators (DSRLRs). Due to the symmetrical structure of the proposed DSRLR, the differential mode (DM), and common mode (CM) equivalent circuits of the resonator are analyzed by using even- and odd-mode methods, and the resonant characteristics can also be obtained. With the proper design of the proposed DSRLR, the first three DM resonant frequencies are utilized to construct the three DM passbands, and their CFs can be adjusted by changing the lengths of the corresponding stubs. Then, the internal coupling between resonators can be controlled independently by the additional capacitive loading microstrip and the interdigital coupling structure. In addition, the so-called frequency dispersion technique is used to enhance CM suppression in the proposed balanced tri-band BPF by loading additional multiple stubs on the symmetry plane. Consequently, the proposed tri-band BPF can realize controllable CFs and FBWs with broadband CM suppression. Prototype balanced filters with CFs of 1.7/2.45/3.4 GHz and FBWs of 2.9%/3.2%/1.8% were designed, manufactured, and measured for verification. The experimental results agree well with the simulated ones.

Automated summary

This study introduces a balanced tri-band superconducting band-pass filter using novel double square ring loaded resonators, allowing controllable center frequencies and fractional bandwidths, with enhanced common mode suppression through frequency dispersion technique.