Quality factor control for near field wireless devices

Abstract

One example discloses a near field wireless device comprising: a near field antenna; a variable current source; a controller coupled to the near field antenna and the variable current source; wherein the controller is configured to measure a transmission quality factor (Qtx) of the near field antenna; and wherein the controller is configured to increase a current sent by the variable current source to the near field antenna when the measured Qtx is below a minimum Qtx.

Description

Technical Field

[0001] This specification relates to systems, methods, apparatus, devices, articles of manufacture, and instructions for quality factor control of wireless near-field devices. Background Technology

[0002] This paper discusses near-field interactions between near-field devices (e.g., on a user's body) and other conductive surfaces and / or other wireless networking devices (e.g., Internet of Things (IoT) devices) based on near-field electromagnetic induction (NFEMI), where the transmitter and receiver are coupled via magnetic (H) and electrical (E) fields. While RF wireless communication is achieved by propagating RF plane waves through free space, NFEMI communication utilizes non-propagating quasi-static H and E fields.

[0003] H-field antennas (i.e., magnetic antennas) are primarily sensitive to magnetic fields and / or primarily enable magnetic fields when driven by current. All E-field components from H-field antennas are greatly reduced (e.g., reduced by -20 dB to -60 dB, depending on the antenna design, by 0.1 to 0.0008 (10% to 0.08%)).

[0004] The small loop antenna is an example H-field antenna and includes a loop antenna whose size is much smaller than the wavelength it uses. The small loop antenna does not resonate at the NFEMI carrier frequency but is tuned to resonate via an external reactance. In some example embodiments, the current in the small loop antenna has the same value at every point in the loop.

[0005] E-field antennas (i.e., electric antennas) are primarily sensitive to electric fields and / or primarily enable electric fields when driven by voltage. All H-field components from E-field antennas are greatly reduced (e.g., reduced by -20 dB to -60 dB, depending on the antenna design, by 0.1 to 0.0008 (10% to 0.08%)).

[0006] The short-loaded dipole antenna is an example E-field antenna and includes a short dipole with a size much smaller than the NFEMI carrier frequency. In some example embodiments, the short-loaded dipole antenna has additional capacitive surfaces at both ends.

[0007] The quasi-static properties of these fields are a result of the combination of the NFEMI antenna size and its carrier frequency. Most of the near-field energy is stored in the form of magnetic and electric fields, while a small amount of RF energy inevitably propagates in free space. The geometry of the small antenna minimizes the radiated waves in free space. Summary of the Invention

[0008] According to an example embodiment, a near-field wireless device includes: a near-field antenna; a variable current source; and a controller coupled to the near-field antenna and the variable current source; wherein the controller is configured to measure the transmission quality factor (Qtx) of the near-field antenna; and wherein the controller is configured to increase the current sent from the variable current source to the near-field antenna when the measured Qtx is lower than a minimum Qtx.

[0009] In another example embodiment, the controller is configured to set an upper limit on the current increase to prevent the near-field antenna from transmitting signals exceeding a set of statutory limits.

[0010] In another example embodiment, the controller is configured to reduce the current sent to the near-field antenna by the variable current source when Qtx is higher than the minimum Qtx.

[0011] In another example embodiment, the variable current source includes a set of amplifiers arranged in parallel with each other.

[0012] In another example embodiment, the current sent to the near-field antenna is increased or decreased by connecting or disconnecting one or more of the amplifiers.

[0013] In another example embodiment, the amplifier is a current amplifier.

[0014] In another example embodiment, half of the current amplifier is coupled to a first feed point of the near-field antenna, and the other half of the current amplifier is coupled to a second feed point of the near-field antenna.

[0015] In another example embodiment, the near-field antenna includes a set of feed points; the device further includes a set of voltage sensing lines coupled from the feed points to the controller; and the controller is configured to measure channel or signal voltages on the voltage sensing lines and calculate the measured Qtx based on the measured channel or signal voltages.

[0016] In another example embodiment, the measured channel or signal voltage is 3 dB lower than the maximum channel or signal voltage.

[0017] In another example embodiment, the voltage on the voltage sensing line is based on the current sent to the near-field antenna by the variable current source.

[0018] In another example embodiment, the controller is configured to acquire a received signal strength (RSS) measurement corresponding to the signal transmitted by the near-field device; and if the measured RSS is higher than a minimum RSS, the controller is configured not to increase the current sent to the near-field antenna by the variable current source even if the measured Qtx is lower than the minimum Qtx.

