WO2012103222A2

WO2012103222A2 - Adaptive impedance matching

Info

Publication number: WO2012103222A2
Application number: PCT/US2012/022554
Authority: WO
Inventors: Finn HAUSAGER
Original assignee: Molex Incorporated
Priority date: 2011-01-27
Filing date: 2012-01-25
Publication date: 2012-08-02
Also published as: TW201238243A; WO2012103222A3

Abstract

A matching circuit for making an impedance at a terminal. The matching circuit comprises at least one variable reactance element for dynamically adjusting values of a plurality of reactive components. The at least one variable reactance element is configured to: (a) perform a first adjustment of a value of one of the reactive components so as to one of minimize and maximize a feedback voltage at least approximately at resonance, and (b) perform a second adjustment of values of multiple reactive components while maintaining a predetermined relationship among the multiple reactive components as a constant so as to match the impedance.

Description

ADAPTIVE IMPEDANCE MATCHING

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims the benefit of U.S. Provisional Patent Application No. 61/436,768 filed January 27, 2011, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] This invention relates generally to the field of transmission line impedance matching and more specifically to adaptive impedance matching.

BACKGROUND OF THE INVENTION

[0003] Loop antennas, such as Near Field Communication (NFC) antennas typically couple portable electronic devices and/or terminals. Fixed tuning of loop antennas, including those used in NFC devices, causes an impedance mismatch that results in power reflections for the NFC transmitter at close range. The resulting reflections, in turn, are likely to cause noise in the receiver, as well as errors in transmission due to hard loading of the transmitter. Detuning caused by nearby metal objects further reduces the transmission range. Additionally, antenna tolerances can cause variation in performance. Similar impedance matching issues exist with respect to inductive coupling for power transmission, such as via a power pad device. Thus, fixed impedance tuning methods are not effective in overcoming impedance mismatching due to nearby metal object detuning, tolerance variation, and other variable factors.

[0004] However existing methods for adaptive impedance matching, such those discussed in Automatic Impedance Matching for 13.56 MHz NFC Antennas, IEEE (Copyright 2008), rely on phase information and, therefore, result in considerable component complexity and expense in implementation (e.g., by requiring a measuring circuit with a phase detector module). Thus, certain individuals would appreciate further improvements in antenna systems. BRIEF SUMMARY OF THE INVENTION

[0005] Systems, methods, and associated circuit topologies are provided for adaptively matching impedance at a terminal by using voltage feedback so as to forego the need to rely on phase information. This results in simplified circuit topologies and reduces cost of

implementation due to relaxed production tolerances, increased reliability due to reduced component count, increased communication range, higher communication reliability, and increased stability of transmitter load conditioning, among other benefits. In various

embodiments, antenna matching circuits are provided that employ a plurality of dynamically tunable reactive components and a voltage feedback circuit for adaptively matching impedance at a terminal connected to an antenna, such as a Near Field Communication (NFC) loop antenna in a mobile device. In various embodiments, the matching circuits described herein rely on voltage feedback and use a split capacitor tuning network for adaptively matching impedance variations of narrow band inductive antennas. Alternatively, split inductor tuning networks are used for adaptive impedance matching of narrow band capacitive antennas. The matching circuit topologies and associated methods described herein may be used for dynamically matching impedance of numerous electrical components associated with inductive or capacitive coupling, including, but not limited to, antennas (including mobile phone antennas subject to impedance mismatch due to user and/or environment interaction), inductive power pads, Radio Frequency Identification (RFID) transducers (such as door key readers), FM transmitters (for instance those connecting portable players to a vehicle radio), and the like.

[0006] In one aspect, a system for matching an impedance at a terminal is provided. The system comprises a signal generator, a matching circuit connected to the signal generator, and a feedback circuit. The matching circuit includes at least one variable reactance element for dynamically adjusting values of a plurality of reactive components, the at least one variable reactance element configured to: (a) perform a first adjustment of a value of one of the reactive components so as to one of minimize and maximize a feedback voltage at least approximately at resonance, and (b) perform a second adjustment of values of multiple reactive components while maintaining a predetermined relationship among the multiple reactive components as a constant so as to match the impedance. The feedback circuit is connected to the matching circuit and is configured for varying the feedback voltage.

[0007] In another aspect, a matching circuit for matching an impedance at a terminal is provided. The matching circuit comprises at least one variable reactance element for dynamically adjusting values of a plurality of reactive components, the at least one variable reactance element configured to: (a) perform a first adjustment of a value of one of the reactive components so as to one of minimize and maximize a feedback voltage at least approximately at resonance, and (b) perform a second adjustment of values of multiple reactive components while maintaining a predetermined relationship among the multiple reactive components as a constant so as to match the impedance.

