| 摘要 |
PROBLEM TO BE SOLVED: To provide a surface acoustic wave filter which is available over a wide range without deteriorating the insertion loss by increasing the width of the electrode fingers that are most adjacent to each other. SOLUTION: The input and output interdigital electrodes (IDT) 12 and 13 are formed opposite to each other on a piezoelectric substrate 11. Then both IDT 12 and 13 are held between the grating reflectors 14 and 15, so that the surface waves excited by the IDT 12 and 13 are shut up between both reflectors 14 and 15. The electrode fingers 16 and 17 are most adjacent to each other between both IDT 12 and 13 at the IDT center part, and the inter-center distance AL between both electrodes 16 and 17 is set larger thanλ/2 (λ: electrode cycle). At the same time, the width of both fingers 16 and 17 are increased and therefore the distance (gap) between both fingers, i.e., a part (a) where no electrode finger exists is set atλ/4. As a result, the propagating surface wave is not converted into a bulk mode from a surface wave mode and therefore a band range can be increased with no insertion loss.
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