[0019] In another example embodiment, if the measured RSS is lower than the minimum RSS, the controller is configured to increase the current sent to the near-field antenna by the variable current source.

[0020] In another example embodiment, the device further includes a receiver circuit; and the controller is coupled to the receiver circuit and configured to acquire the RSS measurement from the receiver circuit.

[0021] In another example embodiment, the minimum RSS is higher than the noise floor of the receiver circuit.

[0022] In another example embodiment, the near-field antenna is a first near-field antenna; the receiver circuit is coupled to a second near-field antenna; and the controller is coupled to acquire the RSS measurement value from the second near-field antenna.

[0023] In another example embodiment, the near-field device is a first near-field device; and the controller is configured to acquire the RSS measurement value by requesting the RSS measurement value from a second near-field device, the second near-field device being configured to receive communication signals transmitted by the first near-field device.

[0024] In another example embodiment, if the second near-field device does not respond to the RSS measurement request from the first near-field device, the controller is configured to assume that the measured RSS is lower than the minimum RSS.

[0025] In another example embodiment, a tuning circuit coupled to the controller is further included; wherein the tuning circuit includes a set of tuning elements configured to adjust Qtx; and wherein the controller is configured to use the tuning circuit to increase Qtx.

[0026] In another example embodiment, the tuning element includes a set of variable resistors; and the controller is configured to use the variable resistors to increase Qtx.

[0027] In another example embodiment, the near-field antenna is configured to receive and transmit non-propagating quasi-static electrical and / or magnetic near-field signals.

[0028] The above discussion is not intended to represent every example embodiment or implementation within the scope of the current or future claims. The following figures and detailed descriptions further illustrate various example embodiments.

[0029] The various exemplary embodiments can be more fully understood by considering the following specific embodiments in conjunction with the accompanying drawings. Attached Figure Description

[0030] Figure 1 This is an example of a first near-field antenna in a first near-field wireless device.

[0031] Figure 2 This is an example of a second near-field antenna in a second near-field wireless device.

[0032] Figure 3 This is an example ideal equivalent circuit for a first near-field antenna configured to transmit communication signals in a first near-field device.

[0033] Figure 4 This is an example ideal equivalent circuit for a second near-field antenna configured to transmit communication signals in a second near-field device.

[0034] Figure 5 This is an example frequency response curve of a second near-field antenna communication channel used to transmit communication signals via a second near-field device.

[0035] Figure 6 This is an example ideal equivalent circuit for a second near-field antenna configured to transmit communication signals in a third near-field wireless device.

[0036] Figure 7 This is an example set of instructions and / or logic circuits for operating a third near-field wireless device.

[0037] While this disclosure allows for various modifications and alternatives, their particularities have been illustrated by way of example in the drawings and will be described in detail. However, it should be understood that other embodiments beyond the specific embodiments described are also possible. All modifications, equivalents, and alternative embodiments falling within the spirit and scope of the appended claims are also covered. Detailed Implementation

[0038] Some wearable devices, such as hearing aids and wireless earbuds, employ near-field magnetic induction (NFMI) as a method of wireless communication. In NFMI wireless communication, two loosely coupled coils transmit signals without radiating radio waves. Current flowing in the transmitting coil generates an H-field, which in turn induces a current in the receiving coil. Wireless communication is thus achieved. A disadvantage is that H-field-based NFMI systems with small antenna coils have a limited range, which may be much smaller than the entire wearer's body. Such H-field communication is sensitive to coil orientation. In terms of the shape factor of hearing aids, H-field-based systems cannot cover the entire human body. However, because the two coils in a hearing aid are always aligned with each other, they are unaffected by human movement.

[0039] Other wearable devices employ near-field induction (NFEI) as a method of wireless communication. NFEI allows electronic devices on and near conductive surfaces (e.g., the human body) to exchange information via E-field coupling (e.g., at 21 MHz). NFEI is sometimes referred to as body-coupled communication (BCC). While the range of E-field-based NFEI signals can be greater than that of H-field-based NFMI signals, the E-field signal strength varies with body posture and is sensitive to body movement. The body can even partially block the capacitive return path, increasing E-field channel loss and making reliable and stable wireless communication impossible.

