Method and system for detecting a marker
First Claim
1. A method of detecting articles containing or carrying markers with a non-linear magnetic characteristic by passing the articles through a surveillance zone in which a first magnetic field of relatively low frequency f1 and a second magnetic field of relatively high frequency f2 are generated, and detecting the harmonic response of said markers;
- characterised in that;
(a) the harmonic response is detected in a frequency bandwidth m.f2 ±
n.f1, where n and m are positive integers, and m is greater than 1;
(b) the harmonic response is detected by phase-sensitive detection means which is locked onto a generated reference frequency p.f2 ±
q.f1, where p and q are positive integers, one of which may be zero; and
(c) the harmonic response at the n.f1 sidebands is analysed by digital signal processing means which activates an alarm if the shape and/or amplitude of the n.f1 sidebands correspond to predetermined values.
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Accused Products
Abstract
An electronic article surveillance system is provided having a transmitter for generating two alternating magnetic fields by way of a single transmitter coil fed with a transmitter signal current, and a receiver that detects harmonics and intermodulation products of the alternating magnetic fields by way of a receiver coil that generates a receiver signal current. The receiver comprises a wide-bandwidth phase detector locked on to a frequency p.f2 ±q.f1, where p and q are positive integers, one of which may be zero, and a digital signal processor adapted to carry out a full time-domain analysis of the waveform of the receiver signal current, and wherein the transmitter signal current corresponds to the linear super position of two alternative currents with a relatively low frequency f1 and high frequency f2 respectively.
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Citations
14 Claims
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1. A method of detecting articles containing or carrying markers with a non-linear magnetic characteristic by passing the articles through a surveillance zone in which a first magnetic field of relatively low frequency f1 and a second magnetic field of relatively high frequency f2 are generated, and detecting the harmonic response of said markers;
- characterised in that;
(a) the harmonic response is detected in a frequency bandwidth m.f2 ±
n.f1, where n and m are positive integers, and m is greater than 1;(b) the harmonic response is detected by phase-sensitive detection means which is locked onto a generated reference frequency p.f2 ±
q.f1, where p and q are positive integers, one of which may be zero; and(c) the harmonic response at the n.f1 sidebands is analysed by digital signal processing means which activates an alarm if the shape and/or amplitude of the n.f1 sidebands correspond to predetermined values. - View Dependent Claims (2, 3, 4)
- characterised in that;
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5. A method of detecting articles containing or carrying markers with a non-linear magnetic characteristic by passing the articles through a surveillance zone in which a first magnetic field of relatively low frequency f1 and a second magnetic field of relatively high frequency f2 are generated, and detecting the harmonic response of said markers;
- characterised in that;
(a) the harmonic response is detected in a frequency bandwidth m.f2 ±
n.f1, where n and m are positive integers, and m is greater than 1; and(b) the amplitude of the first magnetic field is greater than that of the second magnetic field. - View Dependent Claims (6, 7, 8)
- characterised in that;
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9. A method of detecting articles containing or carrying markers with a non-linear magnetic characteristic by passing the articles through a surveillance zone in which a first magnetic field of relatively low frequency f1 and a second magnetic field of relatively high frequency f2 are generated, and detecting the harmonic response of said markers;
- characterised in that;
(a) the harmonic response is detected in a frequency bandwidth m.f2 ±
n.f1, where n and m are positive integers, and m is greater than 1; and(b) the ratio f2 ;
f1 is greater than 150;
1.
- characterised in that;
- 10. An electronic article surveillance system, which system comprises a transmitter which generates two alternating magnetic fields via a single transmitter coil which is fed with a transmitter signal current and a receiver which detects harmonics and intermodulation products of said alternating magnetic fields via a receiver coil which generates a receiver signal current, wherein the receiver comprises a wide-bandwidth phase detector locked onto a frequency p.f2 +q.f1, where p and q are positive integers, one of which may be zero, and a digital signal processor adapted to carry out a full time-domain analysis of the waveform of the receiver signal current and wherein the transmitter signal current corresponds to the linear superposition of two alternative currents with respectively a relatively low frequency f1 and a relatively high frequency f2.
Specification