Electrically-Tunable Ceramic-Waveguide Filter

Ceramic waveguide filters have been popular in 5G wireless system applications. This article proposes an effective solution to enable the ceramic waveguide filter to be electrically tunable. In detail, a new varactor-tunable structure is developed to electrically control the resonant frequency (TE101 mode) of the ceramic waveguide resonator. The novel coupling and feeding configurations are investigated, thus allowing the coupling coefficient and external quality factor $(Q_{e})$ to be flexibly specified according to the prescribed matrix. The element variable coupling matrices (EVCMs) for two-pole, three-pole cascade triplet (CT), four-pole cascade quadruplet (CQ), six-pole CT, and six-pole CQ filters with constant bandwidth are prescribed as the electrically-tunable prototypes. All the designs for the lossless filters are presented, and design results in agreement with the theory ones demonstrate the correctness of the proposed solution. Two six-pole tunable CQ and CT filters are fabricated and assembled with the commercial-available varactors. The measurement results show that the quasi-elliptic responses of two six-pole ceramic filters can be tuned from 3.4 GHz to 3.8 GHz by adjusting the tuning voltage. Finally, the loading impact of the varactors on the unloaded quality factor $(Q_u)$ of the filter is further discussed. A fixed filter and an additional tunable filter are implemented and used to verify the investigation.