Abstract:
A stipline coupling includes a balun-configured stripline, and a transformer-configured stripline coupled to the balun-configured stripline. The balun-configured stripline includes a pair of conductors, one of which includes a pair of coplanar conductive paths. One of the conductive paths is grounded to provide equal magnitude opposite phase signals to the transformer-configured stripline. The transformer-configured stripline impedance matches the opposite phase signals to the balanced transmission line. The transformer-configured stripline includes two conductive trace sections, one of which includes a pair of substantially abutting non-contacting conductive trace portions. The conductive trace portions of the one trace section includes fingers extending from the respective trace portion. The fingers of one of the fingered traces are interlaced with the fingers of the other fingered traces. Links extend between the fingers of the one trace section and the trace portions of the other trace section for cross-coupling the trace portions together.

Description:
FIELD OF INVENTION  
         [0001]    The present invention relates to a transmission line coupling. In particular, the present invention relates to a stripline coupling for effecting electrical signal transmission between a balanced transmission line and un unbalanced transmission line.  
         BACKGROUND OF THE INVENTION  
         [0002]    Most communication systems include either balanced or unbalanced transmission lines. A balanced transmission line may be defined as a transmission line having a pair of conductors configured to carry electrical signals which are 180° out of phase with respect to each other. In contrast, the typical unbalanced transmission line includes only a single conductor, with signal return being provided by a ground return path.  
           [0003]    As will be apparent, unbalanced transmission lines are desirable due their intrinsically low manufacturing costs. On the other hand, balanced transmission lines are desirable for their enhanced ability to transfer power to a load, and their enhanced immunity to noise. Therefore, many communications systems includes both balanced and unbalanced transmission lines, interconnected by a suitable coupling.  
           [0004]    To facilitate an efficient transfer of signal power between a balanced transmission line and an unbalanced transmission line, baluns are often used as the coupling between the adjoining transmission lines. A balun is a form of transformer which splits the unbalanced energy from the unbalanced transmission line into two equal paths, having equal magnitude and opposite phase, for communication with the two inputs of the balanced transmission line. The balun is also advantageous in its ability to match the impedance required by the unbalanced transmission line with the impedance required by the balanced transmission line.  
           [0005]    Although many forms of baluns are presently available, a common limitation is their inability to impedance match over a wide frequency range, such as the range required by VHF and UHF broadcast power amplifiers. The most common solution to this problem has been to cascade a balun with a transmission line transformer. With this arrangement, the balun is used primarily for the separation of the unbalanced energy from the unbalanced transmission line into two equal paths, while the transmission line transformer is used for impedance matching with the balanced transmission line.  
           [0006]    Typically, the balun and the transmission line transformer are each fabricated from sections of flexible or semi-rigid coaxial cable. Although this configuration provides acceptable wideband performance, the available impedance ratio is limited by the variety of coaxial cables presently available. Also, this configuration requires a significant amount of manual labour for assembly, thereby contributing to the manufacturing cost of the balun and the transformer.  
           [0007]    Accordingly, there remains a need for a transmission line coupling for facilitating wideband electrical signal transmission between a balanced transmission line and an unbalanced transmission line in a cost effective manner.  
         SUMMARY OF THE INVENTION  
         [0008]    According to the present invention, there is provided a stripline coupling which addresses deficiencies of the prior art.  
           [0009]    The stripline coupling, according to the present invention, is provided for coupling an unbalanced transmission line to a balanced transmission line, and includes a balun-configured stripline, and a transformer-configured stripline in communication with the balun-configured stripline. The balun-configured stripline is configured for providing a pair of intermediate opposite-phase signals from an unbalanced signal received from the unbalanced transmission line. The transformer-configured stripline impedance matches the intermediate signals to the balanced transmission line.  
           [0010]    The balun-configured stripline includes a pair of conductors, one of the conductors being configured with a pair of coplanar conductive paths. One of the conductive paths is grounded for providing the opposite-phase signals as equal magnitude opposite-phase signals to the transformer-configured stripline.  
           [0011]    The transformer-configured stripline includes two spaced-apart conductive trace sections, at least one including a pair of substantially abutting non-contacting conductive trace portions. The conductive trace portions of one of the trace sections includes at least one finger each extending from the respective trace portion. The fingers of one of the fingered traces are interlaced with the fingers of the other fingered traces. A plurality of links extend between the fingers of the one trace section and the trace portions of the other trace section for cross-coupling the trace portions together.  
