A new SAW convolver which utilizes focused interdigital transducers for beamwidth compression has been demonstrated. Operating at a center frequency of 160 MHz, the device has a terminal convolution efficiency FT = -70 dBm. Here the results of theoretical design considerations together with experimental results from the prototypical device are presented. S I . INTRODUCTION URFACE acoustic wave convolvers have warranted much attention in recent years due to their ability t o perform sophisticated high-speed signal processing. In particular, relatively low-cost versions of these devices have been fabricated utilizing the inherent nonlinear elastic properties of piezoelectric substrates. The most widely used of these piezoelectric substrates has been lithium niobate. Although LiNb03 has a relatively high electromechanical coupling coefficient, its elastic nonlinearities are rather weak, necessitating the use of beamwidth compression techniques in order t o achieve reasonable convolver performance. Beamwidth compression is used in order to obtain highpower densities in the nonlinear interaction region without sacrificing the device bandwidth. Narrow interdigital transducers may be used to generate acoustic beams with highpower densities [ I ] , but these have the limitation that the impedance levels are very high. This makes it difficult to operate the transducers over a sufficiently large bandwidth, although recently narrow chirped transducers have been used with greater success [2] . Various forms of beamwidth compression have been utilized in the design of the elastic convolver [3] , [4] , [5] with the most developed designs incorporating either the multistrip coupler [3] or the parabolic horn compressor [4]. We report here on the use of the focused interdigital transducer as a means of beamwidth compression and report experimental results on a LiNbO, elastic convolver. The basic geometry of the focused transducer elastic convolver is shown in Fig. 1. The focused interdigital transducers and top-plate/waveguiding electrode are defined as a thin film metallization on the piezoelectric substrate. Electrical signals input into each of the focused transducers are converted into surface acoustic waves which counterpropagate each toward Manuscript received July 20, 1982;revised September 27, 1982. This work was supported by the Office of Naval Research under Contract N00014-75-C-0632. .l. R. Green was with Edward L. Ginzton Laboratory. W . W . Hansen Laboratories of Physics. Stanford University. Stanford, CA 94305. He is now with MIT Lincoln Laboratory. Lexington, MA 02173. G. S. Kino is with Edward L. Ginzton Laboratory, W. W. Hansen Laboratories of Physics, Stanford University, Stanford, CA 94305. ,--AV/V WAVEGUIDE METALLIZATION \\ AND TOP-PLATE LECTRODE

SAW Convolvers Using Focused Interdigital Transducers

It is now well e stablished that t he elastic convolver is a simple, compact, and inexpensive structure which promises to perform, with real-time programmability, the matched-filterin g f unction for waveforms of time-bandwidth product greater than 1000. Last year at this conference, we and others described convolvers which provided the major features of programmable matched filters but without the accuracy that is needed for real applications. This paper treats the design of an improved elastic convolver which provides, for the first time, the necessary amplitude, phase and isolation accuracy that is required in a 2200-time-bandwidth-product filter. This device has an efficiency of -69 dBm. A number of second-order effects have been suppressed, most notably the self-convolution , t he long-line effect of the output circuit and the phase dispersion. The emphasis of the discussion will be on (1) the dual-track self-convolution suppression technique, (2) the simp1 ified dualtrack output-combining circuit which minimizes the component count without sacrificing performance and (3) a transducer design with built-in compensatory dispersion.

http://ieeexplore.ieee.org/iel5/10281/32714/01534546.pdf

High Performance Elastic Convolver with Extended Time-Bandwidth Product

Acoustoelectric (AE) convolvers for spread-spectrum communication applications are described with input bandwidth capacities to 200 MHz. These devices offer a unique combination of large processing gain, high dynamic range, small size and weight, and low drive power requirements. The programmable feature of convolvers allows the encoding waveform to be changed from bit-to-bit, thereby providing resistance to repeat jamming and enabling secure communications. The basic concepts of a convolver-based spread-spectrum communications system are reviewed, current convolver capabilities are discussed, and projections are made for future device performance. Deviations from nonideal convolver performance are considered. Special techniques which must be used in the system implementation and evaluation of convolvers are described, and the performance level achieved in a state-of-art convolver subsystem is given.

Acoustoelectric Convolver Technology for Spread-Spectrum Communications

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

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

Disclosed is an improved SAW monolithic convolver using dispersive interdigital transducers designed by a novel technique that allows systematic compensation of phase errors arising in other parts of the convolver. The invention involves a small change in the positional relationship among the electrodes in the input transducers. This change is calculated to be just sufficient to cause a phase error exactly equal to, but of opposite sign to, the aforementioned phase errors.

Saw monolithic convolver using dispersive transducers

http://ieeexplore.ieee.org/iel5/10279/32712/01534241.pdf

R.A. Becker

Papers

Wideband LiNbO 3 Elastic Convolver with Parabolic Horns