WO2021104545A1

WO2021104545A1 - Method for nfc reader antenna tuning using a calibration module

Info

Publication number: WO2021104545A1
Application number: PCT/CZ2019/050054
Authority: WO
Inventors: Vladislav KASÍK; Pavel STANĚK
Original assignee: Y Soft Corporation, A.S.
Priority date: 2019-11-27
Filing date: 2019-11-27
Publication date: 2021-06-03

Abstract

The object of the invention is to provide a method of tuning an NFC reader (2) antenna (1) using a calibration module comprising a reader (2), wherein the reader (2) comprises an antenna (1), a tuning unit (5) and a first calibration unit (6) and wherein the calibration module further comprises a second calibration unit (9) electrically connected to the first calibration unit (6). The tuning unit (5) is electrically connected to the antenna (1) and comprises an EMC filter (3), a matching member (4) electrically connected to the EMC filter (3), a capacitive voltage divider (7) electrically connected to the matching member (4) and the antenna (1 ), and an array of tuning capacitors (8) electrically connected to the matching member (4). The inputs of the first calibration unit (6) are fed to the capacitive voltage divider (7), the outputs of the first calibration unit (6) are fed to the EMC filter (3), and, furthermore, the ports of the first calibration unit (6) are fed to the array of tuning capacitors (8).

Description

Method for NFC reader antenna tuninq usinq a calibration module

Technical field

The invention relates to a method of calibrating an NFC reader antenna operating in the high frequency (HF) region, more specifically, at a frequency of 13.56 MHz, wherein it improves the integrated function of a chip that allows automatic tuning of the antenna. Prior Art

Currently, near-field communication (NFC), a wireless communication method for data transmission between two electronic devices over a short distance, which may be used in, for example, contactless identification and payment systems or during contactless authentication of multifunction printers or copiers, is increasingly being used. NFC technology, which uses signal transmission between a reader and a so- called tag (card, chip), is based on historically older RFID (radio-frequency identification) technology, with which it shares standards including relevant communication protocols, but provides certain additional benefits. Passive NFC tags, i.e. not powered by their own power source but instead using electromagnetic energy from the reader, may be read also by active NFC devices, allowing wireless communication between the devices to be bi-directional. Contactless communication is, in principle, based on the transmission of energy using electromagnetic induction between the tag antenna and the reader antenna, wherein the proposed solution is focused on tuning the reader antenna, which is necessary for the reliability of the signal transmission. Incorrect tuning leads to congestion of the receiver circuits integrated into the reader due to reflection of a portion of the energy from the antenna.

In the prior art, solutions that allow automatic tuning of the antenna of an NFC or RFID reader are known, utilizing assemblies of tuning capacitors. Patent document EP2203909 B1 discloses a device for automatic tuning of an RFID reader, wherein the optimal capacity value is set by tuning capacitors based on the phase of the antenna signal and the value of the current flowing through the antenna. Patent document EP2195880 A1 discloses a similar device for tuning RFID reader antenna, which includes, in addition to the tuning capacitors, a pulse width modulator (PWM), a microprocessor, a coil, and logic gates to communicate with the capacitors and to transmit the coil and the capacitors resonance signal, wherein the setting of the optimal capacitor capacity value is based on comparison of the resonance signal with the source signal. The use of parallel connected tuning capacitors is in addition to patent literature discussed in scientific literature.

However, prior art related to the invention provides the best description of the commercially available chip STM25R3911 , which allows automatic tuning of the NFC reader antenna in terms of antenna voltage amplitude maximization, wherein it provides use in, for example, card readers built into multifunction printers or copiers for the purpose of their contactless authentication. Using the antenna’s automatic tuning function, it is possible to ensure that the reading distance is optimal for all supported technologies, even where ambient conditions are not ideal. Non-ideal conditions may be, for example, the presence of a metal object, and therefore, the need for automatic antenna tuning becomes relevant in for example devices built into printers and copiers. These usually contain a metal frame.

