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United States Patent 3,810,147
Lichtblau May 7, 1974

ELECTRONIC SECURITY SYSTEM

Abstract

An electronic security system especially adapted for use in retail stores and employing a multi-frequency resonant tag circuit having distinct frequencies for detection and deactivation. A transmitting system provides an electromagnetic field within a controlled area at a frequency which is swept through a range including the detection frequency of the resonant circuit. In the presence of a tag circuit within the controlled area and operative at the detection frequency, pulses are detected by a receiver which includes noise rejection circuitry for discriminating true signals from noise. The receipt of a predetermined number of signal pulses within a prescribed interval of time causes an alarm actuation. The resonant circuit is formed by printed circuit techniques as a relatively small tag which can be affixed to an article of merchandise. The tag includes a fusible link integrally formed as part of the circuit and which can be fused upon application of an electromagnetic field of predetermined magnitude at the deactivation frequency of the resonant circuit. Deactivation of the tag destroys the resonant properties of the tag at the detection frequency such that a deactivated tag produces no alarm when passing through a controlled area.


Inventors: Lichtblau; George Jay (New York, NY)
Appl. No.: 05/214,361
Filed: December 30, 1971


Current U.S. Class: 340/572.3 ; 334/39; 334/8; 340/572.4; 340/572.5
Current International Class: G08B 13/24 (20060101); G01s 009/56 ()
Field of Search: 340/280,258R,152T 343/6.5SS,6.5R,726-728

References Cited

U.S. Patent Documents
3521280 July 1970 Janco et al.
3624631 November 1971 Chomet et al.
2535053 December 1950 Ercolino
3689885 September 1972 Kaplan et al.
3169242 February 1965 Davis et al.
3209350 September 1965 Davis et al.
Primary Examiner: Caldwell; John W.
Assistant Examiner: Wannisky; William M.
Attorney, Agent or Firm: Weingarten, Maxham & Schurgin

Claims



1. An electronic security system comprising:

transmitter means for providing an electromagnetic field in a predetermined area at a frequency repetitively swept through a predetermined range;

a multi-resonant tag circuit having a first resonant frequency within said predetermined range of frequencies and a second frequency outside of said predetermined range of frequencies;

receiver means for detecting the presence of said first resonant frequency from a tag circuit present in said predetermined area; and

deactivation transmitter means for providing an electromagnetic field in a predetermined area at said second frequency thereby to induce a current flow in a fusible link of said tag circuit sufficient to destroy the resonant properties of said tag circuit at said first resonant frequency.

2. An electronic security system according to claim 1 wherein said receiver means includes:

an RF front-end for receiving the electromagnetic field from said transmitter means and for providing an RF output signal in response thereto;

detector means operative in response to said output signal to provide output pulses in response to the presence of the first resonant frequency of said tag circuit in said electromagnetic field;

means for suppressing noise received with said output pulses; and

means operative in response to said output pulses for providing an output indication representing the presence of a tag circuit having said first

3. An electronic security system according to claim 2 wherein said noise suppression means includes:

means for passing only the signal spectrum of said RF output signal; and

means for digitally processing said pulses from said detector means to provide output pulses only in response to pulses of predetermined rate and

4. An electronic security system according to claim 1 wherein said transmitter means includes:

an oscillator for providing a modulation signal at a predetermined frequency;

a voltage controlled oscillator operative in response to said oscillator to provide a carrier frequency varied through a predetermined frequency range in response to the modulation signal of said oscillator;

an amplifier coupled to said voltage controlled oscillator; and

an antenna coupled to said amplifier for providing said electromagnetic

5. An electronic security system according to claim 1 wherein said means for electrically deactivating said tag circuit includes:

an antenna for providing said electromagnetic energy at said second resonant frequency; and

transmitter means for driving said antenna to provide said electromagnetic

6. An electronic security system according to claim 5 wherein said antenna includes a balanced loop antenna matched to the output of said transmitter

7. An electronic security system according to claim 5 wherein said antenna includes a balanced loop antenna having one terminal coupled to the output of said transmitter means, said loop antenna being balanced thereto by a balanced lead structure, the other end of said loop antenna being