[0008] In yet another aspect, a method of matching an impedance at a terminal by adaptively adjusting values of a plurality of reactive components of a matching circuit is provided. The method comprises performing a first adjustment of a value of one of the reactive components so as to one of minimize and maximize a feedback voltage at least approximately at resonance, and performing a second adjustment of values of multiple reactive components while maintaining a predetermined relationship among the multiple reactive components as a constant so as to match the impedance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] While the appended claims set forth the features of the present invention with particularity, the invention and its advantages are best understood from the following detailed description taken in conjunction with the accompanying drawings, of which:

[0010] Figures 1A is schematic diagram illustrating circuit topology for adaptively matching impedance of an inductive load , in accordance with an embodiment of the invention;

[0011] Figure IB is a schematic diagram illustrating a Smith chart reflecting an adaptive tuning step performed via the circuit topology of Figure 1A, in accordance with an embodiment of the invention;

[0012] Figure 1C is a schematic diagram illustrating a scattering plot corresponding to the Smith chart of Figure IB, in accordance with an embodiment of the invention; [0013] Figure ID is a schematic diagram illustrating a Smith chart reflecting an additional adaptive tuning step performed via the circuit topology of Figure 1 A, in accordance with an embodiment of the invention;

[0014] Figure IE is is a schematic diagram illustrating a scattering plot corresponding to the Smith chart of Figure ID, in accordance with an embodiment of the invention;

[0015] Figures 2A is a schematic diagram illustrating another circuit topology for adaptively matching impedance of an inductive load, in accordance with an embodiment of the invention;

[0016] Figure 2B is a schematic diagram illustrating a Smith chart reflecting an adaptive tuning step performed via the circuit topology of Figure 2A, in accordance with an embodiment of the invention;

[0017] Figure 2C is a schematic diagram illustrating a scattering plot corresponding to the Smith chart of Figure 2B, in accordance with an embodiment of the invention;

[0018] Figure 2D is a schematic diagram illustrating a Smith chart reflecting an additional adaptive tuning step performed via the circuit topology of Figure 2A, in accordance with an embodiment of the invention;

[0019] Figure 2E is is a schematic diagram illustrating a scattering plot corresponding to the

Smith chart of Figure 2D, in accordance with an embodiment of the invention;

[0020] Figures 3A is a schematic diagram illustrating another circuit topology for adaptively matching impedance of an inductive load, in accordance with an embodiment of the invention;

[0021] Figure 3B is a schematic diagram illustrating a Smith chart associated with the circuit topology of Figure 3A, in accordance with an embodiment of the invention;

[0022] Figure 3C is a schematic diagram illustrating another Smith chart associated with the circuit topology of Figure 3A, in accordance with an embodiment of the invention;

[0023] Figure 3D is a schematic diagram illustrating a scattering plot associated with a state of the tuning circuit of Figure 3A, in accordance with an embodiment of the invention;

[0024] Figure 3E is a schematic diagram illustrating a Smith chart associated with the circuit topology of Figure 3A, in accordance with an embodiment of the invention;

[0025] Figure 3F is a schematic diagram illustrating another Smith chart associated with the circuit topology of Figure 3A, in accordance with an embodiment of the invention; [0026] Figure 3D is a schematic diagram illustrating a scattering plot associated with the tuning circuit of Figure 3A, in accordance with an embodiment of the invention;

[0027] Figures 4A is a schematic diagram illustrating another circuit topology for adaptively matching impedance of an inductive load, in accordance with an embodiment of the invention;

[0028] Figures 4B, 4C are schematic diagrams illustrating Smith charts associated with respective states of the circuit of Figure 4A, in accordance with an embodiment of the invention;

[0029] Figure 4D is a schematic diagram illustrating a scattering parameter plot associated with Figure 4C, in accordance with an embodiment of the invention;

[0030] Figures 4E, 4F are schematic diagrams illustrating Smith charts associated with respective states of the circuit of Figure 4A, in accordance with an embodiment of the invention;

[0031] Figure 4G is a schematic diagram illustrating a scattering parameter plot associated with Figure 4F, in accordance with an embodiment of the invention;

[0032] Figures 5A and 5B are schematic diagrams illustrating a further embodiment of a circuit topology for adaptively matching impedance at a terminal connected to a non-50 Ohm source;

[0033] Figures 5C is a schematic diagram illustrating a Smith chart associated with a state of the circuit topology of Figure 5B, in accordance with an embodiment of the invention;

[0034] Figure 5D is a schematic diagram illustrating a scattering parameter plot associated with a state of the circuit topology of Figure 5B, in accordance with an embodiment of the invention;

[0035] Figure 5E is a schematic diagram illustrating a tuning step associated with the circuit topology of Figure 5B, in accordance with an embodiment of the invention;

