WO2010041189A1

WO2010041189A1 - Antenna with a controllable switching element connecting two antenna portions

Info

Publication number: WO2010041189A1
Application number: PCT/IB2009/054342
Authority: WO
Inventors: Michael Michelitsch
Original assignee: Nxp B.V.
Priority date: 2008-10-09
Filing date: 2009-10-05
Publication date: 2010-04-15
Also published as: EP2335315B1; EP2335315A1; CN102177612A

Abstract

It is described an antenna circuit (120) for a Radio Frequency Identification reader module (140). The antenna circuit (120) comprises a first antenna portion (121) with a first contact node (121a) for connecting the first antenna portion (121) to the Radio Frequency Identification reader module (140), a second antenna portion (122) with a second contact node (122a) for connecting the second antenna portion (122) to the Radio Frequency Identification reader module (140), and a controllable switching element (130), which connects the first antenna portion (121) to the second antenna portion (122). The controllable switching element (130) is adapted to change the ohmic resistance between the first antenna portion (121) and the second antenna portion (122) as a function of a control signal, which is applied to the controllable switching element (130). It is further described a RFID reader system (100), which comprises a RFID reader module (140) and an antenna circuit (120) as described above. Furthermore, it is described a method for adjusting the quality factor of an antenna circuit (120) as described above.

Description

Antenna with a controllable switching element connecting two antenna portions

FIELD OF THE INVENTION

The present invention relates to the technical field of antenna circuits, which are used in particular for Radio Frequency Identification (RFID) devices. Specifically, the present invention related to an adaptive antenna circuit, which comprises an adjustable quality factor. Further, the present invention relates to a RFID reader system, which comprises a RFID reader module and an antenna circuit as described above. Furthermore, the present invention relates to a method for adjusting the quality factor of an antenna circuit as described above.

BACKGROUND OF THE INVENTION Recently, the use of Radio Frequency Identification (RFID) technology to locate and track various types of items has gained a dramatic increase of popularity. One reason for this increase is that the costs associated with manufacturing and implementing readers and tags employing RFID technology has steadily been decreasing. In addition, RFID reader and tags have been manufactured to be even smaller for more densely packed applications.

In order to obtain a good electromagnetic coupling and a reliable signal transmission between a RFID tag and a RFID reader, the antenna in particular of the RFID reader has to be adjusted with respect to its quality factor. If one desires an efficient energy transport between the RFID reader and the RFID tag, a high quality factor needed. Such an efficient energy transport is required for instance if the RFID tag and the RFIC reader are located comparatively far away from each other. If one desires a large bandwidth for the radio transmission between the RFID reader and the RFID tag, a small quality factor is needed. A large bandwidth is required for instance if the amount of data is large, which has to be transmitted from and/or which has to be received by the RFID reader. WO 2007 030 864 Al describes an optimization of a quality factor (Q factor) of a RFID antenna. A relatively high Q factor is needed for a high efficient energy transfer. A relatively low Q factor is demanded when targeting high bandwidth requirements. The described Q-factor optimizing takes place by means of switching capacitors and / or inductors to and/or from the circuit of the RFID antenna. EP 1 739 452 A2 describes an RFID tag having an antenna and a resonant capacitor. In order to allow for an operation in two operational states, namely a normal current state or a low current state, a MOSFET is provided for disconnecting the resonant capacitor from the antenna. US 5,815,355 describes an adjustment of the Q factor of an antenna of a RFID device. The Q factor adjustment is carried out by applying a further load transistor in parallel to a shunt transistor between the two terminals of the RFID antenna circuit. The further load transistor is used to vary the Q factor of a tank circuit depending on data read from the memory. It is further known for instance from the US patent application US

2006/0165039 Al to adjust the quality factor of loop antennas being used for RFID readers by means of shunt resistors, which are selectively connected to an RFID antenna circuit. By connecting and/or disconnecting different shunt resistors to an antenna circuit, the quality factor of the respective RFID antenna circuit can be adjusted appropriately depending on the currently given requirements. However, this procedure is rather cumbersomely because for each quality factor a different circuitry of the RFID antenna circuit has to be established.

There may be a need for improving the adaptiveness of an antenna circuit with respect to the antenna quality factor.

OBJECT AND SUMMARY OF THE INVENTION

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims.

