Nfc reader-writer interface for optimizing energy acquisition and small-size antenna

An NFC reader interface optimized for energy harvesting and small-size antennas

Technical field

0001The invention belongs to the technical field of wireless communication, and specifically relates to an NFC reader interface optimized for energy collection and small-size antennas.

Background technique

0002NFC (Near Field Communication) technology is an emerging short-range secure communication technology in recent years. It uses a magnetic field that decays quickly in space as a medium for transmitting information, and realizes a communication distance of only a few centimeters. It has the advantages of high security and convenient use.

0003As shown in Figure 1, common NFC communication interfaces are divided into two types, namely NFC reader interface (Reader) 101 and NFC tag interface (Tag) 102. Typical NFC communication occurs between these two interfaces. NFC is a half-duplex two-way communication, including two links, namely the reader-to-tag link 103 and the tag-to-reader link 105, each of which is responsible for a communication direction.

0004Reader to tag link 103: The NFC reader interface is responsible for generating an alternating magnetic field 104 with a center frequency of 13.56Mhz, and adjusting the magnetic field to the information to be sent. When the NFC tag interface is located in the alternating magnetic field 104, it receives the energy carried by the magnetic field, demodulates the information carried by the magnetic field, and obtains the information generated by the NFC reader interface.

0005Tag-to-reader link 105: Traditional NFC tag interfaces are passive interfaces and do not actively transmit energy. It uses load modulation to send information. Specifically, the traditional NFC tag interface achieves the purpose of modulating and sending information by changing the load of the receiving antenna to control the amount of energy received from the alternating magnetic field. The NFC reader interface 101 can sense the load change of the NFC tag interface 102 by measuring the current intensity change on the transmitting antenna, and demodulate the information. In order to meet a certain communication bandwidth, the Q value of the receiving antenna of the NFC tag interface cannot be too high (less than 50).

0006Since the NFC reader interface needs to generate energy by itself, it is generally used in devices with more energy, such as smart phones, tablet computers, and POS terminals. The NFC tag interface is widely used in low-power and low-energy devices, such as smart cards, smart tags and other devices.

0007The energy of the alternating magnetic field 104 received by the NFC tag interface 102 can be rectified and stabilized to provide working energy for itself and other devices. NFC energy harvesting is widely used in smart cards and smart tags.

0008Figure 2 is a general structure diagram of a traditional passive NFC tag interface. The antenna 201 is generally composed of one or several turns of coil, which is responsible for receiving the energy and modulation information of the alternating magnetic field; the matching 202 adjusts the impedance of the antenna 201 to a suitable value; the demodulator 203 demodulates the received signal and restores it to Original information; the load modulation 205 changes the load of the antenna 201 according to the information to be sent to realize the modulation of the alternating magnetic field; the data interface 204 communicates with peripheral devices through the data bus, configures the passive NFC tag interface, and exchanges the received and The original NFC information to be sent; the rectifier and stabilizer 206 rectifies and stabilizes the energy of the received alternating magnetic field, and then sends it to the peripheral device through the energy collection interface.

0009Figure 3 is a general structure diagram of a traditional NFC reader/writer interface. The antenna 301 is generally composed of several coils, which is responsible for generating an alternating magnetic field and sending and receiving NFC signals; the matching 302 adjusts the impedance of the antenna 301 to an appropriate value to improve energy transmission efficiency and adjust the transmission power; the modulator 304 controls the transmission The NFC data is modulated into an NFC signal according to the NFC protocol, and is modulated to the antenna 301 through the antenna driver 303; the antenna driver 303 is generally a low output impedance radio frequency amplifier to improve efficiency; the demodulator 309 senses the passive NFC interface through the current intensity of the antenna 301 The signal generated by the load modulation and demodulation; MCU core 307 controls the work of the entire NFC reader interface, and its tasks include organizing NFC data according to the NFC protocol, data verification, communication with off-chip devices, and on-chip components Management and control, etc.; the data interface 305 is the interface for the NFC reader to communicate with off-chip devices, generally a universal serial data interface, such as SPI, I2C, etc.; FIFO306 is the interface between the data interface 305 and the MCU core 307 The bridge is used as a data register for two-way communication; the clock system 308 generates all the clocks required by the NFC reader interface, including the 13.56Mhz carrier frequency; the on-chip power management generates all the power required by the on-chip components.

0010Traditional NFC systems have two main disadvantages. First, the passive NFC interface must use a larger antenna to achieve a reasonable communication distance and communication angle. This is because the NFC reader/writer needs to sense the load modulation of the NFC tag interface to communicate with the reader/writer link. When the antenna is small, the coupling coefficient between the antenna and the NFC reader/writer antenna is low, and its own Q value is low, resulting in low efficiency, resulting in a weak load modulated signal that cannot be reliably received by the NFC reader/writer. Second, because the design of the passive NFC tag interface and the NFC reader interface in the traditional NFC system is directly inherited from RFID, its energy transmission efficiency is very low. The passive NFC tag interface can only obtain a small amount of energy (about 10mW to 20mW) from the NFC reader interface, so it can only maintain the simple operations of the passive device, such as reading and writing internal memory.

