Multilayer Planar Tunable Filter With Very Wide Tuning Bandwidth

In this paper, we propose and develop a partially shielded tunable filter structure using a varactor-loaded split-ring resonator. The novel physical topology of the filter is suitable for vertical stacking of filters and enables a compact multilayer tunable filter for applications requiring a very wide tuning range. The capacitance range and ratio needed to achieve the designated tuning range are presented, and analytical solutions are given for the second and third higher order modes for synthesis in the spurious-free filter operation range. Two prototypes are developed. A partially shielded tunable filter using varactor-loaded split-ring resonators is developed to verify the design method and to demonstrate the wide tuning range of a tunable filter. To further extend the tuning range, a filter bank with three stacked tunable filters at different frequency bands are designed and developed. With slight overlap between neighboring bands, a continuous tuning range of more than 6:1 is achieved.