Rfid transponder and method of operating an rfid transponder

Abstract

The present disclosure relates to an RFID transponder and a method of operating an RFID transponder. According to a first aspect of the present disclosure, a radio frequency identification (RFID) transponder is provided, the RFID transponder comprising a modulator and a controller, wherein: the modulator is configured to generate a modulated signal to be transmitted to an external RFID reader; the controller is configured to adjust an input voltage of the modulator, resulting in an adjusted input voltage, and to feed the adjusted input voltage to the modulator. According to a second aspect of the present disclosure, a corresponding method of operating a radio frequency identification (RFID) transponder is conceived.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a radio frequency identification transponder. Furthermore, the present disclosure relates to a method of operating a radio frequency identification transponder. BACKGROUND

[0002] Today, radio frequency identification (RFID) transponders are widely used in different fields of industry and commerce, as well as for various purposes. For example, RFID transponders can be implemented as so-called RFID tags or RFID cards. It is to be noted that, in the present disclosure, near field communication (NFC) transponders are regarded as a specific type of RFID transponders. Thus, the principles described herein can also be applied to NFC transponders. SUMMARY

[0003] According to a first aspect of the present disclosure, a radio frequency identification (RFID) transponder is provided, the RFID transponder comprising a modulator and a controller, wherein: the modulator is configured to generate a modulated signal to be transmitted to an external RFID reader; the controller is configured to adjust an input voltage of the modulator, resulting in an adjusted input voltage, and to feed the adjusted input voltage to the modulator.

[0004] In one or more embodiments, the controller is configured to adjust the input voltage depending on a power indicator, the power indicator indicating an amount of power available in the RFID transponder.

[0005] In one or more embodiments, the controller is configured to adjust the input voltage depending on at least one of: an operating mode of the RFID transponder, an application domain in which the RFID transponder is used, a configuration of the RFID transponder, and a parameter stored in the RFID transponder.

[0006] In one or more embodiments, the controller is configured to control a voltage level translator, wherein the voltage level translator is configured to combine the adjusted input voltage with a modulator control signal to generate an adjusted control signal for the modulator.

[0007] In one or more embodiments, the modulator comprises a n-channel metal-oxide-semiconductor (NMOS) switch, and the input voltage is a gate voltage of the NMOS switch.

[0008] In one or more embodiments, the RFID transponder additionally comprises a shunt regulator, wherein the controller is configured to use an output of the shunt regulator for adjusting the gate voltage.

[0009] In one or more embodiments, the RFID transponder additionally comprises a current mirror, wherein the current mirror is configured to generate the gate voltage.

[0010] In one or more embodiments, the controller is configured to sample the output of the shunt regulator before the RFID transponder enters the modulation phase.

[0011] In one or more embodiments, the RFID transponder is an RFID tag or an RFID card.

[0012] According to a second aspect of the present disclosure, an illustrative embodiment of a method of operating a radio frequency identification (RFID) transponder is shown, the RFID transponder comprising a modulator and a controller, the method comprising: the controller adjusting an input voltage of the modulator, thereby obtaining an adjusted input voltage, and feeding the adjusted input voltage to the modulator; the modulator generating a modulated signal to be transmitted to an external RFID reader.

[0013] In one or more embodiments, the controller adjusts the input voltage in dependence on a power indicator, the power indicator indicating an amount of power available in the RFID transponder.

[0014] In one or more embodiments, the controller adjusts the input voltage in dependence on at least one of: an operating mode of the RFID transponder, an application domain in which the RFID transponder is used, a configuration of the RFID transponder, and a parameter stored in the RFID transponder.

[0015] In one or more embodiments, the controller controls a voltage level translator, wherein the voltage level translator combines the adjusted input voltage with a modulator control signal to produce an adjusted control signal of the modulator.

[0016] In one or more embodiments, the modulator comprises an n-channel metal-oxide-semiconductor (NMOS) switch, and the input voltage is a gate voltage of the NMOS switch.

