Tunable spiral antenna for security tag

ES 2 322 604 T3

DESCRIPTION

Tunable coiled antenna for security tag.

Background

A radio frequency identification system (RFID) can be used for a variety of applications, such as inventory management, electronic access control, security systems, automatic identification of cars on toll roads, electronic surveillance of articles (EAS), etc. An RFID can comprise an RFID reader and an RFID device. The RFID reader can transmit a radio frequency carrier signal from the RFID device. The RFID device can respond to the carrier signal with a data signal encoded with the information stored by the RFID device.

An RFID device typically includes an antenna to communicate signals between the RFID device and the RFID reader. The antenna must be tuned to operate within a predetermined operating frequency or range of frequencies. Improved techniques for tuning an antenna can increase the performance of an RFID system, as well as reduce associated costs. Accordingly, there may be a need for improved tunable antennas within the RFID system.

WO 2004/027681 A2 discloses a radio frequency identification tag (RFID) antenna system that includes a two-armed logarithmic planar spiral antenna with a small form factor. The two arms are identical to each other but are rotated within the plane by 180 degrees and the electronics can be integrated into the same substrate as the planar antenna, but preferably the electronic circuit is integrated into a second substrate that is mounted, making the relevant electronic corrections , on the antenna support substrate.

Document WO 03/044892 A1 discloses an antenna for use in a remote RFID device or sensor whereby the antenna of the device is a combination of electric and magnetic dipole radiation. The antenna is configured such that the radiation emitted from the antenna is substantially a combination of the radiation patterns of a magnetic dipole and an electric dipole.

US 6,285,342 B1 discloses an RF transponder that includes a spiral dipole antenna. Claim 1 is delimited in relation to this prior art.

Brief description of the drawings

The object considered as the embodiments is especially pointed and clearly claimed in the final part of the specification. The embodiments, however, both with respect to the organization and the operating system, together with their objectives, characteristics and advantages, can be best understood with reference to the subsequent detailed description read in conjunction with the accompanying drawings. , in which:

Fig. 1 illustrates a block diagram of a system according to one embodiment;

Fig. 2 illustrates a side view of a security tag in accordance with one embodiment;

Fig. 3 illustrates a top view of a security tag with an antenna according to one embodiment;

Fig. 4 illustrates a top view of a security tag with an antenna provided with segment points according to one embodiment;

Fig. 5 illustrates a block flow diagram in accordance with one embodiment.

Detailed description

The embodiments can be directed to an RFID system in general. More specifically, the embodiments can be directed to an RFID device, such as a security tag. The RFID device may include a semiconductor integrated circuit (IC) and a tunable antenna. The tunable antenna can be tuned to a desired operating frequency by adjusting the length of the antenna. The range of operating frequencies can vary, although the embodiments may be particularly useful for an ultra-high frequency (UHF) spectrum. Depending on the application and the size of the area available for the antenna, the antenna can be tuned within several hundred Megahertz (MHz) or higher, such as 868 to 950 MHz, for example. In one embodiment, for example, the tunable antenna can be tuned to operate within an RFID operating frequency, such as the 868 MHz band used in Europe, the 915 MHz Industrial, Scientific and Medical (ISM) band. used in the United States, and the proposed 950 MHz band for Japan. It should be appreciated that these operating frequencies are offered by way of example only, and the embodiments are not limited in this context.

ES 2 322 604 T3

The tunable antenna has a unique antenna geometry of an inward spiral pattern useful for RFID applications or EAS applications. The inward spiral pattern can accommodate the antenna locks thereby returning traces back to the origin. This can result in an antenna with similar functionality to a conventional medium wave dipole antenna, but with a smaller overall size. For example, the size of a conventional 915 MHz medium wave dipole antenna would be approximately 16.4 centimeters (cm) long. On the contrary, some embodiments may offer the same performance as the conventional medium wave dipole antenna at the operating frequency of 915 MHz with a length less than about 3.81 cm. Also, the ends of the antenna locks can be modified to tune the antenna to a desired operating frequency. Since the ends of the antenna locks are inward of the antenna perimeter, tuning can be carried out without modifying the antenna geometry.

