HK1089265B - A frequency-division marker for an electronic article surveillance system - Google Patents

Description

Frequency division marker for use in electronic article surveillance systems

Background

Electronic Article Surveillance (EAS) systems are designed to prevent unauthorized removal of articles from a controlled area. A typical EAS system may include a monitoring system and one or more security markers. The monitoring system may generate an interrogation zone at an entry point to the control area. The security tag may be secured to an article, such as an article of clothing. If a marked item enters the interrogation zone, an alarm may be generated indicating unauthorized removal of the marked item from the controlled area.

EAS systems typically utilize the Radio Frequency (RF) spectrum to transmit signals between a monitoring system and a security tag. However, some EAS systems have a limited number of RF spectra that may be used to transmit the signal. Accordingly, there is a need for improvements in EAS systems that provide the advantages of providing a usable RF spectrum.

Disclosure of Invention

According to the present invention there is provided a marker for use in an electronic article surveillance system, comprising: a first resonant circuit including a first planarizing coil having a pair of terminals and a first capacitor connected to the pair of terminals, the first resonant circuit producing a first resonant signal in response to an interrogation signal; and a second resonant circuit including a second planarizing coil having a pair of terminals and a second capacitor connected to the pair of terminals, the second resonant circuit receiving the first resonant signal and producing a second resonant signal having a second resonant frequency, wherein the fields of the first and second resonant circuits are coupled to each other, the marker characterized by: the first and second planarizing coils are separate circuits that are not connected with a wire and partially overlap, wherein an amount of overlap of the first and second planarizing coils corresponds to an amount k of mutual coupling between fields generated by the first and second planarizing coils, the value of k being in a range of 0.0 to 0.6, wherein the second capacitor of the second resonant circuit is a nonlinear capacitor that operates as a voltage-dependent capacitor.

According to the present invention there is also provided an electronic article surveillance system comprising: a transmitter that transmits an interrogation signal operating at a first frequency; a security tag having a frequency-division identifier, the frequency-division identifier comprising: a first resonant circuit including a first planarizing coil having a pair of terminals and a first capacitor connected to the pair of terminals, the first resonant circuit producing a first resonant signal in response to an interrogation signal; and a second resonance circuit including a second planarizing coil having a pair of terminals and a second capacitor connected to the pair of terminals, the second resonance circuit receiving the first resonance signal and generating a second resonance signal having a second resonance frequency, wherein fields of the first and second resonance circuits are coupled to each other, wherein the first planarizing coil and the second planarizing coil are independent circuits that are not connected by a wire and partially overlap, wherein an amount of overlap of the first and second planarizing coils corresponds to an amount k of mutual coupling between the fields generated by the first and second planarizing coils, a value of k being in a range of 0.0 to 0.6, wherein the second capacitor of the second resonance circuit is a nonlinear capacitor operating as a voltage-dependent capacitor; and a detector for detecting the second resonance signal from the marker and generating a detection signal based on the second resonance signal.

Drawings

The gist of which is shown in the form of an embodiment is pointed out with particularity and is clearly described in the concluding portion of the description. However, the embodiments, both as to organization and method of operation, may best be understood by reference to the detailed description which follows, when read in conjunction with the accompanying drawings, along with the objects, features, and advantages thereof.

FIG. 1 illustrates an EAS system suitable for implementing one embodiment;

FIG. 2 shows a block diagram in accordance with one embodiment;

FIG. 3 is a flow diagram of operations performed by a marker, according to one embodiment;

FIG. 4 is a first circuit for implementing a marker according to one embodiment; and

FIG. 5 is a second circuit for implementing a marker according to one embodiment.

Detailed Description

Embodiments are directed to EAS systems in general. In particular, embodiments are directed to markers for EAS security markers. For example, the marker may consist of a frequency division marker for receiving input RF energy. The frequency division marker may condition the received RF energy and emit an output signal having a frequency that is lower in energy than the input RF energy. For example, in one embodiment, the output signal may have a frequency that is half the frequency of the input signal energy. Such frequency-division identifiers may be used in low-bandwidth environments, such as the 13.56 megahertz (MHz) industrial, scientific, and medical (ISM) bands.

