Quad-Band Superconducting Bandpass Filter Using Quad-Mode Stub-Loaded Resonators With Controllable Frequencies and Bandwidths

Two quad-band high-temperature superconducting (HTS) bandpass filters (BPFs) using quad-mode stub-loaded resonators (QMSLRs) and quad-feeding structures are presented. The quad-mode resonators are found to be advantageous because the couplings of four resonant frequencies can be easily adjusted by four coupling areas. The quad-feeding structure, containing two coupled lines and two tapped lines, can offer the adequate degrees of freedom to achieve the desired external quality factor
$Q_{e}$
. Therefore, it can independently control external couplings and coupling coefficients. Moreover, the proposed QMSLRs can conveniently be utilized to tune the four passband frequencies. Both the frequencies and bandwidths of each passband can be easily controlled. The first third-order filter prototype with a fractional bandwidth of 1% is initially designed and implemented to verify the theory. Afterward, the second fourth-order filter with an absolute bandwidth of 10 MHz is realized to certify the capacity of a high-order quad-band BPF. Finally, two HTS BPFs are fabricated, and the measured results agree well with the theory.