Abstract:
A method for adapting an antenna circuit including at least one first capacitive element and an inductive element in series, and at least one second capacitive element having a first electrode connected between the first capacitive element and the inductive element, wherein data representative of the voltage of said first electrode are applied to the second electrode of the second capacitive element.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of France patent application number 12/55447, filed on Jun. 11, 2012, which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND 
       [0002]    1. Technical field 
         [0003]    The present disclosure generally relates to radio frequency communications and, more specifically, to near-field communications based on a short-distance electromagnetic coupling between a terminal and a transponder. The present disclosure more specifically relates to the frequency adaptation of an oscillating circuit of a terminal. 
         [0004]    2. Discussion of the Related Art 
         [0005]    Electromagnetic transponder systems used in near field communications are now well known. They can, for example, be found in portable devices (Smartphones, touch pads, etc.), which are equipped with near field communication (NFC) devices. The operation of such systems is based on the transmission of a radio frequency radiation by a terminal or reader in order to communicate with, and possibly to remotely supply, a transponder present in the field of the terminal. The terminal and the transponder are each equipped with an oscillating circuit (antenna plus capacitive element) and are generally tuned to the same frequency (typically 13.56 MHz for the NFC standard). 
         [0006]    An issue lies in the fact that the components used to form the oscillating circuits undergo drifts due not only to manufacturing tolerances, but also to temperature variations. 
         [0007]    This problem is particularly critical for terminals which have to generate the electromagnetic field. Indeed, integrated circuit technology limits to a few volts the excursion of the voltage that can be provided. To generate a higher voltage, monolithic power components would have to be used, which is not desirable. As a consequence, the resonance should be generated outside of the integrated circuit to be able to reach an excursion of some ten volts (typically on the order of 30 volts). As a result, capacitive elements cannot be integrated. Now, with discrete component, dispersions generally reach approximately 10% and at best approximately 5%. Such tolerances are reflected on the resonance frequency. 
         [0008]    Currently, terminals are most often formed with adjustable capacitive elements. The capacitive element setting is performed in a test or maintenance operation. This, however, does not enable to compensate for possible operating drifts linked, for example, to temperature variations. 
         [0009]    A detuning between the oscillating circuits of the transponder and of the terminal is particularly important since the tuning conditions the quality of the transmission, and especially of the distance at which a transponder must be from a terminal for a communication to be able to occur. 
       SUMMARY 
       [0010]    An embodiment provides an antenna adaptation circuit for an electromagnetic field generation terminal. 
         [0011]    Another embodiment provides an adaptation which does not modify the inductive element of the antenna. 
         [0012]    Another embodiment provides a solution automatically adapting to operating drifts. 
         [0013]    Thus, an embodiment provides a method for adapting an antenna circuit comprising at least one first capacitive element and an inductive element in series, and at least one second capacitive element having a first electrode connected between the first capacitive element and the inductive element, wherein data representative of the voltage of said first electrode are applied to the second electrode of this second capacitive element. 
         [0014]    According to an embodiment, an image of the signal present on said first electrode is applied to said second electrode. 
         [0015]    According to an embodiment, said data are in phase with the signal present on the first electrode. 
         [0016]    According to an embodiment, said data are a voltage proportional to the voltage present on the first electrode. 
         [0017]    According to an embodiment, the method is adapted to an antenna circuit comprising two first capacitive elements and two second capacitive elements, the first two capacitive elements being in series with the inductive element and each second capacitive element having a first electrode connected between the inductive element and one of the first capacitive elements, and data representative of the voltage of each first electrode being applied to second respective electrodes of the second capacitive elements. 
         [0018]    Another embodiment provides an antenna circuit for an electromagnetic field generation terminal capable of implementing the above method. 
         [0019]    According to an embodiment, the circuit comprises a resistive dividing bridge between said or each first electrode and the ground. 
         [0020]    According to an embodiment, the midpoint of said resistive dividing bridge is connected, via an amplifier, to said or each second electrode. 
         [0021]    According to an embodiment, the midpoint of the resistive dividing bridge is applied to a first terminal of an XOR-type logic gate having a second terminal receiving data representative of the signal applied to the antenna circuit and having its output providing data relative to the phase shift between the two electrodes of the first capacitive element. 
         [0022]    According to an embodiment, said data relative to the phase shift are converted into a voltage applied to said or each second electrode. 
         [0023]    Another embodiment provides a terminal for generating an electromagnetic field comprising an antenna circuit such as hereabove. 
