Tuning properties of 2 GHz superconducting microstrip-line filters

We have fabricated a 2 GHz 12-pole superconducting microstrip line filter a nd a 2-pole electrically tunable biter using interdigital electrodes and di electric film. The change of the dielectric constant of a SrTiO3 film, stac ked on the filter, by applying a bias voltage between the electrodes leads to the tunability. The range of the center frequency modulation was 13 MHz (0.7 %) at 60 K, 75 kV/cm. The microwave frequency response was improved by modification of the electrode structure. This tuning method is considered to be promising in that individual resonance elements in multi-pole filters can be tuned independently.