Abstract:
An antenna circuit for near-field communications includes a planar active winding, connected between a first and a second access terminal; an auxiliary winding coplanar with the active winding and connected by a first end to the first access terminal; and a tuning capacitor connected to the second end of the auxiliary winding. The turns of the active and auxiliary windings are interleaved.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Embodiments of the invention relate to Near-Field or NFC communication devices, and more particularly to the antenna circuits used in such devices. 
         [0002]      FIGS. 1A to 1C  represent various known electrical structures of antenna circuits. These antenna circuits are usable in active NFC devices able to create a magnetic field and to detect its modulation (for example an RFID tag reader), and in passive NFC devices (for example an RFID tag) powered by a magnetic field and configured to modulate this field in order to transmit data. 
         [0003]      FIG. 1A  illustrates the structure most generally used in NFC devices. It includes a single winding L 1  connected in parallel with a tuning capacitor C. The antenna circuit is tuned, for example, to 13.56 MHz within the framework of the NFC standards in force (e.g. ISO 14443, ISO 15693). The terminals of the winding form the access terminals A 1 , A 2  of the antenna circuit. 
         [0004]    The turns of the antenna windings of NFC devices are generally produced in the form of metallic tracks etched spirally in a single layer of a flexible printed circuit. The windings are plane and coplanar. This makes it possible to minimize the cost of manufacture, but entails certain difficulties due to the fact that the turns are all of different dimensions and therefore do not have similar characteristics. 
         [0005]      FIG. 1B  illustrates an optimized structure, described for example in Japanese patent application JP 2000-278172. The antenna winding includes a midpoint forming one of the access terminals (A 1 ) of the antenna circuit. It is considered that the antenna winding is formed of two windings connected in series L 1  and L 2 , the access terminals A 1  and A 2  being taken at the terminals of the winding L 1 , which will be referred to as the “active winding”. The winding L 2  will be referred to as the auxiliary winding. These terminals A 1 , A 2  are generally connected to an NFC circuit (not shown), which may be a passive circuit (receiving the magnetic field emitted by another NFC device) or an active circuit (emitting the magnetic field). 
         [0006]    As indicated in the aforementioned document, this structure makes it possible to increase the number of turns of the antenna winding and consequently to increase the energy received or emitted by means of the winding by the NFC circuit connected to the terminals A 1 , A 2 , while decreasing the number of turns seen by the NFC circuit, so that the impedance of the coil seen from the terminals A 1 , A 2  is closer to the output impedance of the NFC circuit, thereby improving the quality factor Q of the antenna circuit. 
         [0007]      FIG. 1C  illustrates an alternative structure to that of  FIG. 1B . The antenna winding includes two midpoints forming the access terminals A 1  and A 2 , also connected to an NFC circuit. It is considered that the antenna winding is formed of three windings connected in series L 1 , L 2  and L 3 , the access terminals A 1  and A 2  being taken at the terminals of the winding L 1 . The winding L 1  is the active winding, and the windings L 2  and L 3  are auxiliary. 
         [0008]      FIG. 2A  schematically represents a conventional spatial configuration of the antenna circuit of  FIG. 1B . The whole antenna winding is coiled in a spiral, in a single plane. To simplify the drawing, it is assumed that the winding L 1  includes one turn and the winding L 2  two turns. The antenna winding thus includes three turns. The inner turn, shown in solid line, is that of winding L 1  and the two outer turns, shown in dotted lines, are those of winding L 2 . 
         [0009]      FIG. 2B  schematically represents a conventional spatial configuration of the antenna circuit of  FIG. 1C . As previously, the whole antenna winding is coiled in a spiral, in a single plane. To simplify the drawing, it is assumed that each winding L 1  to L 3  includes a single turn. The antenna winding thus includes three turns. The inner turn, shown in dotted line, is that of winding L 2  and the outer turn, shown in chain dotted line, is that of winding L 3 . The middle turn, shown in solid line, is that of the active winding L 1 . 
         [0010]    It has been demonstrated that the circuit of  FIG. 1B  achieved according to the spiral spatial configuration of  FIG. 2A  offers, in terms of communication distance, lower performance than that of the circuit of  FIG. 1C  achieved with the same dimensions according to the same spiral spatial configuration ( FIG. 2B ). 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    There is a need to improve the effectiveness of these antenna circuits. 