         [0024]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  very schematically shows an example of an electromagnetic transponder communication system; 
           [0026]      FIG. 2  is a simplified representation of a terminal equipped with a usual antenna circuit; 
           [0027]      FIG. 3  is an electric diagram of an embodiment of the antenna circuit; 
           [0028]      FIGS. 4A ,  4 B, and  4 C illustrate, in the form of timing diagrams, the operation of the circuit of  FIG. 3 ; 
           [0029]      FIG. 5  shows another embodiment of an antenna adaptation circuit; and 
           [0030]      FIG. 6  shows another embodiment adapted to a differential transmission. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    The same elements have been designated with the same reference numerals in the different drawings, where the timing diagrams have been drawn out of scale. For clarity, only those steps and elements which are useful to the understanding of the described embodiments have been shown and will be detailed. In particular, the communications between a terminal and an electromagnetic transponder have not been detailed, the described embodiments being compatible with usual transmissions. Further, the practical forming of the inductive elements of the antennas has not been detailed either, the described embodiments being here again compatible with usual terminals. 
         [0032]      FIG. 1  very schematically shows an example of a near-field transmission system. 
         [0033]    An electromagnetic field generation terminal  1  intended for near-field communications may be formed of a fixed device  1 , for example, a transport ticket reader or validator, an access control terminal, etc. It may also be a mobile device equipped with an electromagnetic field generation terminal. 
         [0034]    An electromagnetic transponder capable of communicating with terminal  1  is, for example, a device  2  of Smart-phone, touch pad, or other type, or a contactless chip card  3 . 
         [0035]    More and more often, Smartphone-type portable devices  2  are capable of operating as transponders (card mode) and communicate with a terminal, or as terminals (reader mode) and communicate with a contactless card  3  or with another Smartphone. 
         [0036]    These are examples only and the embodiments which will be described more generally apply to any type of terminal. 
         [0037]    The operation of near-field communication systems is known and will not be detailed any further. 
         [0038]      FIG. 2  is a simplified electric diagram of a usual electromagnetic field generation terminal, for example, of the type of terminal  1  of  FIG. 1 . One or several integrated circuits  12  integrate electronic circuits (symbolized by a block  14 ) for generating signals to be transmitted and for processing received signals. For a transmission, a signal S to be transmitted is sent by circuit  14  to an amplifier  16  having an output driving an antenna circuit  20  external to integrated circuit  12 . Antenna circuit  20  forms an oscillating circuit formed of a first capacitive element C 1  in series with an inductive element L 1  between output amplifier  16  and the ground, and of a second capacitive element C 2  in parallel with inductive element L 1 . Inductive element L 1  is the device antenna (most often a conductive winding) and its free end is grounded in the example of  FIG. 2 . In a differential embodiment, antenna L 1  (and capacitor C 2 ) connects two capacitive elements C 1  respectively assigned to each of the paths. 
         [0039]    When the antenna circuit is tuned, the signal in the antenna (node B) has a maximum amplitude. The remote supply intended for the transponders is maximum and the phase difference between nodes A (output of amplifier  16 ) and B is 90°. This phase shift is introduced by capacitive elements C 1  and C 2 . 
         [0040]    Capacitive elements C 1  and C 2  have values of several tens, or even several hundreds, of picofarads. These values are capable of varying from one terminal to another due to manufacturing dispersions and, for a same terminal, during the operation, for example, under to the influence of temperature. 
         [0041]    In operation, the value of the inductance may also occur to be modified by environmental factors, which here again causes a detuning of the antenna circuit. 
         [0042]    To date, possible adaptations of the capacitive value of the antenna circuit are performed by using a variable capacitance C 2 . This enables, in end-of-manufacturing tests, to adjust the antenna circuit resonance. However, such a variable capacitance cannot be integrated. Further, an adjustment of the capacitor value at the end of the manufacturing does not solve the problem of drifts in operation. 
         [0043]      FIG. 3  is a representation of an embodiment of an antenna circuit  4 . 
         [0044]    According to this embodiment, instead of being grounded, the electrode of capacitive element C 2 , opposite to node B is connected to a terminal C having a circuit  5  setting a voltage level. The function of circuit  5  is to vary the voltage of node C according to the voltage variations at node B. In other words, the voltage applied to node C is variable (in practice, at the frequency of the signal in the antenna), as an image of the voltage at node B. Preferably, the signal applied to node C is in phase with the signal at node B. 
         [0045]    In the embodiment illustrated in  FIG. 3 , circuit  5  comprises a follower-assembled differential amplifier  52  having its output terminal connected to node C and having a non-inverting input (+) connected to junction point  54  of a resistive dividing bridge comprising two resistors  56  and  58  in series between node B and the ground. The inverting terminal (−) of amplifier  52  is connected to its output. 
         [0046]    The function of resistive bridge  56 ,  58  is to attenuate the voltage excursion at the level of amplifier  52 . Indeed, the potential variation at node B may reach several tens of volts, which is too long for amplifier  52 . 
