Fabrication of Low-Loss Microstrip Band-Pass Filter Made from Double-Sided YBa₂Cu₃Ox Films

A three-pole microstrip band-pass filter, with a center frequency of 9.5 GHz and bandwidth of 2%, has been fabricated using YBa 2 Cu 3 O x (YBCO) films deposited on both sides of (100)MgO substrates by pulsed laser ablation. A double-sided YBCO filter has a much lower passband insertion loss, 0.08 dB at 77 K and less than 0.04 dB below 60 K, compared with the 0.20 dB of a YBCO filter with a Au ground plane, and with the 1.1 dB of a conventional double-sided Au filter at 77 K. The extremely low insertion loss characteristics correspond to the element resonator unloaded quality factor ( Q u ) of 6000 at 77 K and greater than 10000 below 60 K. These results demonstrate that very compact high- Q u superconducting filters with waveguide-like performance can be achieved in planar microstrip configurations.