THE JOURNAL OF THE ACOUSTICAL SOCIETY OF JAPAN

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A flexural piezoelectric bimorph vibrator is practically used as a transducer-resonator of a mechanical filter composed of flexural resonators connected by torsional couplers. When a piezoelectric bimorph vibrator is excited electrostrictively, higher harmonics can be excited ; these higher harmonics except the main resonance act as spurious responses for resonators and mechanical filters. One of the important problems in the design of a single-tuned resonator and a mechanical filter using a bimorph vibrator is to suppress the spurious responses. This paper deals with the suppression of spurious responses originated in a piezoelectric bimorph vibrator and its experimental results. In the first place, the method to suppress the higher harmonics is described that the undesired higher harmonics except the main resonance may be eliminated if the surface of piezoelectric ceramics is covered with the electrode varying functionally with the length (Fig. 1(a), (b)). The shape of a function-shaped electrode is decided by the method that the orthogonality property of the characteristic function is counted in the calculation of electric charge excited in the electrode. Namely, the integrated electric charge excited by the higher harmonics becomes zero by virtue of the orthogonality property, because the shape of electrode is the same as the first characteristic function for a clamped-clamped beam and the excitation electric charge is proportional to the characteristic function for a clamped-clamped beam. Secondly, the equivalent circuit constants and capacitance ratio of a piezoelectric bimorph vibrator with function-shaped electrode are shown ; and the results are compared with the data of vibrators having the normal electrode and the partial one. The capacitance ratio is inversely proportional to the electrode width and the capacitance ratio of a bimorph vibrator with a function-shaped electrode is approximately 1. 19 times as much as the one with the partial electrode, but the function-shaped electrode is advantageous to simplify the equivalent circuit of a bimorph vibrator (Fig. 2, Fig. 3). Thirdly, the frequency responses of three bimorph vibrators with three different electrode shapes are shown (Fig. 5). This result shows that the arrangement of a function-shaped electrode to excite only one particular mode is useful for decreasing spurious responses (Fig. 7, Fig. 8). Finally, the response curves of a mechanical filter composed of bimorph vibrators are shown, and it is shown that the transducer-resonator with a function-shaped electrode is advantageous to decrease spurious responses in the filter characteristics (Fig. 9, Fig. 10).

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