fig1 shows a first embodiment of an external coil assembly 10 inductively coupled with an implant coil 20 . the external coil assembly 10 comprises inductors l 1 , l 3 and l 4 . the implant coil 20 comprises inductor l 2 . throughout the present specification , the various inductors will be described as coils , for exemplary purposes . with reference to the external coil assembly 10 , coil l 1 represents a transmitting coil , and coils l 3 , l 4 represent receiving coils . transmitting coil l 1 allows a forward telemetry ( ft ) signal to be sent to implant coil 20 or l 2 . receiving coils l 3 and l 4 allow a back telemetry ( bt ) signal to be received from the implant coil 20 or l 2 . the concept of ft signals and bt signals is known to the person skilled in the art . see , for example , u . s . pub . app . no . 2005 / 0288734 ( visual prosthesis with operational data telemetry ), incorporated herein by reference in its entirety . in accordance with this embodiment , transmitting coil l 1 is located between receiving coils l 3 and l 4 . transmitting coil l 1 is separated from receiving coils l 3 , l 4 by means of a dielectric , e . g . air . a possible distance between coil l 1 and coil l 4 or coil l 1 and coil l 3 is about 2 mm . in the embodiment of fig1 , coil l 3 is identical to coil l 4 . both coils l 3 and l 4 are placed at a same distance from l 1 . however , embodiments where l 3 is different from l 4 can also be envisaged . fig2 shows an electrical diagram of the first embodiment of fig1 . an ft input signal amplified through power amplifier pa is fed to transmitting coil l 1 and inductively received by implant coil l 2 . implant coil l 2 is part of an implant 30 , further comprising a transceiver or stimulator 40 . implant 30 will not be described herewith in detail . a possible example of implant 30 can be found in u . s . pub . app . no . 2005 / 0222624 ( retinal prosthesis with side mounted inductive coil ), incorporated herein by reference in its entirety . incorporated herein by reference in its entirety . among other functions , transceiver 40 sends a back telemetry ( bt ) signal to coil l 2 through driver 60 . the bt signal sent through coil l 2 is inductively received by receiver coils l 3 and l 4 and sent to a receiver circuit 70 . magnetic coupling coefficients between the various pairs of coils are formed . the coupling coefficient measures the mutual inductance between two inductors . the coupling coefficient between coils l 1 and l 2 is defined as where m 12 is the mutual inductance between coils l 1 and l 2 . for two spiral type coils , the value of their coupling coefficient is determined by their separation distance , coil dimensions , and their alignment . in particular , the coupling coefficient increases as the two coils move closer , and decreases as they move farther . on the external coil assembly side , coupling coefficient k 13 defines the magnetic coupling between l 1 and l 3 . similarly , coupling coefficient k 14 defines the coupling between l 1 and l 4 . when the external coil assembly 10 interacts with the implant coil 20 , further coupling coefficients k 12 , k 23 and k 24 are defined , the meaning of which is identical to the one discussed above . in accordance with the present disclosure k 13 is made to be substantially the same as k 14 , i . e . k 13 ≈ k 14 . in the present embodiment , coils l 3 and l 4 are arranged in a differential configuration in the circuit . in particular , with reference to fig2 , the lower portion of l 3 is in - phase with the upper portion of l 4 , as shown by the position of the dots . therefore , if the coils l 3 and l 4 are arranged so that they receive equal electromagnetic field strength from the l 1 ), the net field strength l 3 and l 4 receive from l 1 is zero , thus obtaining the desired condition of rejecting the interferences from l 1 . in practical realization , the exact fulfillment of the k 13 = k 14 requirement may not be achievable and what is relevant is that k 13 is substantially equal to k 14 , i . e . k 13 ≈ k 14 . with “ substantially equal ” a condition is intended where the direct influence of l 1 on l 3 and l 4 when the bt signal is received from l 2 is substantially reduced . on the other hand , the coupling relation of the bt coils to the implant coil l 2 is different as to l 1 . by positioning l 3 closer to l 2 than l 4 , as shown in fig1 , k 23 is made greater than k 24 , i . e . k 23 & gt ; k 24 . as a result , the front coil l 3 will receive a stronger field strength from implant coil l 2 than the back coil l 4 . the net output produced by the bt coils l 3 and l 4 from the implant coil is the difference between the signals received by l 3 and l 4 individually . the signals transmitted back from the implant 30 will include the bt signals and reflected carrier signals . however , the reflected carrier signals coupled from l 2 are much weaker in strength than the direct transmitting carrier signals coupled from l 1 . therefore , by rejecting the direct transmitting carrier signals from l 1 , the arrangement shown in fig1 and 2 provides the bt signals with a