Microwave characterization of ferroelectric thin films for novel compact tunable BST filters

A microwave characterization of (Ba,Sr)TiO₃ (BST) thin films of different thickness has been carried out by means of a fully comprehensive equivalent circuit applied to experimental data of test capacitors. The voltage-dependent material properties (permittivity, loss tangent, and tunability) have been extracted and used in the development of a specific integrated MIM capacitor that, after a first fabrication test, shows a measured tunability of 72% under 0-15 V bias. This varactor has been exploited in the design of a novel tunable filter implemented by using lumped elements coupled resonators, thus allowing a compact size with respect to conventional λ/4 resonator lines. The analysis of some preliminary experimental results shows a tuning of 88% (657 MHz-1235 MHz) of the filter center frequency and an almost constant fractional bandwidth of 30% upon the application of a 0-15 V bias. The insertion loss is between 3.2 and 5.8 dB, and the return loss is better than 9 dB.