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Capacitive intrusion detector circuitry utilizing reference oscillator drift

  • US 4,169,260 A
  • Filed: 04/11/1978
  • Issued: 09/25/1979
  • Est. Priority Date: 04/11/1978
  • Status: Expired due to Term
First Claim
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1. An electronic security apparatus for detecting intrusion near protected objects connected together to form an antenna means, said apparatus comprising:

  • a voltage controlled oscillator means connected to said antenna means for producing a signal to excite said antenna means, said voltage controlled oscillator signal having a frequency which changes instantaneously when the capacitive reactance of said antenna means changes due to an intrusion;

    a reference oscillator means having inherent drift for producing a reference oscillator signal;

    a phase comparator means connected to said voltage controlled oscillator means and to said reference oscillator means for producing a phase comparator signal representative of the phase difference between said voltage controlled oscillator signal and said reference oscillator signal;

    means to feed back said phase comparator signal to said voltage controlled oscillator means to synchronize said voltage controlled oscillator signal to said reference oscillator signal, said voltage controlled oscillator means and said reference oscillator means having no bidirectional coupling therebetween; and

    deriving means connected to said phase comparator means for activating a signaling circuit means to produce an alarm when a change in said phase comparator signal due to an instantaneous change in said voltage controlled oscillator signal frequency indicates an intrusion;

    whereby, if a frequency generator is connected to said antenna means in an attempt to defeat said apparatus, drift in said reference oscillator signal frequency also causes said phase comparator signal to change so that said deriving means activates said signaling circuit means to produce an alarm.

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