Patent Publication Number: US-2012044034-A1

Title: Symmetrical inductor

Description:
This application claims the priority under 35 U.S.C. §119 of European patent application no. 10173438.2, filed on Aug. 19, 2010, the contents of which are incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a symmetrical inductor. 
     It is well known to use inductors to implement voltage controlled oscillators in integrated circuits (ICs) comprising transceivers. The inductance required for such applications is typically a few nH, although inductor designs should be tailorable to the specific application. It desirable that the quality factor provided by inductors used for these applications should be as high as possible. It is also desirable that the inductor used should produce a low net magnetic field, so as to minimise magnetic coupling with neighbouring components (including other inductors) in the IC. Examples of inductors which seek to produce a low net magnetic field are set out in WO1998005048, WO2004012213, WO2005096328, and WO2006105184. 
       FIGS. 1 and 2  show examples of inductors  2  which are 8-shaped, with the aim of minimising the net magnetic field that is produced. The inductor  2  of  FIG. 1  is a single turn inductor, while the inductor  2  of  FIG. 2  is a two-turn inductor. Each example inductor  2  has a pair of terminals  16 ,  18 , and a conductive track which extends between the terminals  16 ,  18  to define the inductor turn(s)  8 ,  28 . The inductor turn(s)  8 ,  28  define first and second loop portions  4 ,  6 . 
     At the centre of the 8-shaped inductors  2  there is a folding point  12 , at which the conductive track of the inductor turn crosses over itself so that the first loop portion  4  is folded with respect to the second loop portion  6 . In this way, it is the direction of the field passing through the first loop portion is opposite to the direction of the field passing through the second loop portion  6 . These fields have a tendency to appear cancel to zero in the far field. Moreover, the magnitude of the field passing through each loop portion is less than the magnitude required in the case of, for example, a circular inductor not having a folding point  12 . Both of these factors lessen the net magnetic field produced by the inductor. 
     In the case of the two-turn inductor  2  shown in  FIG. 2 , further crossing points can be provided such as the crossing point  32  at which the conductive track crosses over itself, to accommodate the additional inductor turn. The crossing points  32  can be provided with insulation  14 , to electrically isolate the conductive track at the point it crosses over itself 
     One limitation on the performance of an inductor arises from a phenomenon known as the skin effect, which occurs in all conductors carrying high frequency currents. The skin effect occurs due to the fact that the surface of the conductor generally has a lower inductance than the core. Under high frequency operation, the inductive part of impedance of the conductor dominates, and because of this, the current in the conductor chooses the path of minimum inductance. As a result, with increasing frequency, the conductive area decreases and moves further toward the surface of the conductor. This in turn results in an increase of the resistance of the conductor. In the case of an inductor, this increased resistance gives rise to a lowering in quality factor. For example, in the case of an 8-shaped inductor, it is estimated that the skin effect can lead to a reduction in quality factor of 25-30%. 
     The example inductors shown in  FIGS. 1 and 2  are substantially symmetrical. An example of a non-symmetrical spiral inductor is described in WO2003015110. 
     SUMMARY OF THE INVENTION 
     Aspects of the invention are set out in the accompanying independent and dependent claims. Combinations of features from the dependent claims may be combined with features of the independent claims as appropriate and not merely as explicitly set out in the claims. 
     According to an aspect of the invention, there is provided a symmetrical inductor comprising: 
     at least one inductor turn, each inductor turn comprising a plurality of separate conductive paths having substantially equal inductance; and 
     a plurality of crossing points at which some of the conductive paths within a given inductor turn cross over each other to change the order in which they appear within said inductor turn. 
     Compared to known inductor designs, the turns of an inductor according to this invention are divided into a plurality of separate conductive paths. Also, by providing a series of crossing points in the inductor, the layout of the conductive paths can be tailored to ensure that the inductance of each separate conductive path in a given inductor turn is substantially equal. This in turn ensures that current flows in all of the conductive paths, without favouring those conductive paths having lower inductance than the others. 
