Investigations of Surface Acoustic Wave Directional Couplers

Abstract

Three types of directional couplers based on guided surface acoustic wave (SAW) technology have been de- signed, fabricated, and investigated. These types of devices have been studied both theoretically and experimentally at fre- quencies as low as 4 MHz and as high as 100 MHz. The SAW guidance technique employed was gold strip guides on fused and crystalline quartz. This method of guidance was the basis for the three types of couplers reported on here: (1) separated double guide directional coupler based on mode coupling, (2) the contiguous double guide directional coupler whch is the limiting case of type (1) with zero separation, and (3) the Y- junction directional coupler, a symmetric pseudo-four port non-coupled mode device. The separated double guide coupler in principle allows the attainment of arbitrary coupling but the bandwidth is narrow. The contiguous device offers increased bandwidth but a reduced coupling range. The Y-junction coupler gives a 3 dB power split with broad bandwidth. The theoretical determination of coupling lengths, dispersion char- acteristics, bandwidths, and output arm power ratios as func- tions of frequency and guide dimensions have been confirmed experimentally. In addition experiments have determined loss and directivity of the devices. These studies demonstrate that an important circuit function-signal splitting or channelling- can be realized with guided surface acoustic waves. This cir- cuit function is essential for the ultimate realization of a fully integrated surface acoustic wave technology.