Conventional EAS systems do not operate efficiently in the 13.56MHz ISM band. Conventional EAS systems typically utilize a marker comprised of a resonant circuit that combines a single inductor-capacitor (LC) to resonate at a predetermined frequency. Due to the high operating frequency of the 13.56MHz ISM band, the above marker requires an inductance with a small number of turns and a capacitor in the range of 10-100 picofarads (pF). However, detection of such a single resonant marker requires a relatively complex detection system, such as a "swept (swept) RF" or "pulsed" detection system. The swept RF detection system may generate a signal and receive a signal reflected over a frequency range of relevant widths. The pulse detection system may generate an energy pulse at a particular frequency to excite the marker and thereafter detect the ringing signal waveform of the marker. In both cases, the detection system needs to generate energy at a relatively wide frequency spectrum, which is not suitable for use with a 13.56MHz system.

EAS systems utilizing frequency division markers configured to operate in the 13.56MHz ISM band may provide a number of advantages over conventional EAS systems. For example, the 13.56MHz ISM band allows for relatively large transmit powers, which may extend the detection range of EAS systems. In another example, the improved detector is configured to perform continuous detection and may utilize sophisticated signal processing techniques to extend the detection range. In yet another example, a relatively high operating frequency may allow the marker to have a relatively flat geometry and may reduce degradation under restrictive conditions, thereby making the marker more readily suitable for monitoring items.

Numerous specific details are set forth below in order to provide a thorough understanding of embodiments of the invention. However, it will be understood by those skilled in the art that the embodiments of the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the embodiments of the invention. It is contemplated that structural and functional details disclosed herein may represent the scope of the invention, but are not necessarily limiting.

It is worthy to note that any reference in the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment.

Referring now in specific detail to the drawings in which like parts are designated by like reference numerals throughout, there is illustrated in FIG. 1 an EAS system suitable for practicing one embodiment. Fig. 1 is a block diagram of an EAS system 100. For example, in one embodiment, EAS system 100 may be configured as an EAS system operating using the 13.56MHz ISM band. However, EAS system 100 may also be configured to operate with other portions of the RF spectrum expected by a given device. The embodiments are not limited in this context.

As shown in fig. 1, EAS system 100 may include a plurality of nodes. The term "node" as used herein relates to a system, element, module, component, dashboard or device for processing signals representing information. The signal may be, for example, an electrical signal, an optical signal, an acoustic signal, a chemical signal, or the like. The embodiments are not limited in this context.

As shown in FIG. 1, EAS system 100 may comprise a transmitter 102, a security tag 106, a detector 112, and an alarm system 114. The security mark 106 may further include a marker 108. Although fig. 1 shows a limited number of nodes, it is contemplated that any number of nodes may be utilized in EAS system 100. The embodiments are not limited in this context.

In one embodiment, EAS system 100 may comprise transmitter 102. Transmitter 102 may be used to transmit one or more interrogation signals 104 into interrogation zone 116. For example, interrogation zone 116 may include an area between a set of antenna pedestals disposed at the entrance/exit of a control area. Interrogation signal 104 may include an electromagnetic radiation signal having a first predetermined frequency. For example, in one embodiment, the predetermined frequency may be 13.56 MHz. Interrogation signal 104 may trigger a response from a security marker, such as security marker 106.

In one embodiment, EAS system 100 may comprise security tag 106. Security tag 106 may be designed to be attached to an item being monitored. Examples of marked articles may include articles of clothing, Digital Video Disc (DVD) or Compact Disc (CD) boxes, movie rental boxes, packaging materials, and so forth. The security tag 106 may include a marker 108 packaged within a security tag housing. The security tag housing may be hard or soft in texture, depending on the item to which the security tag 106 is attached. The choice of housing may also be determined based on whether the security tag 106 is designed to be disposable or reusable. For example, reusable security tags typically have a hard security tag housing so that they can withstand repeated attachment and detachment operations while remaining rigid. While single-use security markers may have a hard or soft shell that is determined based on, for example, cost considerations, size, type of marked article, visual aesthetics, location of the marker (e.g., source and retail markers), and the like. The embodiments are not limited in this context.

In one embodiment, security mark 106 may include a marker 108. The marker 108 may comprise a frequency division marker device having an RF antenna to receive the interrogation signal 104, for example, from the transmitter 102. The marker 108 may also include an RF sensor to emit one or more marker signals 110 in response to the interrogation signal 104. The identifier signal 110 may comprise an electromagnetic radiation signal having a second predetermined frequency, wherein the second predetermined frequency is different from the first predetermined frequency of the interrogation signal 104. For example, in one embodiment, the first predetermined frequency is 13.56MHz and the second predetermined frequency comprises one-half of 13.56MHz or 6.78 MHz. The identifier 108 may be discussed in more detail with reference to fig. 2-5.