0011The new generation of miniaturized devices such as wearable devices, special-shaped bank cards, Internet of Things devices, and personal mobile devices are small in size and cannot be installed with larger antennas. This greatly restricts the performance of the traditional passive NFC tag interface, resulting in unstable communication performance, very close communication distance or completely unable to communicate, and users have great inconvenience to use. At the same time, for a large number of applications for NFC at present and in the next few years, such as bank dual interface cards, new visual bank cards, smart wearable devices, sensor networks, etc., the passive NFC tag interface has low energy receiving efficiency. The ground limits the functions and performance that these new applications can provide.

0012In order to solve the problem of weak antenna load modulation signal, the existing mainstream solutions mainly use active modulation instead of passive load modulation. Since the actively modulated NFC tag interface is used to actively transmit signals and does not depend on the received alternating magnetic field energy, the use of a small antenna can also allow the NFC reader interface to receive the information sent by the NFC tag interface. The products using this scheme are mainly the NFC boosted frontend product series of AMS and ST. However, the shortcomings of active modulation technology are also very prominent. First, due to the need to actively transmit energy, the signal transmitted by the NFC tag interface must be locked with the frequency and phase of the alternating magnetic field transmitted by the NFC reader interface to avoid signal instability. This scheme requires a complex phase-locked loop and phase-maintaining circuit. This has caused the complexity of the NFC tag interface design and increased costs. Second, the active modulation technology needs to provide energy for the NFC tag interface for transmission. This technology cannot be used on passive devices such as passive smart cards and tags. Third, due to the increased complexity of the active modulation technology to the NFC tag interface, it is difficult for the NFC tag interface using this technology to achieve NFC energy harvesting. None of the existing products using active modulation technology support NFC energy harvesting.

0013In order to alleviate the problem of low energy received by the NFC tag interface, the existing solutions mainly start with reducing the power consumption of the device, so that this small amount of energy can also maintain the normal operation of the device. These solutions include the use of newer manufacturing technologies to reduce power consumption (such as upgrading from a 130nm process to a 90nm process), increase device sleep time, reduce device operating frequency, and so on. However, these methods either increase the cost of the device or limit the performance of the device.

0014Summary of the invention

0015In order to effectively solve the above problems, the present invention provides an NFC reader interface optimized for energy harvesting and small-size antennas.

0016An NFC reader interface optimized for energy harvesting and small-size antennas. The NFC reader interface includes an antenna, an adjustable matching circuit, an antenna drive unit, a modulator, a demodulator, and an MCU core;

0017The antenna, the adjustable matching circuit and the antenna driving unit constitute an antenna resonant loop.

0018Further, the antenna, the adjustable matching circuit, the antenna drive unit, the modulator, and the MCU core are connected in sequence, and the adjustable matching circuit is also connected to the input end of the demodulator, and the output end of the demodulator is connected to the MCU The output end of the MCU core is also connected with an adjustable matching circuit.

0019Further, the NFC reader interface further includes a power management unit, a clock system, and a data interface unit, the data interface unit is connected to the MCU core through a FIFO unit, and the data interface unit is used to receive NFC data;

0020The power management unit supplies power to the components on the NFC reader interface;

0021The clock system is used to generate all the clocks required on the NFC reader interface.

0022Further, the adjustable matching circuit is used to adjust the impedance of the antenna, and the antenna driving unit amplifies the power of the signal generated by the modulator, and transmits the amplified signal to the antenna through the adjustable matching circuit.

0023Further, the antenna resonant loop further includes a Q value adjustment circuit and a Q value adjustment switch.

0024Further, the Q value adjusting switch is connected in series with the Q value adjusting resistor and then connected in parallel with the antenna.

0025Further, the antenna resonant loop includes two working modes, namely a high Q value mode and a low Q value mode, and the two modes can be switched and controlled in real time through a Q value adjustment switch.

0026Further, the Q value adjustment switch controls the opening and closing of the switch in real time by an electric signal.

0027Further, the adjustable matching circuit includes a T-type, PI-type, L-type or a single capacitor, and has a low insertion loss of less than 1 db, and the antenna driving unit has an output impedance of less than 5 ohms.

0028Further, the antenna inductance value is between 1uH and 10uH, and the antenna area is 100mm²Up to 5000mm²In the high-Q mode, the Q value of the antenna resonant loop at 13.56Mhz is above 50, and in the low-Q mode, the Q value of the antenna resonant loop at 13.56Mhz is below 25.

0029The beneficial effects of the present invention: the present invention can greatly improve the wireless energy transmission efficiency of the NFC communication interface, and provide a large amount of electric energy for the device with the NFC tag interface. These additional power will allow the device to provide more functions, higher performance, and a better user experience. At the same time, the present invention allows the NFC communication interface to use a small antenna without affecting the NFC communication performance. The small antenna makes the equipment development very flexible and greatly reduces the difficulty of NFC system development. The present invention greatly expands the application range of the NFC communication interface at present and in the next few years.

Description of the drawings

0030Figure 1 is a schematic diagram of a typical NFC communication.

0031Figure 2 is a schematic diagram of a typical NFC tag interface structure.

0032Figure 3 is a schematic diagram of the interface structure of a typical NFC reader.

0033Figure 4 is a schematic diagram of the coupling coefficient.

0034Figure 5 shows the equivalent circuit diagram of the NFC system.

0035Figure 6 is a schematic diagram of the structure of an NFC reader interface optimized for small-size antennas and energy harvesting.

