| 摘要 |
PURPOSE:To improve the noise resistance characteristics of the instrument by varying the center frequency of the band of the band-pass filter of a receiving circuit by the band-pass filter corresponding to a driving frequency at which a vibrator is driven. CONSTITUTION:An oscillator 1 oscillates at the driving frequency corresponding to the correcting signal of a temperature compensating circuit 15 which inputs the signal of a thermometer 14. A driving circuit 2 drives the vibrator 3 which uses a piezoelectric film according to the oscillation signal and informs a filter circuit 16 of the driving frequency. Then the circuit 16 equalizes the center frequency of the band to the driving frequency. An ultrasonic wave generated by the vibrator 3 is reflected by a liquid surface 4 to be measured to return to the vibrator 3, which converts it into an electric signal; and the signal is amplified by an input amplifier 5 and inputted to the circuit 16 through a resid ual vibration suppressing circuit 6. Here, the frequencies are coincident, so noises are removed. Then the signal is passed through AGC circuits (7, 8 and 9), a waveform shaping circuit 10, and an F/V converting circuit 11 and com pared 12 with a detection threshold value and a generated pulse is inputted to an arithmetic circuit 13. |