In one embodiment, EAS system 100 may comprise detector 112. Detector 112 is operable to detect the presence of security mark 106 within interrogation zone 116. For example, the detector 112 may detect one or more marker signals 110 from the marker 108 in the security mark 106. The presence of marker signal 110 indicates the presence of a valid security mark 106 in interrogation zone 116. In one embodiment, the detector 112 may be used to detect electromagnetic radiation having a second predetermined frequency of 6.78MHz, which is one-half the first predetermined frequency of 13.56MHz generated by the transmitter 102. The detector 112 may generate a detection signal based on the detection of the security mark 106.

Notably, since the marker signal has a different frequency than the interrogation signal, the marker signal can be detected using a single frequency system. The detector 112 can detect the marker signal as long as its front-end circuit (front-end) is not saturated by the introduced 13.56MHz basic signal. The utilization of a single frequency system may increase Digital Signal Processor (DSP) processing time in order to achieve better detection performance.

In one embodiment, EAS system 100 may comprise alarm system 114. The alarm system 114 may be comprised of any kind of alarm system to provide an alarm in response to a detection signal. For example, a detection signal from detector 112 may be received. The alarm system 114 may include a user interface to formulate conditions or rules for triggering an alarm. Examples of alarms include: an audible alarm such as a siren or bell, a flashing light alarm, or a silent alarm. The silent alarm may comprise, for example, a silent alarm that sends information to a monitoring system of a security company. The information may be sent via a computer network, telephone network, pager network, etc. The embodiments are not limited in this context.

In normal operation, EAS system 100 may perform anti-theft operations on a controlled area. For example, transmitter 102 may transmit interrogation signals 104 to interrogation zone 116. Marker 108 may receive interrogation signal 104 when security tag 106 is within the interrogation zone. The marker 108 may generate a marker signal 110 in response to the interrogation signal 104. The identifier signal 110 may have a frequency that is about one-half the frequency of the interrogation signal 104. The detector 112 detects the marker signal 110 and generates a detection signal. The alarm system 114 may receive the detection signal and generate an alarm signal to issue an alarm in response to the detection signal.

FIG. 2 illustrates a marker according to one embodiment. Fig. 2 shows a marker 200. For example, the marker 200 may represent the marker 108. The identifier 200 may comprise one or more modules. Although embodiments may be described using "modules" for ease of description, one or more circuits, components, registers, processors, software subroutines, or any combination thereof may be substituted for one, more, or all of the modules. The embodiments are not limited in this context.

As shown in fig. 2, the marker 200 includes a dual resonant device. More specifically, the marker 200 may include a first resonant circuit 202 coupled to a second resonant circuit 204. Although fig. 2 shows a limited number of modules, it is contemplated that any number of modules may be utilized in the identifier 200.

In one embodiment, the marker 200 may include a first resonant circuit 202. The first resonant circuit 202 may be a resonant LC circuit for receiving the interrogation signal 104. To receive electromagnetic radiation at the first frequency F, the first resonant circuit 202 resonates at the first frequency F. For example, in response to the interrogation signal 110, the first resonant circuit 202 may generate a first resonant signal having a first resonant frequency. For example, the first resonant frequency may be about 13.56 MHz.

In one embodiment, the marker 200 may include a second resonant circuit 204. The second resonant circuit 204 may also be a resonant LC circuit for receiving the first resonant signal from the resonant circuit 202. The second resonant circuit 204 may resonate at a second frequency F/2 to emit electromagnetic radiation at the second frequency F/2, the second frequency F/2 being half the first frequency F. For example, in response to the first resonant signal, the second resonant circuit 204 may generate a second resonant signal having a second resonant frequency. For example, the second resonant frequency may be about 6.78 MHz.

