Abstract:
A security tag includes a combination of a resonant frequency circuit with an adjacent amplification shield for enhancing output signal amplitude. The amplification shield is located adjacent to the resonant frequency circuit and is preferably in the same or substantially the same plane as the resonant frequency circuit or is in a close, generally parallel plane. In an exemplary embodiment, the resonant frequency circuit includes an inductor electrically coupled to a capacitor. The resonant frequency circuit has a center frequency and is arranged to resonate in response to exposure to electromagnetic energy at or near the center frequency, providing an output signal having an amplitude. The amplification shield is arranged to direct a portion of the electromagnetic energy to the resonant frequency circuit to amplify the amplitude of the output signal from the resonant frequency circuit.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to passive security tags detectable by identification and security systems and more particularly to a resonant circuit responsive with an output signal enhanced for improved performance. 
     BACKGROUND OF THE INVENTION 
     The use of inductor capacitor (LC) resonant frequency circuits is well known for applications which include electronic article surveillance (EAS), chip based radio frequency identification (RFID), chipless RFID and other such applications. In such applications there are three key circuit parameters, which are typically employed for quantifying the electrical performance of the circuit, and, in particular, the antenna portion of the circuit. The three parameters are (1) the center frequency of the resonant circuit, (2) the quality factor (Q factor) of the resonant circuit, and (3) the relative output signal amplitude of the resonant circuit. With such circuits, the bandwidth is defined as the difference between an upper frequency and a lower frequency (F 2 ) of the circuit at which the output amplitude response is 3 dB below the passband response. The output signal amplitude is a measured height of response of the circuit based on a fixed position and a fixed incident magnetic field strength. The quality or Q factor is the ratio of the center frequency of the resonant circuit divided by the bandwidth output signal of the circuit. 
     LC resonant frequency circuits are well known in the art. When used for EAS, such circuits are formed into labels or tags which are applied to goods to be protected. As an example, a tag may be formed of a dielectric substrate having first and second generally parallel planar surfaces on opposite sides thereof. A first side of the substrate includes a first conductive pattern in the form of a coil (forming the inductor of the circuit), a first end of which terminates in a generally square or rectangular plate forming a first electrode of the capacitor portion of the circuit. The second surface of the substrate includes a second generally square or rectangular plate forming the second electrode of the capacitor portion of the circuit and a conductive trace extending away from the capacitor plate to a point proximate an edge of the substrate. The distal end of the conductive trace is electrically connected by a weld through or around the edge of the substrate to the second end of the coil to thereby complete the parallel LC circuit. When a tag of this type is exposed to electromagnetic energy at or near the center frequency of the tag, as determined by the values of the inductor and capacitor in accordance with a known formula, the circuit resonates. 
       FIG. 1  shows four typical output signals resulting from the resonance of four respective typical LC tag circuits. A first one of the output signals  2  shown in  FIG. 1  is marked to show the center frequency (Fc), the upper frequency (F 1 ) and the lower frequency (F 2 ) of the respective LC tag having the output signal. Of course, the difference between the upper frequency and the lower frequency establishes the bandwidth. A second one of the output signals  4  shown in  FIG. 1  is also marked to show the amplitude (A 1 ) of the respective output signal from the tag described above. 
     The present invention seeks to improve the performance of a typical LC resonant circuit. In particular, the present invention is aimed at enhancing or amplifying the output signal amplitude response from an LC resonant circuit. 
     SUMMARY OF THE INVENTION 
     The preferred embodiment includes a combination of a LC resonant frequency circuit with an adjacent amplification shield. In one preferred embodiment, a resonant LC circuit device includes a resonant frequency circuit having an inductor electrically coupled to a capacitor. The resonant frequency circuit has a center frequency and is arranged to resonate in response to exposure to electromagnetic energy at or near the center frequency, providing an output signal having an amplitude. The amplification shield directs a portion of the electromagnetic energy to the resonant frequency circuit to amplify the amplitude of the output signal from the resonant frequency circuit. 
     In another preferred embodiment, a resonant LC circuit device includes a passive response member that resonates an output signal having an amplitude in response to exposure of the passive response member to electromagnetic energy at or near a desired frequency. The LC circuit also includes a passive amplification member that directs a portion of the electromagnetic energy to the passive response member to amplify the amplitude of the output signal. 