[0036] Figure 5F is a schematic diagram illustrating various tuning values for a reactance element of Figure 5E, in accordance with an embodiment of the invention;

[0037] Figure 5G is a schematic diagram illustrating a scattering parameter plot associated with the tuning step of Figure 5F, in accordance with an embodiment of the invention;

[0038] Figure 5H is a schematic diagram illustrating another tuning step associated with the circuit topology of Figure 5B, in accordance with an embodiment of the invention; [0039] Figure 51 is a schematic diagram illustrating various tuning values for the reactance elemenst of Figure 5H, in accordance with an embodiment of the invention;

[0040] Figure 5J is a schematic diagram illustrating a scattering parameter plot associated with the tuning step of Figure 5H, in accordance with an embodiment of the invention;

[0041] Figures 6A is schematic diagram illustrating circuit topology for adaptively matching impedance of a capacitive load , in accordance with an embodiment of the invention;

[0042] Figure 6B is a schematic diagram illustrating a Smith chart reflecting an adaptive tuning step performed via the circuit topology of Figure 6A, in accordance with an embodiment of the invention;

[0043] Figure 6C is a schematic diagram illustrating a scattering plot corresponding to the Smith chart of Figure 6B, in accordance with an embodiment of the invention;

[0044] Figure 6D is a schematic diagram illustrating a Smith chart reflecting an additional adaptive tuning step performed via the circuit topology of Figure 6A, in accordance with an embodiment of the invention;

[0045] Figure 6E is a schematic diagram illustrating a scattering plot corresponding to the Smith chart of Figure 6D, in accordance with an embodiment of the invention;

[0046] Figure 7 is a schematic diagram illustrating a general system component layout of the embodiments of the invention; and

[0047] Figure 8 is a flow chart illustrating a method of adaptive impedance tuning in accordance with the embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0048] The following examples further illustrate the invention but, of course, should not be construed as in any way limiting its scope.

[0049] Systems, methods, and associated circuit topologies are provided for adaptively matching impedance at a terminal by using voltage feedback so as to forego the need to rely on phase information. This results in simplified circuit topologies and reduces cost of

implementation due to relaxed production tolerances, increased reliability due to reduced component count, increased communication range, higher communication reliability, and increased stability of transmitter load conditioning, among other benefits, among other benefits. In various embodiments, antenna matching circuits are provided that employ a plurality of dynamically tunable reactive components and a voltage feedback circuit for adaptively matching impedance at a terminal connected to an antenna, such as a Near Field Communication (NFC) loop antenna in a mobile device. In various embodiments, the matching circuits described herein rely on voltage feedback and use a split capacitor tuning network for adaptively matching impedance variations of narrow band inductive antennas. Alternatively, split inductor tuning networks are used for adaptive impedance matching of narrow band capacitive antennas. The matching circuit topologies and associated methods described herein may likewise be used for dynamically matching impedance of various other electrical elements associated with inductive or capacitive coupling, including, but not limited to antennas (including mobile phone antennas subject to impedance mismatch due to user and/or environment interaction), inductive power pads, Radio Frequency Identification (RFID) transducers (such as door key readers), FM transmitters (for instance those connecting portable players to a vehicle radio), and the like. Preferably, a predetermined threshold of the feedback voltage Vfb triggers activation of the embodiments of the matching algorithms described below. Likewise, when the feedback voltage Vfb falls below another predetermined threshold, the variable reactance elements return to predefined initial values.

[0050] Turning to Figure 1A, an embodiment of a circuit topology for adaptively matching impedance at a terminal connected to an inductive element is shown. In this embodiment, the inductive element is an inductive loop antenna, such as an NFC loop antenna used in a mobile device, such as a mobile phone, a smart phone, a tablet, a laptop computer, or the like. In further embodiments, the antenna is a main RF antenna coupling a mobile device with a base station, or a Radio Frequency Identification (RFID) device antenna. In yet further embodiments, the inductive element forms part of an inductively coupled pay terminal, an electronic door key, an inductively coupled power supply, or another device implementing inductive coupling. The circuit topology 100 includes a signal generator 102 connected to a feedback circuit 104. The signal generator 102 includes input impedance R0, such as 50 Ohms (e.g., representing a 50 Ohm input impedance of a mobile device transmitter). One terminal of the signal generator 102 is connected to a first terminal of a resistor 106 of a high impedance feedback circuit 104. The resistor 106 has a value R4, for example 9950 Ohms. The second terminal of the signal generator 102 is connected to ground 108. The second terminal of the resistor 106 is connected to an amplifier 109, which in turn is connected to a second 50 Ohm variable signal source 110.