In one embodiment, the first resonant circuit 202 and the second resonant circuit 204 are disposed opposite one another such that the two circuits are magnetically coupled. The magnetic coupling may allow the first resonant circuit 202 to transfer energy at the first frequency F to the second resonant circuit 204 in response to electromagnetic radiation received by the first resonant circuit 202 at the first frequency F. The second resonant circuit 204 may be configured with a voltage dependent capacitor whose reactance varies as the energy transferred from the first resonant circuit 202 varies. This change may cause the second resonant circuit 204 to emit electromagnetic radiation at the second frequency F/2 in response to the energy emitted by the first resonant circuit 202 at the first frequency F.

FIG. 3 illustrates operations performed on a marker according to one embodiment. Although a specific set of operations is included in FIG. 3 as presented herein, it is contemplated that the above-described operations are merely provided as an example of how the general functionality described herein may be implemented. In addition, the operations presented do not have to be performed in the order presented unless otherwise indicated. The embodiments are not limited in this context.

Fig. 3 illustrates an operational flow 300 for a marker illustrating the operational steps performed by the marker 200 according to one embodiment. As shown in flow 300, at block 302, an interrogation signal may be received at the marker first resonant circuit. At block 304, a first resonant signal having a first resonant frequency is generated in response to the interrogation signal. At block 306, a first resonant signal is received at a second resonant circuit overlapping the first resonant circuit. At block 308, in response to the first resonant signal, a second resonant signal having a second resonant frequency may be generated, wherein the second resonant frequency is different from the first resonant frequency. For example, the second resonant frequency may be about half the first resonant frequency.

FIG. 4 is a first circuit for forming a marker according to one embodiment. Fig. 4 shows a circuit 400. The circuit 400 in the marker 200 may be comprised of a dual resonant structure. In one embodiment, the circuit 400 may include a first resonant circuit 402 and a second resonant circuit 404.

In one embodiment, the circuit 400 may include one or more planarizing coils. The term "planar coil" as used herein refers to a coil having a relatively flat geometry. For example, the planarizing coil can have a thickness of less than 1 millimeter (mm). In another embodiment, the thickness of the planarizing coil can be approximately 0.2mm or 200 microns. The thickness of any given planarizing coil can vary depending on the given equipment, and embodiments are not limited in this context.

In one embodiment, the coil 400 may include a first resonant circuit 402. The first resonant circuit 402 may include an inductor-linear capacitor combination. For example, the first resonant circuit 402 may include a first planarizing coil 406 having a pair of terminals, and a capacitor C1 connected to the pair of terminals. Capacitor C1 may be formed from a linear or non-linear capacitor, depending on the given device. For example, in one embodiment, capacitor C1 is formed from a linear capacitor. When receiving electromagnetic radiation at a first predetermined frequency, the first resonant circuit 402 resonates at the first predetermined frequency. The number of turns of the first planarizing coil 406 can be varied based on the frequency of the interrogation signal 104. With an operating frequency of 13.56MHz, the first planarizing coil 406 has approximately 10 turns, which is sufficient for the resonance and transmitter coupling needed to induce the proper operating voltage. The first resonant circuit stores and amplifies the field as it receives electromagnetic energy from the transmitter 102. This field is imparted to the second resonant circuit 404 by magnetic coupling as described below.

In one embodiment, the circuit 400 may include a second resonant circuit 404. The second resonant circuit 404 may include an inductor-nonlinear capacitor combination. For example, the second resonant circuit 404 may include a second planarizing coil 408 having a pair of terminals and a nonlinear capacitor D1 connected to the pair of terminals. The nonlinear capacitor D1 may be a voltage dependent capacitor. Second resonant circuit 404 may receive the amplified field from first resonant circuit 402 and generate a second resonant signal at a second resonant frequency that is half the frequency of the interrogation signal and the first resonant signal. In one embodiment, the second resonant circuit 404 may generate a second resonant signal at 6.78MHz with a magnetic field threshold of approximately 10mA/m rms.

One advantage of circuit 400 is that it has a lower magnetic field threshold compared to conventional frequency division circuits. The frequency division process has a minimum threshold below which it cannot operate. Thus, the transmitted field at the marker must exceed a minimum magnetic field threshold. The lower the threshold, the more sensitive the marker. Conventional frequency division identifiers using a combination of inductor coil-zener diode have a standard turn-on threshold of about 100mA/m rms. In one embodiment, the circuit 400 may output a marker signal at 6.78MHz with a magnetic field threshold of about 10mA/m rms. As a result, marker 200 utilizing circuit 400 produces a more sensitive marker, thereby improving EAS functionality.

