Patent GB-1406500-A (GB)
Title: Electronic security systems
IPC: H03H5/02
Filed: 1972-12-29
Abstract: 1406500 Inductive signalling G J LICHTBLAU 29 Dec 1972 [30 Dec 1971 14 June 1972] 60234/72 Heading H4U [Also in Division G4] In a security system in which the presence in the field of a radio transmitter of a tag carrying a resonant circuit is detected by means of a receiver, the circuit carried by the tag has two or more resonances. One resonance is used in detection of the tag and a second to allow the use of a separate transmitter operating on its frequency to burn out a fusible link rendering the tag inoperative. Transmitter 10, Fig. 1, generates a field in coil 12 which is picked up by receiving coil 14; the signal passes through R.F. bandpass filter 16 to R.F. amplifier 18 and thence to detector 20. The rectified output is filtered by low-pass filter 22 and the higher frequency components in its output selected by high-pass filter 24 and passed to amplifier 26: the output from amplifier 26 is passed via successive noise rejection circuits 28, 30 to operate alarm 32. The transmitter comprises an amplifier driven from a voltage-controlled R.F. oscillator whose frequency is swept by means of a low-frequency oscillator (Fig. 2, not shown). The tag carries a circuit comprising inductors L1, L2, Fig. 5, with mutual coupling M, capacitors C1, C2 and fusible link 62: the loop comprising inductors L1, L2 and capacitor C2 is resonant at the detection frequency, while the loop comprising inductor L1 and capacitor C1 is resonant at the frequency of the second transmitter used to burn out the link 62. The Specification describes a suitable tag in the form of a printed circuit (Figs. 3, 4, not shown). Noise suppression circuits.-If detector 20, Fig. 1, is an amplitude detector, its signal output is a series of bi-polar pulses, Fig. 7B, each of which occurs upon traversal of the detection frequency by the swept signal carrier. After processing by filters 22, 24, Fig. 1, the demodulation signal is of the form shown in Fig. 9A: noise rejection circuit 28, Fig. 1, comprises a positive pulse height threshold detector 78 producing the pulses of Fig. 9B, from the signal, and a corresponding negative pulse detector producing the pulses of Fig. 9C. The pulse height threshold detectors 78, 80 feed into pulse width discriminators 82, 84 respectively, which produce the pulses of Figs. 9D and 9E, respectively, and these outputs are applied to an OR gate 86 feeding in parallel flip-flop 88, counter 92 and reset multivibrator 90. Flip-flop 88 which is alternately set and reset by the pulses from OR gate 86 provides from its two outputs Q and Q inhbiting signals to the pulse width discriminators 82, 84 respectively; counter 92 is operative to provide an output signal upon receipt of three input pulses, provided they occur within the time interval provided by reset multivibrator 90, which has a period as in Fig. 9F, slightly longer than that corresponding to one traverse of the radio frequency. The pulses from counter 89 pass to a second noise rejection circuit 30, Fig. 1, comprising in cascade a monostable multivibrator 96, Fig. 10, averaging and integraing circuit 98, and threshold level detector 100. The Specification describes an alternative arrangement for the noise suppression circuit 30, Fig. 1, using a staircase wave generator (Figs. 18, 19, not shown). Phase detection arrangement.-The detector 20, Fig. 1, may alternatively comprise a phase detector: signal from R.F. amplifier 18 is applied to a switching phase detector 106 which derives its second input either directly from the transmitter 10 or from an oscillator in a phase-locked loop 108 coupled to the received signal: the following circuits are similar to those used with the amplitude detector. The Specification also describes a transmitter circuit for the tag destruction frequency (Fig. 14, not shown), an arrangement in which the tag detection transmitter and the detector circuit are coupled to the same aerial loop (Fig. 15, not shown) including a modification in which the aerial is untuned (Fig. 16, not shown) and a further arrangement in which coupling between transmitting and receiving loops is normally balanced out by means of a transformer coupling between transmitter and receiver (Fig. 17, not shown).