Patent Publication Number: US-2023152481-A1

Title: Position Sensing Apparatus and Method

Description:
PRIORITY CLAIM 
     The present application claims the benefit of priority of U.S. Provisional Application Ser. No. 62/753,138, titled “Position Sensing Apparatus and Method,” filed on Oct. 31, 2018, which is incorporated herein by reference. The present application claims the benefit of priority of U.S. Provisional Application Ser. No. 62/857,554, titled “Position Sensing Apparatus and Method,” filed on Jun. 5, 2019, which is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates generally to sensing relative position between two members. 
     BACKGROUND 
     Various forms of inductive position sensors are known for detecting the position of a first member relative to a second member. In some instances, one member can carry a transmit aerial and a receive aerial while the other member can carry a coupling element. The transmit aerial and the receive aerial can be arranged so that, in the absence of the coupling element, a magnetic field generated by alternating current flowing through the transmit aerial induces a null signal in the receive aerial (the transmit aerial and the receive aerial are then referred to as balanced). However, in the presence of the coupling element, a magnetic field generated by alternating current flowing through the transmit aerial induces an electromotive force in the receive aerial that generates a current that is dependent on the position of the coupling element relative to the transmit and receive aerials. By analysing this current, the relative position of the two members can be determined. 
     SUMMARY 
     Aspects and advantages of embodiments of the present disclosure will be set forth in part in the following description, or may be learned from the description, or may be learned through practice of the embodiments. 
     One example aspect of the present disclosure is directed to an inductive position sensor configured to detect relative position between a first member and a second member. The inductive position sensor includes a transmit aerial configured to be disposed on the first member. The transmit aerial can have at least one transmit winding. The inductive position sensor can include a receive aerial configured to be disposed on the first member. The receive aerial can include one or more receive windings. The inductive position sensor can include a coupling element operable to be disposed on the second member. The inductive position sensor can include processing circuitry configured to provide one or more signals indicative of the position of the first member relative to the second member based on current induced in the one or more receive windings resulting from an oscillating signal provided to the transmit winding. The inductive position sensor includes at least one electrostatic shield. The electrostatic shield can include a plurality of conductive traces arranged so that no current loops are formed in the electrostatic shield. 
     These and other features, aspects and advantages of various embodiments will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present disclosure and, together with the description, serve to explain the related principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Detailed discussion of embodiments directed to one of ordinary skill in the art are set forth in the specification, which makes reference to the appended figures, in which: 
         FIG.  1    depicts a schematic of an example inductive position sensor according to example embodiments of the present disclosure; 
         FIG.  2    depicts example windings associated with a transmit aerial and a receive aerial for an inductive position sensor according to example embodiments of the present disclosure; 
         FIG.  3    depicts example windings associated with a transmit aerial and a receive aerial for an inductive position sensor according to example embodiments of the present disclosure; 
         FIG.  4    depicts an example first layer of a multilayer structure associated with a transmit aerial and a receive aerial for an inductive position sensor according to example embodiments of the present disclosure; 
         FIG.  5    depicts an example second layer of a multilayer structure associated with transmit aerial and a receive aerial for an inductive position sensor according to example embodiments of the present disclosure; 
         FIG.  6    depicts an example first layer of a multilayer structure associated with a transmit aerial and a receive aerial for an inductive position sensor according to example embodiments of the present disclosure; 
         FIG.  7    depicts an example second layer of a multilayer structure associated with transmit aerial and a receive aerial for an inductive position sensor according to example embodiments of the present disclosure; 
         FIG.  8    depicts an example multilayer structure for an inductive position sensor having a plurality of electrostatic shields according to example embodiments of the present disclosure; 
         FIG.  9    depicts an example electrostatic shield according to example embodiments of the present disclosure; 
         FIG.  10    depicts an example electrostatic shield according to example embodiments of the present disclosure; 
         FIG.  11    depicts an example electrostatic shield according to example embodiments of the present disclosure; 
         FIG.  12    depicts an example electrostatic shield according to example embodiments of the present disclosure; 
         FIG.  13    depicts an example electrostatic shield according to example embodiments of the present disclosure; 
         FIG.  14    depicts an example electrostatic shield disposed on a same layer as a layer associated with a transmit aerial and/or receive aerial of an inductive position sensor according to example embodiments of the present disclosure; 
         FIG.  15    depicts an example electrostatic shield disposed on a same layer as a layer associated with a transmit aerial and/or receive aerial of an inductive position sensor according to example embodiments of the present disclosure; 
         FIG.  16    depicts a schematic of an example transmit aerial portion of an inductive position sensor according to example embodiments of the present disclosure; 
         FIG.  17    depicts example windings associated with a transmit aerial and a receive aerial for an inductive position sensor according to example embodiments of the present disclosure; and 
         FIG.  18    depicts a schematic of an example transmit aerial portion of an inductive position sensor according to example embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference now will be made in detail to embodiments, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the embodiments, not limitation of the present disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments without departing from the scope or spirit of the present disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that aspects of the present disclosure cover such modifications and variations. 
