| 摘要 |
An apparatus for detecting an alcohol content of a liquid with high accuracy regardless of variance in a resonance frequency of a parallel resonance circuit constituted by inductance of a coil, capacity presented by a fluid within a liquid passage of a sensor unit and others. High-frequency voltage signals appearing at both ends of a resistor of a detector circuit connected in series to the coil are compared in respect to phase by a phase comparator, wherein difference signal resulting from the comparison is supplied to a voltage-controlled oscillator through a comparison integrator, the oscillation signal Svco of the oscillator being supplied to one end of the resistor through an amplifier. Through this phase synchronization loop, the oscillation frequency of the oscillator is forced to coincide with the resonance frequency, whereby a signal Sout having a frequency bearing one-to-one correspondence to the resonance frequency is obtained from a frequency divider. By providing a frequency division ratio adjusting circuit for adjusting a frequency division ratio of the frequency divider, alcohol content can be detected with high frequency regardless of apparatus-dependent variance in the resonance frequency.
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