Spatial Uniformity and Broadband Matching in Multiply Tapped SAW Convolvers

Abstrnct-In surface-acoustic-wave (SAW) convolver4 used to process long-duration waveforms, the output plate electrode behaves like a lossy transmission line and the effects of electromagnetic delay and electrode resistivity must be taken into account in evaluating convnlver characteristics. These effects are characterized by a spatial uniformity or weighting function, which results in a device output that is a weighted, instead of a t rue, convolution. When a single tap is used to extract signal from a long cnnvolver, the amplitude and phase nonuniformities due to electromagnetic effects degrade the convolver performance. A common way of overcoming this spatial uniformity problem is to use many taps to extract the output signal. Although it reduces the effect of nonuniformities to acceptable levels, the multiple tapping solution can increase device insertion loss and decrease bandwidth unless special output-matching netwnrks are incorporated. These transmission line effects can be modeled by a simple equivalent circuit which makes it possible to calculate the spatial uniformity functions fnr both single and multiply tapped cnnvolver geometries. Using realistic values for parasitic elements introduced hy honding pads and wires, a multisection circuit for the multiply tapped structure is assembled and analyzed using ac circuit analysis programs. The characteristics of multiply tapped convolvers are presented with particular emphasis on spatial uniformity, system bandwidth, and insertion loss. Examples of the computer-aided design of appropriate output-matching networks which maximize handwidth and output level a r e given.