0036Figure 7 is a simplified circuit diagram that optimizes the resonant loop and auxiliary circuit of the NFC reader interface for small antennas and energy harvesting.

0037Figure 8 is a schematic diagram of a mode control state machine of an NFC reader interface optimized for small-size antennas and energy harvesting.

0038Figure 9 is a schematic diagram of the first NFC tag interface optimized for small-size antennas and energy harvesting.

0039Figure 10 is the first simplified circuit diagram that optimizes the resonant loop and auxiliary circuit of the NFC tag interface for small antennas and energy harvesting.

0040Figure 11 is a schematic diagram of the first mode control state machine of the NFC tag interface optimized for small-size antennas and energy harvesting.

0041Figure 12 is a schematic diagram of the second NFC tag interface optimized for small-size antennas and energy harvesting.

0042Figure 13 is a simplified circuit diagram of the second NFC tag interface resonant loop and auxiliary circuit optimized for small-size antennas and energy harvesting.

0043Figure 14 is a schematic diagram of the second state machine of the NFC tag interface optimized for small-size antennas and energy harvesting.

Detailed ways

0044In order to make the objectives, technical solutions and advantages of the present invention clearer, the following describes the present invention in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, but not to limit the present invention.

0045On the contrary, the present invention covers any alternatives, modifications, equivalent methods and solutions defined by the claims in the spirit and scope of the present invention. Further, in order to enable the public to have a better understanding of the present invention, some specific details are described in detail in the detailed description of the present invention below. Those skilled in the art can fully understand the present invention without the description of these details.

0046The invention focuses on solving the two main problems of how the NFC tag interface effectively uses a small-size antenna and how the NFC system efficiently transmits energy. The following first analyzes the wireless energy transmission problem between the NFC tag interface antenna and the NFC reader interface. For the energy transmission of the NFC system, the highest efficiency the system can achieve η_maxIt can be described by the following formula:

0047<maths num="0001"><img file="WO2017166364A1_D0001.tif" /></maths>

0048in

0049<maths num="0002"><img file="WO2017166364A1_D0002.tif" /></maths>

0050k is the coupling coefficient between the NFC tag interface antenna and the NFC reader interface antenna, Q₁Is the Q value of the NFC reader interface resonant loop at 13.56Mhz, Q₂The Q value of the resonant loop of the NFC tag interface at 13.56Mhz. Coupling coefficient refers to the proportion of magnetic lines of induction generated by one antenna that can pass through another antenna. Generally speaking, the longer the distance between antennas, the smaller the coupling coefficient between them. As shown in FIG. 4, the antenna 402 generates a magnetic field, and the distance between the antenna 405 and the antenna 402 is shorter than the distance between the antenna 404 and the antenna 402. Therefore, the magnetic lines of induction passing through the antenna 405 are more than those passing through the antenna 404, so the coupling coefficient between the antenna 405 and the antenna 402 is higher than the coupling coefficient between the antenna 404 and the antenna 402. At the same time, the coupling coefficient is also related to the relative size of the two antennas. The greater the difference in antenna size, the smaller the coupling coefficient between antennas at the same distance. As shown in Figure 4, when the distance from the antenna 402 of the NFC reader/writer interface is the same, the magnetic lines passing through the small antenna 401 are less than those passing through the large antenna 404, so the distance between the antenna 401 and the antenna 402 is less. The coupling coefficient is lower than the coupling coefficient between the antenna 404 and the antenna 402. The Q value is a parameter that characterizes the frequency selectivity of a circuit and the energy loss at that frequency, and can be calculated by the following formula:

0051<maths num="0003"><img file="WO2017166364A1_D0003.tif" /></maths>

0052Where X is the inductance or capacitive reactance of the circuit, and R is the equivalent series resistance value of the circuit. The higher the Q value, the higher the circuit selectivity and the lower the loss, and vice versa. Specific to the antenna resonant circuit, the higher the Q value, the lower the equivalent series resistance value of the circuit compared to the inductance or capacitive reactance of the circuit, and the smaller the loss during the oscillation process.

0053It should be pointed out that, given the k and Q values of the system, the system load needs to match the impedance of the system energy source to achieve the highest efficiency of the current system.

0054From the above analysis, there are four factors that determine the efficiency of the final wireless energy transmission: the coupling coefficient of the NFC tag interface antenna and the NFC reader interface antenna, the Q value of the NFC reader interface resonant loop at 13.56Mhz, and the NFC tag The Q value of the interface resonant loop at 13.56Mhz, the NFC tag interface load impedance matching degree.

0055Coupling coefficient: It can be seen from formulas (1) and (2) that the higher the coupling coefficient between the NFC tag interface antenna and the NFC reader interface antenna, the higher the transmission efficiency. The coupling coefficient is related to the relative position between the two antennas and the relative size of the two antennas. The closer the antenna spacing, the higher the vertical axis alignment of the two antennas, and the closer the size of the two antennas, the higher the coupling coefficient. These need to be considered in the antenna design stage and the final product industrial design stage.

0056The Q value of the NFC reader interface resonant loop at 13.56Mhz₁: From the above analysis, it can be seen that Q₁The higher the energy transmission efficiency, the higher. It should be noted that Q₁It is the Q value of the entire resonant loop, which is determined by the Q value of the NFC reader interface antenna, the equivalent serial resistance of the antenna drive, and the loss of the matching circuit. In order to achieve a higher Q₁Requires higher NFC reader interface antenna Q value, and very low antenna drive equivalent serial internal resistance and matching circuit loss.