           [0012]    Preferably, the balun-configured stripline and the transformer-configured stripline are fabricated on a common substrate to reduce costs and complexity of manufacture. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The preferred embodiments of the present invention will now be described, by way of example only, with reference to the drawings, in which:  
         [0014]    [0014]FIG. 1 is a perspective view of a conventional stripline, depicting the three substrates and the two conductive traces disposed between the substrates;  
         [0015]    [0015]FIG. 2 is a perspective view of the stripline coupling, according to the present invention, depicting the balun-configured stripline and the 4:1 transformer-configured stripline;  
         [0016]    [0016]FIG. 3 is a magnified view of the link connections between the trace portions of the transformer-configured stripline shown in FIG. 2;  
         [0017]    [0017]FIG. 4 is a schematic diagram of the 4:1 transformer-configured stripline shown in FIGS. 2 and 3;  
         [0018]    [0018]FIG. 5 is a graph depicting the frequency response of a UHF amplifier obtained by transmitting a TV band through the stripline coupling shown in FIGS. 2 and 3;  
         [0019]    [0019]FIG. 6 is a perspective view of a 9:1 transformer-configured stripline, being a variation of the 4:1 transformer-configured stripline shown in FIG. 2;  
         [0020]    [0020]FIG. 7 is a magnified view of the link connections between the trace portions of the transformer-configured stripline shown in FIG. 6; and  
         [0021]    [0021]FIG. 8 is a schematic diagram of the 9:1 transformer-configured stripline shown in FIGS. 6 and 7. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    To aid in the understanding of the stripline coupling, according to the present invention, a conventional stripline will be described first, followed by a description of the stripline coupling. Turning initially to FIG. 1, a conventional broadside-coupled stripline transmission line  10  is shown comprising three stacked planar printed circuit boards  12 ,  14 ,  16  and two transmission lines  18 ,  20  provided between the printed circuit boards  12 ,  14 ,  16 . The transmission lines  18 ,  20  are usually photo-etched onto opposite faces of the centre printed circuit board  14 , and then the printed circuit boards  12 ,  14 ,  16  are typically secured together face-to-face with glue.  
         [0023]    The printed circuit boards  12 ,  14 ,  16  are fabricated from a material having a uniform dielectric constant. The outer surfaces of the printed circuit boards  12 ,  16  are metalized and grounded so as to emulate the characteristics of a coaxial transmission line. As will be apparent, the characteristic impedance, Zo can be adjusted by altering the dimensions of the transmission lines  18 ,  20 , and the dimensions and the dielectric constant of the printed circuit boards  12 ,  14 ,  16 .  
         [0024]    Turning now to FIG. 2, a stripline coupling, denoted generally as  100 , is shown for coupling an unbalanced transmission line to a balanced transmission line. The stripline coupling  100  comprises a balun-configured stripline  102  and a transformer-configured stripline  104 . The balun-configured stripline  102  and the transformer-configured stripline  104  are preferably fabricated together on a common substrate (circuit board  14 ), in accordance with the manufacturing techniques of the conventional broad-side coupled stripline discussed above. However, the characteristic impedance and the coupling of the balun-configured stripline  102  and the transformer-configured stripline  104  can be controlled separately by altering the line width of each transmission line.  
         [0025]    The balun-configured stripline  102  includes a signal input  106   a  for receiving an unbalanced input signal from an unbalanced transmission line, and first and second intermediate signal outputs  108   a ,  108   b  for providing two intermediate output signals to the transformer-configured stripline  104 . The balun-configured stripline  102  is implemented as a broadside-coupled stripline, comprising an upper conductor  110  for receiving the unbalanced input signal, and a lower conductor  112  parallel to and spaced from the upper conductor  110 . The lower conductor  112  is typically grounded and acts as a return current path for the unbalanced input signal.  
         [0026]    The upper conductor  110  comprises first and second coplanar conductive paths  110   a ,  110   b . The first conductive path  110   a  carries the unbalanced input signal, and the second conductive path  110   b  is connected to ground to ensure that the two intermediate signals at the intermediate signal outputs  108  have equal amplitude but opposite phase.  