As shown in fig. 4, the first planarizing coil 406 and the second planarizing coil 408 are disposed in a predetermined overlapping amount with each other, thereby forming a double tuned circuit. The amount of overlap determines the degree of mutual coupling k between the magnetic fields of each resonant circuit. To achieve color separation, the coupling coefficient k between the first planarizing coil 406 in the first resonant circuit 402 and the second planarizing coil 408 in the second resonant circuit 404 should be in the range of 0.0 to 0.6. For example, in one embodiment, k is 0.3 in order to provide sufficient coupling between the fields.

The second resonant circuit 404 may use several different non-linear capacitors as D1. For example, the non-linear capacitor D1 may be obtained using a zener diode, a varactor, a Metal Oxide Semiconductor (MOS) capacitor, or the like. The specific nonlinear capacitor element may be determined based on a number of different factors. For example, one factor may be capacitive nonlinearity (dC/dV). The startup field threshold depends on the dC/dV value at zero voltage bias condition. The higher the dC/dV value, the smaller the threshold. In another embodiment, one factor may be the capacitive loss (Df). The dissipation factor determines the amount of energy that the resonant LC circuit can store. The lower Df, the more efficient the circuit can operate. Other factors such as inductor-capacitor ratio and coil losses can also affect frequency division performance.

MOS capacitors may also be used as non-linear elements. The MOS capacitor can provide excellent dC/dV characteristics. This may greatly improve the sensitivity of the device. In addition, proximity deactivation (proximity deactivation) can be obtained by the breakdown mechanism of the MOS device. By adjusting the thickness of the oxide layer, the MOS breakdown voltage can be controlled. For deactivation, the F/2 frequency can be generated and resonance occurs in the inductor-nonlinear capacitor resonator until the breakdown voltage of the MOS is reached.

FIG. 5 is a second circuit for deriving a marker according to one embodiment. Fig. 5 shows a circuit 500. The circuit 500 in the marker 200 may be composed of different dual resonant structures. In one embodiment, the circuit 500 may be comprised of a first resonant circuit 502 and a second resonant circuit 504. The first resonant circuit 502 and the second resonant circuit 504 are identical to the first resonant circuit 402 and the second resonant circuit 404, respectively. The first resonant circuit 502 includes a first planarizing coil 506 and a linear capacitor C1. The second resonant circuit 504 includes a second planarizing coil 508 and a nonlinear capacitor D1.

In one embodiment, circuit 500 includes a coil arranged to achieve a coupling of 0.3. Circuit 500 may be represented as a dual resonant structure having one LC resonant circuit in another LC resonant circuit. As shown in circuit 500, a second resonant circuit 504 is nested within a first planarizing coil 506 in a first resonant circuit 502. By providing the F resonant circuit outside the F/2 resonant circuit, the sensitivity of this configuration can be improved by increasing the field effective area. While the circuit 500 illustrates a configuration in which the second resonant circuit 504 is nested within the first planarizing coil 506, it is contemplated that the opposite configuration may be utilized and still fall within the scope of the present embodiments. The embodiments are not limited in this context.

Frequency division identifiers such as circuits 400 and 500 can be manufactured in a number of different ways. For example, the metal pattern of the inductor coil may be deposited, etched, stamped or otherwise disposed on the film and flexible substrate. The non-linear capacitor may be soldered to the inductor terminals. Conventional soldering techniques may cause the marker to have a slight bulge due to the layout of the nonlinear capacitor elements. To avoid such a protrusion, an organic semiconductor process may be used. Organic semiconductor processing allows the mass production of conductor patterns and nonlinear elements in a flexible substrate. The embodiments are not limited in this context.

Although the embodiments have been discussed in terms of a dual resonant structure, it is contemplated that a single LC resonant circuit may also be implemented using the principles described herein. For example, a single LC resonant circuit including a non-linear capacitor and a planar coil may operate in the 13.56MHz band. Higher operating frequencies make the geometry smaller and the form factor of a single LC resonant circuit smaller, while still emitting a detectable resonant signal of the appropriate frequency. The embodiments are not limited in this context.