[0040] Figure 1 This is an example of a first near-field antenna 100 in a first near-field wireless device. In this example, antenna 100 is a near-field magnetic induction (NFMI) antenna. Antenna 100 includes a coil (H-field) antenna 105 for the magnetic field. H-field antenna 105 includes a ferrite core 110 wound with wire 115. Feed points 135, 140 of antenna 100 are coupled to various transceiver circuit systems, such as downlink radio transmitter and receiver integrated circuits (RF-ICs), (not shown here). Antenna 100 can be tuned to resonate at the communication frequency by means of reactive elements / assemblies (e.g., capacitors (group C)) integrated in the RF-IC. Similarly, reactive elements / assemblies (e.g., resistors (group R)) can be used to tune the bandwidth and quality factor (Q) of antenna 100.

[0041] Figure 2 This is an example of a second near-field antenna 200 in a second near-field wireless device. In this example, antenna 200 is a near-field electromagnetic induction (NFEMI) antenna. Antenna 200 includes a coil (H-field) antenna 105 for the magnetic field and a short-loaded dipole (E-field) antenna 220 for the electric field. H-field antenna 105 includes a ferrite core 110 wound with wire 115. E-field antenna 220 includes two conductive load surfaces 225 and 230. Feed points 135 and 140 of antenna 200 can also be coupled to various transceiver circuitry, such as a lower-profile radio transmitter and receiver integrated circuit (RF-IC), (not shown here). Antenna 200 can also be tuned to resonate at the communication frequency by means of reactive elements / assemblies (e.g., capacitors (group C)) integrated in the RF-IC. The bandwidth and quality factor (Q) of antenna 200 can be tuned similarly using reactive elements / assemblies (e.g., resistors (group R)).

[0042] When the NFEMI antenna 200 approaches a conductive structure (e.g., a structure, subject, person, object, etc. with one or more conductive surfaces), the magnetic and electric fields will be largely confined to the conductive surface and will not generate significant radiation in free space. This enhances the security and privacy of network communications for such subjects.

[0043] In various example embodiments, the antenna 200 operates at a frequency of 50 MHz or below (e.g., at 30 MHz) to ensure that the field follows the contour of the conductive surface and to ensure that far-field radiation is greatly reduced.

[0044] Figure 3 This is an example ideal equivalent circuit 300 for a first near-field antenna 100 configured to transmit communication signals in a first near-field device. Example 300 includes a tuning circuit 302, a current amplifier 304, and a controller 306.

[0045] The first near-field antenna 100 shows the inductance (L1) of the coil antenna 105 and the resistive loss (R3) within the wire 115. The tuning circuit 302 includes variable capacitors C1 and C2 and variable load resistors R1 and R2. The current amplifier 304 supplies a communication signal modulation current to the near-field antenna 100, thereby generating a voltage across the feed points 135 and 140.

[0046] The controller 306 uses variable capacitors C1 and C2 (i.e., group C) to tune the resonant frequency (e.g., 10.6 MHz) of the antenna 100. The controller 306 uses variable load resistors R1 and R2 (i.e., group R) to tune the bandwidth (e.g., 400 kHz) and quality factor (Q) of the antenna 100.

[0047] Figure 4 This is an example ideal equivalent circuit 400 for a second near-field antenna 200 configured to transmit communication signals in a second near-field device. Example 400 includes a tuning circuit 402, a current amplifier 404, and a controller 406.

[0048] The second near-field antenna 200 shows the inductance (L1) of the coil antenna 105, the resistive loss (R3) within the wire 115, the capacitance (Ca) of the conductive surfaces 225 and 230, and the resistive loss (R4) of the short-loaded dipole (E-field) antenna 220. The tuning circuit 402 includes variable capacitors C1 and C2 and variable load resistors R1 and R2. A current amplifier 404 supplies a communication signal modulation current to the near-field antenna 200, thereby generating a voltage across the feed points 135 and 140.

[0049] The controller 406 tunes the resonant frequency (e.g., 10.6 MHz) of the antenna 200 using variable capacitors C1 and C2 (i.e., group C). The controller 406 tunes the bandwidth (e.g., 400 kHz) and quality factor (Q) of the antenna 200 using variable load resistors R1 and R2 (i.e., group R). The adjustment range of the group C capacitors is limited by the physical size of the capacitors. The adjustment range of the group R resistors is limited by their leakage current.

[0050] Figure 5 A sample frequency response graph 500 is shown for a communication channel 508 of a second near-field antenna 200 used to transmit communication signals via a second near-field device. The horizontal axis 502 displays the frequency in MHz, and the vertical axis 504 displays the voltage across feed points 135 and 140 in dBV. A frequency response curve 506 for channel 508 is also shown in the figure. In this example embodiment, the communication channel 508 is defined by the 3 dBV point in the frequency response curve 506.

