| 摘要 |
To test the function of an ultrasonic alarm system in which ultrasonic vibrations are radiated at a specific frequency (f) into a monitored area and Doppler shifts of the ultrasonic vibrations received by a pick-up (3) are evaluated in comparison with a reference signal supplied by the transmitter (2) for alarm signalling, the reference signal is modified during testing in a characteristic manner, for example by an abrupt change from one frequency to another or by sweeping through the transmission frequency. The evaluation circuit (6) is affected in the same way that it would be by the movement of an intruder in the monitored area. It is also advantageous if the frequency (f) of the transmitted vibrations is modified during testing in the same way as the frequency (f) of the reference signal. In the case of abrupt frequency changes during a short period, the different propagation times of the acoustic and electric signals result in a frequency difference which is indicated by the evaluation circuit (6) when the system is functioning correctly. The method is preferably suited to alarm systems for signalling an intruder in a protected area. <IMAGE>
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