better signal - to - carrier noise ratio than using a single back telemetry receiving coil . the receiver circuit 70 will then condition and decode the bt signal . fig3 shows a further embodiment of the present disclosure , wherein each one of inductors l 3 , l 4 can comprise : a coil layer 100 ( or 110 ), an insulator ( or substrate ) layer 120 ( or 130 ), and an electrical shield layer 140 ( or 150 ). this embodiment can be readily realized with a double - layered printed circuit board ( pcb ) in which the coil and shield are made of copper traces and the insulation is the pcb substrate . as usual , l 4 and l 3 are separated from l 1 by way of an insulator 160 , e . g . air . the presence of the metallic shield layer shunts the stray capacitors around transmitting coil l 1 so that the tuning condition is not affected by the variation of the coil placement relative to the body . it also prevents the displacement current produced by the transmitting coil l 1 from flowing through the human body . incidentally , also the implant coil l 2 can be shielded for the same purpose . in order to reduce the loss of the transmitting power from l 1 caused by eddy currents and thermal effect , thin traces or wires with good conductivity such as copper can be used for the receiving coils l 3 and l 4 and also the electrical shielding . in practical applications , the physical specifications of the coils , such as the coil dimensions , separations between the coils in the assembly , and coil configurations ( turns , pitches , wire diameters etc .) can be optimized for the requirements of power and range of movement between coil l 1 and the implant coil l 2 . fig4 shows a top sectional view of an exemplary configuration of transmitting inductor l 1 , comprising a coil 170 and insulator 180 between the windings of the coil . coil 170 can either be comprised of single layer or multi - layer turns of conductors , such as copper . for applications demanding high power , multi - stranded litz wires can be used for the conductors in coil 170 in order to reduce power loss due to skin effect . fig5 a shows a top sectional view of an exemplary configuration of inductor l 3 or l 4 , comprising a coil 190 , a substrate 200 , and a metallic shield 210 ( shown in dotted lines ). fig5 b shows a partial view of the coil 190 and the substrate 200 . fig5 c shows a partial view of the metallic shield 210 . the conductors ( or traces ) in 210 are arranged in such a way that they do not form any closed circles ( circuits ). the electrical shielding can be connected to a low impedance point and stable electrical potential of the external assembly ( e . g . the rf power ground ) so that the stray capacitances are stabilized and the coupling or displacement path between the coil and the body is shunted . in order to minimize the shielding effects to the magnetic field by the metal materials , low resistivity materials can be used , so that any eddy current formed in the metal consumes a small amount of power . a mesh type structure with thin wires can also be used , without forming any closed circuit in the shield layer that acts like a coil to loop electrical currents . this shield pattern is schematically indicated by reference numeral 210 in fig5 a and 5c . the spaces between the traces shown in fig5 c can be tuned close to the gap distance between the l 1 coil surface and the shield layer for a balance between good shielding performance and low degradation to the magnetic field . fig6 a - 6c show a further embodiment of the present disclosure , where the external coil assembly 10 is provided with one transmitting inductor l 1 and one receiving inductor l 3 , as shown in fig6 a . the electrical behavior of the embodiment of fig6 a can be understood with reference to fig6 c , which is conceptually similar to fig2 . in the embodiment at issue , a single coupling coefficient is formed on the external coil assembly side , i . e . k 13 , which defines the coupling between l 1 and l 3 . when the external coil assembly 10 interacts with implant coil l 2 , further coupling coefficients k 12 and k 23 are defined . in accordance with this additional embodiment , k 13 is made as proximate as possible to zero . this can be obtained , for example , by displacing l 3 with respect to l 1 , when seen from the top , as shown in fig6 b . in particular , l 1 , when seen from the top or from the bottom , defines a bound internal ( substantially circular in the embodiment of fig6 b ) area and an unbound external area . similarly , l 3 also defines a bound internal area . the applicant has noted that k 13 can be reduced to zero by displacing l 3 with respect to l 1 so that the internal area defined by l 3 is spatially associated both with the internal area defined by l 1 and the external area defined by l 1 . in particular , k 13 can be reduced to zero when the magnetic flux b going through dotted area 250 has an intensity which is equal to the intensity of the magnetic flux b going through x &# 39 ; d area 260 . fig7 a - 7c show another embodiment of the present disclosure , where the external coil assembly 10 is