[0051] In this example, the second near-field antenna 200 is tuned at a frequency of 10.6MHz with a channel bandwidth of 400kHz 508, and the second near-field antenna 200 has a transmission quality factor (Qtx), which is defined as:

[0052]

[0053] Where Fr is the communication frequency (MHz) and BW is the channel bandwidth 508 (MHz). In this specific example, the value of Qtx is 26.5. The channel bandwidth of 508 is important for allowing communication signals to be transmitted without distortion.

[0054] The antenna also has an inherent quality factor (Qa) (based on its inherent physical structure, and resistive losses R3 and R4), which is defined as:

[0055]

[0056] Where Fr is the resonant frequency (MHz) and BWa is the channel bandwidth (MHz). For example, a near-field antenna may have an intrinsic quality factor of 50.

[0057] The Qtx and transmission current must not be too high, otherwise the antenna of the near-field device will generate spurious emissions that will exceed legal limits. For example, in Europe, the standard ETSI-EN3003301V1.7.1 (2010-02) describes the limits for far-field spurious emissions. In the United States, the standard FCC-Sub-Part-C15.209 describes the limits for far-field spurious emissions.

[0058] Far-field emissions from near-field antennas can also be caused by small currents induced in the user wearing the near-field device. In some cases, this is because the user's body acts as an efficient antenna in the frequency range of 70 MHz to 150 MHz, and stray currents in this frequency range will generate unwanted far-field emissions. For example, when the communication frequency is 10.6 MHz, stray emissions will occur at all its harmonics (e.g., 21.2 MHz, 31.8 MHz, etc.) and in the frequency range of 70 MHz to 150 MHz.

[0059] Qtx should not be too low, otherwise the received signal strength (RSS) at other near-field devices will decrease, resulting in a signal-to-noise ratio (SNR) that is unsuitable for reliable data communication.

[0060] In various example embodiments, during the near-field device manufacturing process, the current sent to the second near-field antenna 200 is fixed at a maximum value that does not exceed the statutory spurious emission limit, but the current will provide the most stable SNR possible when receiving the near-field device.

[0061] Although the fixed current sent to the second near-field antenna 200 by the current amplifier 404 is related to the idealized / target Qtx (assuming ideal conditions), during actual near-field device operation, the second near-field antenna 200 may periodically couple to multiple conductive structures. Some of these conductive structure couplings are necessary for communication; however, some of these conductive structure couplings are parasitic (e.g., unwanted). These parasitic couplings (non-ideal) cause Qtx to deviate from its idealized / target Qtx.

[0062] If Qtx is too high, the R group of resistors in the tuning circuit 402 can limit Qtx within the specified value. reduce Return to the nominal operating range (e.g., reduce Qtx from 50 to 26.5). Similarly, if Qtx is too low, the R-group resistors in the tuning circuit 402 can reduce Qtx within limits. increase Return to the nominal Q range.

[0063] In some realistic (non-ideal) near-field device operations / applications, Qtx drops low and fast (e.g., depending on the user's body movement), such that the R-group resistors in the tuning circuit 402 cannot keep Qtx low. increase Return to the nominal Q range.

[0064] Examples where the Q factor can drop rapidly and significantly include hearing aid and earbud applications, where users place their smartphones too close to the hearing aids or earbuds. In this case, the NFMI antenna will be close to the conductive structure of the smartphone, and the NFMI antenna's quality factor may inappropriately drop below 26.5, for example, to 15. Another example is when communication devices use NFMI antennas implemented in wristbands. When the user's wrist is near a metal object or behind a person, the antenna's quality factor may drop below the desired value.

[0065] This discussion focuses on specific circuitry and techniques for increasing the transmission quality factor (Qtx) when Qtx is below the nominal operating range, which does not violate any statutory spurious emission limits. These circuitry and techniques can rapidly and significantly increase the current transmitted to the antenna of a near-field device when Qtx is below the nominal Qtx range, and similarly, can rapidly and significantly decrease the current transmitted to the antenna of a near-field device when Qtx returns to the nominal Qtx range, so as not to exceed any statutory spurious emission limits.

[0066] These circuits and techniques are applicable to near-field magnetic induction (NFMI) and near-field electromagnetic near-field (NFEMI) communication devices.

[0067] Figure 6 Example ideal equivalent circuit 600 for a second near-field antenna 200 configured to transmit communication signals in a third near-field wireless device. Example 600 includes a tuning circuit 402, a variable current source 602 (e.g., a set of current amplifiers in parallel), and a controller 604.