     Yet another preferred embodiment includes a method for passively amplifying a response output signal from a LC circuit device. The method includes resonating an output signal having an amplitude from the resonant LC circuit device in response to exposure of the resonant LC circuit device to electromagnetic energy at or near a desired frequency, and directing a portion of the electromagnetic energy to the resonant LC circuit device to amplify the amplitude of the output signal. 
     Further scope of applicability of the present invention will become apparent in the description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. Accordingly, the invention will be described in conjunction with the following drawings, in which like-referenced numerals designate like elements, and wherein: 
         FIG. 1  is a graph showing a trace of the output response signal of a typical LC resonant tag; 
         FIG. 2  is a top plan view of a tag which is engaged with an amplification shield in accordance with a first exemplary embodiment of the present invention; 
         FIG. 3  is a graph showing traces of the output response signals of the tag of  FIG. 2  with and without the amplification shield; 
         FIGS. 4A and 4B  are examples of alternate embodiments of an amplification shield which may be used with a tag in a CD package to enhance tag performance; 
         FIG. 5  is a graph showing traces of the output response signals of a tag with and without the amplification shields of  FIGS. 4A and 4B ; 
         FIG. 6  illustrates a further exemplary embodiment of an amplification shield used in connection with a tag of the type employed in a CD jewel box; 
         FIG. 7  is a diagrammatic illustration of a further embodiment of the present invention which incorporates an amplification shield within product packaging; and 
         FIG. 8  is a graph showing traces of the output response signals of the tag of  FIG. 7  both with and without the amplification shield. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment includes a combination of a LC resonant frequency circuit with an adjacent amplification shield. In the embodiments illustrated and described below, the LC resonant frequency circuit is in the form of an EAS tag of a type well known to those of ordinary skill in the art and described briefly above. It should be appreciated that the present invention is not limited to use in conjunction with an existing EAS tag. That is, the present invention is also useful in other applications, including chip based RFID applications, chipless RFID applications and other applications which are known or will become known to those of ordinary skill in the art. Accordingly, it should be clearly understood that while the following illustrations are directed to the use of an amplification shield in connection with either EAS or RFID LC resonant frequency tags, the examples are only for the purpose of illustrating the inventive concepts and the invention is not limited to the use of an amplification shield with such tags. 
     As stated above, the preferred embodiment includes a combination of an LC resonant frequency circuit and an amplification shield. While not being limited to a particular theory, in the illustrated examples the amplification shields are generally planar and are made of a metal or metallic material, such as steel, aluminum, or the like. There are several benefits regarding the methods and mechanisms that are in play with the shield that result in an increased circuit amplification. One benefit is that the shield absorbs at least some of the magnetic field and magnetically couples the absorbed energy to the resonant LC circuit to increase the circuit amplitude or energy available for resonance. Another benefit is that the shield reflects/refracts at least some of the energy from the magnetic field and redirects the energy field toward the LC circuit. A third benefit incorporates the combination of both the absorption and reflection/refraction of the magnetic field to couple added energy to the LC circuit and likewise increase the signal amplitude of the circuit. The presence of an amplification shield with an LC circuit increases the circuit&#39;s measured signal amplitude and likewise results in an improved detection or read distance of the LC circuit. 
     In some of the illustrated embodiments, the area of the amplification shield is substantially greater than the overall dimensions of the LC resonant frequency circuit or tag. However, in other embodiments ( FIG. 6  for example) the amplification shield is actually smaller than the overall dimensions of the LC resonant frequency circuit. It will be appreciated by those of ordinary skill in the art that the amplification shield could be made of a conductive material or partially conductive material other than the materials described above. It will also be appreciated by those of ordinary skill in the art that while the present embodiments of the amplification shield are all generally planar, the amplification shield could take on a different shape, if desired. Finally, it should be appreciated by those of ordinary skill in the art that the thickness of the amplification shield may vary from application to application depending upon the parameters and use of the LC resonant frequency circuit. 
     While not being limited to a particular theory, preferably, and as illustrated by the embodiments described below, the amplification shield is located adjacent to the LC resonant frequency circuit and is preferably in the same (or substantially the same) plane as the LC resonant frequency circuit, or is in a close, generally parallel plane to the circuit. It will be appreciated by those of ordinary skill in the art that the amplification shield may be spaced from the LC resonant frequency circuit, if desired, and may be in a different plane ( FIG. 7  for example), if desired. The LC resonant frequency circuit could overlap all or a portion of the amplification shield but if there is an overlap it is preferably only a slight overlap. The LC resonant frequency circuit could be spaced from the amplification shield by preferably the spacing is small. While not being limited to a particular theory, both the LC resonant frequency circuit and the amplification shield are passive, in that they are not required to be a source of energy but are responsive to energy in an electromagnetic field. 
