| 摘要 |
PROBLEM TO BE SOLVED: To miniaturize the form size of a composite double mode filter by removing a part of the whole surface electrode in the neighborhood of the center of two double mode piezoelectric filters adjacent to each other. SOLUTION: One main surface of a flat piezoelectric substrate 1 is recessed to form a thin-thickness oscillation part 2 and electrodes 3a, 3b of nearly the same size are closely formed at a prescribed interval in a flat part on a side opposed to the recess to form the divided electrodes. Reed electrodes 4a, 4b are respectively extended toward the end of the substrate 1 from the divided electrodes 3a, 3b. It is the feature of this filter that a gap between backside electrodes 3c and 5c, namely the width of an area including no electrode, along a center boundary adjacent to two sets of double mode filters A, B is made a gap W. In spite of opening the gap W without setting the backside electrode to be the whole surface electrode, the electrode 3c does not contribute to an effect similar to that when the whole surface electrode exists on the backside of the substrate, namely an energy confining effect by an electrode mass, but only the mass of the electrode 3a on the surface contributes to it.
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