0057The Q value of the NFC tag interface resonant loop at 13.56Mhz₂: From the above analysis, it can be seen that Q₂The higher the energy transmission efficiency, the higher. It should be noted that Q₂It is the Q value of the entire resonant loop when the NFC tag interface is not connected to the load, which is determined by the Q value of the NFC tag interface antenna and the loss of the matching circuit. In order to achieve a higher Q₂Requires higher NFC tag interface antenna Q value, and very low matching circuit loss.

0058NFC tag interface load impedance matching degree: k and Q determine the highest efficiency that the system can achieve η_max, And the load impedance matching determines the actual efficiency of the system (always less than the highest efficiency η_max). The precise matching of load impedance and system source impedance allows the system to work near the highest efficiency.

0059Next, analyze the problem of small-size antennas. For most NFC tag interfaces, the bottleneck that affects the communication performance is the performance of the tag to the reader/writer link. In order to facilitate the analysis, the communication principle of the link from the tag to the reader is introduced. Figure 5 is a schematic diagram of a typical NFC communication system. The left half is the NFC reader interface, and the right half is the NFC tag interface. In order to simplify the analysis, the matching circuit of the two interfaces is a single serial capacitor, but the analysis conclusion is applicable to all forms ofmatching circuit. The antenna driver 511 is an RF amplifier with low output impedance, and the resistor 501 is its equivalent output impedance; the capacitor 502 is a matching capacitor, and the antenna 504 is the antenna of the NFC reader interface. The two resonate at 13.56Mhz; the resistor 503 is the capacitor 502 and The combined equivalent series resistance of the antenna 504 at 13.56Mhz; the antenna 505 is the antenna of the NFC tag interface, the capacitor 507 is the matching capacitor, and the resistance 506 is the combined equivalent series resistance of the two; the switch 508 and the resistor 509 constitute a load Modulation circuit. When the NFC tag interface is located in the alternating magnetic field emitted by the NFC reader interface, the NFC tag interface can be regarded as the load of the NFC reader interface. Specifically, the NFC tag interface can be equivalent to a resistor connected in series to the antenna resonant loop of the NFC reader interface. This resistance is called the reflection resistance of the NFC tag interface. The resistance 510 is the reflection resistance of the NFC tag interface. The resistance of the resistor is affected by the coupling coefficient between the two antennas, the antenna Q value, and the antenna load of the NFC tag interface. In NFC communication, when the distance between two specific NFC reader interfaces and the NFC tag interface is certain (that is, the coupling coefficient is fixed), the change of the reflection resistance can only be caused by the change of the antenna load of the NFC tag interface. The change of the load will cause the current of the NFC reader interface antenna to change. The information sent by the NFC tag interface can be decoded by measuring changes in the antenna current.

0060Obviously, the greater the change of the resistance 510 during load modulation, the greater the change of the antenna current flowing through the NFC reader interface, and the stronger the signal generated. Value of reflection resistance 501, Z_r, When the circuit is in resonance, it can be expressed as:

0061<maths num="0004"><img file="WO2017166364A1_D0004.tif" /></maths>

0062Where ω is the signal frequency, M is the mutual inductance between the two antennas, and R₂And R_LThey are the equivalent series resistance of the resonant loop and the resistance of the load modulation. Since ω, M, and R₂It can be regarded as a constant value during NFC communication, Z_rThe change is only made by R_Lcontrol. Obviously, when R_LWhen 0 and positive infinity change, Z_rThe biggest change. Z_rThe maximum and minimum values of can be expressed as:

0063<img file="WO2017166364A1_D0005.tif" />When R_L=0

0064<img file="WO2017166364A1_D0006.tif" />When R_L→+∞

0065The signal strength H of the link from the tag to the reader can be expressed as the ratio of the range of impedance variation on the resonant loop of the NFC reader to the maximum impedance:

0066<maths num="0005"><img file="WO2017166364A1_D0007.tif" /></maths>

0067Where R₁Is the equivalent series resistance of the resonant loop of the NFC reader interface, that is, the sum of resistance 503 and resistance 501; Q₁And Q₂They are the Q values of the resonant loop of the NFC reader interface and the resonant loop of the NFC tag; when H is 1, the signal strength is the largest, and when H is 0, the signal strength is the smallest.

0068The main influence of antenna size on the NFC system is the coupling coefficient k between the NFC tag interface antenna and the NFC reader interface antenna. The smaller the antenna size, the smaller the coupling coefficient between the antennas at the same distance. According to formula (4), the low coupling coefficient between the small antenna and the antenna of the NFC reader interface will directly cause the signal strength H generated by the load modulation to be too small, and the NFC reader interface cannot decode the information.

0069According to the above analysis, when using a small antenna results in a very low coupling coefficient k, there are several ways to improve the signal strength.

0070The Q value of the NFC reader interface resonant loop at 13.56Mhz₁: From the above analysis, it can be seen that Q₁The higher, the higher the signal strength H of the link from the tag to the reader. It should be noted that Q₁It is the Q value of the entire resonant loop, which is determined by the Q value of the NFC reader interface antenna, the equivalent serial resistance of the antenna drive, and the loss of the matching circuit. In order to achieve a higher Q₁Requires higher NFC reader interface antenna Q value, and very low antenna drive equivalent serial internal resistance and matching circuit loss.