     Example aspects of the present disclosure are directed to position sensors (e.g., rotary position sensors) and more particularly, to inductive position sensors having electrostatic protection to reduce interference. The electrostatic interference can be from high voltage sources, such as high voltage electric motors. For instance, high voltage electric motors are being increasingly used in automotive and other applications. These high voltage sources can cause interference in inductive position sensors, such as inductive rotary position sensors. This interference can include, for instance, low frequency electrostatic coupling of the high voltage source(s) to the inductive sensors. 
     For instance, in some embodiments, an inductive position sensor can determine relative position between two members, such as a first member and a second member. The inductive position sensor can include a transmit aerial arranged on the first member. The transmit aerial can include a transmit winding. The inductive position sensor can include a receive aerial arranged on the first member. The receive aerial can include one or more windings, such as a first winding (e.g., a sine winding) and a second winding (e.g., a cosine winding). The inductive position sensor can include a coupling element on the second member. The coupling element can include one or more conductive regions and/or one or more regions of ferromagnetic material. 
     An oscillating signal can be provided to the transmit winding of the transmit aerial. The oscillating signal provided to the transmit aerial can induce electromotive forces in the receive windings(s) of the receive aerial. In the presence of the coupling element on the second member, a magnetic field generated by alternating current flowing through the transmit aerial induces an electromotive force in the receive aerial that generates a current that is dependent on the position of the coupling element relative to the transmit and receive aerials. In this way, the sensor can be used to detect the relative position (e.g., rotary position) between the two members. 
     According to example aspects of the present disclosure, an inductive position sensor (e.g., inductive rotary position sensor) can include one or more electrostatic shields to reduce interference with the inductive position sensor. The electrostatic shield(s) can include a one or more conductive traces arranged so that no current loops are formed in the electrostatic shield(s). The electrostatic shield is grounded. The electrostatic shield can be disposed on a printed circuit board or other substrate associated with the inductive position sensor. For instance, in some embodiments, the electrostatic shield(s) can be disposed on the same layer as windings associated with the inductive position sensor. In some embodiments, the electrostatic shield(s) can be disposed on one or more different layers in the printed circuit board as windings associated with the inductive position sensor. In some embodiments, the electrostatic shield can be integrated into a housing used to house various components of the inductive position sensor. This can provide a low impedance path to ground for the receive winding to reduce interference. 
     In some embodiments, a capacitor can be coupled between a point on the transmit winding and/or a point on the one or more of the receive windings (e.g., sine winding and/or cosine winding) and ground. The capacitor coupled in this manner can reduce interference on the windings, including low frequency interference. 
     One example embodiment of the present disclosure is directed to an inductive position sensor configured to detect relative position between a first member and a second member. The inductive position sensor includes a transmit aerial configured to be disposed on the first member. The transmit aerial can have at least one transmit winding. The inductive position sensor can include a receive aerial configured to be disposed on the first member. The receive aerial can include one or more receive windings. The inductive position sensor can include a coupling element operable to be disposed on the second member. The inductive position sensor can include processing circuitry configured to provide one or more signals indicative of the position of the first member relative to the second member based on current induced in the one or more receive windings resulting from an oscillating signal provided to the transmit winding. The inductive position sensor includes at least one electrostatic shield. The electrostatic shield can include a plurality of conductive traces arranged so that no current loops are formed in the electrostatic shield. 