         [0047]    Actually, circuit  5  copies at node C, while attenuating it, the signal present at node B. This enables to automatically compensate for a possible mismatch of the antenna circuit, which translates as a phase shift different from 90 degrees between nodes A and B, whether this mismatch originates from inductive element L 1  or from capacitive elements C 1  and C 2 . 
         [0048]    Resistor  58  preferably is a variable resistor, as will be seen hereinafter. 
         [0049]      FIGS. 4A ,  4 B, and  4 C illustrate the operation of the antenna circuit of  FIG. 3 . These drawings respectively show example of shapes of signals at nodes A, B, and C. The signal at node A ( FIG. 4A ) for example corresponds to an A.C. signal at the 13.56-MHz frequency generated by the terminal circuits. To simplify the representation, it is assumed that this signal to is not amplitude-modulated. 
         [0050]    In the absence of any correction (left-hand portion of the drawings), the voltage of node C is grounded (at 0 volt) and a phase shift φ can then be observed between the signals present at nodes A and B (on the antenna). Assuming a detuning, phase shift φ is different from 90 degrees. 
         [0051]    In the right-hand portion of  FIG. 4C , the signal at node C is a copy of the signal at node B, attenuated by dividing bridge  56  and  58 . This results in a compensation of the phase shift. The phase shift between the signals at nodes A and B then becomes 90 degrees. 
         [0052]    In a simplified embodiment, the correction performed by the circuit of  FIG. 3  is sufficient. 
         [0053]    Still in a simplified embodiment, the value of variable resistor  58  is selected to obtain the right phase shift with the signal at the system work frequency (in the example, 13.56 MHz) during tests. This enables, among other things, to compensate for the slight phase shift introduced by the delay introduced by amplifier  52  in the signal propagation. 
         [0054]    In a simplified embodiment, this phase-shift is neglected and the use of resistors already is an improvement, since adjustable resistors may be integrated. 
         [0055]    In practice, other phase-shift sources are present, for example, due to a drift of the capacitive elements or to an environmental disturbance of the antenna. 
         [0056]    Preferably, the value of resistor  58  is then matched during the operation. This adaptation is then preferably regularly performed, in a calibration phase, started automatically (for example, periodically) or by an operator. 
         [0057]    The signal for controlling this variable resistor preferentially originates from a measurement of the phase shift between nodes A and B, converted by circuit  14  into a signal CTRL for controlling the value of variable resistor  58  (for example, a voltage level). Various means may be envisaged to form an integrated variable resistor (MOS transistor, for example), as well as to measure the phase shift between nodes A and B. 
         [0058]      FIG. 5  partially shows an adaptation circuit  5 N according to another embodiment. 
         [0059]    It is provided to generate the voltage to be applied to node C by means of a settable-gain differential amplifier  51 , assembled as a follower, having its output connected to node C and which exploits data linked to the phase shift between nodes A and B. For example, a circuit for measuring this phase shift is formed of an XOR-type logic gate  53  having a first input directly connected to node A and having a second input connected to midpoint  55  of a resistive dividing bridge formed of two resistors  57  and  59  of fixed value in series between to node B and the ground. The output of gate  53  is sent to circuit  14  and provides data relative to the value of the phase shift. The circuit then delivers a gain reference value to amplifier  51  according to this phase shift. The non-inverting input of amplifier  51  is connected to midpoint  55  and its output is looped back onto its inverting input. The gain reference value may possibly (according to the nature of the amplifier) be a digital reference. 
         [0060]      FIG. 6  schematically shows another embodiment adapted to a differential transmitter. As compared with the embodiment of  FIG. 3 , a second path formed of an amplifier  16 ′ and of a capacitive element C 1 ′ in series connects circuit  14 ′ to end B′ of antenna L 1 , opposite to its end B. A second capacitive element C 2 ′ connects node B′ to the output of circuit  5 ′ similar to circuit  5 . The operation of the differential embodiment can be deduced from the operation discussed in relation with common-mode embodiments. The digital variation of  FIG. 5  may also apply to the differential mode by measuring the phase shift between nodes A′ and B′. 
         [0061]    An advantage of the described embodiments is that it is now possible to form a self-adaptive antenna circuit in particularly simple fashion. In particular, this circuit can now be integrated and requires no capacitive element of variable value. 
         [0062]    Various embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. Further, the practical implementation of the described embodiments is within the abilities of those skilled in the art based on the functional indications given hereabove and by using tools and components which are also usual. Among the variations that can be envisaged, it may be provided to replace the resistive bridges with capacitive dividing bridges. Further, the correction made at node C may be inverted, that is, it may be negative instead of positive (this amounts to inverting the shape of the right-hand portion of  FIG. 4C ). Further, the sizing of the components is within the abilities of those skilled in the art according to the application and especially to the operating frequency of the system. 
         [0063]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.