Numerous specific details may be set forth herein to provide a thorough understanding of the embodiments. It should be understood by those skilled in the art, however, that the embodiments can be practiced without these specific details. In other cases, well known procedures, practices, components and circuits have not been described in detail so as not to obscure the embodiments. It can be appreciated that the structural and functional details disclosed herein may be representative and do not necessarily limit the scope of the embodiments.

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

Referring now in detail to the drawings of which the same parts are indicated by the same reference numerals throughout, a first system according to one embodiment is illustrated in Fig. 1. Fig. 2 is a block diagram of an RFID system 100. In one embodiment, for example, the RFID system 100 may be configured to operate using an RFID device with an operating frequency in the 868 MHz band, in the 915 MHz band, and in the 950 MHz band. The RFID system 100, however, can also be configured to operate using other parts of the RF spectrum as desired for a given implementation. The embodiments are not limited in this context.

As shown in Fig. 1, the RFID system 100 may comprise a plurality of nodes. The term "node" as used herein can refer to a system, element, module, component, board or device that can process a signal representative of information. The signal can be, for example, an electrical signal, an optical signal, an acoustic signal, a chemical signal, etc. The embodiments are not limited in this context.

As shown in Fig. 1, the RFID system 100 may comprise an RFID reader 102 and an RFID device 106. Although Fig. 1 shows a limited number of nodes, it can be appreciated that any plurality of nodes can be used. in the RFID system. The embodiments are not limited in this context.

In one embodiment, the RFID system 100 may comprise an RFID reader 102. The RFID reader 102 may include a tuned circuit 108 comprising an inductor L1 and a capacitor C1 connected in series. RFID reader 102 may include continuous wave (CW) RF power through tuned circuit 108. This CW RF power can be electronically coupled by AC action to a parallel resonant circuit antenna 112 of RFID device 106. CW RF electromagnetic power can be generically represented by 114.

In one embodiment, the RFID system 100 may comprise an RFID device 106. The RFID device 106 may include a power converter circuit that converts part of the coupled CW RF electromagnetic power 114 into direct current power. by employing by the logic circuits of the semiconductor IC used to implement the RFID operations for the RFID device 106.

In one embodiment, the RFID device 106 may comprise an RFID security tag. An RFID security tag can include a memory for storing RFID information and can communicate the stored information in response to an interrogation signal, such as interrogation signals 104. The RFID information can include any type of information. that can be stored in a memory used by the RFID device 106. Examples of RFID information can include a unique card identifier, a unique system identifier, a monitored object identifier, and so on. The types and amount of RFID information are not limited in this context.

In one embodiment, the RFID device 106 may comprise a passive RFID security tag. A passive RFID security tag does not use an external power source, but rather uses the interrogation signals 104 as the power source. The RFID device 106 can be activated by a direct current voltage that occurs as a result of rectification of the incoming RF carrier signal comprising the interrogation signals 104. Once the RFID device 106 is activated, it can a then transmit the information stored in its memory register via response signals 110.

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In general operation, when the antenna 112 of the RFID device 106 is in proximity to the tuned circuit 108 of the RFID reader 102, an AC voltage is developed across the antenna 112. The AC voltage across the antenna 112 is rectified and when the rectified voltage is sufficient to activate the RFID device 106, the RFID device 106 can begin to send the data stored in its memory register by modulating the signals of interrogation 104 of the RFID reader 102 to form the response signals 110. The RFID reader 102 can receive the response signals 110 and convert them into a bit stream of sensed serial data words of on / off pulses representative of the information from the RFID device 106.

Fig. 2 illustrates a side view of a security tag according to one embodiment. FIG. 2 illustrates a security tag 200. Security tag 200 may be representative of, for example, RFID device 106. As shown in FIG. 2, security tag 200 may include a substrate 202, an antenna 204, a connecting frame 206, a semiconductor IC 208, and a cover material 210. Although Fig. 2 illustrates a limited number of items, it can be appreciated that more or fewer items may be used on the security tag 200. For example, an adhesive and release liner may be added to the security tag 200 to help secure the security tag 200. to an object to be watched. The embodiments are not limited in that context.

