Low-loss acoustic wave filter device

An acoustic wave filter, comprising:

• a substrate of piezoelectric material;
• an electroacoustic interdigital transducer disposed on said substrate and serving as an output transducer;
• a pair of electroacoustic interdigital transducers serving as an output transducer;
• a pair of electroacoustic interdigital transducers serving as input transducers and disposed on said substrate at equal distances on opposite sides of said output transducer, with the transducer fingers of said three transducers parallel to each other and with said three transducers aligned on a common axis and having substantially the same aperture;
• and
• ninety-degree phase shifting means coupled to one of said input transducers;
• wherein one of said input transducers is symmetric and the other is antisymmetric, the two having transfer functions that differ by a ninety-degree phase angle, and wherein said ninety-degree phase shifting means compensates for the phase shift introduced by said antisymmetric transducer and ensures that signals from both of said input transducers reach said output transducer with the same phase angle;
• and whereby signals reflected from said output transducer to said antisymmetric input transducer and back again will return with a 180-degree phase shift and will thereby cancel signals reflected from said output transducer to said symmetric input transducer and back again, and the cancellation effect is relatively independent of frequency.

An acoustic wave filter comprising:

• a substrate of piezoelectric material;
• a symmetric electroacoustic input transducer disposed on said substrate;
• a pair of electroacoustic output transducers disposed on said substrate at equal distances on opposite sides of said input transducer, one of said output transducers being symmetric and the other being antisymmetric, the two having transfer functions that differ by a ninety-degree phase angle and having an aperture substantially equal to that of said input transducer;
• and
• ninety-degree phase shifting means coupled to one of said output transducers, to compensate for the ninety-degree phase shift introduced by said antisymmetric output transducer;
• whereby signals reflected from said antisymmetric output transducer to said input transducer and back again be subject to a 180-degree phase shift and will thereby cancel signals reflected from said symmetric output transducer to said input transducer and back again.