| 摘要 |
In a ultra-sound alarm installation, wherein ultra-sound wave are continously emitted in a room under surveillance (R) and wherein a frequency shift due to an object movement, for example an intruder, is evaluated following a Doppler effect for the emission of an alarm signal, the function control of the installation is carried out by the modulation of ultra-sound wave emitted with a plurality of modulation frequencies which are within the range of frequency shifts evaluated by the emission of the alarm signal, so that the function test may be carried out with the same evaluation circuit (A) as the emission of the alarm signal. Due to the plurality of modulation frequencies, together with the interruption of the emitter signals, the reflected signals arriving at the receiver with a phase shift are not vectorially summed with the emitter signal to give zero, so that a reliable function control is constantly provided. Furthermore, to ascertain modifications in the room under surveillance, the temporary zero setting of the modulation signal in the evaluation circuit (A) after interruption of the modulated signal is compared with a reference signal (R). |