In one embodiment, the security tag 200 may include a substrate 202. The substrate 202 may comprise any type of material suitable for mounting the antenna 204, connecting frame 206, and IC 208. For example, the Material for the substrate 202 may include a base paper, polyethylene, polyester, etc. The particular material implanted in the substrate 202 can affect the RF performance of the security tag 200. More specifically, the dielectric constant and the loss tangent can characterize the dieletric properties of a substrate material suitable for use as a substrate 202.

In general, a higher dielectric constant can cause a wider frequency shift compared to free space with no substrate. Although it may be possible to retune the antenna to the original center frequency by physically modifying the antenna pattern, it may be desirable to have the lowest possible dielectric constant for the label substrate material to improve the headroom reading amplitude. The term "read range" can refer to the operational communication distance between the RFID reader 102 and the RFID device 106. An example of a read range for the security tag 200 may range from 1 to 3 meters, although the embodiments are not limited in this context. The loss tangent can characterize the absorption of RF energy by the dielectric. The absorbed energy may be lost as heat and may be unavailable for use with the IC 208. The energy lost may be the same as the reduction in transmitted power and may reduce the read width accordingly. Consequently, it may be desirable to have as low a loss tangent as possible at substrate 202, since it cannot be "detuned" by adjusting antenna 204. The total frequency shift and RF loss may also depend on the thickness of the substrate 202. As the thickness increases the shift and loss may also increase.

In one embodiment, the substrate 202 can be implanted using base paper. The base paper can have a dielectric constant of 3.3, a loss tangent of 0.135. The base paper can be relatively dissipative at 900 MHz. The embodiments are not limited in this context.

In one embodiment, the security tag 200 may include an IC 208. The IC 208 may comprise a semiconductor IC, such as an RFID chip or an application specific integrated circuit (ASIC) ("RFID chip"). ). The RFID chip 208 may include, for example, an RF or alternating current (AC) rectifier that converts the RF or AC voltage to a DC voltage, a modulation circuit that is used to transmit the stored data up to the RFID reader, a memory circuit that stores the information, and a logic circuit that controls the overall function of the device. In one embodiment, for example, the RFID chip 208 can be implanted using the RFID ASIC of the High Frequency Smart Tag (HSL) of the I-CODE or of the U-CODE manufactured by Philips Semiconductor. The embodiments, however, are not limited in this context.

In one embodiment, the security tag 200 may include a connecting frame 206. A connecting frame may be an element of connected packets, such as the Quad Flat Packet (QFP), the Small Contour Integrated Circuit ( SOIC), the Plastic Connected Chip Carrier (PLCC), etc. Connecting frame 206 may include a die mounting paddle or indicator, and multiple connecting fingers. The die paddle primarily serves to mechanically support the die during package manufacture. The connecting fingers connect the die to external circuitry up to the package. One end of each connecting finger is typically connected to a connecting pad on the die by connecting wire attachments or by automatic strap attachments. The other end of each connecting finger is the connection, which is mechanically and electrically connected to a substrate or a circuit board. The connecting frame 206 may be formed from a sheet of metal by stamping or etching, often followed by a plating, shrinking, and taping type finish. In one embodiment, for example, connecting frame 206 can be implanted using a Sensormatic EAS Microlabel connecting frame manufactured by Sensormatic Corporation, for example. The embodiments, however, are not limited in this context.

ES 2 322 604 T3

In one embodiment, the security tag 200 can include a cover material 210. The cover material 210 can be a semi-finished cover material applied to the top of a finished security tag. Like substrate 202, cover material 210 can also impact the RF performance of RFID device 106. In one embodiment, for example, cover material 210 can be implanted using a semi-finished cover material with a dielectric constant of 3.8 and a loss tangent of 0.115. The embodiments are not limited in this context.