8. An electronic security system according to claim 1 wherein said receiver means includes:

detector means operative in response to said first resonant frequency in a received electromagnetic field from said transmitter means to provide output pulses in response thereto;

filter means for suppressing noise in the presence of such pulses;

means for amplifying said output pulses;

means for shaping said amplified pulses; and

means for digitally processing said shaped pulses and to provide an alarm indication only in response to a predetermined number of shaped pulses

9. An electronic security system according to claim 8 wherein said filter means includes:

a low pass filter operative to pass the signal spectrum of received pulses;

a notch filter operative to remove the signal component caused by said modulating frequency; and

a bandpass filter operative to remove noise components outside of said

10. An electronic security system according to claim 8 wherein said means for digitally processing said detected pulses includes:

means operative in response to said shaped pulses to provide first output pulses of predetermined width greater than the width of expected received pulses;

staircase generating means operative in response to said first output pulses to generate a staircase signal which increases in amplitude in the presence of each of said first output pulses;

means operative in response to said first output pulses to provide counter pulses of a width slightly less than the period of expected signals;

gate means operative in response to said first output pulses for providing a gate signal to said counter means defining an interval of time within which a predetermined number of counter pulses must be received;

counter means operative in response to said counter pulses to provide an output indication in response to the presence of a predetermined number of counter pulses within an interval of time specified by the gate signal of said gate means;

said staircase generating means being operative to provide a reset signal upon the exceedance of a predetermined threshold level by said staircase signal to reset said counter means, said staircase generating means and

11. An electronic security system according to claim 1 wherein said receiver means includes:

detector means operative to provide output pulses in response to the presence of said tag circuit in said electromagnetic field; and

a cross correlation filter operative in response to said output pulses to provide an output signal for indicating the presence of a tag circuit only when the spectral content of said output pulses is of predetermined

12. An electronic security system according to claim 1 wherein said receiver means includes:

means for detecting pulses produced by said multi-resonant tag circuit being swept by the electromagnetic field of said transmitter means;

means for discriminating signal pulses from said tag circuit from spurious signals; and

means for providing an alarm indication in response to receipt of a predetermined number of said signal pulses occurring within a

13. An electronic security system according to claim 1 wherein said multi-resonant tag circuit includes:

a first tuned circuit resonant at said first frequency within said predetermined range;

a second tuned circuit resonant at said second frequency outside of said predetermined range; and

a fusible link in circuit with said tuned circuits and operative to fuse upon application of electromagnetic energy of predetermined power at said

14. An electronic security system according to claim 13 wherein said multi-resonant tag circuit includes:

a planar substrate of electrically insulative material, said first and second tuned circuits being formed on said substrate in planar circuit

15. An electronic security system according to claim 1 wherein said multi-resonant tag circuit includes:

a planar substrate of electrically insulative material;

a first conductive path formed on a surface of said substrate in a configuration to define a first inductor;

a second conductive path formed on said substrate in a configuration to define a second inductor;

a plurality of pairs of conductive areas each pair formed of conductive areas in alignment on respective opposite surfaces of said substrate, the conductive areas on the substrate surface containing said first and second conductive paths being electrically connected thereto at selected points to define a plurality of capacitors for said tag circuit; and

a conductive path of predetermined size provided in circuit with said tag circuit on said substrate and operative to fuse upon application of

16. For use in an electronic security system which includes means for providing an electromagnetic field of a frequency which is swept within a predetermined range, means for detecting a first frequency within said range, and means for providing an electromagnetic field at a second frequency outside of said range, a multi-frequency resonant tag circuit comprising:

a first tuned circuit resonant at said first frequency within said predetermined range;

a second tuned circuit resonant at said second frequency outside of said predetermined range; and

a fusible link in circuit with said first and second tuned circuits and operative to fuse upon application of an electromagnetic field of predetermined power at said second frequency to thereby destroy the

17. The invention according to claim 16 wherein said multi-frequency resonant tag circuit includes:

a planar substrate of electrically insulative material, said first and second tuned circuits being formed on said substrate in planar circuit

18. The invention according to claim 17 wherein said fusible link is also formed in planar circuit configuration with said first and second tuned