One or more embodiments, or portions thereof, may be implemented by altering the structure by varying any number of factors, such as desired computational rate, power levels, thermal tolerances, processing cycle budget, input data rates, output data rates, memory resources, data bus speeds and other operational limitations. For example, a portion of an embodiment may be implemented using software executable by a processor. The processor may be a general-purpose or a special-purpose processor, such as for example a processor of the type described byProcessors manufactured by corporation. Software may include computer program code segments, programming logic, instructions or data. The software may be stored on a medium accessible by a machine, computer or other processing system. Examples of an accessible medium may include a computer readable medium, such as Read Only Memory (ROM), Random Access Memory (RAM), Programmable ROM (PROM), Erasable PROM (EPROM), a magnetic disk, an optical disk, and so forth. In one embodiment, the medium may store programming instructions in a compressed and/or encrypted form, as well as program instructions that have been edited or installed by an installer prior to execution by the processor. In another example, portions of one embodiment may be implemented using dedicated hardware, such as an Application Specific Integrated Circuit (ASIC), Programmable Logic Device (PLD), or DSP, as well as additional hardware structures. In yet another example, by programmedCombinations of conventional computer components and custom hardware components implement portions of one embodiment. The embodiments are not limited in this context.

While certain features of the embodiments of the invention have been described, many modifications, substitutions, changes, and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the embodiments of the invention.

Claims (14)

1. A marker for use in an electronic article surveillance system, comprising:

a first resonant circuit including a first planarizing coil having a pair of terminals and a first capacitor connected to the pair of terminals, the first resonant circuit producing a first resonant signal in response to an interrogation signal; and

a second resonant circuit including a second planarizing coil having a pair of terminals and a second capacitor connected to the pair of terminals, the second resonant circuit receiving the first resonant signal and producing a second resonant signal having a second resonant frequency, wherein fields of the first and second resonant circuits are coupled to each other, the marker characterized by:

the first and second planarizing coils are separate circuits that are not connected with a wire and partially overlap, wherein an amount of overlap of the first and second planarizing coils corresponds to an amount k of mutual coupling between fields generated by the first and second planarizing coils, the value of k being in a range of 0.0 to 0.6, wherein the second capacitor of the second resonant circuit is a nonlinear capacitor that operates as a voltage-dependent capacitor.

2. The marker of claim 1 wherein the turns of said first and second planarizing coils intersect.

3. The marker of claim 2, wherein the value of k is about 0.3.

4. The marker of claim 1, wherein said second capacitor comprises one of a zener diode, a varactor, and a metal oxide semiconductor capacitor.

5. The marker of claim 1, wherein said second resonant frequency is less than said first resonant frequency.

6. The marker of claim 1, wherein said second resonant frequency is approximately one-half of said first resonant frequency.

7. The marker of claim 1, wherein said interrogation signal operates at about 13.56 megahertz.

8. The marker of claim 1, wherein said first resonant signal is approximately 13.56 megahertz and said second resonant frequency is approximately 6.78 megahertz.

9. An electronic article surveillance system comprising:

a transmitter that transmits an interrogation signal operating at a first frequency;

a security tag having a frequency-division identifier, the frequency-division identifier comprising:

a first resonant circuit including a first planarizing coil having a pair of terminals and a first capacitor connected to the pair of terminals, the first resonant circuit producing a first resonant signal in response to an interrogation signal; and

a second resonant circuit including a second planarizing coil having a pair of terminals and a second capacitor connected to the pair of terminals, the second resonant circuit receiving the first resonant signal and generating a second resonant signal having a second resonant frequency, wherein fields of the first and second resonant circuits are coupled to each other,

wherein the first and second planarizing coils are separate circuits that are not wired together and partially overlap, wherein the amount of overlap of the first and second planarizing coils corresponds to an amount k of mutual coupling between the fields generated by the first and second planarizing coils, the value of k being in the range of 0.0 to 0.6, wherein the second capacitor of the second resonant circuit is a non-linear capacitor that operates as a voltage-dependent capacitor; and

and the detector detects the second resonance signal from the marker and generates a detection signal according to the second resonance signal.

10. The system of claim 9 wherein the turns of said first and second planarizing coils intersect.

11. The system of claim 10, wherein k has a value of about 0.3.

12. The system of claim 9, wherein said interrogation signal is approximately 13.56 megahertz.

13. The system of claim 9, wherein the first resonant signal is approximately 13.56 megahertz and the second resonant frequency is approximately 6.78 megahertz.

14. The system of claim 9, further comprising an alarm system coupled to said detector, said alarm system receiving said detection signal and generating an alarm signal in response to said detection signal.