Compensation for Diffraction in SAW Filters

Abstract

It is well known that diffraction can cause serious errors in the response of a SAW transversal filter, and that such errors can be reduced by corrections in the filter tap strengths and positions. In the past these tap perturbations have typically been obtained using calculations made at the center frequency only. However, recent fast methods for the calculation of Fresnel diffraction assuming parabolic anisotropy allow iterative algorithms to be practical for optimizing the response. Herein these methods are utilized in an iterative correction method which improves the frequency response by matching bandlimited responses in the tine domain. Trial example designs which initially had severe errors due to diffraction were greatly improved by corrections obtained using this algorithm, whereas the degree of correction achieved using the center frequency method was much less. Both tap position and amplitude corrections are found to be necessary.