A Non-Electrically-Contacting Tunable Resonator for Wide-Range Bandwidth-Tunable Filter Designs

A class of tunable resonators containing posts (with short ends and open ends) and tuning sleeves for designing wide-range bandwidth-tunable filters with constant center frequencies will be proposed in this presentation. Different from traditional re-entrant coaxial resonators, in the proposed design, the tuning sleeve is driven by a dielectric rod, so that there is no electrical contact between the tuning elements and the cavity. The most interesting feature of this contactless design is the introduction of two load capacitances that change inversely. By carefully adjusting the design parameters, the change in capacitance between the tuning sleeve and the post can cancel out the change in capacitance between the tuning sleeve and the cavity during the tuning process. However, the couplings show significant variations as the tuning sleeve is adjusted. Based on this characteristic, the bandwidths of the designed filters can be tuned over wide ranges while the center frequencies remain almost unchanged. In order to provide a design guideline, a synthesis-based design procedure is first discussed in detail. Several filters are designed according to the theory. As the couplings can be tuned to be very weak, the filters can be switched to the "OFF" state and act as filtering switches. A third-order bandwidth-tunable filter is fabricated and measured as an experimental verification. The measured results demonstrate a wide bandwidth tuning range from 17 to 166.4 MHz (9.8:1, 162.96%) and a good isolation of 80 dB. In addition to the bandwidth-tunable filters, the proposed structure can be adjusted further to design fully tunable filters that incorporate the frequency and bandwidth tuning functions, by making the posts movable. An in-line fourth-order fully tunable filter with independent-tunable center frequency is designed. Frequency tuning while maintaining a CABW can be achieved by tuning only the post positions where the tuning sleeves are fixed. The experimental results well demonstrate the effectiveness of the proposed method.