19. The invention according to claim 16 wherein said multi-frequency resonant tag circuit includes:

a planar substrate of electrically insulative material;

a first conductive path formed on a surface of said substrate in a configuration to define a first inductor;

a second conductive path formed on said substrate in a configuration to define a second inductor; and

a plurality of pairs of conductive areas fromed on said substrate each pair having conductive areas in alignment on respective opposite surface of said substrate to define a capacitor, the conductive areas on the substrate surface containing said first and second conductive paths being electrically connected to said paths at selected points to define said

20. The invention according to claim 16 wherein said multi-frequency resonant tag circuit includes:

a planar insulative substrate having formed on one surface thereof

a first conductive area centrally formed on said substrate surface;

second and third conductive areas formed in adjacent spaced relation along one side of said substrate surface;

a first conductive path arranged on said substrate surface between and in electrical connection with said first and second conductive areas;

a second conductive path formed on said substrate surface between and in electrical connection with said second and third conductive areas;

and wherein the opposite surface of said substrate includes:

fourth, fifth and sixth conductive areas each in alignment and substantially coextensive with said respective first, second and third conductive areas formed on said one substrate surface;

a conductive path connecting said forth and fifth conductive areas;

said first and second conductive paths and plurality of conductive areas comprising said first and second tuned circuits; and

wherein said fusible link includes a conductive path interconnecting said fifth and sixth conductive areas and dimensioned to fuse upon application of an electromagnetic field of predetermined power at said second

21. The invention according to claim 16 wherein said first and second tuned

22. The invention according to claim 16 wherein said first and second tuned

23. An electronic security system according to claim 1 wherein:

said multi-resonant tag circuit has a third resonant frequency operative when the resonant properties of said tag circuit at said first resonant frequency have been destroyed by provision of an electromagnetic field thereto;

and further including:

means for detecting the presence of said third resonant frequency from said

24. The invention according to claim 16 wherein said multi-frequency resonant tag circuit includes:

a third tuned circuit resonant at a third frequency when the resonant properties of said tag circuit at said first frequency have been destroyed by application of said electromagnetic field at said second frequency.
Description



FIELD OF THE INVENTION

This invention relates to electronic security systems and more particularly to a radio frequency system for the reliable detection of a resonant tag circuit within a controlled area and for electronic destruction or alteration of the tag to permit passage of articles through the controlled area.

BACKGROUND OF THE INVENTION

Electronic security systems have been proposed for detecting the unauthorized removal of articles from an area under protection. Such systems have been proposed especially for use in retail stores to prevent the theft of articles from the store and minimize the considerable losses occasioned by shoplifting. Electronic security systems generally include an electromagnetic field provided in a controlled area through which merchandise must pass in leaving the store. A resonant circuit is attached to articles of merchandise and the presence of the resonant circuit in the controlled area is sensed by a receiving system to denote the unauthorized removal of an article. The resonant circuit is removed by store personnel from an article properly leaving the store to permit passage of the article through the controlled area without alarm activation.

One prior art system is shown in U.S. Pat. No. 3,500,373 wherein a single resonant circuit is affixed to an article and its presence in a controlled area detected by interrogation with a swept transmitted frequency which includes the resonant frequency of the circuit. Upon interrogation the resonant circuit absorbs energy which is sensible by a receiver to produce pulses for alarm actuation. This system is, however, quite susceptible to noise which can cause erroneous alarm actuation. A further deficiency of such a system is that the resonant circuit must be physically accessible to be removed prior to passage of the associated article through the controlled area. The accessibility of the resonant circuit also permits its removal by a thief and the system can thereby be circumvented.

Another system of the prior art is shown in U.S. Pat. No. 3,624,631 in which a single resonant circuit includes a fusible link. The resonant circuit is again interrogated by a swept frequency, the presence of the circuit in a controlled area causing energy absorption at the resonant frequency which is detected by a receiver for subsequent alarm actuation. Upon application of a swept frequency of higher energy than that employed for detection, the fusible link of the resonant circuit can be destroyed to deactivate the tuned circuit such that no detection is possible. The utility of this later system in a commercial setting is diminished by several factors. Since deactivation is accomplished by a swept frequency transmitter, the energy level of the radiated field must be very low to meet requirements of the Federal Communications Commission (FCC). The fusible link must therefore be extremely small and made of a material to allow fusing at such low power levels. As a result, the single resonant circuit is relatively costly and difficult to manufacture. Moreover, since the resonant circuit is both detected and deactivated at the same frequency, the energy level of the applied field must be carefully controlled to prevent spurious alarm responses in the receiver of the system or neighboring systems.