[0051] The matching circuit 112 includes adaptively variable capacitors 114, 116 having respective tunable values CI and C2. The variable capacitors 114, 116 are implemented via at least one variable reactance element 118, such as a digital or an analog Microelectromechanical (or Microelectronic) System (MEMS) device, an electronic varactor, a digitally tunable capacitor circuit, a switched capacitor array, or the like. In one embodiment each of the adaptively tunable capacitors CI and C2 is implemented via a dedicated variable reactance element 118.

[0052] Alternatively, a single variable reactance element 118 is configured as an adaptively tunable capacitor network CI, C2, where CI and C2 share a pool of switchable reactive elements. In a capacitor bank implementation, a plurality of reactive elements is configured in series and individually shorted to increase the capacitance as needed, for example. A multi throw switch selects the capacitor split. Alternatively or in addition, each reactive element in the bank is digitally controlled by a processor of a microcontroller executing computer readable instructions stored in memory and comprising an algorithm configured to select a switch configuration based on a feedback signal. In an embodiment, the feedback signal triggers different modes like standby, start tuning to resonance, and go to an impedance tune (split).

[0053] In additional embodiments, the tuning is implemented via voltage controlled dielectric material for capacitors (BST) (or a ferromagnetic tuned inductor with a movable ferrite core, in case of inductive tuning described below in connection with matching capacitive loads), mechanically tuned capacitor with moving conductive plates or dielectric, for example by using a step motor actuator. Alternatively, relay switching, MOSFET or other transistor switching, or a PIN diode switch may be employed.

[0054] In the illustrated embodiment, the first terminal of the variable capacitor 114 is connected to the first terminal of resistor 106 of the feedback circuit 104, while the second terminal of the variable capacitor 114 is connected to the first terminal of the variable capacitor 116, which is further connected to ground. In this embodiment, the inductive antenna 120 is connected across the terminals of the variable capacitor 116 of the matching circuit 118. The inductive antenna 120 is represented by an inductor 122, having a value LI, and a resistor 124 having a value Rl (e.g., 50 Ohms), which represents radiation and thermal losses.

[0055] An embodiment of a tuning algorithm initially involves achieving resonance by minimizing the absolute value of the feedback voltage by way of tuning the value CI of the variable capacitor 114. Once resonance is achieved, both values CI and C2 of variable capacitors 114, 116 are tuned to bring the absolute value of the feedback voltage Vfb to ½ of the absolute value of the generator voltage Vgen (a known value), while keeping the sum of values CI + C2 as a constant. This adaptively matches the change in antenna impedance based on the feedback voltage without the need for phase information. By eliminating the reactive element of impedance, the load impedance is minimized - thus the voltage is also minimized. In an embodiment, an algorithm controlled tuning is implemented via a micro controller with analog input to an analog-to-digital converter (ADC) for the feedback voltage. The microcontroller includes one or more digital output pins for controlling the reactance element switches either in a serial or parallel configuration. In case of varactor as tunable capacitor, the is converted back into the analog domain either controlling an external digital-to-analog (DAC) or using an embedded DAC.

[0056] An embodiment of the adaptive impedance matching associated with a circuit topology of Figure 1 A is described in further detail with additional reference to Figures IB - IE. Initial values for capacitor 114 (value CI) and capacitor 116 (value C2) are based on the followin formulas:

C1 = N * Ctune

C2 = Ctune— CI

[0057] Step one of the adaptive tuning process entails adjusting the value of CI until IVfbl at port 2 of the feedback circuit 104 is minimized, which occurs at resonance where the antenna coil reactance is eliminated by the capacitance of CI + C2, as illustrated by the Smith chart of Figure IB and a corresponding scattering parameter graph of Figure 1C. After this tuning step, Sl l is nearly real (FIG. IB) and Vfb is minimized (FIG. 1C) at the desired frequency, which is 13.56 MHz in this example. Figures IB and 1C show that changing CI by 2 pF achieves resonance, however the impedance match is not yet perfect. The value of IVfbl or IS21I is minimized for Cdelta = 2 pF.

[0058] Step two of the adaptive tuning process entails adjusting both CI and C2, while keeping Ctune = CI + C2 constant, as illustrated by Figures ID and IE. The following setting is made: Cl'= CI + Cdelta if C2' = C2 - Cdelta. The adjustment is performed by using different Cdelta values until IVfbl or IS21I in Figure IE hits 1, which indicates a perfect match. Figures ID and IE show that Cdelta = 4 pF indicates a perfect match since Sl l seen from the generator is very close to the Smith chart center (FIG. ID) and IVfbl = ½ * IVgeneratorl or S21 is very close to 1 (FIG. IE).

Mathematical Proof of an Embodiment of a Matching Method of Figure 1A

[0059] As discussed above, the first step entails using CI to create resonance by minimizing the voltage Vfb across the generator terminals as follows:

[0060] Since CI is the only variable and setting: results in:

[0061] The equation can be minimized only by setting . Resonance does not mean

perfect matching since resonance can mean any real impedance and perfect match occurs only if A = R₀.

