Design of Dual-Mode Dual-Band Superconducting Filters

This paper presents a synthesis design method for superconducting dual-mode dual-band filters. A low-pass to dual-band bandpass frequency transformation is proposed based on lossless lumped-element dual-mode resonator circuits. Formulas for extracting the coupling coefficient k and external quality factor Q
_e
of dual-mode microstrip resonators are derived using the coupling matrix theory and the proposed frequency transformation formula. This makes the design process of dual-mode dual-band filters similar to that of the single bandpass filter. For the demonstration, four dual-band superconducting filters with different dual-mode resonator structures, such as stepped-impedance, stub-loaded, and embedded resonators are designed and fabricated. The measured results match well with the simulations.