SUMMARY OF THE INVENTION

In accordance with the present invention, an electronic security system is provided which is substantially immune to noise and wherein a multi-frequency resonant tag circuit is provided having distinct frequencies for detection and deactivation. The resonant tag circuit is operative at a first frequency to permit detection by electromagnetic interrogation thereof, and is operative at a second frequency to permit the deactivation thereof by an applied electromagnetic field which destroys the resonance of the circuit at its detection frequency. The system is sufficiently sensitive to permit radiation at a detection frequency at levels below the minimum radiation requirements of the Federal Communications Commission (FCC), thereby eliminating the requirement for an operating license. It is a feature of the invention that the deactivation frequency of the resonant tag can be at one of the frequencies designated by the FCC to have unlimited radiated power levels. As a result, sufficient power is readily provided to cause tag deactivation, and further, operation at these designated frequencies does not require an operating license. The tag circuit is composed wholly of passive elements and is typically formed by printed or etched circuit techniques as a small tag or card adapted to be attached to articles being protected. A fusible link is provided which preferably is an integral element of and of the same conductive material as the resonant circuit, permitting the inexpensive and facile manufacture of the tag circuit.

In brief, the system embodying the invention comprises transmitting apparatus for providing an electromagnetic field at a controlled area at a frequency swept through a predetermined range which includes the detection frequency of the tag circuit. Receiving apparatus is provided for detecting pulses caused by the presence of the tag circuit in the radiated field, the pulses being processed to discriminate true signals from noise and to provide an alarm or other suitable output indication in response to a predetermined number of valid signal pulses within a specified interval of time. Deactivation of the tag circuit is accomplished by a separate transmitter operative at the tag deactivation frequency to cause destruction of a fusible link in the tag circuit, and is accomplished at a single transmitted frequency and with sufficient power to alter tap resonance within short and commercially realistic duration.

DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram representation of a security system according to the invention;

FIG. 2 is a block diagram representation of the transmitter of FIG. 1;

FIG. 3 is a pictorial view of one side of a resonant tag circuit embodying the invention;

FIG. 4 is a pictorial view of the opposite side of the resonant tag circuit of FIG. 3;

FIG. 5 is a schematic diagram of the equivalent electrical circuit of the resonant tag of FIGS. 3 and 4;

FIG. 6 is a block diagram representation of an embodiment of the invention employing amplitude modulation detection;

FIGS. 7A thru 7E are a plot of spectral diagrams useful in illustrating operation of the embodiment of FIG. 6;

FIGS. 8A and 8B are a plot of signal diagrams useful in illustrating operation of the invention;

FIG. 9 is a block diagram representation of the digital processing circuitry of FIG. 1;

FIGS. 10A thru 10F are a plot of signal diagrams useful in illustrating operation of the circuitry of FIG. 9;

FIG. 11 is a block diagram representation of an embodiment of the invention employing phase modulation detection;

FIGS. 12A thru 12C are a plot of spectral diagrams useful in illustrating operation of the embodiment of FIG. 11;

FIG. 13 is a diagrammatic representation of the cross correlation filter of the apparatus of FIG. 11; and

FIG. 14 is a diagrammatic representation of a deactivation system embodied in the invention.

DETAILED DESCRIPTION OF THE INVENTION

An electronic security system according to the invention is depicted in block diagram form in FIG. 1 and includes a transmitter 10 coupled to an antenna 12, typically a loop antenna operative to provide an electromagnetic field within a predetermined area to be controlled. A receiving antenna 14, also typically a loop antenna, is arranged at the controlled area to receive energy radiated by transmitting antenna 12 and to couple received energy to an RF front-end which includes an RF bandpass filter 16 and RF amplifier 18. The output of amplifier 18 is applied to a detector 20, the output of which is, in turn, coupled to noise rejection circuitry 22. Output signals from noise rejection circuitry 22 are amplified by amplifier 24 and applied to pulse shaping circuitry 26 and thence to digital processing circuitry 28, the output of which is operative to actuate an alarm 30 or other output utilization apparatus.

