Energy trapping of backward‐wave width‐extensional vibrations in piezoelectric strips and its application to filters

Abstract

For the purpose of extending the useful range of trapped‐energy resonators and filters up to the middle band less than a few MHz, the energy trapping of the width‐extensional vibrations in a thin piezoelectric strip is studied. Since the width‐extensional vibration exhibits a dispersion characteristic of the high‐cut type in most piezoelectric materials, the backward‐wave energy trapping needs to be applied. As realistic methods, two approaches are discussed. In one of them, the areas on both sides of the excitation part at the center of the strip are short‐circuited by the electrode. In the other method, this portion is made unpolarized. In each approach, the energy trapping of the width‐extensional vibration is realized if the electrode in the excitation part is made a partial electrode with its width narrower than the strip width. A clean resonant response with few unwanted modes is obtained. A method for fabrication and experimental results are reported for a ladder filter and a single‐mode three‐terminal filter. Effectiveness of this device for a filter in the middle band is indicated.