Tunable Coaxial Bandpass Filters Based on Inset Resonators

A new class of compact, high-Q, tunable coaxial filters is presented in this paper based on a novel inset resonator concept. The tuning concept is based on the displacement of movable resonators inside a properly modified metallic housing which features wide tuning capabilities and stable high Qfactor performance with minimum variation throughout the tuning window. Various prototypes are designed and implemented to demonstrate and validate the proposed concept. A single tunable inset resonator is first designed and measured showing distinctive results of a 43% tuning range, stable high-Q of 4100±4%, spurious-free band up to 3.8·f0, and volume-saving up to 50% when compared to the conventional combline and half-wavelength structures. The design procedure for constant absolute bandwidth (CABW) tunable filters is then presented, and two different tunable inset filters are designed and implemented. Firstly, a manually tunable four-pole filter is demonstrated with the merits of a wide 39.3% tuning range, while maintaining a constant bandwidth of 116 MHz±6%, and a stable high-Q of 1820±6%. Next, an automatically tunable 3rd-order inset filter is designed and measured using high-accuracy piezomotors. Similarly, the measured results exhibit a wide 1.3 GHz tuning range from 2.65 GHz - 3.95 GHz with a stable insertion loss that is less than 0.35 dB, a return loss that is better than 15 dB, and a good spurious performance up to 2.8·f0. To our own knowledge, the proposed tuning technique and tunable components represent state-of-the-art tuning range and stable high-Q with minimal variation when compared with similar loaded-waveguide designs.


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