In one embodiment, the security tag 200 may include an antenna 204. The antenna 204 may be representative of, for example, the antenna 112 of the RFID device 106. In an example that is not part of the claimed invention, a Antenna 204 may be constituted by a parallel resonant LC circuit, where L is the inductance and C is the capacitance. In one embodiment of the invention, antenna 204 is a tunable antenna. To increase the read width, the antenna 204 can be tuned to the carrier signal so that the voltage across the antenna circuit is maximized. The degree of precision of the tuning circuit is related to the width of the spectrum of the carrier signal transmitted by the transmitter 102. For example, in the United States the Federal Communication Commission may regulate a spectrum band of the RFID security tag. at 915 MHz. Therefore, the transmitter 102 must transmit the interrogation signals 104 at approximately 915 MHz. To receive the interrogation signals 104, the antenna 204 must be closely tuned to the 915 MHz signal. For 915 MHz applications, the inductance L is typically consisting of printed circuit, etched, or wiring. A fixed chip capacitor, a silicon capacitor, a parasitic capacitor which is constituted by the RFID device 106 itself is typically used for the capacitor. These L and C values ​​represent wide variations in tolerance. Therefore, it may be necessary to tune antenna 204 to compensate for tolerance variations of these L and C components. Tuning of an LC resonant circuit can be carried out one or the other by adjusting the values ​​of the L or C components.

In one example, the RFID device 106 may use an induced antenna coil voltage for its operation. This induced AC voltage can be rectified and translated into a DC voltage. When the DC voltage reaches a certain level, the RFID device 106 can begin to operate. By providing a power-up RFID signal via transmitter 102, RFID reader 102 can communicate with a remotely located RFID device 106 that does not have an external power source, such as a battery. Since powering up and communication between the RFID reader and RFID device 106 takes place through antenna 204, it may be important that antenna 204 is tuned for enhanced RFID applications. An RF signal can be effectively radiated or received if the linear dimension of the antenna is comparable to the wavelength of the operating frequency. The linear dimension, however, can be greater than the area available for antenna 204. Consequently, it can be difficult to form a true life-size antenna within a limited space, and this is true for most RFID applications. Consequently, the RFID device 106 can utilize a smaller LC loop antenna circuit that is arranged to resonate at a given operating frequency. An LC loop antenna can comprise, for example, a spiral coil and a capacitor. The spiral coil can be made up of n turns of wire, or n turns of a printed or etched inductor on the dielectric substrate.

In one embodiment, the antenna 204 may be designed such that the complex combination of the total antenna matches the impedance with the complex impedance of the connecting frame 206 and IC 208 at the desired operating frequency, such as 915. MHz, for example. When the RFID device 106 is placed on an object to be monitored, however, the resulting operating frequency may change. Each object can have a substrate material with dielectric properties that affect the RF compartment of antenna 204. As with substrate 202, the object's substrate can cause frequency shifts and RF losses determined by the dielectric constant, the tangent loss and material thickness. Examples of different object substrates can include a chip board which is a material used for part-level cartons, a corrugated fiber board which is the material used for corrugated boxes, video and DVD casings, glass, metal, etc. . Each object substrate can have a significant impact on the RFID device 106's read range.

In one embodiment, antenna 204 may be tunable to compensate for such variations. Since the dielectric constant for many materials is greater than 1, the operating frequency is typically reduced when the security tag 200 is affixed to an object substrate. In order to establish the original frequency, the antenna 204 must be altered somewhat, otherwise the detection performance and the read width may be reduced. In one embodiment, antenna 204 can be altered by trimming the ends of antenna 204. Trimming can be accomplished by severing the antenna conductor and isolating the trimmed antenna segment from the ends that were severed. The trimmed ends do not necessarily have to be removed to enable the tuning operation. Consequently, continuous tuning of the antenna 204 to the desired operating frequency may be possible to enable operation of an RFID device 106 when the RFID device 106 is attached to different objects. RFID device 106 in general, and antenna 204 in particular, can be described in greater detail with reference to Figs. 3 to 5.

