Surface acoustic wave filters.

摘要

A surface acoustic wave filter comprising: a first bidirectional ID transducer (5) of symmetrical form; a second ID transducer (1) acoustically coupled with the first transducer and comprising two parts (1A, 1B) each of which parts is in the form of one half of a respective bidirectional ID transducer (9 or 11) of symmetrical form, the two parts being positioned with respect to one another and the first transducer so that when respectively excited by corresponding signals of different phases they co-operate to produce a unidirectional acoustic signal propagating towards the first transducer from one side thereof; and a third transducer (3) acoustically coupled with the first transducer and comprising two parts (3A, 3B), each of which parts is electrically connected in parallel with a respective one of the two parts of said second transducer, and each of which parts of the third transducer is effectively in the form of the mirror image about the centre line (7) of the first transducer of the other half of the bidirectional transducer whose one half constitutes the part of the second transducer with which it is connected. The invention provides a filter of intrinsically low insertion loss without the use of a cross-over structure as is required in known forms of low loss filter using unidirectional multiphase ID transducers.