[0017] In one or more embodiments, the RFID transponder additionally comprises a shunt regulator, and the controller uses an output of the shunt regulator to adjust the gate voltage. BRIEF DESCRIPTION OF DRAWINGS

[0018] Embodiments will be described in more detail with reference to the drawings, in which:

[0019] Figure 1 An illustrative embodiment of a RFID transponder is shown;

[0020] Figure 2 An illustrative embodiment of a method of operating a RFID transponder is shown;

[0021] Figure 3 An illustrative embodiment of a communication system is shown;

[0022] Figure 4Another illustrative embodiment of an RFID transponder is shown;

[0023] Figure 5 An example of a curve of backscatter versus input power is shown;

[0024] Figure 6 An illustrative embodiment of an adaptive modulation concept is shown;

[0025] Figure 7 An illustrative embodiment of a sampled gate voltage is shown;

[0026] Figure 8 An example of simulation results is shown;

[0027] Figure 9 Another illustrative embodiment of an RFID transponder is shown;

[0028] Figure 10 A graph showing the influence of a changing level shifter voltage on the backscatter signal is shown;

[0029] Figure 11 Another illustrative embodiment of an RFID transponder is shown. DETAILED DESCRIPTION

[0030] Radio frequency identification (RFID) transponders are nowadays widely used in different fields of industry and commerce, as well as for various purposes. For example, RFID transponders can be implemented as so-called RFID tags or RFID cards. It is noted that in the present disclosure near field communication (NFC) transponders are considered as a specific type of RFID transponders. Thus, the principles described herein can also be applied to NFC transponders.

[0031] RFID communication can be based on inductive coupling. Communication between an RFID reader and an RFID transponder, e.g. an RFID tag, is often realized by means of load modulation and can be split into a forward link and a return link. More specifically, an RFID reader can transmit commands to an RFID transponder over the forward link and the RFID transponder can transmit responses to those commands back to the RFID reader over the return link. The RFID transponder contains a modulator which load-modulates a carrier signal. There are different types of load modulation, e.g. active load modulation (ALM) and passive load modulation (PLM). The return link can also be referred to as backscatter signal or more briefly as "backscatter".

[0032] In RFID applications, the return link is an important parameter. In RFID transponders, this backscatter is generated by a modulator, which comprises one or more modulation transistors. The voltage driving this modulator usually has a fixed level or a fixed voltage domain and thus defines the backscatter strength. However, different applications of RFID often require different backscatter strengths. The requirements associated with such applications are often defined in regulations issued by regulatory bodies. Since these requirements can also change, the backscatter strength of a RFID transponder should be easily adjustable. Furthermore, it would be advantageous if a RFID transponder could be used for various applications instead of only for a single application or a limited number of applications. For this reason, too, the backscatter strength of a RFID transponder should be easily adjustable. Adjusting the backscatter strength of a RFID transponder usually requires changing the modulator. However, changing the modulator can be difficult and expensive. Now, a RFID transponder that facilitates adjusting the backscatter strength and a corresponding method of operating a RFID transponder are discussed.

[0033] Figure 1 An illustrative embodiment of a radio frequency identification (RFID) transponder is shown. The RFID transponder 100 comprises a modulator 102 controlled by a controller 104. The modulator 102 is configured to generate a modulated signal to be transmitted to an external RFID reader (not shown). Furthermore, the controller 104 is configured to adjust an input voltage of the modulator, which results in an adjusted input voltage. Furthermore, the controller 104 is configured to feed the adjusted input voltage to the modulator. In this way, the strength of the return link can be easily adjusted.

[0034] Figure 2 An illustrative embodiment of a method 200 of operating a RFID transponder is shown. The method 200 comprises the following steps. At 202, a controller adjusts an input voltage of a modulator and feeds an adjusted input voltage to the modulator. Furthermore, at 204, the modulator generates a modulator signal to be transmitted to an external RFID reader. In this way, the strength of the return link can be easily adjusted.

[0035] Figure 3An illustrative embodiment of an RFID communication system 300 using an RFID transponder 100 is shown. The communication system 300 comprises an RFID transponder 100 and an RFID reader 302 of the kind set forth in communication with each other. The RFID transponder 100 comprises a modulator 102 controlled by a controller 104. Similarly, the RFID reader 302 comprises a demodulator 304 controlled by a controller 306. In operation, the RFID transponder 100 modulates a carrier wave generated by the RFID reader 302 resulting in a modulated signal and transmits the modulated signal to the RFID reader 302. The RFID reader 302 receives the modulated signal and demodulates it using the demodulator 304. In accordance with the present disclosure, the controller 104 is configured to adjust the input voltage of the modulator resulting in an adjusted input voltage. Furthermore, the controller 104 is configured to feed the adjusted input voltage to the modulator.