         [0027]    The transformer-configured stripline  104  is coupled to the balun-configured stripline  102 , and includes two signal outputs  106   b ,  106   c  for providing impedance matched output signals to the balanced transmission line, based on the intermediate signals received from the balun-configured stripline  102 . The transformer-configured stripline  104  is implemented as a two coplanar broadside-coupled striplines, and comprises a first stripline  114   a  coupled to the first intermediate output  108   a  for providing the first output signal at the first signal output  106   b  and a second stripline  114   b  coupled to the second intermediate output  108   b  for providing a second output signal at the second signal output  106   c.    
         [0028]    The first stripline  114   a  comprise a first upper conductive trace  116   a , and a first lower conductive trace  118   a  parallel to and spaced apart from the first upper conductive trace  116   a . Preferably, the first upper conductive trace  116   a  includes a first upper major conductive trace portion  120   a , and a first upper conductive trace end portion  122   a  disposed at a right angle to the first upper major conductive trace portion  120   a . Similarly, preferably the first lower conductive trace  118   a  includes a first lower major conductive trace portion  124   a , and a first lower conductive trace end portion  126   a  disposed at a right angle to the first lower major conductive trace portion  124   a.    
         [0029]    Similarly, the second stripline  114   b  comprises a second upper conductive trace  116   b , and a second lower conductive trace  118   b  parallel to and spaced apart from the second outer conductive trace  116   b . The second upper conductive trace  116   b  is connected to the lower conductor  112  of the balun-configured stripline  102  by a plated through-hole  127  which extends transversely through the substrate (circuit board  14 ), between the second upper conductive trace  116   b  and the lower conductor  112 , but which does not contact the second lower conductive trace  118   b . Preferably, the second upper conductive trace  116   b  includes a second upper major conductive trace portion  120   b , and a second upper conductive trace end portion  122   b  disposed at a right angle to the second upper major conductive trace portion  120   b . Preferably the second lower conductive trace  118   b  includes a second lower major conductive trace portion  124   b , and a second lower conductive trace end portion  126   b  disposed at a right angle to the second lower major conductive trace portion  124   b.    
         [0030]    Preferably, the first and second upper conductive traces  116   a ,  116   b  and the first and second upper conductive trace end portions  122   a ,  122   b  are coplanar with the upper conductor  110  of the balun-configured stripline  102 , and are all fabricated on a common side of the substrate (circuit board  14 ). Similarly, preferably the first and second lower conductive traces  124   a ,  124   b  and the first and second lower conductive trace end portions  126   a ,  126   b  are coplanar with the lower conductor  112  of the balun-configured stripline  102 , and are all fabricated on the opposite side of the substrate  14 .  
         [0031]    As shown in FIG. 3, the first upper conductive trace end portion  122   a  substantially abuts with the second upper conductive trace end portion  122   b . However, the first upper conductive trace end portion  122   a  is spaced from the second upper conductive trace end portion  122   b  and, accordingly, does not contact the second upper conductive trace end portion  122   b.    
         [0032]    Similarly, the first lower conductive trace end portion  126   a  substantially abuts with the second lower conductive trace end portion  126   b . The first lower conductive trace end portion  126   a  is spaced from the second lower conductive trace end portion  126   b  and, accordingly, does not contact the second lower conductive trace end portion  126   b.    
         [0033]    The first lower conductive trace end portion  126   a  includes a plurality of coplanar first fingers  128   a  extending in parallel towards the second lower conductive trace end portion  126   b . Similarly, the second lower conductive trace end portion  126   b  includes a plurality of coplanar second fingers  128   b  extending in parallel towards the first lower conductive trace end portion  126   a . The first fingers  128   a  are interlaced with the second fingers  128   b  but do not contact the second fingers  128   b.    
         [0034]    Alternately, or in addition to the fingers  128   a ,  128   b , in one variation (not shown), the first upper conductive trace end portion  122   a  includes a plurality of coplanar first fingers  128   a ″ extending in parallel towards the second upper conductive trace end portion  122   b , and the second upper conductive trace end portion  122   b  includes a plurality of coplanar second fingers  128   b ′ extending in parallel towards the first upper conductive trace end portion  122   a . The first fingers  128   a ′ are interlaced with the second fingers  128   b ′ and do not contact the second fingers  128   b′.    
