Active microwave bandpass filters using high-temperature superconductors

Passive microwave devices have been demonstrated to be quite feasible with thin films of superconductors, particularly {dollar}rm Ysb1Basb2Cusb3Osb{lcub}7-x{rcub}{dollar} (YBCO). Surface resistance measurements have shown that the YBCO thin films are an order of magnitude better than copper for making high Q resonators. This lower surface resistance allows for the construction of narrow bandpass filters with little insertion loss in the 1-50 GHz range. Passive microwave structures have been designed and tested to verify the properties of the YBCO films for microwave applications. These patterned devices show degraded properties compared to what was expected from the measurements of unpatterned films. Theoretical calculations for the microstrip test structures show that the high Q devices have currents in excess of the critical current for moderate power levels of 0 dBm input power or less.; Tunable high Q 1% bandpass microstripline filters have also been designed and tested for use as a shiftable filter. These tunable filters have a center frequency of 13 GHz, and the center pole is coupled to a DC control line. When the critical current of the control line is exceeded, the filter shifts down in frequency by 50-100 MHz. This shift was found to be due to thermal heating of the substrate that causes a change in the kinetic inductance of the centerpole near the control line. The data, theoretical calculations, computer simulations, and other issues that verify the superconducting microwave switching results are presented.