[0036] In an embodiment, the controller is configured to adjust the input voltage depending on a power indicator, wherein the power indicator indicates an amount of power available in the RFID transponder. Thus, depending on the available power, by changing the voltage used to control the modulator, the backscatter strength can be adapted to meet national regulations. In order to reduce the backscatter strength, a lower input voltage can be fed to the modulator. On the other hand, if the amount of power available is low, the backscatter strength can be increased to improve system performance. In order to increase the backscatter strength, a higher input voltage can be fed to the modulator. Thus, in this context, voltage adjustment refers to the adjustment of the input voltage to the modulator.

[0037] In an embodiment, the controller is configured to adjust the input voltage in dependence of an operational mode of the RFID transponder. For example, the operational mode can be a production mode, an authentication mode and an application mode of the RFID transponder. In this way, the backscatter strength can be easily adjusted to accommodate different operational conditions of the RFID transponder. Further, in an embodiment, the controller is configured to adjust the input voltage in dependence of an application domain in which the RFID transponder is used. In this way, the backscatter strength can be easily adjusted to comply with specific application related requirements. In another embodiment, the controller is configured to adjust the input voltage in dependence of a configuration of the RFID transponder. In this way, the backscatter strength can be easily adjusted according to the configuration of the RFID transponder. For example, the configuration can be a setting of the modulator in one of the available operational modes. This setting can be stored in a memory of the RFID transponder. In an embodiment, the controller is configured to adjust the input voltage in dependence of parameters stored in the RFID transponder. For example, the application domain and the configuration are parameters that can be stored in the RFID transponder. This enables easy updating of the parameters, which in turn enables easy reconfiguration of the RFID transponder and / or makes it possible to reuse the RFID transponder in another application domain.

[0038] Figure 4 Another illustrative embodiment of an RFID transponder 400 is shown. In particular, an RFID transponder 400 is shown that is configured to operate in a plurality of operational modes. The RFID transponder 400 comprises a modulator 402, a controller 404 and a memory 406. The modulator 402 is configured to modulate an input voltage to generate a modulated input voltage. The modulator 402 is configured to modulate the input voltage in dependence of an operational mode of the RFID transponder 400. The controller 404 is configured to adjust the input voltage in dependence of the operational mode of the RFID transponder 400. The memory 406 is configured to store a setting of the modulator 402 in the operational mode. The controller 404 is configured to retrieve the setting of the modulator 402 from the memory 406. The controller 404 is configured to adjust the input voltage in dependence of the setting of the modulator 402. Figure 1An example of an embodiment of an RFID transponder is shown in Fig. 4. The RFID transponder 400 comprises a modulator 402, a voltage regulator 404, a voltage level shifter 414 and a plurality of voltage domains 408, 410. The voltage regulator 404 forms part of a controller of the kind described. Further, the voltage level shifter 414 can be controlled by a controller of the kind described. In some embodiments, the voltage level shifter 414 can also form part of the controller. In a practical and effective embodiment, the voltage level shifter 414 is configured to combine the regulated input voltage, i.e. the voltage output by the voltage regulator 404, with a modulator signal 416. This can be done by varying the amplitude of the modulator signal. It should be noted that the voltage level shifter can also be referred to as a level shifter. The modulator signal 416 can also be generated and provided by the controller. The output of the voltage level shifter 414 is an adjusted modulator control signal, which depends on the input voltage provided by the voltage regulator 404. The adjusted modulator control signal is fed to the modulator. The voltage regulator 404 can regulate the input voltage of the modulator 402 depending on a power indicator 406 and / or an operating mode 412. The power indicator can indicate the amount of power available in the RFID transponder, e.g. the amount of power drawn from the field through the RFp and RFn pins of the antenna (not shown) connected to the transponder. In order to regulate the input voltage of the modulator 402, the voltage regulator 404 can be connected to different voltage domains 408, 410, i.e. power domains. By connecting the voltage regulator 404 to a given voltage domain 408, 410, a predefined input voltage originating from said domain is applied to the voltage level shifter 414. The voltage domains have a given voltage source level, which can be output by a charge pump or can be generated within the integrated circuit to power other components.