The transmitter 10 is illustrated in greater detail in FIG. 2 and includes an oscillator 32 for providing a modulation signal to a voltage controlled oscillator 34, the output of which is applied to a power amplifier 36 which, in turn, drives transmitting antenna 12. Oscillator 32 provides a regularly recurring waveform at a predetermined frequency, typically 1 KHz. The voltage controlled oscillator 34 provides an appropriate carrier frequency which is varied in frequency under the influence of the modulation signal from oscillator 32. For example, the carrier frequency can have a 5 MHz center frequency varied with a 15 percent frequency deviation at a 1 KHz rate. The modulation signal provided by oscillator 32 can be any regularly recurring waveform such as a sine wave, triangle wave, sawtooth, ramp or exponential signal. Amplifier 36 preferably provides a driving current to antenna 12 of a magnitude which varies inversely with frequency and as a result, the signal received by antenna 14 is of an amplitude which is constant with changes in frequency.

In the absence of a resonant circuit in the controlled area, the electromagnetic field provided by antenna 12 is sensed by receiving antenna 14 but no output indication is provided by the receiving system as no pulse response is present to trigger the alarm. When a resonant tag is present in the controlled area, the tag will become maximally coupled to the receiving antenna 14 each time the transmitted sweep frequency passes through the resonant detection frequency of the tag, giving rise to a sensible change in electrical characteristics at the receiving antenna to cause detection of tag presence.

A multi-frequency resonant circuit embodied on a card or tag adapted to be affixed to items of merchandise and the like is illustrated in FIG. 3. The circuit is formed by printed or etched circuit techniques and includes a first conductive path 40 arranged in a generally rectangular path on a surface of an insulative substrate 42 and terminating in respective conductive areas 44 and 46 disposed in adjacent spaced relationship near one edge of substrate 42. A second conductive path 48 is formed as a rectangular spiral on substrate 42 and terminates at its outer end at a junction 50 with path 40, and at its inner end at a conductive area 52 centrally of the spiral. The opposite surface of substrate 42 is illustrated in FIG. 4 and includes a conductive area 54 in alignment and generally coextensive with conductive area 52 on the substrate surface depicted in FIG. 3, and a pair of conductive areas 56 and 58 in alignment and generally coextensive with areas 44 and 46 on the other surface. The conductive areas 56 and 58 are interconnected by a conductive path 62. As will be further described hereinbelow, path 62 is dimensioned to fuse upon energization by a predetermined electromagnetic field to alter the resonant properties of the tag circuit.

The conductive paths 40 and 48 serve as respective inductors of the resonant circuit and these paths are wound with opposite sense to provide a negative mutual coupling coefficient between the inductors. The conductive areas 52 and 54 spaced by the interposed substrate 42 serve as a first capacitor, while second and third capacitors are formed by the conductive areas 44 and 46 on one surface of substrate 42 and cooperative with paths 56 and 58 on the opposite substrate surface. The equivalent electrical circuit of the tag is illustrated in schematic form in FIG. 5 and exhibits two resonant frequencies, one employed for detection of tag presence and the other for alteration or destruction of the detection frequency. The inductor L1 is the outer loop 40, while the inductor L2 is the inner loop 48. Capacitor C2 is formed by conductive areas 52 and 54, while conductive areas 44 and 56 and 46 and 58 serve as capacitors C1 and C3 respectively.

The two loops in series, composed of inductors L1, L2, C2 and C1, are tuned to a detection frequency which may be in any convenient portion of the spectrum. Typically, a detection frequency of 5 MHz is employed. The outer loop, composed of inductor L1 and capacitors C1 and C3 is tuned to a destruction frequency which preferably is one of the frequencies allocated by the FCC for industrial, scientific, and medical purposes known as the ISM frequencies. These ISM frequencies offer the advantage of unlimited radiated power and with no requirement for an operator's license. The ISM frequencies are 13.56, 27.12, 40.00 and 905 MHz, and in the illustrated embodiment a frequency of 27.12 MHz is employed. Destruction of the resonant properties of the tag is readily accomplished by application of energy at the destruction frequency to cause fusing of link 62 such that the tag is no longer sensible by the receiving system. Sufficient power can be applied to the resonant tag at the destruction frequency to permit fusing of the link therein even though the actual resonant point of the tag circuit varies somewhat from the nominal destruction frequency.

