| 摘要 |
Protected objects are connected together to form an antenna. Preferably, the cabinet for the detector circuitry is also included in the antenna. The antenna is excited by a voltage controlled oscillator (VCO). The capacitive reactance of the antenna changes when an intrusion occurs. This causes the frequency of the VCO signal to undergo an instantaneous change. A phase comparator compares the phase of the VCO signal with the phase of a reference oscillator signal. An instantaneous change in the frequency of the VCO signal due to a change in the capactive reactance of the antenna when an intrusion occurs is reflected by a shift in phase between the VCO signal and the reference oscillator signal. This causes the phase comparator signal to change. A deriving means, preferably including a dual differentiator circuit, which derives the rate of the rate of change of the phase comparator signal, and a differential comparator, which is triggered by the dual differentiator circuit in the event of an intrusion, activates a signaling circuit so as to produce an alarm when an intrusion occurs. Since the antenna is connected to the VCO and there is no bidirectional coupling between the VCO and the reference oscillator, the electronic security apparatus is incapable of defeat by connection of a frequency generator to the antenna, because inherent drift in the reference oscillator eventually produces an alarm. Additional features are also disclosed.
|