Fig. 3 illustrates a top view of a partial security tag with an antenna according to one embodiment. Fig. 3 illustrates a top view of certain parts of the security tag 200. As shown in Fig. 3, the security tag 200 may comprise an antenna 204 disposed on

ES 2 322 604 T3 a substrate 202. The substrate 202 may have a substantially rectangular shape, for example. Antenna 204 may be disposed on substrate 202 by die cutting the antenna pattern of the label on substrate 202. Substrate 202 may comprise, for example, metallized paper with paperback. The RFID chip 208 can be connected to a connection frame 206 via a connection frame 206 for ultrasonic bonding to conductive pads located on the RFID chip 208. The RFID chip 208 and the connection frame 206 can be located directly at the geometric center of the dielectric substrate material of the substrate 202. The ends of the connecting frame 206 can be physically and electrically attached to the foil antenna pattern of the antenna 204. The covering material 210 (not shown) can then be applied over the entire upper surface of the security tag 200. to protect the assembly and provide a printable surface, if desired.

In one embodiment, for example, antenna 204 may comprise multiple antenna parts. For example, antenna 204 may comprise a first antenna portion 306 and a second antenna portion 308. The first antenna part 306 may be connected to a first side 206A of the connecting frame 206. The second antenna part 308 may be connected to a second side 206B of the connecting frame 206.

As shown in Fig. 3, the first antenna part 306 has a first antenna end 306A and a second antenna end 306B. Similarly, the second antenna portion 308 has a first antenna end 308A and a second antenna end 308B. The first antenna end 306A of the first antenna part 306 is connected to the connecting frame 206A. The first antenna portion 306 is disposed on the substrate 202 to form an inward spiral pattern from the RFID chip 208 in a first direction, with the second antenna end 306B terminating over the inner loop of the pattern. spiraling inward. Similarly, the first antenna end 308A of the second antenna portion 308 is connected to the connecting frame 206B. The second antenna portion 308 is disposed on the substrate 202 to form an inward spiral pattern from the RFID chip 208 in a second direction, with the second antenna end 308B terminating over the inner loop of the pattern. spiraling inward. In one embodiment, the first and second directions may spiral in a clockwise and clockwise direction, respectively. The embodiments, however, are not necessarily limited in this context.

In one embodiment, the antenna geometry of the antenna 204 can traverse around the perimeter of the substrate 202 and spiral inward. The inwardly directed spiral antenna pattern can provide several advantages. For example, the ends of antenna 204 may be located well within the perimeter of substrate 202. Placing the ends of the antenna 204 within the perimeter of the substrate 202 can allow the ends to be trimmed without using the amount of area used by the antenna 204. In another example, the Q of the antenna 204 can be maximized to that the response of the RFID device 106 only varies by approximately -3 dB at the limits of the ISM band. Using the Chu-Harrington limit of Q = 1 / (ka)³+ 1 / (ka), where k = 2 π / λ and "a" is a characteristic dimension of the antenna 204 so that a sphere of radius "a" could encompass the RFID device 106, for a high Q " ka ”should be << 1. Therefore, Q should be maximized in order to minimize“ a ”to the limits of the operating frequency band.

In one embodiment, the antenna 204 can be tuned to a desired operating frequency by modifying a first length for the first antenna portion 306, and a second length for a second antenna portion 308, after these two parts of the antenna are disposed on the substrate 202. For example, each part of the antenna can be divided into multiple antenna segments at multiple segment points. The first and second antenna lengths can be modified by electrical isolation of at least a first antenna segment from a second antenna segment. The length of the antenna can be modified by sectioning each portion of the antenna into multiple segment points, each segment point corresponding to an operating frequency of the antenna 204. Dividing the first antenna part 306 and the second antenna part 308 into multiple antenna segments results in the shortening of the length of each antenna part, and thereby effectively modifying the total inductance of antenna 204. The antenna segments and segment points can be described in greater detail with reference to FIG. 4.

Fig. 4 illustrates a diagram of a security tag with an antenna with segment points according to one embodiment. FIG. 4 illustrates a top view of portions of the security tag 200 with multiple segment points (SP). The antenna 204 can be tuned to a desired operating frequency by modifying a first length of the first antenna part 306, and a second length of the second antenna part 308, after these antenna parts are arranged. on the substrate 202. For example each part of the antenna can be divided into multiple antenna segments at multiple points from segments SP1 to SP4. The first and second antenna lengths can be modified by electrical isolation of at least a first antenna segment from a second antenna segment. The length of the antenna can be modified by sectioning each portion of the antenna at one or more segment points, each segment corresponding to an operating frequency of the antenna 204. The sectioning can be carried out in many different ways, such as cutting or piercing the antenna trace at a given segment point SP1 to SP4. Sectioning can create a slot at the segment point, such as slots 402 to 412.