The tag circuit can be fabricated by various printed and etched circuit techniques well known in the art. Extremely low cost fabrication can be accomplished by use of a continuous production process to provide a strip of tags which can be separated for use. Typically, in such a continuous process, an insulative strip of material is employed having a conductive foil laminated or extruded on each side thereof. The tag circuit is repetitively printed onto respective sides of the strip and the nonprinted areas are etched away to complete the continuous roll of tags, which can then be cut or otherwise severed from the strip. The strip is typically high density polyethylene of one to two mils thickness having formed on each side thereof an aluminum foil of, typically, one mil thickness.

The resonant circuit depicted in FIGS. 3 through 5 has been found particularly effective in the present system; however, it will be appreciated that the resonant circuit may take a variety of configurations to provide the intended destruction and detection frequencies. In the tag circuit illustrated, the provision of three capacitors allows fabrication of the tag without electrical connection between respective sides of the circuit board. A dual resonant circuit with only two capacitors can be employed by use of a circuit connection between opposite board surfaces, such as by means of a plated-through hole in board 42 interconnecting respective circuit patterns. The fusible link 62 may be placed in circuit otherwise than as illustrated in accomplish deactivation of the tag at the detection frequency, or, as an alternative, to cause complete deactivation of the resonant circuit. The mutual coupling coefficient M between the coils of the resonant circuit can be of either positive or negative sense. When the coupling coefficient is negative, the respective tuned circuits comprising the overall tag can be separately tuned and thus simplify the design of specific tag circuits having intended resonance characteristics. Since the sensitivity of a tuned circuit is related to the total number of coil turns of the circuit inductors, the provision of a positive coupling coefficient will result in greater sensitivity since the number of turns of the circuit inductors, being wound with the same sense, will be additive. In the circuit of FIG. 5, for example, if the mutual coupling (M) is negative, the detection frequency is determined by inductor L1 in series with inductor L2 and capacitors C2 and C1, while the deactivation frequency is determined by inductor L1 in series with capacitors C1 and C3. If, however, the mutual coupling is positive, the resonant frequencies are caused by total interaction of the several inductors and capacitors.

The fusible link can also be placed within the tag circuit such that, upon destruction by an applied electromagnetic field, the tag becomes wholly inoperative. Alternatively, the link can be placed such that upon destruction, the resonant properties of the tag at the detection frequency are destroyed and the tag made operative at a third frequency different than both the detection and destruction frequencies. This third frequency can be ignored during system operation as the system does not operate at the third frequency. In an alternative embodiment, however, the third frequency, which becomes operative upon destruction of the fusible link, may be utilized to ascertain that a tag present at a controlled area has been electrically altered. This latter embodiment may be employed for example at an exit station of a store where the security system is operative to detect tag presence by sensing the detection frequency in order to determine whether items are being improperly removed from the store. Detection of a tag at the third frequency is indicative of an electrically altered tag and the absence of third frequency detection can indicate the presence of an item from which a tag has been removed. The use of such a third tag frequency therefore can provide an added level of security.

The present system is operative to detect amplitude, frequency or phase modulation provided by the presence of a resonant tag within an applied electromagnetic field. An embodiment of the invention employing amplitude modulation is depicted in FIG. 6. The RF front-end is as described above in connection with FIG. 1 and including a receiving antenna 14, RF bandpass filter 16 and RF amplifier 18. Signals from the RF amplifier are applied to a full wave detector 64, the output of which is applied to a low pass filter 66. The output of filter 66 is coupled to a notch filter 68 which, in turn, is coupled to a bandpass filter 70. The output of bandpass filter 70 is then processed in the same manner as in the embodiment of FIG. 1. More particularly, the output of filter 70 is applied via a video amplifier 24 to pulse shaping circuitry 26 and then to digital processing circuitry 28, the output of which is operative to energize a suitable alarm 30.

