| 摘要 |
<p>An ultrasonic anti-theft device is constituted by a transducer-emitter with its excitation circuit, provided in order to establish a substantially stationary acoustic field inside the vehicle. The transducer-receiver is equipped with its receiver circuit. It is followed by a variable-gain amplifier (22), then by a rectifier (23), with, in parallel with these, a servocontrol loop (24,25) for the gain of the amplifier (22), this loop being of a supercritical characteristic. The output voltage from the rectifier (23) can then be the subject of detection by variation (26), then of shaping by a pulse generator (27). The pulse signal available next makes it possible to establish the alarm by a simple counting of pulses. <IMAGE></p> |