0071The Q value of the NFC tag interface resonant loop at 13.56Mhz₂: From the above analysis, it can be seen that Q₂The higher, the higher the signal strength H of the link from the tag to the reader. It should be noted that Q₂It is the Q value of the entire resonant loop when the NFC tag interface is not connected to the load, which is determined by the Q value of the NFC tag interface antenna and the loss of the matching circuit. In order to achieve a higher Q₂Requires higher NFC tag interface antenna Q value, and very low matching circuit loss.

0072Load modulated resistance R_LThe range of change: From the above analysis, the resistance R_LThe greater the change, the higher the signal strength H from the tag to the reader/writer link. In most cases, the resistance R_LThe maximum and minimum values are determined by the RF switch of the load modulation. This requires the RF switch to have very small parasitic capacitance, high isolation, and low insertion loss.

0073It can be seen that how the tag interface effectively uses a small-size antenna and how the NFC system efficiently transmits energy is the Q value of the NFC reader interface and the resonant circuit of the NFC tag interface. However, a noteworthy problem is that the use of a high-Q resonant loop will directly reduce the reader-to-tag link bandwidth. Low bandwidth will cause the NFC signal transmission strength and receiving amplitude to be too low, which will seriously affect the communication performance. In order to solve this problem, the NFC reader interface and the NFC tag interface should use two sets of resonant loops respectively corresponding to different Q value requirements.

0074When the NFC reader interface and the NFC tag interface are far away, in order to avoid the influence of the load of the NFC tag interface on the communication performance (reducing the loop Q value), the energy transmission needs to be disabled. This can be achieved by detecting the magnetic field strength on the NFC tag interface. Only when the signal strength reaches a certain value, the load will be connected to the circuit.

0075NFC reader interface design:

0076Now introduce the interface design of NFC reader. Figure 6 is a structural diagram of an implementation of the NFC reader interface.

0077The antenna 601 is a high-Q coil (Q>100), which is responsible for generating an alternating magnetic field and sending and receiving NFC signals; the adjustable matching circuit 602 adjusts the impedance of the antenna 601 to a suitable value to improve energy transmission efficiency and adjust transmission power; It has two modes, high-Q mode and low-Q mode. The antenna resonant loop can be tuned to a high Q value (Q>50) and a low Q value (Q<25) to optimize performance. The NFC data is modulated into an NFC signal according to the NFC protocol and transmitted to the antenna 601 through the antenna driver 603; the antenna driver 603 is a radio frequency amplifier with a very low output impedance to improve energy transmission efficiency; the demodulator 609 passes the current of the antenna 601 The intensity-sensing passive NFC interface modulates the signal generated by the load and demodulates the signal; the MCU core 607 controls the work of the entire NFC reader interface, and its tasks include organizing NFC data according to the NFC protocol, data verification, and off-chip devices Communication, management and control of on-chip components, etc.; data interface 605 is the interface for the NFC reader to communicate with off-chip devices, generally a general serial data interface, such as SPI, I2C, etc.; FIFO606 is the data interface 605 and MCU The bridge between the cores 607 is used as a data register for two-way communication; the clock system 608 generates all the clocks required by the NFC reader interface, including the 13.56Mhz carrier frequency; the on-chip power management generates all the power required by the on-chip components.

0078The main difference between the NFC reader interface design and the traditional NFC reader interface is the antenna resonant loop. The antenna resonant loop can work in two modes, namely a high-Q mode and a low-Q mode. In the high-Q mode, the antenna resonant loop has high Q and low bandwidth. The 13.56Mhz wireless signal can be radiated by the antenna with very low loss. In the low Q mode, the antenna resonant loop has a low Q and high bandwidth, which is suitable for the transmission of NFC signals. These two modes can be switched in real time, and the switching is controlled by the MCU of the NFC reader interface. Since the interface design of the NFC reader/writer has no change in the communication protocol, it is fully compatible with the existing NFC standard and can communicate with any standard NFC tag.

0079The following describes an implementation scheme of the antenna resonant loop. Figure 7 is a simplified circuit diagram of the scheme. The antenna driver 701 is a low output impedance radio frequency power amplifier, and the output impedance should be less than 5 ohms; the antenna matching circuit 702 changes the impedance of the antenna 705 to control the output power; since the components in the antenna matching circuit 702 will cause energy loss, the circuit should be Take the simplest form possible; the antenna matching circuit 702 shown in Figure 7 is a PI-type matching circuit, but in fact any form of conversion circuit can work, including a matching circuit of L-type and a single capacitive element; Q adjusting resistor 703 cooperates with radio frequency The switch 704 controls the working mode of the antenna resonant loop, the circuit has a high Q value when the switch is off, and a low Q value when the switch is closed; the specific value of the Q adjustment resistor needs to be calculated under the parameters of the impedance matching circuit of the specific antenna.