       FIG.  1    depicts a schematic of an example rotary position sensor according to example embodiments of the present disclosure. As shown in  FIG.  1   , a rotary position sensor  100  has a transmit aerial  102 , a receive aerial  104  formed by a sine winding  103  and a cosine winding  105 , and an intermediate coupling element  107 . The transmit aerial  102  and the receive aerial  104  are formed on a first member (not shown). The intermediate coupling element  107  is formed on a second member (also not shown). The intermediate coupling element  107  can include a conductive material and/or ferromagnetic material. Relative movement (e.g., rotary movement) between the first member and the second member causes a corresponding relative movement between, on the one hand, the transmit aerial  102  and the receive aerial  104 , and on the other hand, the intermediate coupling element  107 . 
     The transmit aerial  102 , the sine winding  103  and the cosine winding  105  are electrically connected to respective terminals of processing circuitry  110 . In this example the processing circuitry  110  is in the form of a semiconductor integrated circuit device, such as an Application Specific Integrated Circuit (ASIC) or an Application Specific Standard Product (ASSP). In other examples, the processing circuitry  110  can use multiple interconnected devices and/or can be implemented using one or more suitable components (e.g., electronic components, such as discrete electronic components). 
     As shown in  FIG.  1   , the processing circuitry  110  includes a TX drive stage  112  which generates an oscillating electrical signal for supply to the transmit aerial  102 . In this example, the TX drive stage  112  can be a free running oscillator that generates an oscillating electrical signal at a drive frequency determined by the inductance of the transmit aerial  102  and the capacitance of one or more capacitor(s)  113  connected in parallel to the transmit aerial  102 . This drive frequency is normally selected to be a few MHz, for example in the range of about 2 MHz to about 5 MHz. As used herein, the use of the term “about” in conjunction with a numerical value is intended to refer to within 20% of the stated numerical value. 
     Supplying an oscillating current to the transmit aerial  102  induces electromotive forces in the sine winding  103  and the cosine winding  105  of the receive aerial  104 , which can cause current to flow in the sine winding  103  and the cosine winding  105 . As shown in  FIG.  1   , the sine winding  103  and the cosine winding  105  are separate windings such that separate currents flow in the sine winding  103  and the cosine winding  105 . The sine winding  103  and the cosine winding  105  are electrically connected to separate terminals of the processing circuitry  110 , with the current flowing in the sine winding  103  being processed to provide a sine output signal  123  and the current flowing in the cosine winding  105  being processed to provide a cosine output signal  125 . The relative magnitudes of the sine output signal  123  and the cosine output signal  125  are indicative of the relative position (e.g., relative rotary position) of the first member and the second member. 
     On entering the processing circuitry  110 , current flowing in the sine winding  103  first goes through an EMC filtering stage  115 , to reduce signal components at frequencies away from the drive frequency. The signal components that are reduced may be caused, for example, by interference from electrical signals generated by other nearby electrical components. According to example aspects of the present disclosure and as discussed in more detail below, the inductive position sensor  100  can include one or more electrostatic shields configured to reduce interference from electrical signals generated by other electrical components, such as by a high voltage electric motor. 
     The filtered electrical signal then goes through a synchronous demodulation stage  117  in which the filtered electrical signal is mixed with a demodulation signal from the TX drive stage  112 . The demodulation signal is in phase with the drive signal. As a result of the intermediate coupling element  107  being formed of a conductive material, the electrical signal from the sine winding  103  can be 180° out of phase with the drive signal. The demodulated electrical signal resulting from the synchronous demodulation accordingly has a baseband component whose magnitude varies in dependence on the relative rotary position of the first and second members and high frequency components at twice the drive frequency and at higher harmonics of the drive frequency. 
     The demodulated electrical signal can pass through a low pass filtering stage  119  to remove the high frequency components corresponding to harmonics of the drive signal, leaving the baseband component. The signal can then pass through gain and output buffer stage  121 , which allows an adjustable gain to be applied before the sine output signal  123  is output by the processing circuitry  110 . 
     As will be apparent from  FIG.  1   , current induced in the cosine winding  105  also undergoes EMC filtering  115 , synchronous demodulation  117 , low pass filtering  119  and gain and output buffering  121  within the processing circuitry  110  before being output as the cosine output signal  125 . 
