Antenna for a backscatter-based RFID transponder

An antenna for a backscatter-based RFID transponder is provided that has an integrated receive circuit having a capacitive input impedance for receiving a radio signal spectrally located in an operating frequency range. The antenna includes two antenna branches that extend outward from a connecting region in which the antenna branches can be connected to the integrated receive circuit, and a yoke-shaped first trace segment that is designed to connect the two antenna branches together. Each antenna branch can have a U-shaped second trace segment connected to the connecting region, and a U-shaped third trace segment connected to the second trace segment and extending parallel to the second trace segment. The invention further relates to a backscatter-based RFID transponder with such an antenna.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an antenna for a backscatter-based RFID (radio frequency identification) transponder, and a backscatter-based RFID transponder having such an antenna.

2. Description of the Background Art

The invention resides in the field of wireless and contactless communication. It resides particularly in the field of radio-based communication for the purpose of identifying objects, animals, persons, etc., as well as the transponders and remote sensors used for this purpose.

While applicable in principle to any desired contactless communication system, the present invention and the problem on which it is based are described below with reference to RFID communications systems and their applications. In this connection, RFID stands for “Radio Frequency Identification.”

In RFID systems, data is transmitted bidirectionally with the aid of high-frequency radio signals between a stationary or mobile base station, which is often also referred to as a reader or read/write device, and one or more transponders that are attached to the objects, animals or persons to be identified.

The transponder, which is also referred to as a tag or label, typically has an antenna for receiving the radio signal emitted by the base station, as well as an integrated circuit (IC) connected to the antenna. In this context, the integrated circuit includes a receive circuit for receiving and demodulating the radio signal and for detecting and processing the transmitted data. In addition, the integrated circuit has a memory for storing the data needed for identification of the corresponding object. Furthermore, the transponder can include a sensor, for example for temperature measurement, which is likewise part of the integrated circuit, for instance. Such transponders are also known as remote sensors.

RFID transponders can be used to advantage anywhere that automatic identification, detection, interrogation, or monitoring is to take place. The use of such transponders makes it possible for objects such as, for example, containers, pallets, vehicles, machines, or pieces of luggage, but also animals or people, to be individually marked and identified in a contactless way without a line-of-sight connection. In the case of remote sensors, it is additionally possible for physical qualities or parameters to be measured and interrogated.

In the area of logistics, containers, pallets and the like can be identified, for example in order to determine their current whereabouts during the course of shipping. In the case of remote sensors, the temperature of the transported goods or products can be regularly measured and stored, for example, and read out at a later point in time. In the area of cloning protection, items such as integrated circuits can be provided with a transponder in order to prevent unauthorized reproduction. In commercial applications, RFID transponders can replace the barcodes often placed on products. Additional applications include, for example, driveaway protection in the automotive field, or systems for monitoring the air pressure in tires, as well as in systems for personal access control.

Passive transponders have no independent energy supply, and extract the energy required for their operation from the electromagnetic field emitted by the base station. Semi-passive transponders, while they do indeed have their own energy supply, do not use the energy provided by it to transmit/receive data, but instead use it to operate a sensor, for example.

RFID systems with passive and/or semi-passive transponders whose maximum distance from the base station is significantly over one meter are operated in particular in frequency ranges in the UHF or microwave range.

In such passive/semi-passive RFID systems with a relatively long range, a backscattering-based method is generally used for data transmission from a transponder to the base station, in the course of which a portion of the energy from the base station arriving at the transponder is reflected (backscattered). In this process, the carrier signal is modulated in the integrated circuit according to the data to be transmitted to the base station and is reflected by means of the transponder antenna. Such transponders are referred to as backscatter-based transponders.

In order to achieve the greatest possible range with backscatter-based transponders, it is necessary to deliver the largest possible fraction of the energy arriving at the transponder from the base station to the integrated receive circuit of the transponder. Power losses of every type must be avoided in this process. On the one hand, this requires transponder antennas with a relatively broad receive frequency range. Such relatively wide-band antennas can have the additional advantage of meeting the requirements of multiple national or regional authorities with only one antenna type. On the other hand, the energy picked up by the transponder antenna must be delivered, with as little reduction as possible, to the integrated receive circuit, which typically has a capacitive input impedance, i.e. an impedance with a negative imaginary part.