0080The antenna 705 needs to meet three conditions to achieve ideal energy transmission efficiency and communication performance. 1. The antenna 705 resonates at 13.56Mhz after connecting and matching 702. Considering that the internal resonance capacitance of the antenna 705 is limited by the actual value, the influence of the parasitic capacitance and the mutual inductance with the receiving antenna, the inductance value of the antenna 705 cannot be too large, otherwise it cannot resonate at 13.56Mhz. However, a too low inductance value will result in a low Q value, so the inductance value is generally between 1uH and 10uH. Second, the antenna 705 has the highest possible Q value. Methods to increase the Q value include using an antenna with a larger cross-sectional area, reducing the parasitic capacitance of the antenna, using antenna materials with lower impedance, and so on. Third, the antenna 705 needs to have a suitable area to provide sufficient coupling with the transmitting antenna. For typical NFC applications, the area of antenna 705 must be 100mm²Up to 5000mm²between.

0081The NFC reader interface needs to switch the working mode in real time to match the current working state. This interface works in high-Q mode most of the time to achieve high-efficiency energy transmission. Figure 8 shows the mode control state machine of the NFC reader interface. The initial mode of this interface after power-on reset is high-Q mode. Reset t₁Seconds later, the radio frequency interface is turned on, and the unmodulated 13.56Mhz carrier is amplified by the antenna power and then sent out through the antenna through the matching circuit. This signal generates a 13.56Mhz alternating magnetic field near the NFC reader interface. The NFC tag interface can receive the energy of the magnetic field to charge itself. After starting to send the carrier, the NFC reader interface needs to wait for t₂Seconds, after the NFC tag receives enough energy to maintain its own operation, data packets can be sent. t₃Seconds later, the NFC reader interface switches to low-Q mode and sends the modulated signal through the antenna. After the signal is sent, the NFC reader switches back to the high-Q mode and waits for at least t₃Seconds in order to receive the NFC tag signal. NFC tags must be t₃Respond within seconds. If there is a tag response, the NFC reader interface receives the signal, and it can be completed after the reception is completed.₆Send the next packet in seconds. If the NFC reader is at t₃If no NFC tag signal is received within seconds, the NFC reader needs to switch to another modulation method at t₅Send again later. The purpose of this is to support tags of a variety of different protocols. If the NFC reader has tried all the modulation methods and there is no tag response, it is considered that there is no tag nearby. At this time, the NFC reader stops the carrier wave transmission and closes the radio frequency interface to save energy. NFC reader at t₄After seconds, restart the RF interface and send the carrier, and repeat the above steps. Above t₁To t₆The setting should refer to the NFC protocol standard setting.

0082NFC tag interface design:

0083Now introduce the design of NFC tag interface. Figure 9 shows the first implementation of the NFC tag interface.

0084The antenna 901 is a high-Q coil antenna, which is responsible for receiving the energy of the alternating magnetic field and the modulated signal sent by the NFC reader interface. Its minimum size is 10mmX10mm; the adjustable matching circuit 902 adjusts the impedance of the antenna 901 to a suitable value; It has two modes, high-Q mode and low-Q mode. The antenna resonant loop can be tuned to high Q (Q>50) and low Q (Q<25) to optimize performance; demodulator 903 will receive The signal is demodulated and restored to the original information; the load modulation circuit 905 changes the load of the antenna 901 according to the information to be sent to realize the modulation of the alternating magnetic field; the data interface 904 communicates with peripheral devices through the data bus, and configures the passive NFC tag Interface, and exchange the received and to-be-sent NFC raw information; the rectification and stabilization circuit 906 rectifies and stabilizes the energy of the received alternating magnetic field, and then sends it to the peripheral device through the energy collection interface 907; the rectification and stabilization circuit 906 There is a switch to control the connection and disconnection of the load.

0085The main difference between the NFC tag interface design and the traditional NFC tag interface is the antenna resonant loop. The antenna resonant loop can work in two modes, namely high-Q mode and low-Q mode. In the high-Q mode, the antenna resonant loop has a high Q value and low bandwidth. The 13.56Mhz wireless signal can be received by the antenna with very low loss. In the low-Q mode, the antenna resonant loop has a low Q value and high bandwidth, which is suitable for receiving NFC signals. These two modes can be switched in real time according to the working status of the NFC tag interface. Since the interface design of the NFC tag does not change the communication protocol, it is fully compatible with the existing NFC standard and can communicate with any standard NFC reader.

0086The following describes an implementation scheme of the antenna resonant loop and auxiliary circuit. Figure 10 is a simplified circuit diagram of this scheme. The antenna 1005 and the matching circuit 1004 form an antenna resonant loop; in order to reduce the loss of the matching circuit, the matching circuit 1004 can be composed of T-type, PI-type, L-type circuits, or even a single capacitor. The matching 1004 shown in Figure 10 is a single parallel capacitor. ; Resistor 1006 and switch 1007 form a Q value adjustment circuit, where resistor 1006 is a fixed resistance capacitor, and switch 1007 is an RF switch; the resistance of resistor 1006 should be calculated according to the parameters of antenna 1005 and matching circuit 1004, after switch 1007 is closed, The Q value of the resonant loop is controlled within 25; the opening and closing of the switch 1007 correspond to the high-Q mode and the low-Q mode of the antenna resonant loop respectively; the resistor 1003 is a modulating load, and the switch 1002 forms a load modulation circuit together; rectification The circuit 1001 rectifies the energy of the resonant loop and sends it to the voltage stabilizing circuit through the switch 908; the rectifier circuit 1001 can use diode bridge rectification or a synchronous rectifier circuit; the switch 1008 controls the connection between the load and the resonant loop; the rectifier circuit The input of 1001 is connected to the decoder input circuit at the same time.