         [0012]    This need may be satisfied by providing an antenna circuit for near-field communications including a planar active winding, connected between a first and a second access terminal; an auxiliary winding coplanar with the active winding and connected by a first end to the first access terminal; and a tuning capacitor connected to the second end of the auxiliary winding. The turns of the active and auxiliary windings are interleaved. 
         [0013]    According to one embodiment, a first part of the turns of the auxiliary winding is situated inside the active winding, and a second part of the turns of the auxiliary winding is situated outside the active winding. 
         [0014]    According to one embodiment, a first part of the turns of the active winding is situated inside the auxiliary winding, and a second part of the turns of the active winding is situated outside the auxiliary winding. 
         [0015]    According to one embodiment, each turn of the active winding is flanked by two turns of the auxiliary winding, adjacent respectively on the inside and on the outside. 
         [0016]    According to one embodiment, the antenna circuit includes a second auxiliary winding coplanar with the active winding, connected between the capacitor and the second access terminal, the active winding comprising a group of turns interleaved with the turns of the first auxiliary winding, and a group of distinct turns interleaved with the turns of the second auxiliary winding. 
         [0017]    According to one embodiment, the auxiliary windings are situated in two disjoint respective zones of one and the same plane. 
         [0018]    A passive near-field communication device may include an antenna circuit of the abovementioned type and a circuit for modulating the impedance between the access terminals of the antenna circuit. 
         [0019]    An active near-field communication device may include an antenna circuit of the abovementioned type and an excitation circuit connected to the access terminals of the antenna circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0020]    The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
           [0021]    Embodiments will be set forth in the following nonlimiting description, given in conjunction with the appended figures among which, in the drawings: 
           [0022]      FIGS. 1A to 1C , previously described, represent electrical structures of conventional antenna circuits; 
           [0023]      FIG. 2A , previously described, represents a spatial configuration of the antenna circuit of  FIG. 1B ; 
           [0024]      FIG. 2B , previously described, represents a spatial configuration of the antenna circuit of  FIG. 1C ; 
           [0025]      FIG. 3  represents an exemplary spatial configuration of an antenna circuit of alternative structure; 
           [0026]      FIG. 4  represents an exemplary spatial configuration for an antenna winding having an even number of turns; 
           [0027]      FIG. 5  represents an exemplary spatial configuration for an antenna winding having an odd number of turns; 
           [0028]      FIGS. 6A to 6C  symbolize several possibilities of interleaving of turns; and 
           [0029]      FIG. 7  represents an exemplary interleaved spatial configuration applicable to an antenna of the type of  FIG. 1C . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    As indicated previously, it has been demonstrated by use that the spatial configuration of the antenna circuit of  FIG. 2B , which implements the electrical circuit of  FIG. 1C , is more effective, in terms of communication distance for equal antenna dimensions, than that of  FIG. 2A , which implements the antenna circuit of  FIG. 1B . 
         [0031]    The structures of the antennas of  FIGS. 1B and 1C  being equivalent from a purely electrical point of view, it is assumed that the difference in effectiveness is related to the difference in the spatial configurations conventionally used to produce these types of antennas. Spatial configurations able to improve the effectiveness are studied here. 
         [0032]      FIG. 3  illustrates a modification of the electrical structure of the antenna of  FIG. 2A  which retains the spatial configuration of  FIG. 2B . The capacitor C of  FIG. 2B  is replaced with an electrical conductor  10 , and a capacitor C is inserted in the connection of turn L 3  to terminal A 2 . 
         [0033]    It turns out that the electrical structure thus obtained is that of  FIG. 1B , with an auxiliary winding L 2  including more turns than the active winding L 1 . Furthermore, a portion L 2   a  of the turns of the winding L 2  (one turn in  FIG. 3 ) is situated inside the winding L 1 , and the remaining portion L 2   b  of the turns of the winding L 2  (one turn in  FIG. 3 ) is situated outside the winding L 1 . 
         [0034]    Stated otherwise, the turns of the windings L 1  and L 2  are interleaved, rather than being coiled in a regular spiral. 
