Microstrip resonators and filters using high-Tc superconducting thin films on LaAlO3

Very low microwave losses have been measured in YBa₂Cu ₃O₇ linear resonators, ring resonators, and bandpass filters. The stoichiometric 1-2-3 compound was deposited on LaAlO₃ substrates, patterned microwave circuits, and overcoated with a passivating LaAlO₃ layer. HTS (high T _c superconductor) linear microstrip resonators demonstrated Q values greater than 1200 at 10 GHz, corresponding to surface resistances less than 300 μΩ. Identical silver resonators showed Q values of 60 and surface resistance of 15 mΩ. The high-frequency transition temperature for these HTS films was greater than 83 K. Q values of 240 were measured in ring resonators at 15 GHz. Finally, the authors designed, fabricated, and tested a two-pole, Chebyshev narrow-bandwidth bandpass filter. The HTS filter was designed to be a 1% bandwidth with 0.2-dB ripple. Insertion loss was 2 dB at 4 K, rising to 3 dB at 77 K. The authors observed a temperature dependence in the filter center frequency which they attribute to kinetic inductance effects. The proposed HTS filters outperform similar cryogenic silver filters, indicating that practical levels of HTS performance have been achieved