An overview of state-of-the-art frequency tunable technologies in the realization of tunable radio frequency (RF) and microwave tunable circuits is presented with focus on filter designs. Those enabling techniques and materials include semiconductors, micro-electro-mechanical systems (MEMS), ferroelectric and ferromagnetic materials. Various performance indicators of one-dimensional tunable filters are addressed in terms of tunability, losses, signal integrity and other aspects. Fundamental limitations of the classical onedimensional tuning method are discussed, which makes use of only one type of tunable elements such as either electric or magnetic tuning/controlling of circuit parameters. Requirements of simultaneous electric and magnetic two-dimensional tuning techniques are highlighted for achieving an unprecedented and advantageous wider modal tuning. It is believed that this emerging scheme will lead its way in the realization of future highly efficient and tunable RF and microwave components and devices.
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