Known from DE 103 93 263 T5, which corresponds to U.S. Pat. No. 6,963,317, is an antenna for an RFID system which has a planar helix structure with two branches. Starting from a central region, each of the two branches extends outward in a helix. The input impedance of this antenna is also capacitive.

A disadvantage here is that the impedance of this antenna differs sharply from the complex conjugate value of the impedance of the chip input circuit, and thus that an additional, separate matching circuit with a coil and a capacitor is required. Because of parasitic resistances of these components, power losses arise in the transponder, disadvantageously reducing the range. Moreover, the separate matching circuit restricts the freedom in placement of the chip and results in more complex and thus more expensive implementations of the transponder.

From the article, “Broadband RFID tag antenna with quasi-isotropic radiation pattern,” by C. Cho, H. Choo and 1. Park, published in Electronics Letters, Vol. 41, No. 20, Sep. 29, 2005, pages 1091-1092, an antenna is known for a UHF RFID system that has two folded dipoles and a twin-T matching network. The area required by this antenna is 79 mm×53 mm. A region of 1.7 m to 2.4 m is given as the range of the RFID system.

However, for many applications only a relatively small area is available. In addition, elongated antennas having a relatively small width of approximately 35 mm and a length of up to 100 mm are advantageous for some applications, and for simple manufacture of the antenna on a strip. Moreover, many applications require greater range.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an antenna for a backscatter-based RFID transponder with an integrated receive circuit (IC) for receiving a radio signal spectrally located in an operating frequency range, that permits simpler and more economical implementations while still permitting very wide-band reception of high-frequency radio signals as well as having directional characteristics that are as omnidirectional as possible. It is a further object of the invention to provide a backscatter-based RFID transponder that is simple and inexpensive to implement and that has a relatively long range with very wide-band, omnidirectional reception of high-frequency radio signals.

In an embodiment of the present invention, the antenna includes: a) two antenna branches that extend outward from a connecting region in which the antenna branches can be connected to the integrated receive circuit, b) a yoke-shaped first trace segment that is designed to connect the two antenna branches together, wherein c) each antenna branch has a U-shaped second trace segment connected to the connecting region, and d) each antenna branch has a U-shaped third trace segment connected to the second trace segment and extending parallel to the second trace segment.

The backscatter-based RFID transponder can have an integrated receive circuit with a capacitive input impedance and an antenna according to the invention connected with the integrated receive circuit.

In an embodiment, two U-shaped trace segments are placed parallel or substantially parallel to one another and are connected to one another (contacting) in each of the two antenna branches. This makes possible antennas and transponders that require only a very small, e.g. elongated, area and that can be implemented in a simpler and more economical manner. At the same time, such an antenna permits greater ranges while still allowing very wide-band and largely omnidirectional reception of high-frequency radio signals.

In an embodiment, the second and third trace segments are designed such that the antenna can have an input impedance in the operating frequency range with an inductive reactance whose frequency response has an inflection point and/or a local minimum value and/or a local maximum value in the operating frequency range. To this end, a trace length along the second and third trace segments is selected such that this requirement on the frequency response is met. This permits very long ranges and a particularly wide-band and largely omnidirectional reception of high-frequency radio signals.

In another embodiment, the second and third trace segments can each be piecewise linear in design. In this way, better area utilization by the antenna can be achieved for a given rectangular or square area.

In another embodiment, the first trace segment is designed so that the antenna can have an inductive impedance in the operating frequency range that approximates the complex conjugate values of the capacitive impedance in such a manner that no circuit arrangement is needed for impedance matching between the antenna and integrated receive circuit. The first trace segment24can be designed such that the antenna has an inductive impedance in the operating frequency range whose real component is below 35 ohms and whose imaginary component has a magnitude above 170 ohms. This results in particularly long ranges as well as transponders that are particularly simple to implement.

Each antenna branch can have a serpentine fourth trace segment that is designed to connect the connecting region to the second trace segment of the antenna branch. In this way, it is advantageously possible to reduce the overall length of the area occupied by the antenna. Preferably, the fourth trace segments in this context have a third trace width that is smaller than a first trace width of a second or third trace segment. By this means, small effective resistances of the antenna impedance can advantageously be achieved.