     The effect of providing multiple conductive paths having substantially equal inductance in each inductor turn is to mitigate against the skin effect described above. This is because high frequency currents in the inductor turn(s) are provided with a larger area effectively used for current flow, owing to the fact that the combined area of the separate current paths is greater than that of inductors of the kind described herein in relation to  FIGS. 1 and 2 . The decreased AC resistance at high frequencies associated with this larger effective area for current flow more than compensates for the fact that the less conductive material is used in the inductor turns (compared to inductors of the kind shown in  FIGS. 1 and 2 ) and leads to an increase in the quality factor of the inductor. 
     In one embodiment, the separate conductive paths can terminate at a common terminal or terminals. This can facilitate a construction in which the separate conductive paths are equivalent, having the required substantially equal inductances. In some examples, the terminal or terminals can be positioned to allow connection to a centre tap. 
     In one embodiment, the conductive paths have equal inductance to within a tolerance of 0.1%. More particularly, the conductive paths can have equal inductance to within a tolerance of 0.001%. 
     The conductive paths in each inductor turn can run substantially parallel, to avoid any local variations in current or field density. 
     The inductor can be an 8-shaped inductor. The inductor can include a plurality of inductor turns. 
     In some examples, the inductor can be provided with a centre tap. 
     Example inductors having large numbers of separate conductive paths require a layout having more crossing points. This is to allow the paths to retain substantially equal inductances, thereby to maintain the overall symmetry of the inductor. However, crossing points introduce additional capacitance between the conductive paths. Accordingly, for optimal performance, there is a balance to be held between increasing the number conductive paths to counteract the skin effect, and avoiding a proliferation of crossing points. In accordance with an embodiment of the invention, it has been found the optimal number of conductive paths in each inductor turn is four. 
     According to another aspect of the invention, there is provided a voltage controlled oscillator (VCO) comprising an inductor of the kind described above. 
     According to a further aspect of the invention, there is provided a transceiver comprising an inductor of the kind described above. 
     According to another aspect of the invention, there is provided an integrated circuit comprising an inductor of the kind described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will be described hereinafter, by way of example only, with reference to the accompanying drawings in which like reference signs relate to like elements and in which: 
         FIG. 1  shows a known kind of single turn inductor; 
         FIG. 2  shows a known kind of two-turn inductor; 
         FIG. 3  shows a single turn inductor according to an embodiment of the invention; 
         FIG. 4  shows a two-turn inductor according to an embodiment of the invention; 
         FIG. 5  shows a single turn inductor according to an embodiment of the invention; and 
         FIG. 6  shows a two-turn inductor according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are described in the following with reference to the accompanying drawings. 
     A first embodiment of the invention is shown in  FIG. 3 . In this embodiment, there is provided a symmetrical, 8-shaped inductor  2  having a single turn  80 . The layout of the inductor  2  in this embodiment may be thus compared with the layout of the know conductor shown in  FIG. 1 . The inductor  2  includes the pair of terminals  16 ,  18  at which the inductor turn  80  terminates at either end. In common with the known inductor described above in relation to  FIG. 1 , the inductor  2  in this embodiment includes a folding point  12 , at which the inductor turn  80  crosses over itself, so that a first loop portion  4  of the inductor  2  is folded with respect to a second loop portion  6 . These two loop portions together form the “8” shape of the inductor  2 . 
     As described herein, in accordance with the invention, an inductor is provided in which each inductor turn includes a plurality of separate conductive paths having substantially equal inductance. This mitigates against the skin effect described above, as a high frequency currents in the inductor turn(s) are provided with a larger area for current flow. The decreased AC resistance associated with this large area for current flow leads to an increase in the quality factor of the inductor. 
     In the example shown in  FIG. 3 , the inductor turn  80  is divided into four separate conductive paths  81 ,  82 ,  83 ,  84 . As shown in  FIG. 3 , each of the conductive paths  81 ,  82 ,  83 ,  84  terminates at the terminals  16  and  18 . 
     Also as shown in  FIG. 3 , in this example, the conductive paths  81 ,  82 ,  83 ,  84  are substantially parallel throughout the inductor  2 , thereby to ensure homogeneity of the currents and fields associated with each conductive path. By providing conductive paths that are substantially parallel, that have appropriately placed crossing points, and that terminate at the common terminals such as terminals  16 ,  18  shown in  FIG. 3 , it becomes possible to manufacture a symmetrical inductor having separate conductive paths with substantially equal inductance to within a desired tolerance. The tolerance level to within which the conductive paths are effectively equivalent may depend upon the intended application for the inductor and/or the manufacturing process that is used. In one example, the conductive paths in an inductor of the kind shown in  FIGS. 3-6  of this application may have equal inductances to within a tolerance of 0.1%. In another example, in which higher tolerance levels are required for a particular application, the conductive paths may have equal inductances to within a tolerance of 0.001%. 
