Abstract:
Nesting an active NFC coil with an open-circuited coil can boost the effective range over which the NFC coil can communicate.

Description:
FIELD 
       [0001]    The invention relates to wireless communication using near field communication (NFC) techniques. 
       BACKGROUND 
       [0002]    NFC is a form of wireless communication in which a communications channel is formed by creating a magnetic coupling between an antenna structure in a transmitting device and an antenna structure in a receiving device. Typically, the antenna structures of the transmitting and receiving devices need to be closer than about 40 cm in order for the magnetic coupling to be strong enough to support communications at a data rate that is sufficiently high to be considered worthwhile. 
         [0003]    The performance of an NFC antenna is in part determined by its size. That is to say, the larger the antenna, the better NFC performance becomes. NFC antennas are often constrained to fit within the form factor of a cell phone. Due to that requirement, the largest practical NFC antenna is about credit card sized and the smallest is about one quarter of that size. That range translates to an antenna structure having an area in the range 4600 to 1100 mm 2 . It is normal to describe NFC antennas in terms of their area since they are usually, but not always, two dimensional structures. 
         [0004]      FIG. 1  shows the effect that antenna size has on NFC performance.  FIG. 1  plots magnetic coupling strength (k) between a transmitting NFC antenna and a receiving NFC antenna as a function of the separation of the transmitting and receiving antennas. For an NFC link to be considered viable, the lower limit on the magnetic coupling strength is about 10 −2 . All of the plots in  FIG. 1  relate to arrangements in which the transmit and receive antennas are flat, planar, rectangular coils located in parallel planes with the centres of the coils lying on a common axis. Therefore, “antenna separation” in  FIG. 1 , which is the parameter assigned to the chart&#39;s horizontal axis, is the separation of the coils&#39; centres along that common axis. In  FIG. 1 :
       plots  76  and  74  show, respectively, the experimentally measured and theoretically predicted variation in magnetic coupling strength versus antenna separation for the case where both of the transmit and receive antennas are A4 sized;   plots  80  and  78  show, respectively, the experimentally measured and theoretically predicted variation in magnetic coupling strength versus antenna separation for the case where one of the transmit and receive antennas is A4 sized and the other is credit card sized; and   plots  84  and  82  show, respectively, the experimentally measured and theoretically predicted variation in magnetic coupling strength versus antenna separation for the case where both of the transmit and receive antennas are credit card sized.       
 
         [0008]    The plots  74  to  84  do indeed show that a larger antenna size generally leads to increased NFC performance. By “A4 size”, an antenna taking up the two dimensional area of a piece of A4 paper (so approximately 62000 mm 2 ) is meant. 
         [0009]    It has been suggested that incorporating ferrite into an NFC antenna structure allows the size of the antenna structure to be decreased while maintaining performance. However, such an advantage would be accompanied by a disadvantage in that the cost of the bill of materials for the antenna structure will increase. 
       SUMMARY 
       [0010]    According to one aspect, an embodiment of the invention provides a wireless communications device comprising an antenna comprising a first coil that is open-circuited and a second coil, wherein one of the first and second coils is nested inside the other one of the first and second coils and wherein the device further comprises at least one of a demodulator coupled to the second coil and arranged to demodulate data from NFC signals picked up by the second coil and a modulator coupled to the second coil and arranged to modulate data onto NFC signals and then supply said signals for the second coil to transmit. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0011]    By way of example only, certain embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
           [0012]      FIG. 1  is a chart plotting magnetic coupling strength versus antenna separation for various pairings of receive and transmit antennas forming an NFC link; 
           [0013]      FIG. 2  is a block diagram of an NFC transceiver and its NFC subsystem; 
           [0014]      FIG. 3  illustrates schematically the antenna structure of the NFC transceiver of  FIG. 2 ; 
           [0015]      FIG. 4  is a cross sectional view along line C-C in  FIG. 3 , viewed in the direction of arrows D; 
           [0016]      FIG. 5  is another chart plotting magnetic coupling strength versus antenna separation for various pairings of receive and transmit antennas forming an NFC link; 
           [0017]      FIG. 6  illustrates a cross section on line A-A in  FIG. 3 , viewed in the direction of arrows B, 
           [0018]      FIG. 7  is a cross sectional view along line A-A, viewed in the direction of arrows B, in a variant of the antenna structure of  FIG. 3 ; 
           [0019]      FIG. 8  is a repeat of  FIG. 7  that has been relabelled to emphasise another feature of the geometry of the elements shown in the Figure; 
           [0020]      FIG. 9  is a cross sectional view along line C-C, viewed in the direction of arrows D, in a variant of the antenna structure of  FIG. 3 ; and 
           [0021]      FIG. 10  is a schematic illustration of an alternative to the antenna structure of  FIG. 3 ; 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Some of the drawings in the document describe variants of earlier drawings in the document. Where that is the case, elements carried over from one drawing to another retain the same reference signs. 