     The transmit aerial  102 , the sine winding  103  and the cosine winding  105  are formed by conductive traces. The conductive traces can be on a printed circuit board or other substrate that is on disposed on the first member. The intermediate coupling element  107  includes a conductive material (e.g., pattern of conductive material) located on the second member. 
       FIG.  2    depicts one example configuration of a transmit aerial  102 , sine winding  103  and cosine winding  105  of an inductive position sensor  100  according to example embodiments of the present disclosure. The transmit aerial  102 , sine winding  103  and cosine winding  105  can be disposed on a substrate, such as a printed circuit board. As shown in  FIGS.  4  and  5   , the transmit aerial  102 , sine winding  103 , and cosine winding  105  can be configured as a multilayer structure with different components of the transmit aerial  102 , sine winding  103 , and cosine winding  105  being arranged in different layers of a printed circuit board or other substrate. 
     More particularly,  FIG.  4    depicts a first layer  202  of the inductive position sensor of  FIG.  2    according to example aspects of the present disclosure. The first layer  202  includes conductive traces associated with the transmit aerial  102 , sine winding  103 , and the cosine winding  105 .  FIG.  5    depicts a second layer  204  of the inductive position sensor of  FIG.  2    according to example aspects of the present disclosure. The second layer  204  includes conductive traces associated with the sine winding  103  and cosine winding  105 . The first layer  202  and the second layer  204  can be disposed, for instance, on opposite sides of a printed circuit board or other substrate. In some embodiments, the first layer  202  and the second layer  204  can be disposed on different substrates. The first layer  202  and the second layer  204  can be connected, for instance, by vias. 
       FIG.  3    depicts another example configuration of a transmit aerial  102 , sine winding  103  and cosine winding  105  of an inductive position sensor  100  according to example embodiments of the present disclosure. The transmit aerial  102 , sine winding  103  and cosine winding  105  can be disposed on a substrate, such as a printed circuit board. As shown in  FIGS.  6  and  7   , the transmit aerial  102 , sine winding  103 , and cosine winding  105  can be configured as a multilayer structure with different components of the transmit aerial  102 , sine winding  103 , and cosine winding  105  being arranged in different layers of a printed circuit board or other substrate. 
     More particularly,  FIG.  6    depicts a first layer  202  of the inductive position sensor of  FIG.  3    according to example aspects of the present disclosure. The first layer  202  includes conductive traces associated with the transmit aerial  102 , sine winding  103 , and the cosine winding  105 .  FIG.  7    depicts a second layer  204  of the inductive position sensor of  FIG.  3    according to example aspects of the present disclosure. The second layer  204  includes conductive traces associated with the sine winding  103  and cosine winding  105 . The first layer  202  and the second layer  204  can be disposed, for instance, on opposite sides of a printed circuit board or other substrate. In some embodiments, the first layer  202  and the second layer  204  can be disposed on different substrates. The first layer  202  and the second layer  204  can be connected, for instance, by vias. 
     According to example aspects of the present disclosure, an inductive position sensor can include one or more electrostatic shields disposed proximate to the transmit aerial and receive aerial to reduce electromagnetic interference. In some embodiments, the electrostatic shield(s) can be disposed on different layer(s) relative to the layers associated with the transmit aerial and receive aerial. 
     For instance,  FIG.  8    depicts an example multilayer structure for a transmit aerial and receive aerial of an inductive position sensor  100  according to example embodiments of the present disclosure. The multilayer structure  100  can include a first layer  202  and a second layer  204 . The first layer  202  can include conductive traces associated with the transmit aerial and/or the receive aerial. For instance, the first layer  202  can be the first layer  202  illustrated in  FIG.  4    or  FIG.  6   . The second layer  204  can include conductive traces associated with the transmit aerial and/or the receive aerial. For instance, the second layer  204  can be the second layer  204  illustrated in  FIG.  5    or  FIG.  7   . 
     The inductive position sensor can include a first electrostatic shield layer  210  and a second electrostatic shield layer  212 . The first electrostatic shield layer  210  can be grounded. The second electrostatic shield layer  212  can be grounded. The first electrostatic shield layer  210  and the second electrostatic shield layer  212  can be arranged in the multilayer structure such that the layer(s) associated with the transmit aerial and the receive aerial (e.g., layers  202  and  204 ) are located between the first electrostatic shield layer  210  and the second electrostatic shield layer  212 . 
