| 摘要 |
PURPOSE: To avoid the formation of unwanted spectrum and the generation of noise by controlling control voltage so that the amplitude of an ultrasonic sensor signal is damped in accordance with a specified envelope curve to damping duration. CONSTITUTION: A generator 4 is provided with a source 20 of low ohmic resistance besides a source 1 of high ohmic resistance. The source 20 is parallel- connected to the output side of the source 1 through a diode 21, and the output side of the generator 4 is connected to a control part 3. In the first part of damping connection time of a damping curve (envelope curve) obtained by this circuit, voltage drops to the voltage value U1 until a time point t1, but when voltage is supplied to a capacitor C2 by the source 20 through the diode 21 at this time, the envelope curve becomes almost constant from time t1 to time T, and amplitude in this time interval becomes markedly smaller than the maximum value of amplitude. output is thereby reduced, and radiated unwanted spectrum does not generate noise.
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