In an embodiment of the inventive RFID transponder, the integrated receive circuit is arranged in the connecting region of the antenna. This permits very simple implementations of the transponder.

In another embodiment, each antenna branch includes a thin conductive layer that is formed on a substrate, and the integrated receive circuit is formed on this substrate.

DETAILED DESCRIPTION

FIG. 1schematically shows an example of an RFID system. The RFID system10has a base station11and at least one inventive transponder15. By means of high-frequency radio signals, the base station11exchanges data with the transponder or transponders15in a contactless and bidirectional fashion.

The base station11has at least one antenna12for transmitting and receiving radio signals in an operating frequency range fB, a transmitting/receiving unit13connected to the antenna(s) for transmitting and receiving data, and a control unit14connected to the transmitting/receiving unit for controlling the transmitting/receiving unit13.

The backscatter-based passive or semi-passive transponder15has an antenna16for receiving the radio signal spectrally located in the operating frequency range fB, and has, connected to the antenna, a receive circuit17for demodulating the received radio signal and for detecting the data contained therein. The receive circuit17here is part of an integrated circuit (IC) that is not shown inFIG. 1, for example an ASIC (application specific integrated circuit) or an ASSP (application specific standard product), which normally has in addition a memory for storing the data required for identification of the corresponding object. If applicable, the transponder15or the integrated circuit contains additional components that are not shown inFIG. 1, such as a sensor for temperature measurement, for example. Such transponders are also known as “remote sensors.”

The explanation below assumes that the operating frequency range fB is in the UHF frequency band, specifically in a frequency range between approximately 840 MHz and approximately 960 MHz. Alternatively, the operating frequency range can also range in the ISM (industrial, scientific, medical) band, which is available almost everywhere in the world, between 2.4 and 2.5 GHz. Additional alternative operating frequency ranges are found at 315 MHz, 433 MHz and 5.8 GHz.

As a result of differences in existing requirements of regulatory authorities with respect to the maximum permissible transmit power in the frequency range between 840 MHz and 960 MHz, ranges of approximately 5 m for the European market (500 mW ERP) and of approximately 11 m for the USA (4 W EIRP) are desired in read operation.

The integrated receive circuit17or the input circuit of the IC has a complex-valued input impedance Z1with a real component (effective resistance) R1and an imaginary component (reactance) X1. In order to minimize power losses, the effective resistance R1here is preferably relatively small. The reactance X1is generally capacitive (X1<0) and in particular has a larger magnitude than the effective resistance, |X1|>R1, for small values of the effective resistance R1.

Integrated receive circuits17developed by the applicant have input impedances Z1with effective resistances R1in the range of approximately 4 to 35 ohms, and have capacitive reactances X1whose absolute values are greater than approximately 170 ohms. The magnitude of the imaginary component (|X1|) thus significantly exceeds the real component (R1): |X1|>4*R1. With advances in integrated circuit production technology and the associated decreases in structure sizes, capacitive reactances X1with further increases in magnitude are to be expected.

The antenna16of the transponder15has antenna branches that extend outward from a connecting region in which the antenna branches are connected (contacted) to the integrated receive circuit17. The antenna branches and the integrated receive circuit17are preferably embodied on a common substrate. Example embodiments of the antenna16are described below.

FIG. 2shows a top view of a first example embodiment of an inventive antenna for a backscatter-based RFID transponder15in accordance with the description above.

The antenna has exactly two antenna branches21and22which extend outward from a connecting region23in which the antenna branches are connected to the integrated receive circuit17(FIG. 1). The branches21,22here are connected together by means of a yoke-shaped trace segment24. Each antenna branch21,22has a serpentine trace segment25that is connected to the connecting region23, a U-shaped trace segment26connected to and adjoining the segment25, and another U-shaped trace segment27connected to and adjoining the segment26that extends parallel to the segment26.

Each leg of the U-shaped segment26here is located parallel to a respective adjacent leg of the U-shaped segment27of the same antenna branch, so that the three legs of the segment26extend parallel to and at a uniform (constant) spacing d from the three legs of the segment27of the same antenna branch. Moreover, in each branch, the segment26is located in an inner area surrounded by the segment27, wherein the openings of the two U-shaped segments face in the same direction.