         [0035]    The stripline coupling  100  includes a plurality of first conductive links  130   a , fabricated as plated through-holes, which extend transversely through the substrate  14  between the first upper conductive trace end portion  122   a  and the second lower conductive trace end portion  126   b  for electrically coupling together the first upper conductive trace  116   a  with the second lower conductive trace  118   b . The stripline coupling  100  also includes a plurality of second conductive links  130   b , fabricated as plated through-holes, which extend transversely through the substrate  14  between the second lower conductive trace end portion  122   b  and the first upper conductive trace end portion  126   a  for electrically coupling together the second upper conductive trace  116   b  with the first lower conductive trace  118   a . Preferably, the through-holes are equidistantly spaced so that the links  130  are substantially parallel to each other. As will be appreciated, the foregoing arrangement electrically cross-couples the conductive trace portions  116 ,  118  together at the end portions  122 ,  126 .  
         [0036]    The stripline coupling  100  also includes a short-circuit link  136  connected between the first inner conductive trace end portion  126   a  and the second inner conductive trace end portion  126   b  for electrically short-circuiting the conductive trace portions  124   a ,  124   b  together at the end opposite the end portions  126   a ,  126   b . The short-circuit link  136  is coplanar with the first and second lower conductive traces  124   a ,  124   b , the first and second lower conductive trace end portions  126   a ,  126   b , and the lower conductor  112  of the balun-configured stripline  102 . As will be appreciated, the resulting transformer-configured stripline  104  mimics the operation of the 4:1 transmission line transformer shown in FIG. 4, with the balanced signal outputs  106   b ,  106   c  of the transformer  104  preferably being tapped from the first and second upper conductive trace end portions  122   a ,  122   b . However, the present invention results in a larger bandwidth and higher impedance transformer ratios than those which can be achieved with a coaxial cable-based 4:1 transmission line transformer, and without a significant increase in complexity. For convenience, the constituent elements of the 4:1 transformer shown in FIG. 4 are denoted, in brackets, with the reference numerals of the corresponding elements of the transformer-configured stripline  104 .  
         [0037]    In one implementation of the stripline coupling  100 , the printed circuit boards are fabricated from G200 with a dielectric constant of 4. The upper and lower printed circuit boards  12 ,  16  are 0.125 inches thick, and the middle printed circuit board  14  is 0.025 inches thick. The transmission lines  18 ,  20  comprising the balun-configured transformer  102  are 0.155 inches in width, while the transmission lines  18 ,  20  comprising the transformer-configured transformer  104  are 0.125 inches in width. The transmission and reflection obtained with the transmission of a UHF TV band through the stripline coupling  100  is shown in FIG. 5.  
         [0038]    A variation of the transformer-configured stripline  104  is shown in FIG. 6. The transformer-configured stripline  204 , shown in FIG. 6 is implemented as a broadside-coupled stripline, and comprises a first transmission line  216  coupled to the first intermediate output  108   a  and a second transmission line  218  coupled to the second intermediate output  108   b . As above, the first and second transmission lines  216 ,  218  are fabricated on opposite sides of a common substrate (circuit board  14 ), so that the first transmission line  216  is parallel to and spaced apart from the second transmission line  218 .  
         [0039]    The first transmission line  216  is configured as a spiral conductive trace, and comprises a first upper conductive trace portion  220 , a second upper conductive trace portion  222 , a third upper conductive trace portion  224 , a first upper short-circuit trace end portion  226 , a second upper short-circuit trace end portion  228 , and a third upper short-circuit trace end portion  230 . The first upper conductive trace portion  220  includes a first end  220   a  for receiving a first balanced input signal to the transformer  204 , and a second end  220   b  opposite the first end  220   a . Similarly, the second upper conductive trace portion  222  includes a first end  222   a  and a second end  222   b  opposite the first end  222   a , and the third upper conductive trace portion  224  includes a first end  224   a , and a second end  224   b  opposite the first end  224   a . Preferably, the first, second and third upper conductive trace portions  220 ,  222 ,  224  are coplanar and oriented parallel to each other.  