         [0023]      FIG. 2  is a block diagram schematically illustrating an NFC transceiver  10 .  FIG. 2  illustrates only those components of the NFC transceiver  10  that are most closely concerned with providing a detailed description of an embodiment of the invention. Persons skilled in the art of wireless communication device design will readily appreciate that a communications device includes many elements besides those shown in  FIG. 2 . As shown in  FIG. 2 , the NFC transceiver  10  comprises a processor  12 , a modulator  14 , a demodulator  16 , and an antenna structure  20 . 
         [0024]    When NFC transmission from the antenna structure  20  is required, the processor  12  sends an electrical signal conveying data that needs to be transmitted over connection  22  to the modulator  14 . The modulator  14  converts the electrical signal that it receives on connection  22  into a transmittable form and supplies the converted electrical signal via connection  25  to the antenna structure  20  for transmission from the NFC transceiver  10  as a transmitted NFC signal  26 . 
         [0025]    The NFC transceiver  10  can also use the antenna structure  20  to receive NFC signals, such as signal  28 , that are transmitted to the NFC transceiver  10 . NFC signals that are received by the antenna structure  20  are delivered over connections  25  and  30  to the demodulator  16 . The demodulator  16  recovers data that may be contained in the signals received over connection  30  and sends that data as an electrical signal over connection  32  to the processor  12  so that the processor can make use of that data. 
         [0026]    The processor  12  is further connected to the modulator  14 , the demodulator  16  by means of connections  34  and  36 , respectively. The connections  34  and  36  are for delivering control signals from the processor  12  that control the operation of the modulator  14 , the demodulator  16  respectively. The details of the control exerted by the processor  12  on the modulator  14 , the demodulator  16  and the switch  18 , and the details of the modulation and demodulation schemes applied respectively by the modulator  14  and the demodulator  16 , are beyond the scope of this document and in any event are conventional and tangential as regards describing the invention is concerned. 
         [0027]      FIG. 3  shows the antenna structure  20  in more detail. As shown in  FIG. 2 , the antenna structure comprises a substrate  40  on which two coils  42  and  44  are provided. The substrate  40  may be, for example, a flexible plastic membrane or a printed circuit board (PCB). The substrate  40  might also support other elements of the NFC transceiver  10 , for example antenna matching components. The coils  42  and  44  are each shown as rectangular coils, each having three turns. In practice, the coils  42  and  44  might have a shape other than a rectangle and could easily have a different number, typically a higher number, of turns. The coils  42  and  44  are each made of a rectangular spiral of conductive material, typically a metal. Typically the two spirals making up the coils  42  and  44  are printed or etched onto the substrate  40 . The coil  44  is nested within the coil  42 . In the configuration shown, coils  42  and  44  have a common centre. However, in practice, the centres of coils  42  and  44  could be offset relative to one another. The coils  42  and  44  are nested in the sense that coil  44  is enclosed by coil  42 . 
         [0028]    Coil  42  is an open circuited coil. That is to say, the two ends  46  and  48  of the rectangular spiral track that makes up coil  42  are not connected to anything. On the other hand, the ends  58  and  60  of coil  44  provide the connection  25  of  FIG. 1  so that the coil  44  can be driven by modulator  14  and so that demodulator  16  can recover data from wireless signals that are picked up by coil  44 . In order to bridge the turns of coil  42 , end  58  is connected to the outer turn of coil  44  through vias  50  and  54  and end  60  is connected to the inner turn of coil  44  through vias  52  and  56 . 
         [0029]    The surface of the substrate  40  that supports the coils  42  and  44  is planar such that the coils  42  and  44  are flat.  FIG. 4  is a cross sectional view along line C-C in the direction of arrows D. The flat, planar, supporting surface  61  of the substrate is readily apparent in  FIG. 4 , as is the coplanar relationship of the turns of the two coils  42  and  44 . 
         [0030]    By nesting the connected coil  44  within the open-circuited coil  42 , an improvement in antenna performance is achieved, in that the strength of the magnetic coupling formed with a cooperating antenna is boosted at larger distances. This effect is illustrated in  FIG. 5 , which plots magnetic coupling strength (k) between a transmitting NFC antenna and a receiving NFC antenna as a function of the separation of the transmitting and receiving antennas. The plots in  FIG. 5  relate to arrangements in which the transmit and receive antennas are flat, planar, rectangular structures located in parallel planes with the centres of their rectangular structures lying on a common axis. Therefore, “antenna separation” in  FIG. 5 , which is the parameter assigned to the chart&#39;s horizontal axis, is the separation of the rectangular structures&#39; centres along that common axis. Because the magnetic field around these antennas is toroidal in shape, and therefore not sharply directional, the curves would have similar shapes to those of  FIG. 5  if measured off-axis. 
