Microwave superconducting filters

This paper reports on high-temperature superconducting (HTS) planar strip filters used in applications where small size and low loss are desirable, such as radar and communications. A major factor for these devices is packaging, since not only thermal considerations are important but also those having to do with HTS material integrity. Four-pole, parallel-coupled microstrip line filters were fabricated using epitaxial YBa2Cu3O7 (YBCO) films deposited by off-axis sputtering on LaAlO3 substrates. Filters were made with and without superconducting ground planes. Because it is not possible as yet to grow in-situ epitaxial films on both sides of a substrate, two separate substrates were used, one for the filter and one for the ground plane. The devices were packaged so that the microstrip-to-ground plane distance was the thickness of one of the substrates (0.43 mm). Passband characteristics and input power and temperature dependence were measured, as well as thermal cycling stability