The operation of the system of FIG. 6 is best described in conjunction with the spectral diagrams of FIG. 7 and the signal diagrams of FIG. 8. The carrier frequency illustrated in FIG. 8A varies in a regularly recurring manner through a predetermined frequency range, typically .+-. 15% of the carrier center frequency. After processing by RF bandpass filter 16 (FIG. 1) the received signal has a spectral content 72 as depicted in FIG. 7A centered about the carrier fc. After full wave detection by detector 64, the spectrum present is as in FIG. 7B and includes a carrier spectrum 74 centered about twice the carrier frequency 2 fc, and a signal spectrum 76 which includes the modulation frequency fm. Noise components 78 are also present, as illustrated. The signal output from detector 64 is a series of bipolar pulses, depicted in FIG. 8B, each of which occurs upon traversal of the resonant detection frequency of the tag circuit by the swept carrier. It will be noted that adjacent pulses are of opposite sense in response to respective traversal of the resonant detection frequency in positive and negative directions by the swept carrier.

The low pass filter 66 is operative to remove the double carrier spectrum 74, as illustrated in FIG. 7C, leaving the signal spectrum 76, and carrier frequency fc and associated noise component 78. Notch filter 68 is operative to remove the frequency component at twice the modulation frequency (2 fm) caused by the response of RF bandpass filter 16 to the received swept carrier frequency and provides the signal spectrum of FIG. 7D. The bandpass filter 70 is operative to pass only the signal spectrum 76, as shown in FIG. 7E, rejecting noise above and below this spectrum. After amplification in video amplifer 24, signals are applied to pulse shaping circuitry 26 which typically includes pulse height and pulse width discriminating circuits to provide a train of pulses of standardized height and width for subsequent digital processing. Digital processing circuitry 28 is operative to discriminate true signal pulses from noise and spurious signals and to energize an alarm 30 upon detection of a predetermined number of valid signal pulses within a selected interval of time.

The pulse processing circuitry 28 is depicted in greater detail in FIG. 9. Signals from pulse shaping circuitry 26 are applied to a first monostable multivibrator 80, the output of which is applied simultaneously to a staircase generator and comparator 82, a second monostable multivibrator 84 and a third monostable multivibrator 86. The output of multivibrator 84 is applied to a counter 88, the output of which is a pulse train for alarm energization. Staircase generator 82 is operative to apply a reset signal to counter 88 and to multivibrator 86. Multivibrator 86 is operative to apply a reset signal to counter 88 and to staircase generator 82.

Operation of the circuitry of FIG. 9 will be described in conjunction with the signal diagrams of FIG. 10. The pulses from pulse shaping circuitry 26 are depicted in FIG. 10A. These pulses are applied to a monostable multivibrator 80 which is operative to produce output pulses, as depicted in FIG. 10B, of predetermined width T1 which is selected to exceed the duration of expected received signals. The standardized pulse signals from multivibrator 80 are applied to respective inputs of a staircase generator and comparator 82 and monostable multivibrators 84 and 86. The multivibrator 84 has a period equal to approximately the modulation rate, and in response to pulses from multivibrator 80, provides pulses as shown in FIG. 10D to advance the counter 88 by one count for each pulse applied thereto. The pulses T2 from multivibrator 84 commence upon the leading edge of corresponding pulses from multivibrator 80 and terminate slightly before the commencement of a subsequent pulse from multivibrator 80, and are operative to inhibit spurious input signals from advancing counter 88 at a rate faster than that of expected signals.

The output from staircase generator and comparator 82 is depicted in FIG. 10C and is seen to be a staircase voltage which rises during the duration of each pulse from multivibrator 80. A threshold circuit is provided within the staircase generator and comparator 82 and upon exceedance of a predetermined threshold level by the staircase signal, circuit 82 is operative to provide an output pulse employed to reset counter 88 and multivibrator 86 and also to reset the staircase generator itself. Noise pulses which occur at a repetition rate greater than the modulation rate cause the staircase voltage to rise to a level above the threshold to reset the system. The counter is thus reset before it has advanced to its final count so that an alarm will not be provided in response to fast noise inputs. For true signals occurring at the modulation rate, the staircase voltage does not exceed the threshold level within the interval of time in which a predetermined number of signal pulses is received for processing.