In one embodiment, each segment point may correspond to an operating frequency of antenna 204. FIG. 4 illustrates four (4) segment points SP1 through SP4 by way of example. SP1 can tune antenna 204 for an operating frequency of approximately 868 MHz when RFID device 106 is in a

ES 2 322 604 T3 free space and not fixed to an object. SP2 can tune the antenna 204 for an operating frequency of approximately 915 MHz when the RFID operative 106 is in free space and not attached to an object. The SP3 can tune the antenna 204 for an operating frequency of approximately 915 MHz when the RFID device 106 is attached to a VHS cassette case. The SP4 can tune the antenna 204 for an operating frequency of approximately 915 MHz when the RFID device is attached to ordinary gray cardboard. It can be appreciated that the number of segment points and corresponding operating frequencies of antenna 204 may vary according to a given implementation. The embodiments are not limited in this context.

Fig. 5 illustrates a block flow diagram according to one embodiment. Security tag 200 can be displayed in a number of different ways. Fig. 5 illustrates a block flow diagram 500, which is an example of a way to exploit the security card 200. As shown in Fig. 5, an integrated circuit may be connected to a connection frame in the block 502. An antenna may be arranged on a substrate at block 504. The connecting frame can be connected to the antenna at block 506.

In one embodiment, the antenna can be tuned for use with an operating frequency at block 508. Tuning can be accomplished by modifying a length of the antenna. The length can be modified by sectioning the antenna into multiple antenna segments at a segment point corresponding to an operating frequency. Sectioning can electrically disconnect a first antenna segment from a second antenna segment, thereby effectively shortening the length of the antenna.

As previously described, the unique antenna geometry of an inward spiral pattern can be useful for RFID applications when connected to an RFID chip. The unique antenna geometry shown in Figs. 3 and 4, however, can also be useful for an EAS system. In one embodiment, for example, the RFID chip 208 can be replaced by a diode or other non-linear passive device in which the voltage and current characteristics are non-linear. The antenna of the diode or other passive non-linear EAS device may have the same geometry as shown in Figs. 3 and 4, and may be clipped to tune the antenna to the transmitter operating frequency used to transmit the interrogation signals destined for the EAS system. Similar to the RFID system 100, the range of operating frequencies can vary, although the embodiments may be particularly useful for the UHF particle spectrum, ranging from 868 to 950 MHz, for example. The embodiments are not limited in this context.

Some embodiments can be implemented using an architecture that can vary according to a plurality of factors, such as the desired computational speed, power levels, thermal tolerances, budget for processing cycles, transmission speeds. input data, output data transmission speeds, memory resources, data bus speeds and other performance constraints. For example, one embodiment can be implemented using software run by a general-purpose or special-purpose processor. In another example, an embodiment can be implemented as dedicated hardware, such as a circuit, an ASIC, a programmable logic device (PLD) or a digital signal processor (DSP), etc. In yet another example, an embodiment may be implemented using any combination of general purpose programmed computer components and custom hardware components. The embodiments are not limited in this context.

Some embodiments can be described using the terms "coupled" and "connected" together with their derivatives. It should be understood that these terms are not intended to be synonymous with one another. For example, some embodiments can be described using the term "connected" to indicate that two or more elements are in direct physical or electrical contact with each other. In another example, some embodiments may be described using the term "coupled" to indicate that two or more elements are in direct physical or electrical contact. The term "coupled", however, can also mean that two or more elements are not in direct contact with each other without thereby ceasing to cooperate or interact with each other. The embodiments are not limited in this context.

Although certain features of the embodiments have been illustrated and described herein, those skilled in the art will appreciate many modifications, substitutions, changes, and equivalents.