     As described herein, an effect of dividing the inductor turn  80  into a plurality of conductive tracks  81 ,  82 ,  83 ,  84  is to increase the surface area available within the inductor turn  80  for current flow, thereby to counteract the skin effect. 
     Although splitting the inductor turn  80  into a plurality of separate paths  81 , 82 , 83 , 84  has the benefit of counteracting the skin effect, it does complicate the layout of the inductor  2 . In particular, as the inductor  2  is a symmetrical inductor, the symmetry of the inductor turn  80  and the conductive paths  81 ,  82 ,  83 ,  84  within the inductor turn  80  should be ensured. In particular, the layout of the inductor  2  must be designed such that the conductive paths  81 ,  82 ,  83 ,  84  have substantially equal inductance, whereby current can flow substantially equally within those paths. If the inductances of the conductive paths within a given inductor turn were not equal to within a given tolerance, then current would tend to flow only in those paths having lower inductance, effectively limiting the amount of current which can flow in the inductor turn. 
     In order to produce such a layout, in accordance with an embodiment of the invention, the inductor  2  and in particular the conductive paths  81 ,  82 ,  83 ,  84  of the inductor  2  are provided with one or more crossing points  32 . Crossings points are points at which a conductive path in the inductor turn  80  crosses over another conductive path in the inductor turn  80 . Additionally, at each crossing point  32 , the ordering of the plurality of the conductive paths changes to some degree. In particular, at each crossing point, the two conductive paths associated with the crossing point may effectively swap places within the inductor turn  80 . It is noted that folding points or crossing points of the kind described in relation to  FIGS. 1 and 2  are merely points at which the (entire) inductor turn crosses over itself—these points are not therefore crossing points at which conductive paths within an inductor turn cross over each other. 
     As shown in  FIG. 3 , since each crossing point  32  comprises a location at which a first conductive path crosses over a second conductive path, insulation  14  may be provided at the crossing point  32  to prevent conductive contact being made between the two conductive paths. It will be appreciated that because the conductive paths associated with each crossing point  32  come into close proximity with each other at the location of the crossing point  32 , each crossing point  32  has associated therewith an increase in capacitance between those conductive paths. The increase in capacitance from the crossing points  32  in the inductor  2  sum up to increase the overall capacitance of the inductor  2 , which may have a detrimental effect on the Q-factor of the inductor  2 . 
     Accordingly, although it is possible to provide crossing points  32  of the kind described herein, it is not desirable to provide too many crossing points  32 , or to provided unnecessary crossing points. Optimally, a sufficient number of crossing points should be provided to allow the resistance and inductance of the conductive paths to be equal to the extent that the inductor retains it symmetry to within a given tolerance, without overly increasing mutual capacitance between the conductive paths in the inductor turn  80 . 
     As mentioned above, the purpose of the crossing points  32  is to allow the conductive paths in the inductor turn  80  to be laid out in such a way to ensure symmetry in the inductor  2 . The number of crossing points which can be used in this way depends upon the number of inductor turns  80  provided with the inductor and also on the number of the conductive paths provided within each inductor turn. 
     In the example of  FIG. 3 , two crossing points  32  are provided (in addition to, of course, the folding point  12 ) to ensure symmetry. In a single turn 8-shaped inductor having four conductive paths  81 ,  82 ,  83 ,  84 , this is the minimum number of crossing points  32  that can be used in this way to ensure symmetry. 
     As described above, it has been found that the optimal number of conductive paths in each inductor turn  80  is four. For this reason, each of the examples shown in  FIGS. 3 ,  4 ,  5  and  6  of this application include inductor turns having four conductive paths. Inductors having different numbers of conductive paths per inductor turn are nevertheless envisaged. 