According to a first aspect of the invention there is provided an antenna circuit, in particular a loop antenna circuit for a Radio Frequency Identification reader module. The provided antenna circuit comprises (a) a first antenna portion comprising a first contact node for connecting the first antenna portion to the Radio Frequency Identification reader module, (b) a second antenna portion comprising a second contact node for connecting the second antenna portion to the Radio Frequency Identification reader module, and (c) a controllable switching element, which connects the first antenna portion to the second antenna portion. Thereby, the controllable switching element is adapted to change the ohmic resistance between the first antenna portion and the second antenna portion as a function of a control signal, which is applied to the controllable switching element.

This first aspect of the present invention is based on the idea that the quality factor of an antenna coil which comprises the first and the second antenna portion can easily be adjusted simply by applying an appropriate control signal to the controllable switching element. Thereby, an appropriate ohmic resistance, which can be seen as a damping element within the antenna circuit, can be selected for connecting the first and the second antenna portion with each other.

When operating the described antenna circuit there may be a desire to improve the bandwidth of the antenna circuit for instance for increasing the maximal data volume which can be transmitted and/or received by the antenna circuit. In this case the quality factor, which is often also called Q-factor or coil Q-factor, must be decreased. This requires an increase of the ohmic resistance of the controllable switching element connecting the first and the second antenna portion.

By contrast thereto, there may be another desire to reduce the bandwidth of the antenna circuit for instance for increasing the transmitting power in the transmitting case and/or for increasing the sensitivity of the antenna circuit in the receiving case. In this case the quality factor will have to be increased. This requires a decrease of the ohmic resistance of the controllable switching element.

According to an embodiment of the invention the controllable switching element is adapted to change its ohmic resistance in a continuous manner. This may provide the advantage that any arbitrary value for the ohmic resistance can be adjusted. According to a further embodiment of the invention the antenna circuit comprises a symmetric layout, wherein, when operating the antenna circuit in a symmetrical manner, a virtual electrical ground point is formed at the intersection between the first antenna portion and the second antenna portion.

By contrast to an antenna circuit, which comprises an asymmetric layout, the described symmetric layout allows the antenna circuit to be operated in an in particular interference-insusceptible manner. Thereby, in the virtual electrical ground point the antenna voltage has a voltage level of at least approximately zero Volts. Further, the virtual electrical ground point is insusceptible against putting the antenna circuit out of tune.

The described symmetric antenna circuit can be realized for instance by splitting a usual loop antenna at the centre point and inserting the controllable switching element in between the two antenna portions, which result from the splitting procedure.

According to a further embodiment of the invention the control signal is a voltage signal. This may provide the advantage that the ohmic resistance of the controllable switching element can be adjusted in a simple and effective manner. According to a further embodiment of the invention the ohmic resistance of the controllable switching element is adjustable within a resistor range between zero Ohm and infinite Ohm.

Specifically, when the ohmic resistance is zero the first antenna portion and the second antenna portion are directly connected with each other. In this case the quality factor of the antenna circuit will be maximal and, as a consequence, the frequency bandwidth will be minimal. By contrast thereto, when the ohmic resistance is infinite, the first antenna portion and the second antenna portion are electrically not connected with each other. In this case the described antenna circuit comprises two separate antenna element, which may be operated independent from each other.

According to a further embodiment of the invention the controllable switching element comprises a semiconductor device. In particular, the semiconductor device can be a transistor. Thereby, a control signal, which is applied to the base of the transistor, can modify the current, which in response to a potential difference across the transistor, flows through the transistor.

In this respect it is mentioned that in particular when the transistor is located at or at least close to the above described virtual electrical ground point, the voltage between the transistor source terminal and the transistor drain terminal will be small such that the transistor is operated in a non saturated regime with respect to its ohmic behaviour. Hence, the transistor will act as an ohmic resistor, wherein the resistor value can be adjusted in a controllable manner.

According to a further embodiment of the invention the controllable switching element is a metal-oxide-semiconductor field-effect transistor. This may provide the advantage that when controlling and/or when adjusting the controllable switching element respectively the metal-oxide-semiconductor field-effect transistor (MOSFET), the current, which is drawn by the MOSFET, can be reduced significantly to a minimum current. Therefore, almost perfect voltage control for the ohmic resistance between the first antenna portion and the second antenna portion can be realized in a simple and in an efficient manner.