Nevertheless, the automatic tuning function integrated into the STM25R3911 chip does not work entirely correctly and presents a number of practical disadvantages. T uning based on the maximum antenna voltage provides optimal reading distance only for certain technologies (e.g. IS015693), while other technologies (e.g. IS014443-A) are not read correctly or not at all. In the relevant demonstration application ST25R3911 B, it is also possible to perform tuning based on a specific phase and amplitude value, but even this method is in principle not entirely correct, as it does not directly take into consideration the amount of signal reflected from the antenna. Incorrect and inaccurate antenna tuning may lead, in addition to the congestion of the receiver circuits integrated into the reader, also to the failure of the device in electromagnetic compatibility tests (so-called EMC test), which determine resistance to potential interference effects due to the presence of another electrical device. In this case, inaccurate tuning would result in the presence of higher harmonic frequencies corresponding to the frequency of 13.56 MHz. Moreover, the STM25R3911 chip also allows to measure the signal amplitude and phase, providing the possibility of implementing its own tuning algorithm. A practical disadvantage, however, is the fact that the chip resolution does not allow to measure phase values outside the 30-150° interval, which means that in some cases, a truly optimal phase value is not found, since it may lie outside the said interval as well.

Summary of the Invention

Said drawbacks are at least partially overcome by a method of tuning a NFC reader antenna using a calibration module comprising a reader, wherein the reader comprises an antenna, a tuning unit, wherein the tuning unit is electrically connected to the antenna and comprises an EMC filter, a matching member electrically connected to the EMC filter, a capacitive voltage divider electrically connected to the matching member and the antenna, and an array of tuning capacitors electrically connected to the matching member, and a first calibration unit, wherein the inputs of the first calibration unit are fed to the capacitive voltage divider, the outputs of the first calibration unit are fed to an EMC filter, and, furthermore, the ports of the first calibration unit are fed to the array of tuning capacitors, the essence of the method being that the calibration module further comprises a second calibration unit electrically connected to the first calibration unit, wherein the method for NFC reader antenna tuning comprises the following steps.

First, the desired value of the impedance matching of the antenna is set, wherein this value is controlled by changing the total capacity of the array of tuning capacitors.

Furthermore, a first set of phase and amplitude value pairs of the received signal is measured using the first calibration unit, wherein within this first set of pairs, a first subset of phase and amplitude value pairs, which includes phase values lying in the interval determining the resolution capacity of the first calibration unit, and, furthermore, a second subset of phase and amplitude value pairs, which includes phase values lying outside the interval determining the resolution capacity of the first calibration unit, is defined. In another step, a second set of phase and amplitude value pairs of the received signal is set using the second calibration unit, wherein within this second set of pairs, a first subset of phase and amplitude value pairs, which includes phase values lying in the interval determining the resolution capacity of the first calibration unit, and, furthermore, a second subset of phase and amplitude value pairs, which includes phase values lying outside the interval determining the resolution capacity of the first calibration unit, is defined. The first subset of pairs shares the same phase values with the first subset of pairs within the first set of phase and amplitude value pairs, and the second subset of pairs shares the same amplitude values with the second subset of pairs within the first set of phase and amplitude value pairs, whereas the second quantity of each pair is selected so that the individual amplitude and phase value pairs lie on an ellipse when represented in the Cartesian coordinate system. The interval determining the resolution capacity of the first calibration unit is 30-150°.

Subsequently, a phase and amplitude values pair are selected from the second set of phase and amplitude pairs using the second calibration unit so that the resulting tuning corresponds to the maximum real antenna voltage value. Such tuning may be considered as the only tuning fundamentally correct, since it corresponds to the minimum value of the standing wave ratio (SWR) coefficient, which is defined as the ratio of the maximum and minimum amplitude of the standing wave produced by interference of the forward and backward-reflected signals and as such corresponds to the amount of loss of useful signal. The SWR parameter is closely related to the antenna's reflection coefficient, and if all the signal is captured by the antenna, it is equal to a threshold minimum value of 1 , and if all the signal is reflected from the antenna, it grows infinitely. The correct tuning of the antenna therefore consists in minimizing the SWR factor.

In the next step, the selected phase and amplitude value pair is set to the first calibration unit.