If the two ends of the U-shaped trace segment26are labeled26aand26b, and those of the U-shaped segment27are labeled27a,27b, then in each antenna branch21,22an outer end26bof the segment26is connected to an outer end27aof the segment27, so that the U-shaped segments26,27of the same antenna branch are each connected together at a respective outer (“first”) end26b,27ain an electrically conductive manner. Here, an “outer” end is understood to mean the (“first”) end of the relevant segment that is further separated (in terms of path length) along the trace segment from the connecting region23than the other, inner (“second”) end of the same segment. The “outer” end thus corresponds to the end facing away (along the segment) from the connecting section23.

Furthermore, in each antenna branch21,22an inner end26aof the segment26is connected to an inner end27bof the segment27, so that the U-shaped segments26,27of the same antenna branch are also connected to one another in an electrically conductive manner at the other inner (“second”) end26a,27b.

Moreover, in each antenna branch21,22, the inner end26aand the inner end27bare connected to the connecting region23, specifically through an outer end25bof the segment25, which is to say one facing away from the connecting region23, and through this segment25itself. In this way, the U-shaped segments26,27of the same antenna branch are each connected to the outer end25bof the serpentine segment25of the same antenna branch at the other inner (“second”) end (26a,27b) in an electrically conductive manner.

The yoke-shaped trace segment24connects the serpentine segments25of the two antenna branches21,22together, and forms a parallel inductor connected between the antenna branches21,22. The yoke-shaped trace segment24preferably has two first subsections24aparallel to one another, and a second subsection24bthat is arranged perpendicular to the first subsections and connects them to one another. Proceeding from the connecting region23, the yoke-shaped trace segment24preferably extends into an unoccupied region between the outer ends26b,27aof the upper antenna branch21and the outer ends26b,27aof the lower antenna branch22.

Each serpentine segment25forms a series inductor inserted in its antenna branch.

In addition to the segments24-27, the antenna20preferably has an additional trace segment28that connects the two U-shaped segments27of the two antenna branches21,22to one another. In this regard, the segment28connects, in an electrically conductive manner, the two inner ends27bof the segments27of the two antenna branches21,22, and thus also connects the two inner ends26aof the segments26of the two antenna branches, as well as the two outer ends25bof the serpentine segments25of the two antenna branches.

The trace segments24and26-28are preferably designed to be piecewise linear or polygonal, as can be seen inFIG. 2. The angles between the straight subsections here are each preferably 90 degrees. In other embodiments, “corners” of the traces are rounded or beveled, e.g., with 45-degree or 135-degree angles.

The two antenna branches21,22are preferably designed to be symmetrical to one another in shape. The antenna branch22shown at the bottom inFIG. 2represents a mirror image of the antenna branch21, shown at the top, reflected at a horizontal axis or plane S passing through the connecting region23—and vice versa.

In addition, the antenna branches21,22are preferably planar in design and lie in a common plane (drawing plane inFIG. 2).

The two antenna branches21,22preferably each include a thin conductive layer, e.g. of copper, silver, etc., formed on a common substrate, for example of polyimide, or on a printed circuit board. The integrated receive circuit17(FIG. 1) of the transponder is also preferably formed on this substrate. Alternatively, the thin conductive layer can be applied to a film on which the integrated receive circuit is arranged using flip-chip technology. The transponder, having at least the antenna and integrated receive circuit, is ultimately applied to the object to be identified.

The antenna branches21,22make contact with the integrated receive circuit17of the transponder15(FIG. 1) in the connecting region23. The receive circuit17is preferably arranged directly in the connecting region23. This advantageously simplifies the implementation of the transponder.

As is evident fromFIG. 2, the trace segments24-28have a trace width that is piecewise constant along the subsections. The trace width preferably remains constant in each straight subsection, but changes “abruptly” from subsection to subsection. Starting from the connecting region23, the first subsection can have a first width, the next straight subsection can have a second, larger width, and the third subsection can have a third, larger width (in comparison, in turn, to the second width), etc.

