Two-pole tunable bandpass filter based on YBCO plated single crystal KTO disk resonators

This paper addresses design and experimental characterization of a two-pole 1.5% bandpass tunable filter operating at 0.9 GHz and 77 K. The filter is based on two YBCO plated KTaO₃ disc resonators (0.5-mm-thick, 10 mm in diameter) and utilizes the TM₀₂₀ mode. The tunability is achieved through the nonlinear DC electric field dependence of the relative dielectric constant of KTaO₃. About 5% tuning range is achieved at 500 V driving voltage. Although the insertion losses are relatively high (about -5 dB at 500 V) the shape of the passband is essentially stable. A rather good agreement between simulated and experimental performances is found.