| 摘要 |
PROBLEM TO BE SOLVED: To suppress spuriousness which is intrinsic to a vertical mode resonator type surface acoustic wave filter. SOLUTION: This filter includes interdigital transducers(IDT) 4 and 6 at input and output sides, is formed by covering a thin film such as aluminum, etc., on a piezoelectric substrate 8 consisting of a crystal, and is arranged at an interval on the substrate 8. The widths of the respective electrode fingers 12 of IDTs 4 and 6 are 1/4 of the length of surface acoustic wave at resonance. When a reflection coefficient per one electrode finger is defined asεand the total logarithm of the electrode fingers 12 constituting IDTs 4 and 6 at the input and output signal sdes is sides as N, the reflection coefficientεand the total logarithm N are set so as to establish Nε>=0.55. Also, when the opening length of a surface acoustic wave transmission path is W(wavelength), the center frequency of the filter is f(Hz) and the film thickness of the electrode fingers is H(m), the opening length W, the center frequency f and the film thickness of the electrode fingers H are set so as to establish fH <=-17.5W+210.
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