| 摘要 |
PROBLEM TO BE SOLVED: To improve the group delay time ripples of a two-step cascaded transversal SAW(surface acoustic wave) filter by making the transmission time and center frequency of 1st and 2nd filters satisfy each specific condition. SOLUTION: IDT electrodes 2 and 3 are placed on the primary surface of a piezoelectric substrate 1, in the transmitting direction of a surface wave and with a distance L1 (1st transmission line) secured between the centers of both electrodes. Meanwhile, IDT electrodes 4 and 5 are placed separated by a distance L2 (2nd transmission line) secured between the centers of both electrodes, so as not to cause the mutual interference with the electrodes 2 and 3 respectively. The cycles of electrodes 2 to 5 are set at P, and the difference between the 1st and 2nd transmission lines (L2-L1) is set at 1/4 of IDT electrode cycle P. Then|T1-T2|=1/(4f0 )±n/(2f0 )±1/40 (n=0, 1, 2, etc.), is defined, where T1 and T2 are the transmission times of the 1st and 2nd filters respectively and f0 is the center frequency for both filters.
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