[0068] In various example embodiments, controller 604 includes a set of voltage sensing lines 606, 608 for monitoring the voltage across near-field antenna 200, and is coupled to receive RSS measurements 610 from local receiver circuitry, and / or to receive communication from another near-field device that receives communication signals transmitted by the near-field device of example 600.

[0069] Controller 604 controls such as Figure 4 The tuning circuit 402 described herein; however, the controller 604 also controls the variable current source 602 (e.g., by connecting or disconnecting one or more of the current amplifiers (e.g., A1 to A5, B1 to B5) arranged in parallel with each other) to further change the total amount of current sent to the second near-field antenna 200.

[0070] This greater degree of current control allows the controller 604 to compensate for a significant drop in Qtx by sending a much larger current to the second near-field antenna 200, and also to quickly compensate for a significant increase in Qtx to avoid violating legal emission limits by reducing the current sent to the second near-field antenna 200.

[0071] The increased current causes an increase in the voltage across the feed points 135 and 140 of the second near-field antenna 200. In some examples, the controller 604 includes a DSP (digital signal processor).

[0072] Figure 7 This is an example set of instructions and / or logic circuitry for operating the controller 604 of the third near-field wireless device. Unless otherwise specifically stated, the order in which the instructions are discussed does not limit the order in which they are implemented in other example embodiments. Additionally, in some embodiments, the instructions are implemented in parallel.

[0073] The first example instruction set 700 of the controller 604 begins at 702, wherein if the measured transmission quality factor (Qtx) is higher than the minimum Qtx, the controller 604 will reduce the current flowing to the second near-field antenna 200.

[0074] Next, in step 704, if the measured Qtx is lower than the minimum Qtx but within the first adjustment range, Qtx is increased via the variable resistor tuning (R group) circuit. Then, in step 706, if the measured Qtx is lower than the minimum Qtx but within the first adjustment range, Qtx is increased by increasing the current sent to the second near-field antenna 200, but this operation does not cause spurious emissions exceeding legal limits. Controller 604 changes the current.

[0075] Therefore, although the tuning circuit 402 (e.g., groups C and R) can initially attempt to compensate by increasing or decreasing Qtx, when the decrease in Qtx is too large, the tuning circuit cannot sufficiently increase Qtx to above the minimum Qtx, and the controller 604 must adjust the variable current source 602 (e.g., connect more current amplifiers) to enhance the current delivered to the second near-field antenna 200.

[0076] The second example instruction set 708 for controller 604 begins at 710, wherein if the measured transmission quality factor (Qtx) is higher than the minimum Qtx, controller 604 will reduce the current flowing to the second near-field antenna 200.

[0077] Next, in 712, if the measured Qtx is lower than the minimum Qtx but within the first adjustment range, Qtx is increased by the variable resistor tuning (R group) circuit. Subsequently, in 714, if the measured Qtx is lower than the minimum Qtx but within the first adjustment range, the controller 604 acquires the received signal strength (RSS) measurement corresponding to the transmitted signal and Qtx of the near-field device.

[0078] The controller 604 can acquire RSS measurements in various ways. For example, the transmitting near-field device can request an RSS measurement corresponding to a communication signal transmitted by another receiving near-field device. Furthermore, when the other near-field device does not respond to the RSS measurement request, the transmitting near-field device assumes that the RSS is below a minimum RSS. For example, the minimum RSS may be 5 to 10 dB higher than the noise floor of the near-field device receiver.

[0079] In another example, antenna 200 switches between transmission tuning circuit 402 and receiver circuit (not shown), which then measures RSS.

[0080] In another example, the transmitting near-field device may have another near-field antenna coupled to the receiving circuitry of the transmitting near-field device itself to receive RSS measurements locally.

[0081] Subsequently, in step 716, if the RSS falls below the minimum RSS, the controller 604 increases Qtx by increasing the current flowing to the second near-field antenna 200, but limits this to a higher current level that does not cause spurious emissions exceeding statutory limits. In some example embodiments, any reduction in the quality factor below a threshold reference level (e.g., the minimum Qtx) can be linearly compensated by increasing the current flowing to the antenna 200.

[0082] The circuits and techniques described above can be applied in various embodiments to wireless earbuds, hearing aids, wireless body networks, smart lock systems, identification industrial measurement systems, and IoT systems.

