Abstract

The system includes a transmitter producing an electromagnetic field at a frequency repetitively swept through a predetermined range at a predetermined sweep frequency. A receiver senses a resonant frequency produced by a resonant tag circuit when the tag circuit is within the electromagnetic field and produces an output signal in response to the resonant frequency. A first noise rejection circuit is provided which accepts the output of the receiver and produces a pulse in response to selected output signals from the receiver. The selected output signals include an initial output signal and successive output signals which occur at an interval from the previous selected output signal which is at least as great as the period of the sweep frequency. The circuit then compares the frequency of the pulses produced with the sweep frequency and produces an alarm signal when the pulse frequency and sweep frequency are substantially equal. Additional circuitry is provided which produces an inhibit pulse coincident with a known disturbance signal. A gate inhibits the production of the alarm signal in response to the inhibit pulse.