Tunable filters using wideband elastic resonators

Currently, an ultra-wideband resonator is greatly needed to realize a tunable filter with a wide tunable range, because mobile phones with multiple bands and cognitive radio systems require such tunable filters to simplify their circuits. Although tunable filters have been studied using SAW resonators, their tunable range was insufficient for the filters even when wideband SAW resonators with a bandwidth of 17% were used. Therefore, the fabrication of wider-bandwidth resonators has been attempted with the goal of realizing tunable filters with wide tunable ranges. In this study, an SH₀- mode plate wave resonator in a 27.5°YX-LiNbO₃ plate with an ultra-wide bandwidth of 29.1%, a high impedance ratio of 98 dB, and a high Q (Q_r = 700 and Q_a = 720) was realized. Two types of tunable filters were constructed using such SH₀-mode resonators and capacitors. As a result, tunable ranges (bands) of 13% to 19% were obtained. The possibility of applying the SH₀-mode resonator in the high-frequency gigahertz range is discussed.