Patent ID: 7762124

Claim:
A sensor for detecting a substance in liquid, the sensor comprising: a sensing oscillation circuit including a sensing surface acoustic wave element and a first amplifier circuit, the sensing surface acoustic wave element including a piezoelectric substrate, at least one interdigital transducer disposed on the piezoelectric substrate, and a reaction film arranged so as to cover the interdigital transducer and to react with a substance in liquid, the first amplifier circuit being connected to the sensing surface acoustic wave element; and a reference oscillation circuit including a reference surface acoustic wave element and a second amplifier circuit, the reference surface acoustic wave element including a piezoelectric substrate, at least one interdigital transducer disposed on the piezoelectric substrate, and including no reaction film, the second amplifier circuit being connected to the reference surface acoustic wave element; wherein the piezoelectric substrate of the sensing surface acoustic wave element and the piezoelectric substrate of the reference surface acoustic wave element are made of substantially the same piezoelectric material, and, where an electromechanical coupling coefficient of each of the piezoelectric substrates is k 2 , an oscillation frequency of the sensing oscillation circuit and an oscillation frequency of the reference oscillation circuit are separated by at least about 200×k 2 .