Novel Tunable Bandpass Filter Realized Using Barium–Strontium–Titanate Thin Films

A novel three-pole electrically tunable narrow bandpass filter has been fabricated from bi-layer YBCO/Ba_0.05Sr_0.95TiO₃ thin films. At 77 K, and without bias, the filter has a center frequency of 3.50 GHz and 5.8% bandwidth. Independent application of dc bias to each of the resonators of 120, 182, and 100 V shifted the center frequency to 3.68 GHz. The insertion loss was less than 1.4 dB and return loss was better than 20 dB. A similar filter was made using bi-layer Ag/Ba_0.5Sr _0.5TiO₃ thin films and measured at room temperature. The insertion loss of the filter was, as expected, much higher, although it showed improved tunability as compared with the YBCO version.