The trace width of the U-shaped segments26preferably matches the trace width of the U-shaped segments27and, if applicable, the trace width of the segment28. This trace width, which is labeled Wb2inFIG. 2, takes on a value of 2.0 mm, for example. In contrast thereto, the trace widths in the yoke-shaped segment24and the serpentine segments25are preferably smaller than in the segments26,27. InFIG. 2the segments24and25have the same trace width Wb1by way of example. It takes on a value of 0.5 mm, for example.

The antenna20shown inFIG. 2occupies an area with an overall length L of approximately 87 mm and an overall width W of approximately 23 mm, so that this antenna is especially suitable for production on a strip (W<approximately 35 mm) and/or for applications in which an elongated area is available for the antenna. The largest geometric dimension (L) of this antenna for all wavelengths λ=c/f of the operating frequency range fB (with f=840 . . . 960 MHz) is below the value λ/π=99 mm, so that the antenna20is an “electrically small” antenna as defined by Wheeler (1975). The antenna20is thus especially space-saving, permitting especially simple and economical transponder implementation.

The complex-valued input impedance of the antenna20is designated below as Z2=R2+j*X2, where R2is the effective resistance and X2is the reactance of the antenna.

Preferably the U-shaped trace segments26,27are designed such that the antenna20has an input impedance Z2with an inductive reactance X2>0 in the operating frequency range fB, whose frequency response X2(f) has an inflection point in the mathematical sense in the operating frequency range fB.

Moreover, the yoke-shaped trace segment24is preferably designed such that the antenna20has values of an inductive input impedance Z2in the operating frequency range fB that is matched to the complex conjugate value Z1′ of the capacitive input impedance Z1of the integrated receive circuit17such that no circuit arrangement for impedance matching is needed between the antenna and the integrated receive circuit (seeFIG. 1).

This state of affairs is described below in detail with reference toFIG. 3.

FIG. 3schematically shows the frequency response of the input impedance Z2of an inventive antenna as in the exemplary embodiment described above. In the top part of the figure, the reactance X2, which is to say the imaginary component of Z2, is plotted over the frequency f, while the effective resistance R2, which is to say the real component of Z2, is shown in the bottom part. The above-mentioned operating frequency range fB between approximately 840 MHz and approximately 960 MHz is emphasized inFIG. 3.

It is evident from the frequency response X2(f) of the reactance that the reactance X2reaches a high inductive value of over 200 ohms already at the lower limit of the operating frequency range fB, which is to say at approximately 840 MHz. With increasing frequency values, the reactance X2rises to a local maximum value32of approximately 214 ohms, then declines slightly to a local minimum value33of approximately 208 ohms, and finally rises again until a value of approximately 215 ohms is reached at the upper limit of the operating frequency range fB, which is to say at approximately 960 MHz. An inflection point31of the frequency response X2(f) is located at approximately the center of the operating frequency range fB, i.e. at approximately 900 MHz.

The U-shaped trace segments26,27of the above-described antenna20are designed such that the reactance X2of the antenna is inductive (X2>0) in the entire operating frequency range fB and has a frequency response X2(f) that has an inflection point31as well as a local maximum value32and a local minimum value33in the operating frequency range fB, each of which is not located at an edge of the operating frequency range fB. To this end, inFIG. 2the trace length Lu, in particular, along the trace segments26,27, i.e. the sum of the path lengths of the U-shaped segments26,27is chosen such that the inflection point31and the local maximum and minimum values32,33lie within the operating frequency range fB.

In other embodiments of the antenna, the U-shaped segments are designed such that the frequency response X2(f) in the operating frequency range fB has only an inflection point, but no local extreme values, or else has an inflection point and either a local maximum value or a local minimum value.

The values of the inductive reactance X2of the antenna20shown inFIG. 3, in the operating frequency range fB, correspond to a good approximation to the absolute values |X1| of the capacitive reactance X1of the integrated receive circuit17specified above with reference toFIG. 1.

