Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2000-12-01
2002-11-12
Lee, Benjamin C. (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S572800, C343S741000
Reexamination Certificate
active
06480110
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to radio frequency identification tags, and more particularly, to inductively tunable antenna circuit for a radio frequency identification tag.
2. Description of the Related Technology
Radio frequency identification (RFID) tags have been used in managing inventory, electronic access control, security systems, automatic identification of cars on toll roads, electronic article surveillance (“EAS”), etc. By utilizing the advantages of radio frequency, RFID tags will work under more hostile environmental conditions than optical bar-code labels since RFID tags are capable of being read through non-metallic substances such as paint, water, dirt, dust, human bodies, concrete, and even through the tagged item itself.
RFID tags are used in conjunction with a radio frequency tag reader (“Interrogator”) which transmits a radio frequency (“RF”) carrier signal and detects data signals from the RFID tag. Passive RFID tags do not use external power sources, rather they use incoming RF carrier signals from the reader (“Interrogator”) as a power source. The passive RFID tag is activated by a DC voltage that is developed as a result of rectifying the incoming RF carrier signal. Once the RFID tag is activated, it transmits the information (data) stored in its memory register.
A typical RFID tag comprises a silicon integrated circuit (IC) and an antenna circuit. The silicon IC may include an RF (or AC) rectifier that converts RF (or AC) voltage to DC voltage, a modulation circuit that is used to transmit stored data to a reader (“Interrogator”), a memory circuit that stores information, a logic circuit that controls overall function of the device, etc. The antenna circuit for a typical RFID tag may be formed by a parallel resonant LC circuit, where L is inductance and C is capacitance, as illustrated in FIG.
1
.
A limiting factor of the RFID application is its reading range, which is defined as the communication operating distance between the reader and tag. The reading range of a typical RFID tag may be less than one meter. In order to maximize the reading range, the RFID tag's antenna circuit must be tuned precisely to the carrier signal so that the voltage across the antenna circuit is maximized. The degree of preciseness of the tuning circuit is related to the spectrum width of the reader's carrier signal. For example, the Federal Communication Commission regulates the RFID tag devices spectrum to 13.56 MHz±7 KHz. Therefore, the reader must transmit the 13.56 MHz carrier signal within ±7 KHz bandwidth. To receive this signal, the tag's antenna circuit must be narrowly tuned to the 13.56 MHz±7 KHz signal. For 13.56 MHz application, the inductance L is typically formed by printed, etched, or wired circuit (
FIG. 1
shows various prior art antenna circuits for RFID.). A typical value of the inductance for this frequency band (13.56 MHz) is a few hundred nanohenries to a few microhenries. A fixed chip capacitor, silicon capacitor, or parasitic capacitor that is formed by the tag itself is typically used for the capacitor. These L and C values have wide variations in tolerance. Therefore a tuning circuit is often needed to compensate for the tolerance variations of these L and C components. The tuning of an LC resonant circuit can be accomplished by either adjusting the L or C component values.
A typical passive RFID tag utilizes an induced antenna coil voltage for operation. This induced AC voltage is rectified and results in a DC voltage. As the DC voltage reaches a certain level, the RFID device starts operating. By providing an energizing RF signal, an RFID reader can communicate with a remotely located RFID device that has no external power source such as a battery. Since the energizing and communication between the reader and tag is accomplished through antenna circuit, it is important that the RFID device be equipped with a properly tuned antenna circuit for successful RFID applications. An RF signal can be radiated or received effectively if the linear dimension of the antenna is comparable with the wavelength of the operating frequency. However, the wavelength at 13.56 MHz, for example, is 22.12 meters. Therefore, it is difficult to form a true full size antenna in a limited space, and this is true for most RFID applications. Alternatively, a small LC loop antenna circuit that resonates at the operating frequency is used.
The small LC loop antenna may comprise a spiral coil and a capacitor, the spiral coil is formed by n-turns of wire, or n-turns of printed or etched inductor on dielectric substrate. FIG.
3
illustrates an LC antenna circuit having a spiral inductor on dielectric substrate. The inductor (b) may be formed by n-turns of wire and the inductor (a) may be formed by printed circuit techniques, etc.
Generally, the RFID tag antenna may be tuned using trimming capacitors. This capacitive tuning method, however, requires the capacitor electrodes (metallization) to be on both the top and bottom sides of the substrate, which in turn requires the tag's antenna circuit to be formed with a double sided circuit. A double-sided circuit structure generally requires a more complex manufacturing process than does a single sided circuit which results in a higher cost product.
Therefore, what is needed is an inexpensive, simple and effective way of tuning an RFID tag antenna circuit without requiring a double sided circuit structure for the antenna circuit.
SUMMARY OF THE INVENTION
The invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies in RFID applications by changing the inductance of a spiral coil of an RFID tag antenna circuit disposed on a circuit substrate. The variable inductance spiral coil on the substrate comprises a step-tunable inductor for frequency tuning of the antenna circuit of the RFID tag. The step-tunable inductor may resonant with a discrete capacitor connected to the inductor, or a capacitor that is part of and internal to a semiconductor integrated circuit RFID tag device. A series resonant circuit antenna is also contemplated and within the scope of the present invention.
The substrate may be, for example but not limited to; PET, Mylar, paper, plastic, Kapton, ceramic, polyimide, polyvinylchloride (PVC), etc., and combinations thereof. A RFID tag device semiconductor integrated circuit die is attached to the substrate, preferably on the same side as the antenna inductor and is electrically connected thereto. Connection to the RFID tag device semiconductor integrated circuit die may be by wire bonding, flipchip (C4), etc., or any combination thereof. The dielectric substrate may also have other connection pads that may be used for testing and/or programming the RFID tag. The coil material is electrically conductive and may be, for example but not limited to; metal such as copper, aluminum, gold, plated metal, electrically conductive organic and inorganic materials, etc.
In one embodiment of the invention, a parallel resonant circuit antenna is formed on only one side of a substrate. The parallel resonant circuit antenna, which includes a step-tunable inductor, may be printed as metallic traces on the one side of the substrate. The step-tunable inductor is adapted for adjustment to a desired inductance value for resonating the tuned frequency of the parallel resonant antenna circuit of the RFID tag.
In yet another embodiment of the invention, the step-tunable inductor consists of various inductive tuning branches, i.e., these branches are capable of being trimmed to adjust the step-tunable inductor to a desired inductance value. In other words, the length and spacing of the step-tunable inductor can be adjusted by trimming the branches. The branches may be trimmed by laser, ablation or mechanically cutting.
In another embodiment, fusible conductive links may be used to trim the branches that comprise the step-tunable inductor. Series connected turns may be disconnected by ca
Furey Lee
Lee Youbok
St. Amand Roger
Baker & Botts L.L.P.
Lee Benjamin C.
Microchip Technology Incorporated
Nguyen Phung T
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