[0062] The second matching step entails changing both CI and C2 simultaneously in a way that maintains resonance. Resonance occurs if:

[0063] Since the Q is high, we know that Ij ωLll >> IRll, which allows for an approximation neglecting Rl. Then we get:

[0064] Defining Ctune and keeping it constant resonance is maintained

hence if C2 is changed by AC, CI is also changed by - AC. At resonance, the network impedance Z_IN (antenna and matching capacitors) is real, as shown below:

[0065] Then if the coil is high Q (roLl>>Rl):

[0066] The feedback voltage across the generator terminals:

[0067] And then using the constant Ctune = C1 + C2 = we get:

[0068] Solving this equation when it is equal to ½ provides a perfect match:

or

which is equal to the approximate formulas discussed above in connection with Figures 1A-1E.

[0069] Turning to Figure 2A, another embodiment of a circuit topology for adaptively matching impedance at a terminal connected to an inductive element, such as an antenna, is shown. The circuit topology 200 is similar to that described with respect to Figure 1A above with the exception that the first terminal of resistor 106 of the high impedance feedback circuit 104' is connected to the first terminal of the variable capacitor 116, which further connects to ground 108. The inductive antenna 120 is connected between the first terminal of resistor 106 and ground 108. The feedback voltage Vfb associated with this circuit topology is across the parallel capacitor C2.

[0070] In this embodiment, the tuning algorithm is generally performed in two steps. First, resonance is achieved by maximizing voltage feedback by tuning value CI of the variable capacitor 114. Tuning CI eliminates the reactive element seen from generator and thus maximizes current drawn from generator. Then voltage across the antenna terminal is maximized. Thus, the second step generally involves again maximizing voltage feedback by tuning the values of both CI and C2 while keeping CI + C2 constant. Therefore, no phase information is required to adaptively match the antenna 120. In the illustrated embodiment, the generator 102 has an impedance of 50 Ohms.

[0071] An embodiment of the adaptive impedance matching associated with a circuit topology of Figure 2A is described in further detail with additional reference to Figures 2B - 2E. In this embodiment, initial values for capacitor 114 (value CI) and capacitor 116 (value C2) are bas llowing formulas:

C1 = N * Ctune

C2 = Ctune— CI

[0072] Step one of this embodiment of the tuning process entails adaptively adjusting the value of CI until IVfbl at port 2 of the feedback circuit 104' is maximized, which occurs at resonance where the antenna coil reactance is eliminated by the capacitance of CI + C2, as illustrated by the Smith chart of Figure 2B and a corresponding scattering parameter graph of Figure 2C. After this tuning step, Sl l is nearly real (FIG. 2B) and Vfb is maximized (FIG. 2C). Figures 2B and 2C show that changing CI by 2 pF achieves resonance, however the impedance match is not yet perfect. The value of IVfbl or IS21I is maximized for Cdelta = 2 pF.

[0073] Step two of the illustrated embodiment of the adaptive tuning process entails adjusting both CI and C2, while keeping Ctune = CI + C2 constant, as illustrated by Figures 2D and 2E. The following setting is made: Cl'= CI + Cdelta if C2' = C2 - Cdelta. The adjustment is performed by using different Cdelta values until IVfbl or IS21I in Figure 2E is maximized, which indicates a perfect match.

[0074] Mathematical Proof of an Embodiment of a Matching Method of Figure 2A

[0075] Using CI to create resonance by maximizing voltage Vfb across C2: v_FB

and then letting: which does not include CI, results in:

[0076] Varying only CI results in a simple first order equation. Vfb is maximized by having thus eliminating the reactive part of the denominator. The impedance seen from the

generator is Z which is a real number indicating resonance.

[0077] Resonance does not mean perfect matching since resonance can mean any real impedance and a perfect match occurs if also A = R0.

[0078] The second matching step entails changing both CI and C2 simultaneously, while maintaining resonance.

[0079] Since the Q is high we know that jω L1>>R1, which allows for an approximation neglecting Rl. In this case:

[0080] Resonance is maintained by defining Ctune = C1 + C2 = and keeping it

constant. Hence a change in the value of C2 by AC requires a change in the value of CI by AC. At resonance the network impedance Zin is real:

[0081] Then if the coil is high Q ( ωL1>>R1):

[0082] If we then look at the feedback voltage across C2:

1

[0083] Using the constant Ctune = C1 + C2 = we get:

[0084] Differentiating this equation for CI and setting it to equal to zero (0) to find maximum and minimum, we get:

or

CI = ±Ctune *

For which Vfb is minimized or maximized.