It is evident from the frequency response R2(f) of the effective resistance that the effective resistance R2takes on a small value of approximately 5 ohms at the lower limit of the operating frequency range fB. With increasing frequency values, the value of the effective resistance R2also increases, until a maximum value34of approximately 22 ohms is reached approximately in the center of the operating frequency range fB at approximately 900 MHz. As frequency values continue to rise, the effective resistance R2then falls again, reaching a value of approximately 8 ohms at the upper limit of the operating frequency range fB. Thus, a local maximum value34of R2(f) is located within the operating frequency range fB.

Because of the shallow slopes of the frequency responses R2(f), X2(f) in the operating frequency range fB, the antenna20has a wide bandwidth. The bandwidth of the overall system (transponder) depends strongly on the impedance of the integrated receive circuit, the antenna substrate carrier, and the support surface to which the transponder is applied. Investigations carried out by the applicant have yielded bandwidths for the overall system of approximately 80 MHz.

The values shown inFIG. 3of the effective resistance R2of the antenna20, in the operating frequency range fB, correspond to a good approximation to the values R1of the effective resistance R1of the integrated receive circuit17specified above with reference toFIG. 1.

Under the boundary conditions explained above with reference toFIG. 1, the input impedance Z2=R2+j*X2of the antenna20in the operating frequency range fB thus approximates the complex conjugate values Z1′=R1−j*X1of the input impedance Z1=R1+j*X1of the integrated receive circuit17sufficiently closely. Advantageously, no separate circuit arrangement for impedance matching is required. The yoke-shaped trace segment24of the antenna20is designed appropriately for this purpose.

Especially the trace length along the subsections24a,24b, but also the trace width Wb1is chosen for this purpose such that the ideal case Z2=Z1′ is approximated as closely as possible in the operating frequency range fB. Thus, for example, a lengthening of the subsections24aby 1 mm results in an increase of |X2| by approximately 5 ohms, and a lengthening by 2 mm results in an increase of approximately 10 ohms, so that fine adjustment of the impedance matching can be accomplished by such modification.

In this way, power losses in the transponder are reduced so that large ranges result, and wide-band and omnidirectional reception is possible in the entire operating frequency range fB. Investigations carried out by the applicant produced ranges in read operation of approximately 10 m for the USA (4 W EIRP) and approximately 5 m for the European market (500 mW ERP). Moreover, as a result the integrated receive circuit17can advantageously be placed directly in a connecting region of the antenna16without limitations by separate components for impedance matching, thus permitting especially simple and economical, but nonetheless powerful, transponder implementations.

How closely the inductive input impedance Z2of the antenna can be made to approach the likewise inductive impedance Z1′ in general depends on many boundary conditions, but especially the following: a) the frequency location and width of the desired operating frequency range fB, b) the value of the capacitive input impedance Z1of the receive circuit17and its curve in the operating frequency range, and c) the precise design of the inventive antenna.

As is evident fromFIG. 2, the U-shaped trace segments26,27and the yoke-shaped trace segment24are advantageously designed such that optimum use is made of the area W×L occupied by the antenna. Thus, inFIG. 2the horizontal extent of the outer U-shaped trace segments27corresponds essentially to the horizontal extent of the antenna in the region of the yoke-shaped trace segment24, which segment in turn corresponds essentially to the overall width W of the antenna. Moreover, the sum of the lengths of the two right vertical subsections of the U-shaped segments27and the subsection24bcorresponds to the overall length L of the antenna, with the exception of vertical minimum distances that must be observed between the outer ends26b,27aof the U-shaped segments and the subsections24a. In the U-shaped segments26,27, and also in the yoke-shaped trace segment24as well, the total trace length required in each case is thus advantageously divided up among the individual horizontal and vertical subsections such that the antenna makes the fullest possible use of the smallest possible area.

In other example embodiments, the inventive antenna has no serpentine segments. Instead, for example, the U-shaped trace segments are designed such that the antenna occupies a more elongated area. This is advantageous in applications in which the overall width W of the antenna is strictly delimited in the upward direction by a small maximum value, while the value of the overall length is of secondary importance.

Even though the present invention has been described above on the basis of example embodiments, it is not restricted thereto, but can instead be modified in multiple ways. Thus, for example, the invention is neither restricted to passive or semi-passive transponders, nor to the specified frequency bands or the specified impedance values of the integrated receive circuit, etc. Rather, the invention can be used to advantage in an extremely wide variety of contactless communications systems.