0087The antenna 1005 needs to meet three conditions to achieve ideal energy transmission efficiency and communication performance. 1. The antenna 1005 resonates at 13.56Mhz after being connected and matched with 1004. Considering that the internal resonance capacitance of the antenna 905 is limited by the actual value, the influence of the parasitic capacitance and the mutual inductance with the receiving antenna, the inductance value of the antenna 1005 cannot be too large, otherwise it cannot resonate at 13.56Mhz. However, a too low inductance value will result in low received power, so the inductance value is generally between 1uH and 10uH. Second, the antenna 1005 has the highest possible Q value. Methods to increase the Q value include using an antenna with a larger cross-sectional area, reducing the parasitic capacitance of the antenna, using antenna materials with lower impedance, and so on. Third, the antenna 1005 needs to have a suitable area to provide sufficient coupling with the transmitting antenna. For the antenna size of a typical NFC reader interface, the area of the antenna 1005 must be 100mm²Up to 5000mm²between.

0088The load modulation circuit needs to maximize the load variation range to improve the communication performance when using a small antenna. The switch 1002 needs to have the characteristics of high isolation and low insertion loss. The resistor 1003 is used to limit the current passing through the switch 1002 to prevent it from exceeding the maximum power and being damaged. The selection of the resistor 1003 should be based on the maximum allowable power consumption and internal resistance of the switch 1002, and the maximum received power of the NFC tag interface.

0089The working mode of the antenna resonant loop needs to be selected in real time according to the working status. Energy harvesting and load modulation need to work when the antenna resonant loop is at a high Q value, while NFC signal reception needs to work when the resonant loop is at a low Q value. Figure 11 shows the mode control state machine of the NFC tag interface. The initial mode of this interface after power-on reset is low-Q mode. After reset, the interface repeatedly detects the NFC carrier. If the NFC carrier is detected, check whether the carrier strength is greater than A_t1. If it is greater than, switch to high-Q mode and connect the load to the resonant loop to collect energy; otherwise, continue in the low-Q mode and disconnect the load. This is to prevent forcibly connecting the load when the magnetic field strength is too low to reduce the communication performance. Then the interface detects whether there is a modulated signal sent by the NFC reader interface. If it is not detected, keep in the current mode and return to the carrier detection step. If it is detected, it will immediately switch to the low-Q mode and receive the NFC signal. After the reception is complete, immediately switch to the high-Q mode, and₁After seconds, the response signal is sent through load modulation. After the load modulation is completed, check whether the carrier exists in the high-Q mode, if not, switch to the low-Q mode to continue the detection, otherwise continue to work in the high-Q mode and repeat the above steps.

0090A_t1With A_t2It needs to be set according to the actual value of the NFC tag interface and the NFC reader interface when they are close to each other, but A_t1Need to be greater than A_t2To prevent the NFC tag interface from oscillating between the two modes. t₁It should be set according to the standard value of the NFC interface protocol.

0091Because this scheme detects that the NFC reader interface signal is in the high-Q mode with poor reception performance when the energy harvesting is in progress, and the actual signal reception is in the low-Q mode, if the signal modulation rate is high, the mode conversion is completed Several modulation symbols may have been missed later, so this kind of scheme can only use the NFC low-rate protocol (&lt;=106Kbps) during energy harvesting.

0092Figure 12 shows the second implementation scheme of the NFC tag interface.

0093The solution includes two antennas, a first antenna 1201 and a second antenna 1205, and two antenna resonant loops, a first resonant loop and a second resonant loop. The first antenna 1201 and the first matching circuit 1202 form a first antenna resonant loop, which is responsible for signal reception from the reader to the tag link; the second antenna 1205 and the second matching circuit 1206 form a second antenna resonant loop, which is responsible for energy Receiving and load modulation. The first resonant loop has a low Q value (<25) at 13.56Mhz, and the second resonant loop has a high Q value (Q>50) at 13.56Mhz. The receiver 1203 demodulates the signal received by the first resonant loop and sends it to the data interface 1204; the load modulation circuit 1208 modulates the load of the second antenna 1205 according to the data to be sent obtained from the external bus by the data interface; the data interface 1204 serves as a channel for the receiver 1203 and the load modulation circuit 1208 to exchange data with peripheral devices, and obtains and transmits data through the external bus connection with the peripheral devices, and configures the passive NFC tag interface; the alternating magnetic field received by the rectifier and voltage regulator circuit 1207 After the energy is rectified and stabilized, it is sent to peripheral devices through the energy harvesting interface 1209; the rectification and stabilization circuit 1207 has a switch that can control the connection and disconnection of the load.

0094The main difference between the NFC tag interface design and the traditional NFC tag interface and the first implementation scheme is that it includes the first resonant loop and the second resonant loop, two antenna resonant loops with a low Q value and a high Q value respectively. . The low Q value of the first resonant loop can provide a wide receiving bandwidth, and the high Q value of the second resonant loop can provide high energy transmission efficiency. The two resonant loops can work at the same time, so the complicated switching circuit and control logic in the first implementation scheme can be omitted. Since there is no need to switch, the second implementation scheme can support the full transmission rate of the existing NFC. Since the interface design of the NFC tag does not change the communication protocol, it is fully compatible with the existing NFC standard and can communicate with any standard NFC reader.