       FIG.  8    depicts an example embodiment having two electrostatic shield layers for purposes of illustration and discussion. Those of ordinary skill in the art, using the disclosures provided herein, will understand that any number of electrostatic shield layers can be used without deviating from the scope of the present disclosure. For instance, the inductive position sensor can have a single electrostatic shield layer arranged next to the first layer  202  or the second layer  204 . 
     Each of the electrostatic shields  210 ,  212  can include a plurality of conductive traces that are connected to ground. The conductive traces can be arranged such that no current loops are formed on the electrostatic shield  210 ,  212 . Example configurations of electrostatic shields  210 ,  212  are set forth in  FIGS.  9 - 13   . 
     For instance,  FIG.  9    depicts an example configuration of an electrostatic shield  210 ,  212  according to example embodiments of the present disclosure. The electrostatic shield  210 ,  212  includes an arcuate first conductive trace  220  that extends from a first end to a second end in an arcuate manner. The electrostatic shield  210 ,  212  further includes a plurality of second conductive traces  222 . Each of the second conductive traces  222  extends in a direction away from the first conductive trace  220 . The second conductive traces  222  can each have the same length or different lengths. 
       FIG.  10    depicts an example configuration of an electrostatic shield  210 ,  212  according to example embodiments of the present disclosure. The electrostatic shield  210 ,  212  includes a linear first conductive trace  230  that extends from a first end to a second end in a linear manner. The electrostatic shield  210 ,  212  further includes a plurality of second conductive traces  232 . Each of the second conductive traces  232  extends in a direction away from the first conductive trace  230 . The second conductive traces  232  can each have the same length or different lengths. 
       FIG.  11    depicts an example configuration of an electrostatic shield  210 ,  212  according to example embodiments of the present disclosure. The electrostatic shield  210 ,  212  includes a linear first conductive trace  240  that extends from a first end to a second end in a linear manner. The electrostatic shield  210 ,  212  further includes a plurality of second conductive traces  242 . Each of the second conductive traces  242  extends in a first direction away from the first conductive trace  240 . The second conductive traces  242  can each have the same length or different lengths. The electrostatic shield  210 ,  212  can further include a plurality of third conductive traces  244 . Each of the third conductive traces  244  extends in a second direction away from the first conductive trace  240 . The second direction can be different than the first direction. The third conductive traces  244  can each have the same length or different lengths. 
       FIG.  12    depicts an example configuration of an electrostatic shield  210 ,  212  according to example embodiments of the present disclosure. The electrostatic shield  210 ,  212  includes a first conductive trace  250  that extends from a first end to a second end in an arcuate manner. The electrostatic shield  210 ,  212  further includes a plurality of second conductive traces  252 . Each of the second conductive traces  252  extends in a direction away from the first conductive trace  250 . The second conductive traces  252  can each have a length such that the ends of the second conductive traces  252  form an arcuate shape. 
       FIG.  13    depicts an example configuration of an electrostatic shield  210 ,  212  according to example embodiments of the present disclosure. The electrostatic shield  210 ,  212  includes a linear first conductive trace  260  that extends from a first end to a second end in a linear manner. The electrostatic shield  210 ,  212  further includes a plurality of second conductive traces  262 . Each of the second conductive traces  262  extends in a first direction away from the first conductive trace  260 . The second conductive traces  262  can each have a length such that the ends of the second conductive traces  262  form an arcuate shape. The electrostatic shield  210 ,  212  can further include a plurality of third conductive traces  264 . Each of the third conductive traces  264  extends in a second direction away from the first conductive trace  260 . The second direction can be different than the first direction. The third conductive traces  264  can each have a length such that the ends of the second conductive traces  264  form an arcuate shape. 
     The configurations of the electrostatic shields  210 ,  212  depicted in  FIGS.  9 - 13    are provided for example purposes. Other configurations of electrostatic shields can be used without deviating from the scope of the present disclosure. 