[0039] In one or more embodiments, the modulator comprises an n-channel metal-oxide-semiconductor (NMOS) switch and the input voltage is a gate voltage of the NMOS switch. This results in a feasible and effective implementation of the modulator. Moreover, in this way, the backscatter strength can be easily adjusted. In one or more embodiments, the RFID transponder additionally comprises a shunt regulator and the controller is configured to use an output of the shunt regulator to adjust the gate voltage. The shunt regulator can be comprised in a contactless front-end of the RFID transponder. The output of the shunt regulator provides a suitable indication of the amount of power available in the RFID transponder. Thus, in this way, the input voltage can be easily adjusted depending on the amount of power available. Moreover, in one or more embodiments, the RFID transponder comprises a current mirror configured to generate the gate voltage. This results in a feasible and effective implementation. Furthermore, in one or more embodiments, the controller is configured to sample the output of the shunt regulator before the RFID transponder enters the modulation phase. In this way, it is ensured that a current can be mirrored during the modulation phase of the RFID transponder, as will be explained in more detail below.

[0040] Figure 5 An example of a curve 500 of backscatter versus input power is shown. During normal communication, the transponder will typically be powered by a field generated by a reader (which can also be referred to as an interrogator). This field is used to generate a local internal chip power supply. The transponder typically responds to the reader by modulating this field, e.g. by means of a switch, which is typically an NMOS device. This process can also be referred to as backscatter. In this example, the strength of the backscatter depends on the gate voltage of the switch and the size of the switch. If the gate voltage is set to a maximum value, the strength of the backscatter will only depend on the size of this switch. Thus, if the backscatter strength should be adjusted, e.g. to comply with certain predefined requirements, the switch has to be changed. According to the present disclosure, the gate voltage of the NMOS switch can be varied depending on the strength of the input field. In this way, the backscatter strength can be easily adjusted. This can be necessary to comply with requirements defined by, e.g., the European Telecommunications Standards Institute (ETSI). In order to comply with the ETSI requirements, the backscatter strength and thus the electromagnetic interference (EMI) should be reduced at higher input power levels (e.g. +18 dBm to -20 dBm). In contrast, at low input power levels (e.g. -22 dBm to 0 dBm), a maximum backscatter strength is required. With a constant gate voltage and size of the NMOS switch, it is not possible to reduce the EMI at and above one point and to achieve the maximum backscatter strength at lower input power levels.

[0041] In Figure 5This problem is illustrated in Fig. 5, where three curves are shown: a curve 502 of maximum backscatter versus input power, a curve 504 of backscatter versus input power according to a predefined requirement (i.e. a curve of a predefined requirement that should be achieved to comply with e.g. the aforementioned ETSI requirements, and a curve 506 of backscatter versus input power after scaling down the NMOS switch. In particular, in order to avoid adversely affecting the backscatter strength at lower input power levels, it is necessary to achieve the curve 504. An adaptive control based on the strength of the input field helps to achieve this curve.

[0042] Figure 6 An illustrative embodiment of the adaptive modulation concept 600 is shown. The output of the shunt regulator 602 can be indicative of the strength of the input, more specifically of the amount of power available to the RFID transponder. The shunt regulator 602 can form part of the contactless front-end of the RFID transponder. The shunt regulator 602 is typically used to regulate the chip supply by shunting excess current "I". Since the shunt current "I" has its characteristic that it scales with the input field strength, it is a suitable control parameter for the kind of adaptive control that is set forth. Furthermore, a part of the shunt current can be mirrored by the current mirror 608 to invert the characteristic using the resistor 606 to generate the gate voltage of the modulator switch 612. This inverted characteristic can be represented by the equation Vg = VDDA - I * k * R, where VDDA represents the internal chip supply, I represents the limiter current, k represents a scaling factor, and R represents the bias resistor for inverting the characteristic.