0095The following describes an implementation scheme of the antenna resonant loop and auxiliary circuit. Figure 13 is a simplified circuit diagram of this scheme. The antenna 1310, the capacitor 1311 and the resistor 1309 form the first resonant loop; the function of the resistor 1309 is to reduce the Q value of the first resonant loop when the Q value of the antenna 1310 is too high; the Q value of the first resonant loop should be controlled at Below 25; the output of the first resonant loop is connected to the NFC receiver; the antenna 1306 and the matching circuit 1304 form the second resonant loop; in order to reduce the loss of the matching circuit, the matching circuit 1304 can be made of T-type, PI-type, L-type circuits, or even It is composed of a single capacitor. The matching circuit 1304 shown in Figure 13 is a single capacitor in parallel; the Q value of the second resonant loop should be controlled above 50; the resistor 1303 is a modulating load, and the switch 1302 forms a load modulation circuit together; the rectifier circuit 1301 The energy of the resonant loop is rectified and sent to the voltage stabilizing circuit through the switch 1308; the rectifier circuit 1301 can use diode bridge rectification or a synchronous rectifier circuit; the switch 1308 controls the connection between the load and the resonant loop; the rectifier circuit 1301 The input is also connected to the decoder input circuit.

0096In this NFC tag interface design scheme, since only the energy received by the second resonant loop can be collected and used (the energy received by the first resonant loop will eventually be converted into heat and lost), in order to increase the energy received power, the first A resonant loop needs to reduce the received energy as much as possible (only need to meet the receiver sensitivity). Specifically, it is necessary to reduce the voltage of the first resonant loop when it receives a 13.56Mhz signal as much as possible. The first resonant loop must have at least one of the following properties to reduce the received energy. First, it has a low Q value (&lt;25) at 13.56Mhz. This can be achieved by adjusting the resistor 1309, or adjusting the value of the capacitor 1311 to make the resonant frequency of the first resonant circuit deviate from 13.56Mhz. Second, low antenna inductance (&lt;4uH). The low antenna inductance value can reduce the mutual inductance value of the antenna with the NFC reader interface, thereby reducing the oscillation voltage value of the first resonant loop. This can be achieved by using an antenna with a small number of turns, reducing the antenna area, and so on. Third, low coupling with the antenna of the NFC reader interface. Low coupling can also reduce the mutual inductance value of the antenna with the NFC reader interface, thereby reducing the oscillating voltage value of the first resonant loop. This can be achieved by reducing the antenna area, adjusting the antenna position, and increasing the distance from the transmitting antenna.

0097The load modulation circuit needs to maximize the load variation range to improve the communication performance when using a small antenna. The switch 1302 needs to have the characteristics of high isolation and low insertion loss. The resistor 1303 is used to limit the current through the switch 1302 to prevent it from being damaged by exceeding the maximum power. The selection of the resistance 1303 should be calculated and selected based on the maximum allowable power consumption and internal resistance of the switch 1302, and the maximum received power of the NFC tag interface.

0098For the second resonant circuit, in order to enable it to receive as much energy as possible, the antenna 1306 and the matching 1304 need to meet four conditions. 1. The antenna 1306 resonates at 13.56Mhz after being connected and matched with 1304. Taking into account the limitation of the actual value of the internal resonance capacitance and electro-sensing of the matching 1304, the influence of parasitic capacitance and the mutual inductance with the NFC reader interface antenna, the inductance value of the antenna 1306 cannot be too large, otherwise it cannot resonate at 13.56Mhz. However, a too low inductance value will result in a low received voltage and reduce the overall efficiency. Therefore, the inductance value is generally suitable between 1uH and 10uH. Second, the second resonant circuit has the highest possible Q value. This requires the antenna 1306 to have the highest possible Q value and the matching 1304 to have the lowest possible loss. The methods to improve the antenna Q value include using an antenna with a larger cross-sectional area, reducing the parasitic capacitance of the antenna, using antenna materials with lower impedance, and so on. The main method to reduce the loss of matching 1304 is to use the simplest matching circuit, because the more complicated the matching, the more ways to produce loss. Third, the output impedance of the antenna 1306 after connection and matching 1304 needs to be matched with the load impedance to ensure maximum power transmission. This can be achieved by adjusting the parameters of the matching 1304. Fourth, the antenna 1306 needs to have a suitable area to provide sufficient coupling with the NFC reader interface antenna. For the antenna size of a typical NFC reader interface, the area of the antenna 1306 must be 100mm²Up to 5000mm²between.

0099Figure 14 shows the control state machine of the NFC tag interface. This interface waits for NFC carrier in low power consumption state after power-on reset. If the NFC carrier is detected, check whether the carrier strength is greater than A_t. If it is larger, connect the load into the resonant loop to collect energy; otherwise, disconnect the load. This is to prevent forcibly connecting the load when the magnetic field strength is too low to reduce the communication performance. Then the interface detects whether there is a modulated signal sent by the NFC reader interface. If it is not detected, return to the carrier detection step. If detected, the NFC signal is received. After the reception is complete, at t₁After seconds, the response signal is sent through load modulation. After the load modulation is completed, return to the step of detecting the carrier and repeat the above steps. A_tIt needs to be set according to the actual value of the NFC tag interface and the NFC reader interface when they are close to each other to ensure that the communication stability will not be affected after the load is connected. t₁It should be set according to the standard value of the NFC interface protocol.