     A second example inductor  2  in accordance with an embodiment of the invention is shown in  FIG. 4 . The inductor  2  shown in  FIG. 4  is an 8-shaped inductor having two inductor turns  80 ,  280 . As described above in relation to  FIG. 3 , the inductor  2  in  FIG. 4  includes terminals  16 ,  18 , at which the inductor turn  80  and the conductive paths  81 ,  82 ,  83 ,  84  of the inductor turn  80  terminates. In order to incorporate the second inductor turn  280  into the inductor  2 , the layout includes a first folding point  12  (which is similar to the folding point described above in relation to  FIG. 3 ), and a second folding point  34 . In addition to the folding points  12  and  34 , as shown in  FIG. 4 , a number of crossing points  32  are provided. In particular, two crossing points are provided for each the inductor turn  80 ,  280  in the inductor  2 , (whereby a total of four crossing points  32  are provided). 
     As with the example described above in relation to  FIG. 3 , the purpose of the four crossing points  32  in the inductor  2  shown in  FIG. 4  is to allow the layout of the conductive paths in the inductor turns of the inductor  2  to be determined in such a manner that the resistance and inductance of each conductive path is substantially equal to the other conductive paths in that inductor turn. 
     As shown in  FIG. 4 , the inductor turn  80  in the inductor  2  includes four conductive paths  81 ,  82 ,  83 ,  84 . The second inductor turn  280  in  FIG. 4  includes four further conductive paths  281 ,  282 ,  283 ,  284 . In total therefore, the inductor  2  in  FIG. 4  includes eight conductive paths, four in each inductor turn. As shown also in  FIG. 4 , insulating material may be provided at each crossing point  32 , at the folding points  12  and  34  and also in the vicinity of the terminals  16  and  18 , to ensure that there is no short circuiting between the conductive paths where they cross over each other or come into close proximity with other conductive portions of the inductor  2 . 
     Two further examples of inductors in accordance with embodiments of this invention are shown in  FIGS. 5 and 6 . The example in  FIG. 5  is a single turn 8-shaped inductor  2 . The example in  FIG. 6  is an 8-shaped inductor  2  having two inductor turns. Both examples in  FIGS. 5 and 6  include a centre tap  40 . Moreover, both inductors shown in  FIGS. 5 and 6  include modifications to the layout of the inductor turns and conductive paths therein to accommodate the incorporation of the centre tap  40 . These modifications may be seen by comparing the layout of the conductor in  FIG. 5  with the layout of the conductor in  FIG. 3  and also by comparing the layout of the inductor in  FIG. 6  with the inductor in  FIG. 4 . 
     A first modification in both of the inductors shown in  FIGS. 5 and 6  is that the layouts of the conductive paths in those inductors include a greater number of crossing points  32 . The increase in the number of crossing points  32  results from the increase in complexity of the layout of the inductor implicated by the inclusion of the centre tap  40 . As described above, a proliferation of crossing points  32  in the inductor  2  may result in an increase in the overall mutual capacitance of the conductive paths in the inductor  2 , leading to a reduced Q-factor for the inductor  2 . However, the examples of  FIGS. 5 and 6  at least show that it is possible to incorporate centre taps into single turn and multiple turn 8-shaped inductors having a plurality of conductive paths in each inductor turn, in accordance with the embodiments of this invention. 
     A further modification, which is made in the case of the inductor of  FIG. 5 , is the provision of a further terminal  50 . As shown in  FIG. 5 , the terminal  50  is a point at which each of the conductive paths  81 ,  82 ,  83 ,  84  in the inductor turn  80  terminate. Moreover, the terminal  50  is connected to the centre tap  40 . Accordingly therefore, the terminal  50  allows each of the conductive paths  81 ,  82 ,  83 ,  84  to be connected to the centre tap  40  in such a way that their path lengths remain substantially equal (which in turns helps to ensure the symmetrical nature of the inductor  2 ). 
     Inductors of the kind described herein may be incorporated into devices such as voltage controlled oscillators (VCOs) in transceivers. For example, the inductors described herein may be incorporated into integrated circuits associated with a transceiver or VCO. 
     Accordingly, there has been described a symmetrical inductor having at least one inductor turn. Each inductor turn has a plurality of separate conductive paths having substantially equal inductance. The inductor also comprises a plurality of crossing points. At each crossing point, some of the conductive paths within a given inductor turn cross over each other to change the order in which they appear within the inductor turn. 
     Although particular embodiments of the invention have been described, it will be appreciated that many modifications/additions and/or substitutions may be made within the scope of the claimed invention.