     In some embodiments, the electrostatic shield(s) can be formed on a same layer as one or more conductive traces associated with the transmit aerial and/or receive aerial. For example,  FIG.  14    depicts an electrostatic shield  300  that is formed in the same layer as conductive traces associated with a sine winding and cosine winding of an inductive position sensor. In particular, the electrostatic shield  300  is formed in the second layer  204  associated with the inductive position sensor  100  of  FIG.  3   . The second layer  204  without the electrostatic shield  300  is shown in  FIG.  7   . 
     The electrostatic shield  300  of  FIG.  14    is grounded and includes a plurality of conductive traces that are arranged so that no current loops are formed in the electrostatic shield  300 . The electrostatic shield  300  includes a first conductive trace  302  that at least partially surrounds the conductive traces associated with the sine winding  103  and the cosine winding  105 . A plurality of second conductive traces  304  extend from the first conductive trace  302 . The plurality of second conductive traces  304  can extend from the first conductive trace  302  such that each second conductive trace is disposed between a trace associated with the sine winding  103  and a trace associated with the cosine winding  105 . A plurality of third conductive traces  306  can extend from the second conductive traces  304 . The plurality of third conductive traces  306  can extend in any direction and can have the same or different lengths. The third conductive traces  306  can also be arranged so as not to overlap or intersect with any traces associated with the sine winding  103  or the cosine winding  105 . 
       FIG.  15    depicts an electrostatic shield  300  that is formed in the same layer as conductive traces associated with a sine winding and cosine winding of an inductive position sensor. In particular, the electrostatic shield  300  is formed in the second layer  204  associated with the inductive position sensor  100  of  FIG.  2   . The second layer  204  without the electrostatic shield  300  is shown in  FIG.  5   . 
     The electrostatic shield  300  of  FIG.  15    is grounded and includes a plurality of conductive traces that are arranged so that no current loops are formed in the electrostatic shield  300 . The electrostatic shield  300  includes a first conductive trace  312  that extends in an arcuate manner from a first end and a second end. A plurality of second conductive traces  314  extend from the first conductive trace  312 . The plurality of second conductive traces  314  can extend from the first conductive trace  312  such that the second conductive traces  314  at least partially surround conductive traces associated with the sine winding  103  and the cosine winding  105 . 
       FIG.  16    depicts a schematic of an example transmit aerial according to example embodiments of the present disclosure. The transmit aerial  102  configuration is similar to that of  FIG.  1   , except that a point  102 . 5  of the transmit aerial winding is connected to ground, for instance via a capacitor  114 . This can reduce electrostatic fields coupling through the coils, reducing low frequency electrostatic interference. This same principal can be applied to the receive aerial windings, such as the sine winding  103  and/or the cosine winding  105 . 
       FIG.  17    depicts another example of a transmit aerial  102 , sine winding  103  and cosine winding  105  of an inductive position sensor  100  according to example embodiments of the present disclosure. In some implementations, the electrostatic shield  400  discussed above with reference to  FIG.  14    can be integrally formed with the transmit aerial  102 . For instance, the transmit aerial  102  can include a first conductive trace  402  that at least partially surrounds the conductive traces associated with the sine winding  103  and the cosine winding  105 . The transmit aerial  102  can include a plurality of second conductive traces  404 . As shown, each of the plurality of second conductive traces  404  can extend from the first conductive trace  402  such that each of the plurality of second conductive traces  404  is disposed between a trace associated with the sine winding  103  and a trace associated with the cosine winding  105 . The transmit aerial  102  can include a plurality of third conductive traces  406  that extend from each of the second plurality of second conductive traces  404 . The plurality of third conductive traces  406  can extend in any direction and can have the same or different lengths. The plurality of third conductive traces  406  can also be arranged so as to not overlap or intersect with any traces associated with the sine winding  103  or the cosine winding  105 . 
       FIG.  18    depicts a schematic of an example transmit aerial  102  according to example embodiments of the present disclosure. The transmit aerial  102  configuration of  FIG.  18    is similar to the transmit aerial configuration of  FIG.  16   . However, the transmit aerial  102  configuration of  FIG.  18    does not include the capacitor  114  ( FIG.  16   ) coupled between ground and the point  102 . 5  ( FIG.  16   ) of the transmit aerial  102 . 
     While the present subject matter has been described in detail with respect to specific example embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing may readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, the scope of the present disclosure is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations and/or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art.