         [0035]      FIG. 4  represents a spatial configuration based on the same principle as that of  FIG. 3 , having a two-turn winding L 1 , and a four-turn winding L 2 . The winding L 2  is separated into two portions L 2   a  and L 2   b,  the portion L 2   a  comprising two adjacent turns inside the active winding L 1 , and the portion L 2   b  comprising two adjacent turns outside winding L 1 . The partial winding L 2   a  and winding L 1  are coiled clockwise one after the other in a regular spiral from the inside towards the outside. At the inside, the end of winding L 2   a  is connected by conductor  10  to an end of winding L 2   b.  Starting from this end, the winding L 2   b  is coiled anti-clockwise from the outside towards the inside. The winding L 2   b  terminates upon capacitor C connected to terminal A 2 . 
         [0036]    Through this choice of the winding directions, the series current in the antenna winding flows in the same direction in all the turns. According to an alternative of the configuration of  FIG. 4 , retaining the direction of the current, the partial winding L 2   b  could be coiled clockwise from the inside towards the outside, from the end connected to the conductor  10 . 
         [0037]    In practice, preferably, all the turns are produced in one and the same layer of a substrate  15 , such as a flexible printed circuit, and the conductor  10 , which crosses the turns radially, is produced in a different layer of the substrate and connected to the corresponding ends of the turns by vias. The same goes for conductors serving to offset the access terminals A 1  and A 2  towards the periphery of the structure, and for the connections of capacitor C. 
         [0038]    The terminals A 1  and A 2  are connected to a circuit  20  for managing the NFC device, which integrates the antenna circuit. It may be a passive device, such as an RFID tag, in which case the circuit  20  is designed to draw its power supply from the magnetic field sensed by the antenna and to modulate the impedance between the terminals A 1  and A 2  in order to transmit information. In the case where the NFC device is active, the circuit  20  is designed to excite the antenna through the terminals A 1  and A 2  so as to create a magnetic field, and to detect modulations of this field that are caused by other NFC devices within the range of the field. 
         [0039]    Trials have been conducted on two antennas of like characteristics in terms of dimensions and total number of turns, one produced according to the spatial configuration of  FIG. 4 , and the other according to the spatial configuration of  FIG. 2B  (with three windings having two turns). These two antennas have turned out to be equivalent, for equal dimensions, in terms of communication distance with various conventional types of RFID tags. 
         [0040]    The effectiveness of an antenna of the type of  FIG. 1B  is therefore successfully improved by modifying its spatial structure in such a way that the turns of the windings L 1  and L 2  are interleaved. 
         [0041]    An antenna of the type of  FIG. 1B ,  3  or  4 , is easier to produce and to implement than an antenna of the type of  FIG. 1C . Indeed, it exhibits three points of connection to the windings instead of four, counting the two connections to the access terminals A 1 , A 2  and the connection of the auxiliary winding L 2  to a first terminal of the capacitor. The second terminal of the capacitor and the terminal A 2  are grounded instead of being floating. 
         [0042]    The antenna windings described up till now have a ratio 1:3 (between the number of turns of the active winding L 1  and the total number of turns of the antenna winding). It is desirable to produce other ratios. In the structure of  FIG. 4 , it is accordingly possible to vary the number of turns of the partial windings L 2   a  and L 2   b  independently of the number of turns of winding L 1 , or vice versa, without affecting the generic configuration of the antenna circuit. 
         [0043]      FIG. 5  illustrates an alternative configuration suited to an odd total number of turns, notably for the case where winding L 1  has one turn less than winding L 2 . The example represented corresponds to a winding L 1  of two turns and a winding L 2  of three turns. In this case, rather than separating the winding L 2  into two unequal parts (which nevertheless remains conceivable in the structure of  FIG. 4 ), each turn of winding L 1  is spatially alternated with a turn of winding L 2 . Each turn of the winding L 1  is thus flanked by two turns of winding L 2 , adjacent respectively on the inside and on the outside. 
         [0044]      FIGS. 6A to 6C  summarize various elementary possibilities of interleaving of turns offered according to the numbers of turns of windings L 1  and L 2 . The vertical lines represent the spatial arrangement of the turns. From left to right the turns are represented from the outside towards the center of the antenna winding. The solid lines correspond to the turns of the active winding L 1 , and the dotted lines correspond to the turns of the auxiliary winding L 2 . The numbers under the lines designate the order of winding. 
