Patent ID: 6335667
Filing Date: 2002-01-01
Classification: H03H

Abstract:
A multi-longitudinal-mode-coupled resonator type SAW filter comprising, on a piezoelectric substrate:a first interdigital transducer that excites a surface acoustic wave; a second interdigital transducer that receives the surface acoustic wave excited by the first interdigital transducer; a third interdigital transducer interposed between the first interdigital transducer and the second interdigital transducer that controls an amplitude of the excited surface acoustic wave; and a pair of reflectors arranged with the first interdigital transducer, the second interdigital transducer and the third interdigital transducer interposed therebetween in a direction of propagation of the surface acoustic wave, the reflectors, the first interdigital transducer, the second interdigital transducer and the third interdigital transducer being formed of parallel metallic conductors periodically arranged on the piezoelectric substrate, a distance between a reflector and a closest parallel conductor of the first interdigital transducer being set to be equal to a width of a spacing of the first interdigital transducer with one period length thereof having alternating spacing and line, a distance between a reflector and a closest parallel conductor of the second interdigital transducer being set to be equal to a width of a spacing of the second interdigital transducer with one period length thereof having alternating spacing and line, each period length PT1 of the parallel conductors of the first interdigital transducer and period length PT2 (=PT1) of the parallel conductors of the second interdigital transducer being set to be shorter than a period length PT3 of the parallel conductors of the third interdigital transducer (PT3>PT1, PT2), each of the period length PT1 and the period length PT2 being set to be shorter than a period length PR of the parallel conductors of the reflectors, the first interdigital transducer, the second interdigital transducer and the third interdigital transducer having a total reflective coefficient &Ggr; set to be 10>&Ggr;>0.8, and being energy trapped type resonators, and a two-longitudinal-mode-coupled resonator filter being formed of a fundamental wave symmetrically longitudinal mode S0 having a displacement amplitude function generally symmetrical with respect to a central position in the direction of propagation X in which the surface acoustic wave is standing, and a fundamental wave anti-symmetric mode A0 having a displacement amplitude function generally anti-symmetrical with respect to the central position.