Small-Aperture Focusing Chirp Transducers vs. Diffraction-Compensated Beam Compressors in Elastic SAW Convolvers

Abstmct-High performance has been achieved in SAW convolvers using diffraction-corrected multistrip beam compressors. However, two different levels of metalization are required for optimum performance. A significant simplification of device fabrication was achieved with the design of convolvers using small-aperture chirp transducers, which require only one metalization layer. A design procedure is described that makes the transducers focusing to minimize diffraction loss. Experimental results are presented, which show that performance of this new design surpasses conventional designs in several respects. I. INTRODUCTION M ONOLITHIC acoustic surface wave convolvers on YZ-LiNb03 combine high-speed signal processing capability with ruggedness, small size, and relatively low cost of the device-thanks to the simplicity of its construction. However, the intrinsic nonlinearity of the substrate material polarization, which is used to generate the mixing product of the two input signals, is low and limits the efficiency of the device. High efficiency is crucial to achieve high dynamic range and economize input power. Since the introduction of beamwidth compression [l] to boost device efficiency, the elastic convolver has been developed to high performance in several laboratories [2], [3], [4] by optimization of its various components. The aim of such optimization was to increase the device efficiency by eliminating sources of loss in the acoustic path and providing good match at all electrical ports, while tailoring the device characteristics to minimize signal distortion and suppress spurious signals [5]. This means that the convolution efficiency vs. frequency should be reasonably flat and smooth in magnitude and linear in phase over the desired bandwidth. Notably reflections of acoustic waves in the device have to be suppressed to avoid selfconvolution of one input signal. The distributed output signal has to be collected from the integrating electrode in such a way as to avoid electromagnetic propagation loss or destructive interference of various signal components due to long-line effects. By these considerations one arrives at a standard design scheme as shown in Fig. 1. Interdigital transducers, which convert the input signals, typically are unapodized to avoid