[0043] As mentioned above, in one or more embodiments, the controller is configured to sample the output of the shunt regulator 602 before the RFID transponder enters the modulation phase. In this way, it can be ensured that the current can be mirrored during the modulation phase of the RFID transponder. In particular, the limiter current drops during modulation, since all current is shunted through the modulator switches 612 and 614. This problem can be overcome by sampling this current (and thus the voltage) on the sampling capacitor 610 before the modulation phase and by driving the gate of the switch 612 with this sampled voltage.

[0044] In Figure 6In the middle, when the signal MOD is high, modulation is performed. Before modulation, i.e. when MOD_B is high, the limiter 602 current (and thus voltage) is sampled on the sampling capacitor 610. During this phase, the switch 612 is not conductive. During the modulation phase, i.e. when MOD_B is low and MOD is high, this sampled voltage is held by the sampling capacitor 610 and applied to the gate of the switch 612 that is modulated. The minimum sampling capacitor size is determined by the instantaneous charge to be provided to the parasitic gate capacitor of the switch 612 (and any logic between the switches 612, if present) and the longest modulation pulse duration. The maximum sampling capacitor is determined by the area limit and time constant defined by the sampling capacitor 610 and the bias resistor 606 relative to the modulation period. Selecting the correct values of k and R can be used to achieve the ETSI specification at +18dBm. For example, by selecting a gate voltage that is less than the threshold of the switch 612, the backscatter strength can be reduced at 18dBm and above, where only the switch 614 remains active during modulation.

[0045] Figure 7 An illustrative embodiment of the sampled gate voltage 700 is shown. In particular, a plot 702 is shown that represents the sampled gate voltage relative to the input power. As shown, at 18dBm and above, the sampled gate voltage is less than the threshold (VTHn) of the switch 612.

[0046] Figure 8 An example of simulation results 800 is shown. In particular, according to the present disclosure, a plot 802 represents simulation results without adaptive modulation, while a plot 804 represents simulation results with adaptive modulation. As shown, the plot 804 is similar to the desired plot 504 shown in the middle. Figure 5 Thus, by using adaptive modulation, the predefined requirements (e.g. ETSI requirements) can be met.

[0047] Figure 9 Another illustrative embodiment of an RFID transponder 900 is shown. The RFID transponder includes a controller 902, a voltage domain 904, and a level shifter 906. In particular, Figure 9 One possible implementation of using the level shifter 906 (which can also be referred to as a voltage level translator) to change the control voltage of the modulator is shown. The controller 902 is configured to adjust the backscatter strength by switching the voltage domain 904 of the voltage level translator 906.

[0048] Figure 10 A plot 1000 is shown that illustrates the effect of changing the level shifter voltage on the backscatter signal. In particular, Figure 10 The voltage level used by the level shifter can be used to change the strength of the backscatter signal is shown.

[0049] Figure 11 Another illustrative embodiment of an RFID transponder 1100 is shown. The RFID transponder 1100 includes a controller 1102 and a level shifter 1106. In particular, Figure 11 An implementation is shown in which a power indicator 1104 is used to change the level of the level shifter 1106. This power indicator 1104 dynamically and adaptively changes the voltage of the level shifter 1106, thereby also changing the strength of the backscatter signal.

[0050] It should be noted that the above described embodiments can have been described with reference to different subjects. In particular, some embodiments can have been described with reference to claims of the method type and others can have been described with reference to claims of the device type. However, a skilled person will appreciate from the above that, unless otherwise stated, any combination of features with reference to different subjects, in particular a combination of features of claims of the method type and features of claims of the device type, is considered to be disclosed with this document, in addition to any combination of features belonging to one type of subject matter.

[0051] Furthermore, it is noted that the drawings are schematic. Like numbers refer to like or similar elements throughout. Additionally, it should be noted that for the sake of brevity it can have been omitted to describe all details of conventional techniques and some implementations. It is understood that where such process parameters are used they are used to enable or optimize the application, and are not used to limit the application. It should be further noted that the description as provided above is merely illustrative and that numerous modifications can be made by those skilled in the art without departing from the true spirit and scope of the application. It is understood that the terms used are indicative of selected embodiments and are used to enable a clear disclosure of the application. In this context, words ending in "er" or "or" are intended to mean "and / or", for example, the term "a component" means "one or more components". Words using the singular or plural number of an element shall include that and that of other members of an equivalent range for the element. The terms "comprise", "comprising", "comprises" and "comprising" specify the presence of stated features but do not preclude the presence or addition of one or more other features. The term "a" or "an" means "one or more".

