The Excitation and Detection of Surface-Generated Bulk Waves

Abstract

Absrruct-The performance of a conventional pair of surface acoustic wave interdigital transducers (IDT’s) used to generate bulk acoustic waves is analyzed. An IDT located on the upper surface of a thick, rotated Ycut quartz crystal acts as the acoustic equivalent of an array antenna, launchingacollimated beam into the crystal bulk. The beam is reflected off the crystal bottom and redirected towards the upper crystal surface where it is intercepted by a second receiving IDT. Closed-form expressions are derived for the important electrical and acoustical delay line characteristics and comparison is made with experimental measurements. The analyzed characteristics of the reflected bulk wave (RBW) delay line are significantly different from either the SAW or SSBWand include a) a frequency bandwidth directly proportional to transducer length, b) a center frequency determined not by finger-to-finger spacing but rather by the ratio of transducer separation to crystal thickness, c) a nonresonant radiation resistance, d) the capability to synthesize filter designs in which the frequency response is proportional to the correlation of the individual transducer finger weightings, and e) the capability to modify temperature stability by change of transducer separation and crystal thickness. ULK ACOUSTIC wave radiation generated by interdigital B transducers has historically been considered as spurious, often degrading or interfering with the performance of devices based upon surface acoustic wave (SAW) excitation. Within the last several years, however, there has been increased interest in utilizing the bulk waves generated by IDT’s for practical microwave applications. Such surface-generated bulk waves make use of the IDT as an acoustic equivalent of a phasedarray antenna, with the electrode fingers constituting the array elements. The phasing between these elements is controlled by the applied signal frequency thereby allowing the direction of the acoustic beam radiated into the bulk to be scanned.