         [0045]      FIG. 6A  corresponds to the case of  FIG. 5 , that is to say an odd total number of turns. The two turns of winding L 1  are wound consecutively in spatial positions  2  and  4 , and the winding of the three turns of winding L 2  is continued in spatial positions  5 ,  3  and  1 . 
         [0046]      FIG. 6B  corresponds to the case of  FIG. 4 , that is to say an even total number of turns. The two turns of winding L 1  are wound consecutively in spatial positions  3  and  4 , and the winding of the four turns of winding L 2  is continued in spatial positions  5 ,  6 ,  1  and  2 . 
         [0047]      FIG. 6C  is an alternative of  FIG. 6B . The two turns of winding L 1  are separated spatially by two turns of winding L 2 . More specifically, the turns of winding L 1  are coiled in spatial positions  2  and  5 , and then the turns of winding L 2  are coiled in spatial positions  6 ,  4 ,  3  and  1 . 
         [0048]    The interleaved spatial configurations illustrated by  FIGS. 6A to 6C  tend to reduce the mean radial distance between the turns of the auxiliary winding L 2  and the turns of the active winding L 1 , the consequence of which is an improvement in the coupling coefficient between the active and auxiliary windings. For this same reason, the alternatives of  FIGS. 6A and 6C  offer better effectiveness than that of  FIG. 6B , since the turns of the active winding L 1  are better distributed among the turns of the auxiliary winding L 2 . 
         [0049]    The orders of winding indicated in  FIGS. 6A to 6C  are given by way of example. What matters is the spatial position of the turns, so that the turns of the windings L 1  and L 2  are interleaved. The order of winding will be chosen, preferably, so as to decrease the number of turn crossings, which require conductors in a second substrate layer and vias. 
         [0050]    Depending on the ratios to be obtained and the numbers of turns of the windings, it will be possible to duplicate the patterns of  FIGS. 6A to 6C  or else combine them within one and the same antenna winding. Each line shown in the Figs. can furthermore represent multiple turns of one and the same winding and the number of turns per line may be different for the two windings. 
         [0051]      FIG. 7  represents an exemplary application of the interleaving to an antenna winding of the type of  FIG. 1C , comprising three windings L 1 , L 2  and L 3 , each of two turns. The winding L 1  is the active winding, and the windings L 2  and L 3  are auxiliary. This structure is similar to that of  FIG. 6C , the two halves of which correspond respectively to the auxiliary windings L 2  and L 3 . The turns are coiled clockwise starting from the inside. Going from the inside towards the outside are found, successively, a first turn of winding L 2 , a first turn of winding L 1 , the second turn of winding L 2 , a first turn of winding L 3 , the second turn of winding L 1 , and then the second turn of winding L 3 . 
         [0052]    The free ends of the first turn of winding L 2  (at the center) and of the second turn of winding L 3  (on the outside) are linked by capacitor C. The two turns of winding L 1  are connected by a radial conductor in a second layer. The two ends of winding L 1  are connected to the access terminals A 1  and A 2 . 
         [0053]    Thus, a first group of turns of winding L 1  is interleaved with the turns of winding L 2 , and a second group formed of the remaining turns of winding L 1  is interleaved with the turns of winding L 3 . This spatial configuration improves the effectiveness of the antenna with respect to the conventional spatial configuration of  FIG. 2B . By contrast, the auxiliary windings do not include any interleaved turns—they are situated in disjoint zones of the plane of the antenna winding. 
         [0054]    It is appreciated that each of the windings L 2  and L 3 , associated with the group of turns of winding L 1  which is assigned to it, plays the same role as the lone winding L 2  associated with winding L 1  as a whole in the two-winding configurations. Thus, it is possible to apply combinations of the configurations of  FIGS. 6A to 6C  to each of the windings L 2  and L 3 , and the associated half of the winding L 1 . 
         [0055]    Numerous variants and modifications of the embodiments described here will be apparent to the person skilled in the art. A tuning capacitor connected to the terminals of the antenna winding as a whole has for example been described. The capacitor could also be connected to the terminals of the auxiliary winding. Moreover, in the diverse examples described, the auxiliary winding has more turns than the active winding, thereby corresponding to the more common use cases. It is nonetheless conceivable for the active winding to have more turns than the auxiliary winding, in which case the spatial roles of these two windings are reversed in the examples described. 
         [0056]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.