A piezoelectric double-mode filter using the complex-branch-type energy trapping

A filter using complex-branch-type energy trapping which is caused by contributions from the complex branches of dispersion curves is proposed. Theoretical and experimental results are presented for the width-extensional modes in a thin piezoelectric ceramic strip. Characteristics of the symmetric and antisymmetric trapped-energy modes in this filter have some interesting features that are different from those of the usual trapped-energy modes, such as noticeable undulations in the resonant frequency spectrum and sinusoidally oscillating decay of displacement in the unelectroded region. These results were used to construct double-mode filters with a center frequency of about 1 MHz and a 3-dB fractional bandwidth of 3%