| 摘要 |
PROBLEM TO BE SOLVED: To detect the presence of the crack of a piezoelectric substrate by simultaneously measuring the electrostatic capacity across an input electrode and an earth electrode and that across an output electrode and the earth electrode and calculating the difference value between them. SOLUTION: An earth terminal 29 is connected to an earth electrode 4 and an input terminal 27 is connected to an input electrode 1 and an output terminal 28 is connected to an output electrode 2. The capacity across the input electrode 1 and the earth electrode, that across the output electrode 2 and the earth electrode 4, that across the input electrode 1 and a coupling electrode 3, that across the output electrode 2 and the coupling electrode 3 and that across the coupling electrode 3 and the earth electrode are respectively set to C1, C5, C2, C4 and C3. When the shape of a filter is made symmetric left and right centering around the earth electrode 4 and C1≈C5 and C2≈C4 are set, the respective synthetic capacities across the earth terminal 27, the input terminal 27 and the output terminal 28 are also equal. A low frequency signal is supplied from the earth terminal 29 to measure capacities from the outputs of the input terminal 27 and the output terminal 28 and the difference value between the capacities is calculated to detect the capacity difference between an input side and an output side.
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