The advantage of the proposed solution over the standard method of automatic tuning, which is an integrated function of the STM25R3911 chip, in other words, the first calibration unit, is not only providing fundamental tuning accuracy by minimizing the SWR parameter but also the elimination of the disadvantage resulting from the limited resolution of this chip. It is because the proposed method allows to determine phase values also outside the 30-150° phase interval, which defines the chip resolution, and therefore, the optimal tuning is sought throughout the entire range of phase values. Description of the drawings:

A summary of the invention is further illustrated using exemplary embodiments, which are described with reference to the accompanying drawings, in which:

Fig. 1 shows a diagram of the calibration module for calibrating the NFC reader antenna,

Fig. 2 shows a connection diagram of the individual components of the NFC reader.

Exemplary embodiments of the invention

The invention will be further clarified by means of the exemplary embodiments with reference to the respective drawings.

A method of tuning an NFC reader 2 antenna 1_using a calibration module, the diagram of which is shown in Fig. 1 , comprising a reader 2, wherein the reader 2 comprises the antenna 1, a tuning unit 5, wherein the tuning unit 5 is electrically connected to the antenna 1 and comprises an EMC filter 3 , a matching member 4 electrically connected to the EMC filter 3, a capacitive voltage divider 7 electrically connected to the matching member 4 and the antenna 1, and an array of tuning capacitors 8 electrically connected to the matching member 4, and a first calibration unit 6, wherein the inputs of the first calibration unit 6 are fed to the capacitive voltage divider 7, the outputs of the first calibration unit 6 are fed to the EMC filter 3, and, furthermore, the ports of the first calibration unit 6 are fed to the array of tuning capacitors 8, comprising the steps described below, wherein the calibration module in the exemplary embodiment further comprises a second calibration unit 9 electrically connected to the first calibration unit 6.

In the exemplary embodiment of Fig. 1, the NFC reader 2 antenna 1 forms, together with the first calibration unit 6 and the tuning unit 5, a wireless communication system, which preferably occurs at the frequency of 13.56 MFIz, wherein the output signal of the first calibration unit 6 is routed via the EMC filter 3 and the matching member 4 to the antenna 1 and the signal received by the antenna 1 is fed in the backward direction to the inputs of the first calibration unit 6, preferably via the capacitive voltage divider 7, which limits the value of the voltage signal from the antenna 1 to the inputs of the first calibration unit 6.

In the exemplary embodiment of the NFC reader 2 with the individual components connected, which is shown in Fig. 2, the capacitive voltage divider 7 is realized using a pair of series connected capacitors Cs and C6, which are at one end electrically connected to the antenna 1 and grounded at the other end, wherein the electrical connection to the RFI1 input of the first calibration unit 6 is realized at a location between the two capacitors Cs and C6. Alternatively, series of multiple connected capacitors may be used. In the preferred embodiment of Fig. 2, the series capacitors Cs, C6, Cz and Cs forming the capacitive voltage divider 7 are connected in symmetrical connection so that each pair of the capacitors, i.e. Cs, C6 and Cz, Cs is at one end electrically connected to the antenna 1 and grounded at the other end, and the electrical connection to the individual RFI1 and RFI2 inputs, respectively, of the first calibration unit 6 is realized at a location between the two capacitors within the given pair. The EMC filter 3 is, in this exemplary embodiment, realized as a low-pass filter consisting of a series coil Li and a parallel capacitor Ci, which are at one end connected to the RF01 output of the first calibration unit 6 and, at the other end, to the input of the matching member 4, wherein in the preferred configuration of Fig. 2, the connection at a location between the respective symmetrically located capacitors Ci, C2 is grounded, the coil Li is connected in series to the first RF01 output of the first calibration unit 6, and the coil L2 is connected in series to the second RF02 output of the first calibration unit 6. The advantageous connection of the EMC filter 3 allows to filter out unwanted higher harmonic frequencies of the output signal of the first calibration unit 6, wherein the optimal value of the top threshold (so-called cut-off) frequency of the EMC filter 3 differs from the particular application, nevertheless, optimally, it should not exceed the value of about 17, and similarly, it is not desirable for the threshold frequency value to be too close to the frequency value of the transmitted signal, namely 13.56 MHz. In such a case, the quality of the entire resonant system is significantly reduced.

