Surface wave acoustic device with compensation for diffraction effects

In a surface acoustic wave device of the type including a first acoustic path, sending transducer means positioned to emit a surface acoustic signal on said path and a diffracted surface acoustic wave at an angle to said path, a second acoustic path offset laterally from said first path, means coupling said paths to transfer said signal from said first path to said second path, and receiving transducer means positioned to receive said signal after it has been transferred to said second path and to receive said diffracted wave arriving directly from said sending transducer means;

• the improvement comprising:
• compensating transducer means positioned to receive said diffracted wave and including finger pairs having respective lengths interdigitated with each other whereby to function as respective elemental electro-acoustic transducers;
• said interdigitated lengths of said finger pairs being oriented substantially parallel to the wave fronts of said diffracted wave;
• and an electrical circuit including means electrically connecting said receiving and compensating transducer means so that their instantaneous outputs add phasorially;
• said compensating transducer means being positioned relative to said receiving transducer means so as to be substantially in electrical phase opposition thereto whereby the response of said compensating transducer means to said diffracted wave tends to compensate for the response of said receiving transducer means thereto.

An apodized surface acoustic wave transducer having a plurality of pairs of fingers and defining an active area within which the fingers of each said pair are interdigitated and an inactive area within which they are not interdigitated;

• first and second primary bus bars electrically connected to first and second fingers respectively of each said pair;
• a window region within said inactive area in which said interdigitated lengths of said first fingers are disconnected from said first primary bus bar;
• means including a first auxiliary bus bar electrically connecting said disconnected interdigitated lengths of said first fingers of said receiving transducer means to said first primary bus bar;
• compensating transducer means located within said window region;
• said compensating transducer means having interdigitated finger pairs;
• a first finger of each said finger pair of said compensating transducer means being electrically connected to said first primary bus bar;
• and means including second auxiliary bus bar electrically connecting a second finger of each said finger pair of said compensating transducer means to said second primary bus bar.

A surface acoustic wave filter having a passband and trap frequencies above and below said passband respectively;

• said filter bedng adapted to be employed in an environment where it receives a surface acoustic signal including a desired frequency component within said passband, said desired frequency component having wave fronts substantially parallel to a selected direction, and respective undesired frequency components at said trap frequencies, at least one of said undesired frequency components having wave fronts at a substantial angle to said selected direction;
• said filter comprising;
• a primary array and a secondary array of finger pairs, each of said pairs having respective lengths interdigitated with each other whereby to function as respective elemental electro-acoustic transducers;
• said interdigitated lengths of said finger pairs of said primary array being oriented substantially parallel to said selected direction, and those of said secondary array being oriented substantially parallel to said wave fronts of said one undesired frequency component;
• and an electrical circuit including means electrically connecting said primary and secondary arrays so that their instantaneous outputs add phasorially;
• said secondary array being positioned relative to said primary array so as to be substantially in electrical phase opposition thereto whereby the response of said secondary array to said one undesired frequency component tends to compensate for the response of said primary array thereto;
• said primary array being substantially more responsive to said one undesired frequency component than to the other of said undesired frequency components;
• said secondary array being so arranged as to minimize its response to said other undesired frequency component, at which compensation is unnecessary and undesirable.

A surface acoustic wave filter having a passband and trap frequencies above and below said passband respectively;

• said filter being adapted to be employed in an environment where it receives a surface acoustic signal including a desired frequency component within said passband, said desired frequency component having wave fronts substantially parallel to a selected direction, and respective undesired frequency components at said trap frequencies, at least one of said undesired frequency components having wave fronts at a substantial angle to said selected direction;
• said filter comprising:
• a primary array and a secondary array of finger pairs, each of said pairs having respective lengths interdigitated with each other whereby to function as respective elemental electro-acoustic transducers;
• said interdigitated lengths of said finger pairs of said primary array being oriented substantially parallel to said selected direction, and those of said secondary array being oriented substantially parallel to said wave fronts of said one undesired frequency component;
• and an electrical circuit including means electrically connecting said primary and secondary arrays so that their instantaneous outputs add phasorially;
• said secondary array being positioned relative to said primary array so as to be substantially in electrical phase opposition thereto whereby the response of said secondary array to said one undesired frequency component tends to compensate for the response of said primary array thereto;
• said secondary array being so arranged as to minimize its response to said desired frequency component.