A solution method based on the coupled‐mode theory for natural single‐phase unidirectional surface acoustic wave transducers

Abstract

A method for analysis of natural single‐phase unidirectional transducer (NSPUDT), which is one of unidirectional surface acoustic wave interdigital electrodes, is proposed in which the mode coupling theory and the finite‐element method are used. the finite‐element method is used for calculations of the upper and lower limit frequencies of the stop band, the potential standing wave distribution at these frequencies, and the static capacitance per electrode pair for the short and open gratings corresponding to the electric terminals either shorted or open. In the substrate used for NSPUDT, the locations of the maximum and minimum of the standing wave in general vary in the depth direction of the substrate. Hence, it is not possible to use the standing wave pattern in the substrate as is used for a substrate surface where electrodes are present. the variation of the potential in the depth directional of the substrate is incorporated into the mode coupling theory in this research, so that the depth dependence of the standing wave profile is qualitatively evaluated. Hence, it is made possible to confirm the locations of the maximum and minimum of the standing wave at the substrate surface from the standing wave profile in the substrate. Specifically, the present method is applied to NSPUDT on an ST‐cut 25deg; X‐propagation quartz substrate so that its validity and usefulness are confirmed.