Variable frequency RF electronic surveillance system
First Claim
1. An electronic article surveillance system including(a) transmitter means for creating within an interrogation zone, bursts of electromagnetic energy at discretely different radio frequencies within a predetermined range of frequencies, each burst being spatially separated from the next by a quiescent period during which the transmitter means does not transmit,(b) receiver means for receiving electromagnetic signals at said radio frequencies during said quiescent periods and for activating alarm means when said received signals exceed a predetermined level, and(c) marker means adapted to be affixed to an article, the presence of which within said interrogation zone is to be monitored, said marker means comprising an inductive-capacitive (LC) circuit resonant at a frequency within said range of frequencies such that when said marker means is in said interrogation zone RF transmitted energy is absorbed by said circuit and is reemitted at said resonant frequency during the subsequent quiescent period for receipt by the receiver means,wherein in the improvement, said system comprises a plurality of marker means, each adapted to be affixed to an article and each comprising a said LC circuit including an inductive-capacitive-resistive combination designed to have a Q-factor of not less than 50, a nominal resonant frequency f, and an associated bandwidth (BW) centered about said resonant frequency f, all as defined by the expression Q=f/BW, said transmitter means comprises means for creating within said interrogation zone bursts of a sufficient number of different RF frequencies, such that there are bursts of at least three different frequencies sufficiently close to the resonant frequency of each of said LC circuits so as to fall within the bandwidth BW thereof, and said receiver means comprises means at least responsive to frequencies extending through the bandwidth BW of all said LC circuits for activating said alarm means when signals exceeding said predetermined level and corresponding to at least three frequencies are detected.
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Accused Products
Abstract
An electronic article surveillance system is disclosed, having a transmitter means for producing in an interrogation zone sequences containing a plurality of discrete different radio frequencies thereby causing a circuit present within the zone to resonate at its resonant frequency in response to energy absorbed at at least three different frequencies. A receiver means is provided to cause an alarm in the event of detection of three such resonances over two successive sequences.
66 Citations
44 Claims
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1. An electronic article surveillance system including
(a) transmitter means for creating within an interrogation zone, bursts of electromagnetic energy at discretely different radio frequencies within a predetermined range of frequencies, each burst being spatially separated from the next by a quiescent period during which the transmitter means does not transmit, (b) receiver means for receiving electromagnetic signals at said radio frequencies during said quiescent periods and for activating alarm means when said received signals exceed a predetermined level, and (c) marker means adapted to be affixed to an article, the presence of which within said interrogation zone is to be monitored, said marker means comprising an inductive-capacitive (LC) circuit resonant at a frequency within said range of frequencies such that when said marker means is in said interrogation zone RF transmitted energy is absorbed by said circuit and is reemitted at said resonant frequency during the subsequent quiescent period for receipt by the receiver means, wherein in the improvement, said system comprises a plurality of marker means, each adapted to be affixed to an article and each comprising a said LC circuit including an inductive-capacitive-resistive combination designed to have a Q-factor of not less than 50, a nominal resonant frequency f, and an associated bandwidth (BW) centered about said resonant frequency f, all as defined by the expression Q=f/BW, said transmitter means comprises means for creating within said interrogation zone bursts of a sufficient number of different RF frequencies, such that there are bursts of at least three different frequencies sufficiently close to the resonant frequency of each of said LC circuits so as to fall within the bandwidth BW thereof, and said receiver means comprises means at least responsive to frequencies extending through the bandwidth BW of all said LC circuits for activating said alarm means when signals exceeding said predetermined level and corresponding to at least three frequencies are detected.
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37. An electronic article surveillance system including
(a) transmitter means for creating within an interrogation zone, bursts of electromagnetic energy at discretely different radio frequencies within a predetermined range of frequencies, each burst being separated from the next by a quiescent period during which the transmitter means does not transmit, (b) receiver means for receiving electromagnetic signals at said radio frequencies during said quiescent periods and for activating alarm means when said received signals exceed a predetermined level, and (c) marker means adapted to be affixed to an article, the presence of which within said interrogation zone is to be monitored, said marker means comprising an inductive-capacitive (LC) circuit resonant at a frequency within said range of frequencies such that when said marker means is in said interrogation zone RF transmitted energy is absorbed by said circuit and is reemitted at said resonant frequency during the subsequent quiescent period for receipt by the receiver means, wherein in the improvement, said transmitter means comprises means for providing a plurality of bursts at each of said different radio frequencies, and wherein said receiver means comprises means activated during a first interval of time occurring relatively early in each of said quiescent periods when a signal produced by a resonating marker circuit would likely be present for providing a marker signal in response to electromagnetic signals received during said first interval, means activated during a second interval of time occurring relatively late in each of said quiescent periods when no signals produced by resonating marker circuits would likely be present and which would represent ambient background noise for providing a noise signal in response to electromagnetic signals received during said second interval, and means for comparing said marker signal and said noise signal produced following each burst of each of said different radio frequencies and for providing a detector signal in the event the net marker signal corresponding to at least two different frequencies exceeds a corresponding net noise signal by a predetermined amount.
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41. An electronic article surveillance system including
(a) transmitter means for creating within an interrogation zone, bursts of electromagnetic energy at discretely different radio frequencies within a predetermined range of frequencies, each burst being separated from the next by a quiescent period during which the transmitter means does not transmit, (b) receiver means for receiving electromagnetic signals at said radio frequencies during said quiescent periods and for activating alarm means when said received signals exceed a predetermined level, and (c) marker means adapted to be affixed to an article, the presence of which within said interrogation zone is to be monitored, said marker means comprising an inductive-capacitive (LC) circuit resonant at a frequency within said range of frequencies such that when said marker means is in said interrogation zone RF transmitted energy is absorbed by said circuit and is reemitted at said resonant frequency during the subsequent quiescent period for receipt by the receiver means, wherein in the improvement, said transmitter means includes means for generating said bursts as repetitive sequences of discretely different frequencies, each burst at a given frequency being repeated at least twice, and wherein said receiver means further includes detector means for activating said alarm means upon the detection of received signals exceeding said predetermined level corresponding to at least two transmitted frequencies during at least two successive sequences.
Specification