Design of Superconducting Hex-Band Bandpass Filter Using New External Quality Factor Adjustment Method

A compact high-performance superconducting hex-band bandpass filter (HB-BPF) using a new external quality factor adjustment method has been developed. It consists of three five-pole dual-band bandpass filters (DB-BPFs) with center frequencies of 2.0 and 3.0 GHz, 2.5 and 3.5 GHz, and 2.74 and 4.0 GHz and, passband bandwidths of 2.0%. The DB-BPFs use stub-loaded dual-band resonators. Additional microstrip lines between the resonators enables flexible control of the coupling coefficient of each passband. To avoid affecting the external quality factors of the other two DB-BPFs, that of each DB-BPF is adjusted by placing the resonator on the feedline and changing the first (last) resonator configuration without changing the feedline configuration. A compact (20.9 mm × 40 mm; 0.36λ
_g
× 0.68λ
_g
) HB-BPF was fabricated using YBa
₂
Cu
₃
O
_y
thin film on a 0.5-mm-thick sapphire substrate. The measured frequency responses agreed well with the simulated ones.