Tunable Coaxial Bandpass Filters Based on Inset Resonators

A new class of compact, high-<inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula>, tunable coaxial filter is presented in this article 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 <inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula>-factor 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-<inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> of 4100% ± 4%, spurious-free band up to <inline-formula> <tex-math notation="LaTeX">$3.8\times f_{0}$ </tex-math></inline-formula>, and volume-saving up to 50% when compared with 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. First, 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-<inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> of 1820% ± 6%. Next, an automatically tunable third-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 to 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 <inline-formula> <tex-math notation="LaTeX">$2.8\times f_{0}$ </tex-math></inline-formula>. To our own knowledge, the proposed tuning technique and tunable components represent state-of-the-art tuning range and stable high-<inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> with minimal variation when compared with similar loaded-waveguide designs.