Tunable HTS microwave filters using strontium titanate thin films

High performance HTS microwave filters stand to benefit from the capability to electrically tune their frequency of operation. This function may be performed with tunable dielectric materials such as SrTiO₃ (STO). We have optimized the growth of STO thin films by pulsed laser ablation and by reactive coevaporation. Loss tangents on the order of 0.001 at 65 K are possible, while maintaining acceptable tunability. We have integrated STO films into lumped-element YBCO microwave resonators and filters in order to tune their center frequencies. We describe methods of doing so, the effect of coupling strength, and the performance of these devices.