The matching member 4 is, in the exemplary embodiment of Fig. 2, realized using an assembly of one series capacitor Csi or Cs2, respectively, and two parallel capacitors CPI and Cp3,or CP2 and CP4, respectively, in each branch of the symmetrical connection, wherein the series capacitors Csi and Cs2 are connected to the respective outputs of the EMC filter 3 and via the parallel capacitors CPI and CP3, or CP2 and CP4, respectively, the signal is routed to the antenna 1. The electrical connection at location between the respective symmetrically placed capacitors CPI and CP2, or CP3 and CP4, respectively, is grounded. Alternatively, the matching member 4 may be realized using an array of series capacitors Csi and Cs2 and only one stage of the parallel capacitors CPI and CP2, wherein the second stage of the parallel capacitors CP3 and CP4 may be omitted. The matching member 4 serves to set the desired value of the impedance matching of the antenna 1, wherein this value is controlled by changing the total capacity value of the array of tuning capacitors 8, which is connected to the matching member 4 in both of the symmetrical connection branches at a location between the series capacitor Csi and the parallel capacitor CPI , or at a location between the series capacitor Cs2 and the parallel capacitor CP2, respectively. The outputs of the array of tuning capacitor 8 fed to both symmetrical connection branches are designated in Fig. 2 as TRIM1 , or TRIM2, respectively. The signal from the individual ports of the first calibration unit 6 which include switches for the controlled connection of the respective capacitors allowing the automatic tuning function of the NFC reader 2_antenna 1, is fed to the array of the tuning capacitors 8.

The antenna 1 of the NFC reader 2 is, in the first exemplary embodiment, realized as a magnetic loop antenna, preferably implemented in the form of a printed coil or flexible printed circuits. Alternatively, the antenna 1 may be embodied in the form of a winding wire. In Fig. 2, the antenna 1 is shown by means of a substitution diagram using parallel connection of the capacitor Cant, the coil L_ant, and the resistor Rant, wherein series damping resistors Ri, or R2, respectively, are connected at a location between the antenna 1 and the matching member 4 in both of the branches to match the quality of the antenna 1 and the tuning unit 5. The antenna 1 is designed to have a higher intrinsic quality factor than its tuning unit 5, since using additional connection of the damping resistors R-i, or R2, respectively, in series between the matching member 4 and the antenna 1, the quality factor of the antenna 1 may be only reduced. In an alternative embodiment, the damping resistor Ri, or R2, respectively, may be connected in parallel to the antenna 1_.

The method for the NFC reader 2 antenna I uning using the calibration module described above comprises the following steps in the order in which they are presented below.

First, the desired value of the impedance matching of the antenna 1 is set using the array of tuning capacitors 8, wherein this value is controlled by changing the total capacity of the array of tuning capacitors 8 due to the controlled connection of the respective capacitors to the individual ports of the first calibration unit 6 using switches integrated into each port.

In the next step, a first set of phase and amplitude value pairs of the received signal is measured using the first calibration unit 6, wherein within this first set of pairs, a first subset of phase and amplitude value pairs, which includes phase values lying in the interval determining the resolution capacity of the first calibration unit 6, and, furthermore, a second subset of phase and amplitude value pairs, which includes phase values lying outside the interval determining the resolution capacity of the first calibration unit 6, is defined. In a particular embodiment, the interval determining the resolution capacity of the first calibration unit 6 is defined using a phase range of 30- 150°.

In the next step, a second set of phase and amplitude value pairs of the received signal is set by the second calibration unit 9, wherein within this second set of pairs, a first subset of the phase and amplitude value pairs, which includes phase values lying in the interval determining the resolution capacity of the first calibration unit 6, and, furthermore, a second subset of phase and amplitude value pairs, which includes phase values outside the interval determining the resolution capacity of the first calibration unit 6, is defined. The first subset of pairs shares the same phase values with the first subset of pairs within the first set of phase and amplitude value pairs, and the second subset of pairs shares the same amplitude values with the second subset of pairs within the first set of phase and amplitude value pairs, whereas the second quantity of each pair is selected so that the individual amplitude and phase value pairs lie on an ellipse when represented in the Cartesian coordinate system.