The multivibrator 86 specifies the maximum time in which the predetermined number of pulses must be received in order to provide an output pulse to the alarm circuitry. For purposes of illustration, 32 pulses are employed in the present embodiment to cause alarm actuation. The output signal from multivibrator 86 is depicted in FIG. 10E and is effectively a long gate which commences upon receipt of the first pulse from the tag circuit and which terminates at a selected time after the occurrence of the 32nd pulse. The multivibrator 86 thus has a period longer than 32 times the modulation rate, and the output pulse thereof is operative to reset counter 88 to its initial state. The counter will not generate an output pulse to the alarm circuitry if 32 pulses from multivibrator 84 are not received within the interval specified by the gate provided by multivibrator 86. Counter 88 provides an output pulse, depicted in FIG. 10F, upon receipt of 32 pulses from multivibrator 84 at the modulation rate, and, as described above, noise signals will not, statistically, be operative to cause a counter output for alarm actuation.

A system for phase detection of the presence of the resonant tag within an applied electromagnetic field is illustrated in FIG. 11. Signals from the RF front-end (FIG. 1) are applied to a phase detector 90 such as a phase-locked loop, the output signal of which is a series of demodulated pulses which occur each time the transmitted carrier frequency sweeps through the resonant detection frequency of the tag. The phase shift resulting from the tag being present is detected by comparing the phase of the received signals with that of a local oscillator which is phase synchronized to the transmitted signal. In the phase-locked loop, the received signal is automatically synchronized to the local oscillator and the phase of the transmitted signal is compared with that of the local oscillator providing an error signal which represents the derivitive of the phase error between received and local oscillator signals.

The output of the phase detector 90, depicted in FIG. 12A, comprises a signal spectrum 91 which includes the modulation frequency fm together with noise components 93. Output signals from detector 90 are applied to a bandpass filter 92 which passes the signal spectrum 91, removing noise components above and below this spectrum, as depicted in FIG. 12B. The filtered output of bandpass filter 92 is applied to cross correlation filter 94 which is operative in response to spectral energy of predetermined configuration to provide an output signal, and to provide effectively no output signal for other input conditions. The output of the cross correlation filter is typically as shown in FIG. 12C and is a pulse of predetermined spectral content 95 in response to a unique spectral input condition. The cross correlation filter, itself well known in the art, is shown in FIG. 13 and includes a tapped delay line 96, each output tap of which is connected via a respective resistor 98 to an input of summing circuit 100. The resistors 98 are illustrated as adjustable resistors and are each of a value selected to provide a predetermined weighted signal to summing circuit 100. In response to input pulses of predetermined spectral characteristics an output pulse is provided by summing circuit 100 which is then processed as described above by amplifier 24, pulse shaping circuitry 26 and digital processing circuitry 28.

The apparatus for providing an electromagnetic field for destruction or alteration of the resonant tag circuit is shown in FIG. 14 and includes a transmitter 102 operative to provide an output at the destruction frequency of the tag circuit, and which is coupled to a balanced loop antenna 104 which is arranged to provide the destruction field for a tag in the vicinity of this field. The transmitter 102 is typically of 50 to 100 watts output power and is matched to the loop antenna 104 by means of a balanced lead configuration. One output terminal of transmitter 102 is coupled to a lead of loop antenna 104. The other output terminal of transmitter 102 is coupled to a lead 106 which follows the path of the antenna and which is connected to the antenna at a selected point 108. This lead 106 serves as an impedance matching loop and follows the configuration of the antenna loop to prevent flux leakage between the input leads and the associated antenna. The other end of antenna 104 is grounded via a variable capacitor 110. The geometry of the input leads provides effectively a tapped inductor having an intended input impedance to match the output impedance of transmitter 102 to the input impedance of antenna 104.

In an alternative embodiment of the invention, subcarrier modulation can be employed such that different subcarrier frequencies can be provided in systems operating in proximity to one anoher to prevent crosstalk between such systems. It will also be appreciated that although the invention finds primary utilization as a security system for stores and other establishments of trade, it is also useful in various other environments. For example, the system can be employed for automatic identification by use of a multi-resonant tag circuit such as described above but which does not include a fusible link. The resonant frequencies of the tag are selected to be within a frequency range swept by an interrogating signal. The resonant frequencies of the interrogated tag are sensed by a receiver such as described above but operative at each resonant frequency, the detection of all tag frequencies being employed to identify the presence of the tag in a prescribed area. It should be evident that the invention can be implemented by various circuits other than those disclosed to accomplish the intended purpose. Accordingly, it is not intended to limit the invention by what has been particularly shown and described except as indicated in the appended claims.

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