The utilization of a MOSFET further provides the advantage that the controllable switching element can be realized by means of a well established

Complementary Metal-Oxide-semiconductor (CMOS) technology. This may provide the advantage that a whole Radio Frequency Identification (RFID) reader system, which comprises an RFID reader module and an RFID antenna circuit as described in this application, can be realized by applying well known CMOS procedures. According to a further embodiment of the invention the antenna circuit further comprises a filter circuit, which is connected to the controllable switching element and which is adapted to provide the control signal to the controllable switching element. This may provide the advantage that unwanted oscillations on the control signal can be eliminated in an easy and efficient manner.

According to a further aspect of the invention there is provided a Radio Frequency Identification (RFID) reader system. The RFID reader system comprises (a) a Radio Frequency Identification reader module and (b) an antenna circuit as described above. Thereby, the antenna circuit is electrically connected to the RFID reader module. Also this further aspect of the invention is based on the idea that the quality factor of an antenna coil, which comprises the first and the second antenna portion, can easily be adjusted simply by applying an appropriate control signal to the controllable switching element. Thereby, an appropriate ohmic resistance, which can be seen as a damping element within the antenna circuit, can be selected for connecting the first and the second antenna portion with each other.

According to a further aspect of the invention there is provided a method for adjusting the quality factor of an antenna circuit. The provided method comprises (a) applying a control signal to a controllable switching element, which is connected between a first antenna portion of the antenna circuit and a second antenna portion of the antenna circuit, wherein the controllable switching element is adapted to change the ohmic resistance between the first antenna portion and the second antenna portion as a function of the control signal, and (b) adjusting the quality factor of the antenna circuit by selecting an appropriate strength for the control signal.

Also this further aspect of the invention is based on the idea that the quality factor of the antenna circuit can be adjusted by applying an appropriate control signal to the controllable switching element. Thereby, an appropriate ohmic resistance, which can be seen as a damping element within the antenna circuit, can be selected for connecting the first and the second antenna portion with each other.

It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to method type claims whereas other embodiments have been described with reference to apparatus type claims. However, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters, in particular between features of the method type claims and features of the apparatus type claims is considered as to be disclosed with this application.

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic illustration of a RFID reader system comprising a split antenna circuit with two antenna portions being connected with each other via a controllable switching element.

Figure 2 shows simulation of the RFID reader system depicted in Figure 1, which simulation was carried out with the electronic design software program OrCAD.

DESCRIPTION OF EMBODIMENTS

The illustration in the drawing is schematically. It is noted that in different figures, similar or identical elements are provided with the same reference signs or with reference signs, which are different from the corresponding reference signs only within the first digit.

Figure 1 shows a schematic illustration of a Radio Frequency Identification (RFID) reader system 100. The RFID reader system 100 comprises an antenna circuit 120 and a RFID reader module 140. The RFID reader module 140 is a standard module. Since such modules are widely known in the field of RFID technology, for the sake of conciseness of this application hereinafter the configuration of the RFID reader module 140 will not be described in detail.

The antenna circuit 120 comprises two antenna portions, a first antenna portion 121 and a second antenna portion 122. A controllable switching element 130 connects the two antenna portions 121 and 122 with each other. Further, the antenna circuit 120 comprises a first contact node 121a being assigned to the first antenna portion 121 and a second contact node 122a being assigned to the second antenna portion 122.

According to the embodiment described here the controllable switching element is a metal-oxide-semiconductor field-effect transistor (MOSFET) 130. The gate of the MOSFET 130 is connected to a control node 130a.

As can be seen from Figure 1 the antenna circuit 120 comprises a symmetric design, which is split by the controllable switching element 130. Further, the RFID reader module 140 operates the antenna circuit 120 in a symmetric mode, such that between the two antenna portions 121 and 122 a virtual electrical ground point 125 develops. As can be further seen from Figure 1, the controllable switching element 130 is located directly next to the virtual electrical ground point 125.

In this respect it has to be mentioned that the described symmetric operation mode and the resulting development of the virtual electrical ground point 125 can be realized both in the transmitting case, wherein the antenna circuit 120 transmits radio signals in particular to one or more RFID tags, and in the receiving case, wherein the antenna circuit 120 picks up radio signals in particular from one or more RFID tags.

