| 摘要 |
PROBLEM TO BE SOLVED: To provide an SAW(surface acoustic wave) filter which has a wide band, a small loss and the sharp damping characteristic by placing grating electrodes having each specific number of digits among three IDTs of almost equal logarithms of a primary/tertiary vertically coupled triple mode filter. SOLUTION: A vertically-coupled tertiary mode SAW filter is excited intensively in the vertical primary, tertiary and quintuple modes, since the surface waves excited by the IDT 2, 3 and 4 are confined between reflectors 5a and 5b and acoustically coupled together. The band width of the SAW filter is substantially decided by the frequencies of the vertical primary and quintuple modes. The almost equal electrode logarithms are set among the IDT 2, 3 and 4, and a ratio between the total electrode digit number L of IDTs and the total electrode digit number M of gratings 6 is set as 0.25</M/L<0.80. Then the aluminum electrode film thickness is selectively set 3% to 12% (desirably 6%) of a wavelengthλof an exciting surface wave, and the center spaceαis set asα=λ/4 between the innermost electrode digits facing each other of the gratings 6a and 6b. As a result, the sharpness of damping characteristic is improved greatly for the SAW filter.
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