High-Q Tunable Bandpass Filters With a Wide Tuning Range Using a Minimum Number of Tuning Elements

This article reviews the evolution of three-dimensional (3D) tunable microwave bandpass filters over the past decade, providing an outline of the tuning elements employed and their benefits and limitations. The major challenges of realizing 3D tunable filters with a minimum number of tuning elements over a wide tuning range, without compromising the filter performance are addressed. The development of 3D tunable filters has progressed through three stages: i) using tuning elements only for the resonators, ii) using a single tuning element for the entire filter, and iii) exploiting techniques to widen the tuning range while using a single element. The article summarizes the work reported in the literature over the past decade in these three stages.