A novel K-band tunable microstrip bandpass filter using a thin film HTS/ferroelectric/dielectric multilayer configuration

We report on YBCO/strontium titanate (STO) thin film K-band tunable bandpass filters on lanthanum aluminate substrates. The 2 pole filters were designed for a center frequency of 19 GHz and 4% bandwidth. Tunability is achieved through the nonlinear DC electric field dependence of the relative dielectric constant of STO (/spl epsiv//sub rSTO/). Center frequency shifts greater than 2 GHz were obtained at a 400 V bipolar DC bias at temperatures below 77 K, with minimal degradation in the insertion loss of the filters.