When operating the antenna circuit 120 in the described symmetric mode, the virtual electrical ground point 125 has an electric potential of zero Volts. Further, the virtual electrical ground point 125 is insensitive against putting the antenna circuit 120 out of tune. Due to the low voltage in the region of the virtual electrical ground point 125 the source-drain voltage of the MOSFET 130 will also be very small in case the symmetric antenna circuit 120 is operated in a symmetric mode. Therefore, the MOSFET 130 will be operated in a non saturated regime with respect to its ohmic behaviour. Hence, the MOSFET 130 will act as an ohmic resistor between the two antenna portions 121 and 122, wherein the resistor value can be adjusted in a controllable manner by applying an appropriate voltage signal to the control node 130a. This voltage signal may be the gate-source voltage of the MOSFET 130.

According to the embodiment described here the ohmic resistor can take in a continuous manner any value within a wide ohmic range. Preferably, the ohmic range is between zero Ohm and infinite Ohm. As has already been described above, the resistor value of the ohmic resistor respectively of the MOSFET 130 is directly related to the quality factor (Q-factor) of the antenna circuit 120. Therefore, the described antenna circuit 120 represents an adaptive antenna circuit. For instance, if the frequency the bandwidth of the antenna circuit 120 has to be increased in particular in order to increase the maximal data volume, which can be transmitted and/or received by the antenna circuit 120, the quality factor must be decreased. This requires an increase of the ohmic resistance of the MOSFET 130.

By contrast thereto, if for instance the bandwidth of the antenna circuit 120 has to reduced in particular in order to increase the transmitting power in the transmitting case and/or to increase the sensitivity of the antenna circuit 120 in the receiving case, the quality factor will have to be increased. This requires a decrease of the ohmic resistance of the MOSFET 130. Figure 2 shows simulation of the RFID reader system 100, which is now denominated with reference numeral 200. The simulation was carried out with the electronic design software program OrCAD, which is a proprietary software tool suite used primarily for electronic design automation.

The illustrated simulation refers to the transmitting case, wherein the RFID reader system 200 transmits radio signals to an RFID tag. However, it has been approved by the inventor that the depicted design of the RFID reader system 200 works also for the receiving case, wherein the RFID reader system 200 receives radio signals from at least one RFID tag.

The RFID reader system 200 comprises an antenna circuit 220, a RFID reader module 240 and a filter circuit 260.

As has already been mentioned above, the antenna circuit 220 comprises first antenna portion 221 having a first contact node 221a, a second antenna portion 222 having a second contact node 222a and a metal-oxide-semiconductor field-effect transistor (MOSFET) 230 connecting the first antenna portion 221 and the second antenna portion 222 with each other at a virtual electrical ground point 230. By means of a control node 230a an appropriate voltage can be applied to the gate of the MOSFET 230 in order to adjust an appropriate ohmic resistance between the first antenna portion 221 and the second antenna portion 222.

According to the embodiment described here the voltage provided at the control node 230a is filtered in order to suppress unwanted high frequency oscillations of the antenna circuit 220. Therefore, the filter circuit 260 comprises voltage generator 262 connected to the virtual electrical ground point 225 and to the control node 230a via an electrical ladder structure comprising four resistors Rl 1, R12, R13 and R14 and one capacitance Cl 1. As can be seen from Figure 2, the negative output of the voltage generator 262 is connected to ground GND. In the equivalent circuit used for the OrCAD simulation a capacitance CO represents the parasitic capacitance of antenna loop comprising the two antenna portions 221, 222. CO' represents a capacitance used for matching the antenna circuit 220 to the output of the RFID reader module 240. Further, a first matching capacitor Cl being assigned to the first antenna portion 221 and a second matching capacitor C2 being assigned to the second antenna portion 222 have been used for the OrCAD simulation procedure.

An inductance Ll represents the parasitic inductivity of the first antenna portion 221. A resistor Rl represents the parasitic ohmic resistance of the first antenna portion 221. Accordingly, an inductance L2 represents the parasitic inductivity of the second antenna portion 222 and a resistor R2 represents the parasitic ohmic resistance of the second antenna portion 222.