provided with receiving inductors l 3 , l 4 arranged on the same side of the transmitting inductor l 1 . the inductors l 3 , l 4 can be seen as two distinct inductors or as a single inductor comprising portions l 3 , l 4 . the electrical behavior of the embodiment of fig7 a can be understood with reference to fig7 c , which is conceptually similar to fig4 and 6c . in the embodiment at issue , coupling coefficients k 13 and k 14 are formed on the external coil assembly side and coupling coefficients k 12 , k 23 and k 24 are formed when the external coil assembly 10 interacts with implant coil l 2 . given that both inductors l 3 and l 4 are located on the same side of inductor l 1 , the in - phase arrangement of l 3 and l 4 is different from that shown in fig2 , as shown by the position of the dots in fig7 c . therefore , in this case , minimization of the combined coupling coefficient inside arrangement 10 is obtained by designing l 3 and l 4 so that k 13 =− k 14 . this can be obtained , for example , by displacing l 3 and l 4 with respect to l 1 , when seen from the top , as shown in fig7 b . each of l 3 and l 4 , when seen from the top or the bottom , defines an internal area . the applicant has noted that k 13 can be made equal and opposite to k 14 by displacing l 3 , l 4 with respect to l 1 so that the internal areas defined by each of l 3 and l 4 are spatially associated both with the internal area and the external area defined by l 1 . in particular , k 13 + k 14 can be made equal to zero when the intensity of the magnetic flux b going through dotted areas 270 , 280 is equal to the intensity of the magnetic flux b going through x &# 39 ; d areas 290 and 300 . in other words , the magnetic field created by l 1 is distributed in such a way that a ) the direction of the field enclosed by the coil turns is opposite to the direction of the field surrounding them and b ) the intensity of the field enclosed by the coil turns is equal to the intensity of the field surrounding the coil turns . a qualitative explanation of this concepts is shown in fig8 a and 8b , where fig8 a shows a cross sectional view of the magnetic field distribution in the case at issue , and fig8 b shows a top view thereof . (+) field lines are outward field lines , while (−) field lines are inward field lines . the net magnetic fluxes passing through the receiving coils l 3 , l 4 are minimized to zero , at which condition the effective coupling between the external transmitting coil l 1 and the external receiving coils l 3 , l 4 is negligent . in this way , the effect of the power carrier during reception is minimized while , at the same time , the forward coupling between the implant coil l 2 and the external receiving coils l 3 , l 4 still follows a size and distance relationship . to maximize the receiving sensitivity , the receiving coil or coils should be tuned to the carrier frequency of the bt signal . the positioning of the receiving coil relative to the transmitting coil can be tuned by driving the transmitting coil with the transmitting coil with the power carrier frequency and monitoring the receiving signal strength of the same frequency at the receiving coil in the same time . the optimal position is where the signal strength is the lowest . in case of an inductively powered implant , the power carrier component received by the external receiver coil is composed of two parts : a part originating from the transmitting coil l 1 and a part reflected from the implant coil l 2 with a phase shift . therefore , the position of the receiver coil l 3 or l 3 , l 4 should be tuned in the presence of the implant . the embodiment shown in fig7 a - 7c is a differential embodiment . similar embodiments are shown in fig9 - 11 . the person skilled in the art will understand that those embodiments are for exemplary purposes only and that similar or different embodiments can be envisaged . in the embodiments of fig9 - 11 , the windings of the wires of the receiving coil are not shown , for the sake of clarity . in the embodiment of fig9 , the receiving coil l 3 is comprised of two substantially circular portions connected by an elongated portion . in fig1 , the receiving coil l 3 is comprised of two substantially mushroom - shaped portions connected by an elongated portion . in fig1 , the receiving coil l 3 is comprised of four substantially while several illustrative embodiments of the invention have been shown and described in the above description , numerous variations and alternative embodiments will occur to those skilled in the art . such variations and alternative embodiments are contemplated , and can be made without departing from the scope of the invention as defined in the appended claims .
methods and devices for inductively coupled implants on the human or animal body are disclosed . an external coil assembly to be used with the implant has a transmitting coil and one or more receiving coils . the number of the receiving coils , their distance from the transmitting coil and their shape is chosen to reduce the influence of a noise signal received by the external coil assembly .