Structure of surface acoustic wave filter

A surface acoustic wave filter having an input terminal and an output terminal, said filter comprising:

• a piezoelectric substrate for propagating a surface acoustic wave and having an electromechanical coupling coefficient k.sup.2 ;
• a plurality of first surface-acoustic-wave exciting transducers connected to the input terminal and formed on said piezoelectric substrate in a first direction of propagation of a first surface acoustic wave;
• a plurality of first surface acoustic wave receiving transducers electrically isolated from said first surface acoustic wave exciting transducers and formed on said piezoelectric substrate in the first direction of propagation so that said first surface acoustic wave receiving transducers and said first surface acoustic wave exciting transducers are interdigitated;
• a plurality of second surface acoustic wave exciting transducers electrically connected to said first surface acoustic wave receiving transducers respectively and formed on said piezoelectric substrate in a second direction of propagation of a second surface acoustic wave;
• and
• a plurality of second surface acoustic wave receiving transducers electrically isolated from said second surface acoustic wave exciting transducers, and formed on said piezoelectric substrate in the second direction of propagation so that said second surface acoustic wave receiving transducers and said second surface acoustic wave exciting transducers are interdigitated, connected to the output terminal;
• wherein
• one of said first surface acoustic wave receiving transducers and said second surface acoustic wave exciting transducers each comprise first, second, and third interdigital transducers having electrode finger pairs in numbers of N1, N2 and N3, respectively;
• said finger pairs being formed on said piezoelectric substrate in at least one of the first and second direction of propagation in order of the first, second and third interdigital transducers;
• said first interdigital transducer being electrically connected to said third interdigital transducer in parallel;
• said second interdigital transducer being electrically connected to each of said first and third interdigital transducers in series;
• and
• wherein said numbers of electrode finger pairs N1, N2 and N3 being related so that 0.5.times.N2<(N1+N3)<3.times.N2, and so that 2/k.sup.2 .gtoreq.(N1+N2+N3).gtoreq.0.8/k.sup.2.

A surface acoustic wave filter having an input terminal and an output terminal, comprising:

• a piezoelectric substrate for propagating a surface acoustic wave and having an electromechanical coupling coefficient k.sup.2 ;
• a plurality of first surface acoustic wave exciting transducers connected to the input terminal and formed on said piezoelectric substrate in a first direction of propagation of a first surface acoustic wave;
• a plurality of first surface acoustic wave receiving transducers electrically isolated from said first surface acoustic wave exciting transducers, and formed on said piezoelectric substrate in the first direction of propagation;
• a busbar;
• said first surface acoustic wave receiving transducers and said first surface acoustic wave exciting transducers being interdigitated and electrically connected to said busbar for temporarily equalizing their potentials;
• a plurality of second surface acoustic wave exciting transducers electrically connected to said busbar and formed on said piezoelectric substrate in the second direction of propagation;
• a plurality of second surface acoustic wave receiving transducers electrically isolated from said second surface acoustic wave exciting transducers, respectively and formed on said piezoelectric substrate in the second direction of propagation;
• said second surface acoustic wave receiving transducers and said second surface acoustic wave exciting transducers being interdigitated and connected to the output terminal;
• one of said first surface acoustic wave receiving transducers and said second surface acoustic wave exciting transducers each comprise first, second and third interdigital transducers having numbers of electrode finger pairs N1, N2 and N3, respectively and being formed on said piezoelectric substrate in one of the first and second directions of propagation in order of the first, second and third interdigital transducers;
• said first interdigital transducer being electrically connected to said third interdigital transducer in parallel;
• said second interdigital transducer being electrically connected in series to said first and third interdigital transducers;
• and
• said numbers of electrode finger pairs N1, N2 and N3 being related to so that 0.5'N2<N1+N3<3'N2 and so that 2/k.sup.2 .gtoreq.(N1+N2+N3).gtoreq.0.8/k.sup.2.

A surface acoustic wave filter having an input terminal and an output terminal, comprising:

• a piezoelectric substrate for propagating a surface acoustic wave and having an electromechanical coupling coefficient k.sup.2 ;
• a plurality of surface acoustic wave exciting transducers connected to the input terminal and formed on said piezoelectric substrate in a direction of propagation of the surface acoustic wave;
• a plurality of surface acoustic wave receiving transducers electrically isolated from said surface acoustic wave exciting transducers connected to the output terminal, respectively, and formed on said piezoelectric substrate in the direction of propagation;
• said surface acoustic wave receiving transducers and said surface acoustic wave exciting transducers being interdigitated and connected to the output terminal;
• said surface acoustic wave exciting transducers and said surface acoustic wave receiving transducers each comprise first, second and third interdigital transducers having numbers of electrode finger pairs N1, N2 and N3, respectively, and which pairs are formed on said piezoelectric substrate in the direction of propagation in order of said first, second, and third interdigital transducers;
• said first interdigital transducer being electrically connected to said third interdigital transducer in parallel;
• said second interdigital transducer being electrically connected in series with said first and third interdigital transducers;
• and
• said numbers of electrode finger pairs N1, N2 and N3 being related so that 0.5'N2<(N1+N3)<3'N2 and so that 2/k.sup.2 .gtoreq.(N1+N2+N3).gtoreq.0.8/k.sup.2.