[0052] Finally, it is noted that a person of ordinary skill in the art would be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The indefinite articles "a" and "an" shall not be construed as excluding the presence of zero or more than one of the mentioned elements - i.e. "a" or "an" can mean "one or more" if the context allows. The implementation of the measures recited in the claims can be performed by means of hardware comprising several distinct elements, and / or by means of a processor, specially adapted for executing software, wherein the software can comprise several sub-software's. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that different claims enumerate

[0053] List of reference signs

[0054] 100 RFID transponder

[0055] 102 modulator

[0056] 104 controller

[0057] 200 method of operating an RFID transponder

[0058] 202 controller regulates the input voltage of the modulator and feeds the regulated input voltage to the modulator

[0059] 204 modulator generates a modulated signal to be transmitted to an external RFID reader

[0060] 300 communication system

[0061] 302 RFID reader

[0062] 304 demodulator

[0063] 306 controller

[0064] 400 RFID transponder

[0065] 402 modulator

[0066] 404 voltage regulator

[0067] 406 power indicator

[0068] 408 voltage domain 1

[0069] 410 voltage domain n

[0070] 412 operating mode

[0071] 414 voltage level converter

[0072] 416 modulator signal

[0073] 500 backscatter vs. input power

[0074] 502 maximum backscatter vs. input power

[0075] 504 backscatter vs. input power according to requirements

[0076] 506 backscatter vs. input power after scaling NMOS switch

[0077] 600 adaptive modulation concept

[0078] 602 shunt regulator or limiter

[0079] 604 inverter

[0080] 606 bias resistor

[0081] 608 current mirror

[0082] 610 sampling capacitor

[0083] 612 modulator switch

[0084] 614 modulator switch

[0085] 700 sampled gate voltage

[0086] 702 sampled gate voltage vs. input power

[0087] 800 simulation results

[0088] 802 simulation results without adaptive modulation

[0089] 804 simulation results with adaptive modulation

[0090] 900 RFID transponder

[0091] 902 controller

[0092] 904 voltage domain

[0093] 906 level shifter

[0094] 1000 effect of altered level shifter voltage on backscatter signal

[0095] 1100 RFID transponder

[0096] 1102 controller

[0097] 1104 power indicator

[0098] 1106 level shifter

Claims

1. A radio frequency identification (RFID) transponder, characterized by comprises: a modulator configured to generate a modulated signal to be transmitted to an external RFID reader, the modulator comprising: a shunt regulator transistor and a current mirror transistor coupled to provide a mirrored output in response to an input voltage; a first switch having a second terminal and a first terminal coupled to receive the mirrored output, the first switch responsive to a first modulation signal; a sampling capacitor coupled to the second terminal of the first switch; a second switch having a second terminal and a first terminal coupled to the second terminal of the first switch, the second switch responsive to a second modulation signal; and a modulator transistor having a first terminal coupled to provide the modulated signal, a second terminal coupled to ground, and a gate coupled to the second terminal of the second switch.

2. The RFID transponder of claim 1, wherein, the modulator is configured to adjust the input voltage depending on at least one of: an operating mode of the RFID transponder, an application domain in which the RFID transponder is used, a configuration of the RFID transponder, and a parameter stored in the RFID transponder.

3. The RFID transponder of claim 2, wherein, the modulator is configured to control a voltage level translator, wherein the voltage level translator is configured to combine the adjusted input voltage with a modulator control signal to generate an adjusted control signal for the modulator.

4. The RFID transponder of claim 1, wherein, is an RFID tag or an RFID card.

5. A method of operating a radio frequency identification (RFID) transponder, characterized by, the RFID transponder is configured to generate a modulated signal to be transmitted to an external RFID reader, the method comprising: providing a mirrored output in response to an input voltage using a shunt regulator transistor and a current mirror transistor; conducting the mirrored output to a sampling capacitor in response to a first modulation signal; and conducting stored charge on the sampling capacitor to a gate of a modulator transistor to turn the modulator transistor on in response to a second modulation signal, wherein the modulator transistor is coupled between ground and a radio frequency antenna to provide the modulated signal.

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