Subsequently, a phase and amplitude value pair is selected from the second set of phase and amplitude pairs using the second calibration unit 9 so that the resulting tuning corresponds to the maximum real voltage value on the antenna 1, i.e. to the minimum value of the SWR coefficient, which corresponds to the amount of loss of useful signal.

Finally, the selected phase and amplitude value pair is set by the second calibration unit 9 to the first calibration unit 6, wherein this value pair serves as input data for the optimal tuning of the antenna 1 under the given circumstances taking into consideration the influence of the surrounding objects.

Industrial applicability

The device according to this invention may be used for tuning the NFC reader antenna, which is one of the basic elements of NFC technologies which are utilized for example in contactless identification and payment systems or in contactless authentication of multifunction printers or copiers.

List of reference numerals

1 - antenna

2 - NFC reader 3 - EMC filter

4 - matching member

5 - tuning unit

6 - first calibration unit

7 - capacitive voltage divider 8 - array of tuning capacitors

9 - second calibration unit

Claims

1. Method for NFC reader (2) antenna (1) tuning using a calibration module comprising a reader (2), wherein the reader (2) comprises the antenna (1), a tuning unit (5), wherein the tuning unit (5) is electrically connected to the antenna (1) and comprises an EMC filter (3), a matching member (4) electrically connected to the EMC filter (3), a capacitive voltage divider (7) electrically connected to the matching member (4) and the antenna (1), and an array of tuning capacitors (8) electrically connected to the matching member (4), and a first calibration unit (6), wherein the inputs of the first calibration unit (6) are fed to the capacitive voltage divider (7), the outputs of the first calibration unit (6) are fed to the EMC filter (3), and, furthermore, the ports of the first calibration unit (6) are fed to the array of tuning capacitors (8), characterized in that the calibration module further comprises a second calibration unit (9) electrically connected to the first calibration unit (6), wherein the method for NFC reader (2) antenna (1) tuning comprises the steps of:

- setting the desired value of the impedance matching of the antenna (1),

- measuring a first set of phase and amplitude value pairs of the received signal using the first calibration unit (6), wherein within this first set of pairs, a first subset of the phase and amplitude value pairs, which includes phase values lying in the interval determining the resolution capacity of the first calibration unit (6), and, furthermore, a second subset of the phase and amplitude value pairs, which includes phase values lying outside the interval determining the resolution capacity of the first calibration unit (6), is defined,

- setting a second set of phase and amplitude value pairs of the received signal using the second calibration unit (9), wherein within this second set of pairs, a first subset of phase and amplitude value pairs, which includes phase values lying in the interval determining the resolution capacity of the first calibration unit (6), and, furthermore, a second subset of phase and amplitude value pairs, which includes phase values lying outside the interval determining the resolution capacity of the first calibration unit (6), is defined, wherein this first subset of pairs of the second set shares the same phase values with the first subset of pairs within the first set of the phase and amplitude value pairs and the second subset of pairs of the second set shares the same amplitude values with the second subset of pairs within the first set of phase and amplitude value pairs, whereas the second quantity of each pair is selected such that individual amplitude and phase value pairs lie on an ellipse when represented in the Cartesian coordinate system, - selecting a phase and amplitude value pair from the second set of phase and amplitude pairs using the second calibration unit (9) so that the resulting tuning corresponds to the maximum real voltage value of the antenna (1),

- setting the selected phase and amplitude value pair to the first calibration unit (6).

2. The method for NFC reader (2) antenna (1) tuning according to claim 1 , characterized in that the value of the impedance matching of the antenna is controlled by changing the total capacity of the array of tuning capacitors (8).

3. The method for NFC reader (2) antenna (1) tuning accordingly to any one of claims 1 and 2, characterized in that the phase and amplitude values are selected such that they lie on a circle when represented in the Cartesian coordinate system.