         [0031]    In  FIG. 5 :
       plot  86  shows the variation of magnetic coupling strength versus antenna separation for the case where both the transmit and receive antennas are credit card sized rectangular coils;   plot  88  shows the variation of magnetic coupling strength versus antenna separation for the case where one of the transmit and receive antennas is a credit card sized rectangular coil and the other one is an A4 sized rectangular coil;   plot  90  shows the variation of magnetic coupling strength versus antenna separation for the case where both the transmit and receive antennas are A4 sized rectangular coils; and   plot  92  shows the variation of magnetic coupling strength versus antenna separation for the case where one of the transmit and receive antennas is a credit card sized rectangular coil and the other one is a structure of the kind shown in  FIG. 3  in which the area bounded by the outer open-circuited coil is credit card sized.       
 
         [0036]    From an inspection of  FIG. 5 , it will be apparent that, upwards of an antenna separation of about 1000 mm, the nested coil arrangement of plot  92  is the best performing of all, and that, upwards of about 100 mm, the nested coil arrangement of plot  92  is better performing than the credit card size to credit card size arrangement of plot  86  and the credit card size to A4 size arrangement of plot  88 . For conventional NFC, this extra level of coupling will probably not be too beneficial as normal NFC operation requires a coupling &gt;10 −2  for reasonable power transfer. Other magnetically coupled systems such as NFC Peer to Peer mode and NULEF where communication occurs between two active units will benefit greatly from this new arrangement due to increased range. 
         [0037]    Some variations of the embodiment described above will now be discussed. 
         [0038]    It was indicated earlier that the supporting surface  61  for coils  42  and  44  is planar. However, this need not be the case in all embodiments.  FIG. 6 , shows a cross section along line A-A in  FIG. 3 , viewed in the direction of arrows B, and reiterates that the surface  61  is flat. The turns of coil  44  are indicated  62 ,  64  and  66  in  FIG. 6 .  FIG. 7  shows how the same cross section looks according to another embodiment. In  FIG. 7 , the surface  68  of the substrate  40  is not flat, and in this example undulates sinusoidally. In  FIG. 7 , the coil  44  is not flat, since the two outer turns  62  and  66  lie in minima on the surface whilst the inner turn  64  runs along a local maxima. Thus, the turns of a coil need not be coplanar. 
         [0039]    Whilst the coil  44  has been described as having a profile that follows a surface, the surface that the turns of the coil follow need not be a physical surface. In fact, it is only convenient to talk in terms of a physical surface because in the embodiments of  FIGS. 6 and 7  the turns lie on a substrate. It is, however, possible instead to refer to the turns of the coil or the profile of the coil as following a notional surface. As an illustration of this,  FIG. 8  repeats the cross sectional view of  FIG. 7  and adds a dashed line defining a notional surface  70  which is a plane on which the turns of the coil  44  lie, albeit that not all of the turns lie on the same side of the notional surface  70 . It is indeed possible to go further, and think of the turns of the coil as defining the notional surface  70 . 
         [0040]    The foregoing discussion of the turns of a coil following or defining a non-flat surface focused on the turns of coil  44 . For the sake of completeness, it is observed that  FIGS. 6 to 8  and the associated discussion could equally will have related to a cross section along line E-E in  FIG. 3 , i.e. to the profile of coil  42 . In other words, any part of either or both of the coils  42  and  44  could be locally non-planar. Moreover, although the example of a sinusoidally undulating surface was given above, the coils  42  and  44  could follow or define almost any other type of non flat surface, for example a parabolic or otherwise dished surface. 
         [0041]    The coils  42  and  44  need not be coplanar. In the embodiment of  FIGS. 3 and 4 , the surface  61  of the substrate  40  is planar. However, that need not necessarily be the case.  FIG. 9  relates to an example where the surface  72  of the substrate  40  is crowned rather than flat and shows what a cross section on line C-C of  FIG. 3  in the direction of arrows D might then look like. As shown in  FIG. 9 , the surface  72  of substrate  40  is curved and the turns of coil  42  lie at one region on the surface  72  whilst the turns of coil  44  lie at another region on the surface  72 , and it is apparent that the turns of the coils  42  and  44  do not lie in a common plane. 
         [0042]    In the embodiments discussed thus far, the outer coil  42  is open-circuited and the inner coil  44  is connected to the modulator  14  and the demodulator  16 . However, enhanced performance of the antenna structure  20  arises even if the roles of the coils  42  and  44  are reversed.  FIG. 10  shows a variant  94  of the antenna structure  20  in which this reversal has been implemented. As shown in  FIG. 10 , the ends  96  and  98  of coil  44  are left open-circuited, whereas the ends  100  and  102  of coil  42  provide the connection  25  to the rest of the NFC transceiver  10 . The inner turn of coil  42  is connected to end  102  through vias  104  and  106  that allow the intervening turns of coil  42  to be bridged.