[0085] Choosing CI = Ctune since C1=0 gives the minimum and CI must be

positive, we then use the constraint C1 + C2■ to determine that

matching has taken place:

[0086] Turning to Figure 3A, another embodiment of a circuit topology for adaptively matching impedance at a terminal connected to an inductive element, such as an antenna, is shown. The circuit topology 300 includes a signal generator 302 connected to a high impedance envelope feedback circuit 304. An embodiment of the signal generator 302 has input impedance R0, such as 50 Ohms. One terminal of the signal generator 302 is connected to a first terminal of a resistor 306 of the feedback circuit 304. An embodiment of the resistor 306 has a value R4, for example 9950 Ohms. The second terminal of the signal generator 302 is connected to ground 308. The second terminal of the resistor 306 is connected to an amplifier 309, which in turn is connected to a second 50 Ohm variable signal source 310.

[0087] The matching circuit 312 includes adaptively variable capacitors 314, 316 having respective tunable values C3 and C4. As described above, the variable capacitors 314, 316 are implemented via at least one variable reactance element 318, such as a digital or an analog Microelectromechanical (or Microelectronic) System (MEMS) device, an electronic varactor, a digitally tunable capacitor circuit, a switched capacitor array, or the like. In the illustrated embodiment, the first terminal of the variable capacitor 316 is connected to the first terminal of resistor 306 of the feedback circuit 304, while the second terminal of the variable capacitor 316 is connected to ground 308. The first terminal of the variable capacitor 316 is also connected to the first terminal of the variable capacitor 314. In this embodiment, the inductive antenna 120 is connected between the second terminal of the variable capacitor 314 and ground 308. The inductive antenna 320 is represented by an inductor 322, having a value L2, and a resistor 324, having a value R2 (e.g., 50 Ohms), which represents radiation and thermal losses.

[0088] In this embodiment of the tuning algorithm, resonance is approached by tuning the value C4 of the variable capacitor 316 to maximize voltage feedback. Next, the values of both capacitors 316 (C4) and 314 (C3) are tuned while keeping the ratio 1/(1/C3+1/C4) as a constant until the feedback voltage is equal to half of the generator voltage (i.e., Vfb = ½*Vgenerator). Therefore, no phase information is required to achieve a match.

[0089] In more detail initial values for C3 and C4 are found as follows:

[0090] In this example, the tuning results in offsetting C4 by 30 percent and adding 20 pF to C3. First, value of C4 is adjusted until IVfbl at port 4 (S42) of the feedback circuit 304 is maximized which occurs very close to resonance where the antenna coil reactance is eliminated by the serial capacitance:

[0091] Figure 3B illustrates a Smith chart of an initial match, while Figures 3C and 3D illustrate the effect of reducing C4 by 110 pF, which occurs very close to resonance, and maximizing Vfb. [0092] Next, both C3 and C4 are adjusted while maintaining the relation

constant, thus maintaining resonance, until the feedback S42 becomes 1, corresponding to Vfb=l/2*Vgen. Figure 3E depicts a Smith chart prior to this step. Figures 3F and 3G, respectively, show a Smith chart and a scattering parameters plot pursuant to this step when C3 is reduced by 20 pF and C4 is adjusted while maintaining resonance. Figure 3F shows that very close to a perfect match is achieved when C3 is reduced by 20 pF where Vfb=l/2*Vgen (i.e., S42=l in the scattering parameters plot of Figure 3G).

[0093] Turning to Figure 4A, another embodiment of a circuit topology for adaptively matching impedance at a terminal connected to an inductive element, such as an antenna, is shown. The circuit topology 400 is similar to that described with respect to Figure 3A above with the exception that the first terminal of resistor 306 of the high impedance feedback circuit 304' is connected to the second terminal of the variable capacitor 316.

[0094] In this embodiment, initial values of C3 and C4 are based on the following formulas:

[0095] First, the value of C4 is adjusted until IVfbl at the antenna terminals is maximized, which occurs at resonance where the antenna coil reactance is eliminated by the capacitance of 1/(1/C3+1/C4). Figures 4B-4D illustrate the foregoing step, where Figure 4B shows a Smith chart prior to this step, while Figures 4C-4D illustrate a Smith chart and a corresponding scattering parameters plot where resonance is obtained by changing C4 until IVfbl is maximized (Sl l is nearly real). Specifically, it is shown that resonance, however not a perfect match, is achieved by reducing C4 by 140 pF. In this embodiment, IVfbl or IS42I is maximized for Cdelta = -140 pF.

