Small ripple surface acoustic wave filter with low insertion loss

Claim 1

A surface acoustic wave filter comprising substrate means having at least a major surface layer of a piezoelectric material; an input transducer for transforming an electrical input signal into an acoustical signal, said input transducer including a first electrode responsive to an input signal of positive polarity and a second electrode responsive to an input signal of negative polarity, each electrode being recessed into said substrate means to be essentially coplanar with said major surface layer and comprising a plurality of electrode fingers, said positive and negative electrode fingers being interleaved to form an interdigitated electrode finger configuration and disposed to form a plurality of finger groups capable of reducing reflection between adjacent electrode fingers, each group including a set of N equal length finger electrodes of both positive and negative polarity, said input transducer being apodized such that adjacent finger groups comprise electrode fingers of different lengths perpendicular to the direction of travel of a surface acoustic wave, wherein the different lengths are determined in accordance with a predetermined apodization equation (W(t,T)); and an output transducer including a first electrode of positive polarity and a second electrode of negative polarity, each electrode responsive to the acoustic signal generated by said input transducer for providing as an output an associated electrical signal, each electrode being recessed into said substrate means to be essentially coplanar with said major surface layer and comprising a plurality of electrode fingers, said positive and negative electrode fingers being interleaved to form an interdigitated electrode finger configuration and disposed to form a plurality of finger groups capable of reducing reflection between adjacent electrode fingers, each group including a set of M fingers of both positive and negative polarity, M being different from N.