       FIG. 2 , illustrates a first example of a preferred embodiment of a LC resonant frequency circuit  5  in the form of an exemplary LC resonant tag  10  and a surrounding amplification shield  12 . While not being limited to a particular theory, the amplification shield  12  is comprised of a planar layer of metal foil with a thickness of about 38 microns and is generally in the configuration of a square. While the thickness of about 38 microns is a preferred thickness, the thickness of the amplification shield may vary within the scope of the invention. For example, a thinner shield may be preferable for lower costs, while a thicker shield may be preferable for structural integrity. The shield  12  includes a center portion  14  having an aperture of a size which approximates the dimensions of the tag  10 . The center portion  14  has been removed from the amplification shield  12  to provide an open area within which the tag  10  is inserted. In this manner, the tag  10  is in the same or substantially the same plane as the plane of the amplification shield  12  with the amplification shield surrounding the tag on all sides. 
     Still referring to  FIG. 2 , the tag  10  and amplification shield  12  are electromagnetically coupled, as the tag and shield preferably are not in physical contact, but are substantially in the same plane. Electrical current flows through and around the amplification shield  12 , and couples magnetically with the tag  10  via a coil or capacitor of the tag. Preferably, the amplification shield  12  includes a break or slotted groove  16 , which extends between the cutout center portion  14  and an outer edge  18  of the amplification shield. The groove  16  provides an open loop to eliminate inductive short-circuiting of the coil portion of the tag  10 . In the illustrated embodiment shown in  FIG. 2 , the tag  10  is generally square, preferably with sides of approximately four inches and the amplification shield  12  is generally square with sides of about 8 inches with the open center portion  14  being square and about four inches. It will be appreciated by those of ordinary skill in the art that while it is preferable to have a shield  12  with dimensions that are greater than that of the tag  10 , the specific dimensions employed in connection with the illustrated embodiment are not meant to be limiting. 
       FIG. 3  shows the output characteristics or traces of the response signals from the tag  10  of  FIG. 2  both with and without the amplification shield  12 . The trace labeled “A” shows the output signal of the tag  10  without the amplification shield  12 , in which the tag  10  has a center frequency of approximately 12.852 MHz, a quality factor of 71.7 and a signal amplitude of 6.85 dB. The trace labeled “B” illustrates the output characteristics of the same tag  10  but with the amplification shield  12  attached, as shown, for example, in  FIG. 2 . As can be seen from Trace B of  FIG. 3 , the center frequency is shifted upwardly to 13.57 MHz, the quality factor is slightly diminished to 60.3, but the signal amplitude has nearly doubled to 12.15 dB. 
       FIGS. 4A and 4B  illustrate more examples of the preferred embodiment in the form of two slightly different amplification shields  20 ,  22 . The amplification shields  20 ,  22  may preferably be used in conjunction with a DVD or CD package. In the embodiment illustrated in  FIG. 4A , the amplification shield  20  is a splint shield comprised of two generally semicircular shaped components  20 A and  20 B which are spaced apart by a slight distance  20 C. In the embodiment illustrated in  FIG. 4B , the amplification shield  22  is generally circular and continuous. While not specifically shown in  FIG. 4B , the amplification shield  22  may also include a break or slot as exemplified by the slotted groove  16  described above during the discussion of  FIG. 2 . Both amplification shields  20 ,  22  are adapted to fit within a standard CD “jewel case” package or similar package employed for a CD, DVD or the like. 
     Although not illustrated in  FIGS. 4A and 4B , a tag  10  can be positioned within the standard CD “jewel case” package, either adjacent to or slightly overlapping either of the amplification shields  20 ,  22 . In the illustrated embodiment, the amplification shields  20 ,  22  are formed of a thin layer of aluminum foil. However, other conductive materials could alternatively be employed. In the illustrated embodiment, the amplification shields  20 ,  22  have a thickness in the range of about 1–100 microns. However, other thicknesses may alternatively be employed. 