[0096] Next, both C3 and C4 are adjusted while keeping Ctune = 1/(1/C3+1/C4) constant. Specifically, different values of Cdelta are used until IVfbl or IS42I in Figures 4F and 4G, respectively, is maximized. This indicates a nearly perfect match. Inductive Antenna Matching with Non-50 Ohm Generator

[0097] Turning to Figures 5A-5J, an embodiment of a circuit topology for adaptively matching impedance at a terminal connected to an inductive antenna using a non-50 Ohm generator is shown. The signal level at the feedback point is calculated by loading the transmitter with a RO = 50 Ohm antenna, where generator power and impedance are known. Specifically, if the generator delivers Vg and the generator impedance is Rg instead of R0; then Vfb is Vg*R0/(R0+Rg). The tuning steps are then first changing CI to minimize Vfb, which brings the antenna and matching into resonance. Secondly, the impedance is changed keeping C1+C2 constant (reducing CI by AC and increasing C2 by AC, or vice versa) until Vfb equals Vg*R0/(R0+Rg). As shown in Figures 5A-5D, using the foregoing method, matching is nearly perfect to a 50 Ohm load irrespective of the generator impedance.

[0098] In an adaptive design embodiment, Vfb is minimized for CI delta of 56 pF giving Cla = 80 pF (FIGS. 5E-5G). With this value the antenna and matching network go into resonance at 13.56 MHz. In the subsequent step, both CI and C2 vary, while CI + C2 = Cm. This means adding 1 pF to CI means subtracting 1 pF from C2, for example (FIGS. 5H-5J).

[0099] In the illustrated embodiment, to obtain a 50 Ohm match for 0 dBm from a 10 Ohm transmitter, a 50 Ohm load corresponds to a Vfb of 0.236 Vp. Changing Clb from 80 pF down by 32 pF, resulting in 48 pF, and similarly adding 32 pF to C2b, resulting in 287 pF, Vfb = 0.242 Vp.

[00100] Capacitive Antenna Matching using Inductors

[00101] Turning to Figure 6A, an embodiment of a circuit topology for adaptively matching impedance at a terminal connected to a capacitive element, such as a capacitive antenna, is shown. A capacitive antenna is modeled as a series resistor Ra, representing radiation and loss, and a capacitor Ca, representing a reactance. In the illustrated embodiment, the matching circuit comprises two variable inductors. In one embodiment, a tunable inductor or a positive reactive impedance is achieved using an inductor element. For instance, by moving a ferrite core into a coil, the reactance is increased (or if a conductive surface is brought closer, e.g., mechanically, to an inductor, the reactance is decreased due to an increase in parasitic capacitance).

Alternatively, a parallel tunable capacitor may be added to a large inductor, in which case the parasitic capacitance from the capacitor can then be tuned to bring the effective inductance down. In an embodiment, the tunable inductors are in series. At higher frequencies, a tunable capacitor connected through a quarter wave transmission line may be employed so as to act as a positive reactive element such as an inductor.

[00102] The impedance seen by the generator circuit is:

[00103] Resonance is achieved if following relation is met:

[00104] If Ra « l/(coCa) at resonance, then Q is high and we can reduce the relation by neglecting the much smaller component Ra. Once resonance resonance is achieved, the impedance is tuned while maintaining resonance by keeping the relation LI + L2 = constant intact.

[00105] Specifically, to tune the circuit, parallel inductor L2 is iteratively adjusted to obtain resonance and then the impedance is tuned while keeping the above relation intact - adjusting both LI and L2 by iteratively adding dL to LI and subtracting dL from L2, thereby maintaining resonance.

[00106] The process is controlled by using a feedback from the antenna terminal and maximizing the feedback amplitude. First, by bringing circuit in resonance using L2 and then matching the impedance and keeping resonance by adjusting both LI and L2 simultaneously and keeping the LI + L2 = constant. During both tuning steps, the feedback amplitude is maximized.

[00107] Referring to Figures 6B-6E, the feedback voltage is maximized by bringing the network impedance as close as possible to the center of the illustrated Smith charts. For instance, for LI = 1.25uH, decreasing L2 brings the circuit in resonance at L2 of approximately 152 nH and maximizes the amplitude of the feedback circuit (FIGS. 6B-6C). Next, both LI and L2 are adjusted while maintaining LI + L2 = 1.25uH + 152nH. As shown in Figure 6E, the feedback amplitude is maximized by moving 60nH from LI to L2. [00108] Turning to Figure 7, a diagram illustrating a general system component layout of the foregoing embodiments of the invention is shown. The system 700 includes a source/generator 702, a load 706, as well as the matching/feedback circuit 704 connecting the generator to the load and operating in accordance with the embodiments of the adaptively tunable circuit topologies and associated tuning algorithms described above.