[0083] Various circuits, logic gates, modules, computers, etc., can host these instructions. Such systems may include input / output data interfaces, processors, storage devices, and non-transitory machine-readable storage media. The machine-readable storage media includes instructions that control how the processor receives input data and how it converts the input data into output data using data within the storage device. In alternative example embodiments, the machine-readable storage medium is a non-transitory computer-readable storage medium. In other example embodiments, the instruction set described above can be implemented using logic gates, dedicated chips, firmware, and other hardware forms.

[0084] Example embodiments of the materials discussed in this specification may be implemented, in whole or in part, via networks, computers, or data-based devices and / or services. These may include cloud, internet, intranet, mobile devices, desktop computers, processors, lookup tables, microcontrollers, consumer devices, information infrastructure, or other enableable devices and services. The following non-exclusive definitions are provided as may be used herein and in the claims.

[0085] It is readily understood that the components of the embodiments generally described herein and illustrated in the accompanying drawings can be arranged and designed in a variety of different configurations. Therefore, the specific implementations of the various embodiments illustrated in the figures are not intended to limit the scope of this disclosure, but merely to illustrate various embodiments. While various aspects of the embodiments are presented in the drawings, the drawings are not necessarily drawn to scale unless specifically indicated.

[0086] The invention may be practiced in other specific forms without departing from the spirit or essential characteristics thereof. The described embodiments should be considered illustrative rather than restrictive in all respects. Therefore, the scope of the invention is indicated by the appended claims rather than by the specific embodiments. All variations within the meaning and scope of the equivalents of the claims are covered within its scope.

[0087] References to features, advantages, or similar language throughout this specification do not imply that all features and advantages achievable with the invention should be included in or in any single embodiment of the invention. Rather, references to such features and advantages should be understood to mean that a particular feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the invention. Therefore, the discussion of features and advantages and similar language throughout this specification may (but is not necessarily) refer to the same embodiment.

[0088] Furthermore, the features, advantages, and characteristics described in this invention can be combined in one or more embodiments in any suitable manner. Those skilled in the art will recognize that, in view of the description herein, the invention can be practiced without one or more of the specific features or advantages of particular embodiments. In other instances, additional features and advantages that may not be present in all embodiments of the invention can be identified in certain embodiments.

[0089] Throughout this specification, the terms "an embodiment," "embodiment," or similar language mean that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the invention. Therefore, the phrases "in one embodiment," "in an embodiment," and similar language throughout this specification may, but not necessarily all, refer to the same embodiment.

Claims

1. A near-field wireless device, characterized in that, include: Near-field antenna; Variable current source; A controller coupled to the near-field antenna and the variable current source; The controller is configured to measure the transmission quality factor (Qtx) of the near-field antenna. and The controller is configured to increase the current sent to the near-field antenna by the variable current source when the measured Qtx is lower than the minimum Qtx; The controller is configured to acquire a received signal strength (RSS) measurement corresponding to the signal transmitted by the near-field device; and If the measured RSS is higher than the minimum RSS, the controller is configured not to increase the current sent to the near-field antenna by the variable current source even if the measured Qtx is lower than the minimum Qtx.

2. The apparatus according to claim 1: Its features are, The controller is configured to set an upper limit for current increase to prevent the near-field antenna from transmitting signals exceeding a set of statutory limits.

3. The apparatus according to claim 1: Its features are, The controller is configured to reduce the current sent to the near-field antenna by the variable current source when Qtx is higher than the minimum Qtx.

4. The apparatus according to claim 1: Its features are, The variable current source includes a set of amplifiers arranged in parallel with each other.

5. The apparatus according to claim 4: Its features are, The current sent to the near-field antenna can be increased or decreased by connecting or disconnecting one or more of the amplifiers.

6. The apparatus according to claim 4: Its features are, The amplifier is a current amplifier.

7. The apparatus according to claim 6: Its features are, Half of the current amplifier is coupled to the first feed point of the near-field antenna, and the other half of the current amplifier is coupled to the second feed point of the near-field antenna.

8. The apparatus according to claim 1: Its features are, The near-field antenna includes a set of feed points; The device further includes a set of voltage sensing lines coupled from the feed point to the controller; and The controller is configured to measure the channel or signal voltage on the voltage sensing line and calculate the measured Qtx based on the measured channel or signal voltage.

9. The apparatus according to claim 1: Its features are, Additionally, it includes a tuning circuit coupled to the controller; The tuning circuit includes a set of tuning elements configured to adjust Qtx; and The controller is configured to use the tuning circuit to increase Qtx.

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