| 摘要 |
<p>Method and Circuitry for the Function Control of Ultrasonic Alarm Systems In an ultrasonic alarm system, in which ultrasonic waves are continuously radiated within a surveilled enclosure (R), and a frequency deviation caused by a moved object e.g. an intruder, is by virtue of the Doppler Effect determined to be cause for an alarm, the function control of the system occurs by means of modulating the emitted ultrasonic waves with many modulation frequencies present in the zone of frequency deviations determined to be cause for the giving of an alarm, so that the function test can be carried out by the same test circuit (A) as is the signal indication. The multitude of modulation frequencies together with the turning off of the transmitter signal causes the reflected waves of varying phase condition arriving at the receiver not to combine victroially with the transmitter signal to make zero, so that a consistently reliable function control is guaranteed. In order to identify changes in the enclosure under surveillance, the time lag of the modulation signal is compared with a reference signal (R) after the modulation signal has been turned off.</p> |