[00109] Finally, Figure 8 is a flow chart illustrating a method of adaptive impedance tuning in accordance with the embodiments of the invention. In step 800, a first adjustment of a value of one of the reactive components is performed in order to either minimize or maximize a feedback voltage in accordance with the corresponding embodiments of the foregoing circuit topologies described above. In step 802, the adjustment continues until the circuit is at least approximately at resonance. If so, in step 802, a second adjustment of values of multiple reactive components is made while maintaining a predetermined relationship among the multiple reactive components as a constant so as to match the impedance, as described in detail above.

[00110] All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

[00111] The use of the terms "a" and "an" and "the" and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms "comprising," "having," "including," and "containing" are to be construed as open-ended terms (i.e., meaning "including, but not limited to,") unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non- claimed element as essential to the practice of the invention.

[00112] Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims

WHAT IS CLAIMED IS:

1. A system for matching an impedance at a terminal, the system comprising:

a signal generator;

a matching circuit connected to the signal generator, the matching circuit comprising at least one variable reactance element for adaptively adjusting values of a plurality of reactive components, the at least one variable reactance element configured to: (a) perform a first adjustment of a value of one of the reactive components so as to one of minimize and maximize a feedback voltage at least approximately at resonance, and (b) perform a second adjustment of values of multiple reactive components while maintaining a predetermined relationship among the multiple reactive components as a constant so as to match the impedance; and

a feedback circuit connected to the matching circuit, the feedback circuit configured to vary the feedback voltage.

2. The system of claim 1 wherein the terminal is connected to a transducer.

3. The system of claim 2 wherein the transducer is an antenna.

4. The system of claim 3 wherein the antenna is an inductive antenna.

5. The system of claim 3 wherein the antenna is a capacitive antenna.

6. The system of claim 3 wherein the antenna is a Near Field Communication (NFC) antenna.

7. The system of claim 3 wherein the antenna is a loop antenna.

8. The system of claim 1 wherein the plurality of reactive components are capacitors.

9. The system of claim 1 wherein the plurality of reactive components are inductors.

10. The system of claim 1 wherein the at least one variable reactance element adaptively adjusts the values of the plurality of reactive components in response to detecting an increase in the feedback voltage above a predetermined threshold.

11. The system of claim 1 wherein the at least one variable reactance element resets the values of the plurality of reactive components to predetermined initial values in response to detecting a decrease in the feedback voltage below a predetermined threshold.

12. A matching circuit for matching an impedance at a terminal, the matching circuit comprising at least one variable reactance element for adaptively adjusting values of a plurality of reactive components, the at least one variable reactance element configured to: (a) perform a first adjustment of a value of one of the reactive components so as to one of minimize and maximize a feedback voltage at least approximately at resonance, and (b) perform a second adjustment of values of multiple reactive components while maintaining a predetermined relationship among the multiple reactive components as a constant so as to match the impedance.

13. The matching circuit of claim 12 wherein the terminal is connected to a transducer.

14. The matching circuit of claim 13 wherein the transducer is an antenna.

15. The matching circuit of claim 14 wherein the antenna is an inductive antenna.

16. The matching circuit of claim 14 wherein the antenna is a capacitive antenna.

17. The matching circuit of claim 14 wherein the antenna is a Near Field Communication (NFC) antenna.

18. The matching circuit of claim 14 wherein the antenna is a loop antenna.

19. The matching circuit of claim 12 wherein the plurality of reactive components are capacitors.

20. The matching circuit of claim 12 wherein the plurality of reactive components are inductors.

21. The matching circuit of claim 12 wherein the at least one variable reactance element adaptively adjusts the values of the plurality of reactive components in response to detecting an increase in the feedback voltage above a predetermined threshold.

22. The matching circuit of claim 12 wherein the at least one variable reactance element resets the values of the plurality of reactive components to predetermined initial values in response to detecting a decrease in the feedback voltage below a predetermined threshold.

23. A method of matching an impedance at a terminal by adaptively adjusting values of a plurality of reactive components of a matching circuit, the method comprising:

performing a first adjustment of a value of one of the reactive components so as to one of minimize and maximize a feedback voltage at least approximately at resonance; and

performing a second adjustment of values of multiple reactive components while maintaining a predetermined relationship among the multiple reactive components as a constant so as to match the impedance.

24. The method of claim 23 wherein the terminal is connected to a transducer.

25. The method of claim 24 wherein the transducer is an antenna.

26. The method of claim 25 wherein the antenna is a Near Field Communication (NFC) antenna.

27. The method of claim 23 wherein the plurality of reactive components are capacitors.

28. The method of claim 23 wherein the plurality of reactive components are inductors.

29. The method of claim 23 wherein the at least one variable reactance element adaptively adjusts the values of the plurality of reactive components in response to detecting an increase in the feedback voltage above a predetermined threshold.

30. The method of claim 23 wherein the at least one variable reactance element resets the values of the plurality of reactive components to predetermined initial values in response to detecting a decrease in the feedback voltage below a predetermined threshold.