| 摘要 |
A security fence, in which contact sensors produce corresponding signals when the security wires are touched. Each sensor is attached to two low-pass filters, the cutoff frequency of one filter being above, and below the cutoff frequency of the other filter being below the frequency spectrum of a sensor signal which occurs when the wire is deflected by a person. The output signals of the low-pass filters are subtracted in a subtraction circuit. If the subtracted output signal of the subtraction circuit exceeds a predetermined threshold, an alarm signal is released. Deflections of the wire due to other circumstances and, therefore being in another frequency spectrum do not result in the alarm being released.
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