As can be seen from Figure 2, the RFID reader module has been simulated with a signal generator 242, wherein one output of the signal generator 242 is connected to ground GND and to the second contact node 222a. The other output of the signal generator 242 is connected to the first contact node 221a via an adjustment resistor RO. According to the embodiment described here the adjustment resistor RO has a resistor value of 50 Ohm.

It should be noted that the term "comprising" does not exclude other elements or steps and "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

Reference Numerals:

100 RFID reader system

120 antenna circuit

121 first antenna portion

121a first contact node

122 second antenna portion

122a second contact node

125 virtual electrical ground point

130 controllable switching element / semiconductor d semiconductor field-effect transistor

130a control node

140 RFID reader module

200 RFID reader system

220 antenna circuit

221 first antenna portion

221a first contact node

222 second antenna portion

222a second contact node

225 virtual electrical ground point

230 metal-oxide-semiconductor field-effect transistor

230a control node

240 RFID reader module (transmitting case)

242 signal generator

260 filter circuit

262 voltage generator

CO parasitic capacitance of antenna loop

CO' matching capacitance

Cl matching capacitance

C2 matching capacitance

Ll parasitic inductivity of first antenna portion

Rl parasitic ohmic resistance of first antenna portion

L2 parasitic inductivity of second antenna portion R2 parasitic ohmic resistance of second antenna portion

Rl 1 resistor of filter circuit

Rl 2 resistor of filter circuit

Rl 3 resistor of filter circuit

Rl 4 resistor of filter circuit

CI l capacitor of filter circuit

GND ground potential

RO adjustment resistor

Claims

CLAIMS:

1. An antenna circuit, in particular a loop antenna circuit (120), for a Radio Frequency Identification reader module (140), the antenna circuit (120) comprising: a first antenna portion (121) comprising a first contact node (121a) for connecting the first antenna portion (121) to the Radio Frequency Identification reader module (140), a second antenna portion (122) comprising a second contact node (122a) for connecting the second antenna portion (122) to the Radio Frequency Identification reader module (140), and a controllable switching element (130), which connects the first antenna portion (121) to the second antenna portion (122), wherein the controllable switching element (130) is adapted to change the ohmic resistance between the first antenna portion (121) and the second antenna portion (122) as a function of a control signal, which is applied to the controllable switching element (130).

2. The antenna circuit as set forth in the preceding claim, wherein the controllable switching element (130) is adapted to change its ohmic resistance in a continuous manner.

3. The antenna circuit as set forth in any one of the preceding claims, wherein the antenna circuit (120) comprises a symmetric layout, wherein, when operating the antenna circuit (120) in a symmetrical manner, a virtual electrical ground point (125) is formed at the intersection between the first antenna portion (121) and the second antenna portion (122).

4. The antenna circuit as set forth in any one of the preceding claims, wherein the control signal is a voltage signal.

5. The antenna circuit as set forth in any one of the preceding claims, wherein the ohmic resistance of the controllable switching element (130) is adjustable within a resistor range between zero Ohm and infinite Ohm.

6. The antenna circuit as set forth in any one of the preceding claims, wherein the controllable switching element comprises a semiconductor device (130).

7. The antenna circuit as set forth in the preceding claim, wherein the controllable switching element is a metal-oxide-semiconductor field-effect transistor (130).

8. The antenna circuit as set forth in any one of the preceding claims, further comprising a filter circuit (260), which is connected to the controllable switching element (130, 230) and which is adapted to provide the control signal to the controllable switching element (130, 230).

9. A Radio Frequency Identification reader system (100) comprising: a Radio Frequency Identification reader module (140) and an antenna circuit (120) as set forth in any one of the preceding claims, wherein the antenna circuit (120) is electrically connected to the Radio Frequency Identification reader module (140).

10. A method for adjusting the quality factor of an antenna circuit (120), the method comprising applying a control signal to a controllable switching element (130), which is connected between a first antenna portion (121) of the antenna circuit (120) and a second antenna portion (122) of the antenna circuit (120), wherein the controllable switching element (130) is adapted to change the ohmic resistance between the first antenna portion (121) and the second antenna portion (122) as a function of the control signal, and adjusting the quality factor of the antenna circuit (120) by selecting an appropriate strength for the control signal.