       FIG. 5  illustrates the response of the tag  10  with and without an amplification shield as described in greater detail below. Trace A of  FIG. 5  shows the output response of the tag  10  by itself. Trace B of  FIG. 5  shows the output response of the tag  10  when placed in a CD “jewel case” package without an amplification shield. Trace C of  FIG. 5  shows the output response of the same tag  10  in the CD package but with the amplification shield  22  of  FIG. 4B  placed within the CD package so that the tag at least partially overlaps the amplification shield. Trace D of  FIG. 5  illustrates the output response of the split amplification shield  20  of  FIG. 4A  with the same tag  10  that is used for Traces A–C and similarly placed within the CD package. As can be seen from a comparison of Trace B with Traces C and D, the use of either of the amplification shields  20 ,  22  results in a significant increase in amplitude from approximately 0.525 dB to approximately 0.750 dB for amplitude gain of about 43%. Trace E of  FIG. 5  illustrates the output response of the tag  10  with component  20 A, which is one half of the amplification shield  20  of  FIG. 4A . Trace E shows an output response amplitude gain over the output response amplitude of the tag  10  used without a shield (Trace B). 
       FIG. 6  illustrates another example of the preferred embodiment as a generally rectangular tag  30  having a resonant circuit  31  and a generally arcuate amplification shield  36 . The resonant circuit  31  includes a first electrical member formed as an inductor coil  32  and a second electrical member formed as a capacitor  34  having two conducting plates, with one of the plates coupled to the inductor coil. The amplification shield  36  is positioned within the generally open central area  38  of the tag  30 , such that the amplification shield  36  is in the same or substantially the same plane as the tag  30 . The resonant circuit  31  and amplification shield  36  are attached to a substrate  37 , preferably by bonding with an adhesive or heat. The bonding holds the resonant circuit  31  and amplification shield  36  in a spatial relationship as configured. While not being limited to a particular theory, the amplification shield  36  is connected to one of the conduction plates of the capacitor  34 . A tag  30  with an amplification shield  36  as shown in  FIG. 6  also exhibits enhanced output response signal amplitude. 
       FIG. 7  shows a container  40  having a parallelepiped shaped box  41  and a display panel  42  outwardly extending from a back wall  40 A of the box  41 . The display panel  42  is typically employed for advertising and display purposes. In the exemplary embodiment illustrated in  FIG. 7 , the container  40  includes a generally rectangular amplification shield  44  positioned along the back wall  40 A of the container  40  and a tag  46  positioned either in or on an outwardly extending display panel  42 . Preferably the amplification shield  44  is located along an interior side of the entire back wall  40 A for protection, and the tag  46  is located so that the tag and the amplification shield are generally aligned in substantially the same plane and adjacent to but slightly spaced from each other. While not being limited to a particular theory, the amplification shield  44  is preferably attached to the back wall  40 A via an adhesive or heat bonding, and the tag  46  is attached to the display panel  42  via an adhesive or held in place between layers of the panel, as readily understood by a skilled artesan. 
       FIG. 8  illustrates the performance of the tag  46  of  FIG. 7 , both without the amplification shield  44  (trace A) and with the amplification shield  44  (trace B). As can be seen, the use of the amplification shield  44  increases the response output signal amplitude from the tag  46  from about 0.4 dB to about 0.48 dB for an amplitude gain of about 20%. It is understood that the amplitude could be even further increased by minimizing the spacing between the tag  46  and the amplification shield  44 . 
     As can be seen from the foregoing discussion and examples, the use of an amplification shield provides significant improvement in the output signal amplitude of a LC resonant frequency circuit. Thus, an amplification shield can be employed for applications in which a higher output signal amplitude is desired without the need for increasing the size of the resonant frequency circuit. The application of such an amplification shield is much more efficient and cost effective than developing and producing a larger resonant frequency circuit or tag. The use of amplification shields also provide greater flexibility for meeting an array of resonant frequency circuit applications with the same basic circuit design and construction but with amplification shields of differing sizes, shapes or configurations. 
     From the foregoing it can be that the present invention comprises an improved LC resonant frequency circuit which includes an amplification shield for enhancing output signal amplitude. It will be appreciated by those of ordinary skill in the art that changes or modifications could be made to the various embodiments described above. For example, the embodiment exemplified in  FIG. 6  could be modified with the capacitor  34  positioned outside the inductor coil  32 , and one plate of the coil extended to form an amplification shield. It should be understood, therefore, that the present invention is not limited to the particular described embodiments, but instead, is meant to encompass all modifications within the spirit and scope of the disclosed inventive concept.