         [0040]    The first upper short-circuit trace end portion  226  includes a first end  226   a  and a second end  226   b , and the second upper short-circuit trace end portion  228  includes a first end  228   a  and a second end  228   b . The first and second upper short-circuit trace end portions  226 ,  228  are in series with each other, and are provided between the first and second upper conductive trace portions  220 ,  222 , at the second ends  220   b ,  222   b , for short circuiting the first and second upper conductive trace portions  220 ,  222  together at the second ends  220   b ,  222   b . The third upper short-circuit trace end portion  230  is provided between the second and third upper conductive trace portions  222 ,  224  at the first ends  222   a ,  224   a , for short circuiting the second and third upper conductive trace portions  222 ,  224  together at the first ends  222   a ,  224   a . The first transmission line  216  also includes an upper junction  232 , disposed at the point of common connection of the second ends  226   b ,  228   b  of the first and second upper short-circuit trace end portions  226 ,  228 , for providing the first balanced output signal of the transformer  204 .  
         [0041]    Similarly, the second transmission line  218  is configured as a spiral conductive trace, and comprises a first lower conductive trace portion  220 ′, a second lower conductive trace portion  222 ′, a third lower conductive trace portion  224 ′, a first lower short-circuit trace end portion  226 ′, a second lower short-circuit trace end portion  228 ′, and a third short-circuit trace end portion  230 ′. The first upper conductive trace portion  220 ′ includes a first end  220   a ′ and a second end  220   b ′ opposite the first end  220   a ′. Similarly, the second upper conductive trace portion  222 ′ includes a first end  222   a ′ for receiving a second balanced input signal to the transformer  204 , and a second end  222   b ′ opposite the first end  222   a ′. The third upper conductive trace portion  224 ′ includes a first end  224   a ′ and a second end  224   b ′ opposite the first end  224   a ′. Preferably, the first, second and third upper conductive trace portions  220 ′,  222 ′,  224 ′ are coplanar and oriented parallel to each other.  
         [0042]    The first lower short-circuit trace end portion  226 ′ includes a first end  226   a ′ and a second end  226   b ;, and the second lower short-circuit trace end portion  228 ′ includes a first end  228   a ′ and a second end  228   b ′. The first and second lower short-circuit trace end portions  226 ,  228  are in series with each other, and are provided between the first and second lower conductive trace portions  220 ′,  222 ′, at the second ends  220   b ′,  222   b ′, for short circuiting the first and second lower conductive trace portions  220 ′,  222 ′ together at the second ends  220   b ′,  222   b ′. The third lower short-circuit trace end portion  230 ′ is provided between the second and third lower conductive trace portions  222 ′,  224 ′ at the first ends  222   a ′,  224   a ′, for short circuiting the second and third lower conductive trace portions  222 ′,  224 ′ together at the first ends  222   a ′,  224   a ′. The second transmission line  218  also includes a lower junction  232 ′, disposed at the point of common connection of the second ends  226   b ′,  228   b ′ of the first lower short-circuit trace end portion  226 ′ and the second lower short-circuit trace end portion  228 ′, for providing the second balanced output signal of the transformer  204 .  
         [0043]    As shown in FIG. 7, the second end  224   b  of the third upper conductive trace portion  224  terminates in an upper triangular-shaped end portion  234 . The first upper short-circuit trace end portion  226  is tapered adjacent the second end  226   b , and the second upper short-circuit trace end portion  228  is similarly tapered adjacent the second end  228   b  so as to define together an upper triangular-shaped cut-out portion  236  shaped to receive the upper triangular-shaped end portion  234 . The upper triangular-shaped end portion  234  is coplanar with the first and second upper short-circuit trace end portions  226 ,  228  and is positioned in a substantially abutting manner with the upper triangular-shaped cut-out portion  236 . However, consistent with the previous embodiment, the upper triangular-shaped end portion  234  of the third upper conductive trace  224  is spaced from the first and second upper short-circuit trace end portions  226 ,  228  at the upper cut-out portion  236 . Accordingly, the third upper conductive trace portion  224  does not contact either of the first or second upper short-circuit trace end portions  226 ,  228 .  
         [0044]    Similarly, the second end  224   b ′ of the third lower conductive trace portion  224 ′ terminates in a lower triangular-shaped end portion  234 ′. The first lower short-circuit trace end portion  226 ′ is tapered adjacent the second end  226   b ′, and the second lower short-circuit trace end portion  228 ′ is similarly tapered adjacent the second end  228   b ′ so as to define together a lower triangular-shaped cut-out portion  236 ′ shaped to receive the lower triangular-shaped end portion  234 ′. The lower triangular-shaped end portion  234 ′ is coplanar with the first and second lower short-circuit trace end portions  226 ′,  228 ′ and is positioned in a substantially abutting manner with the lower triangular-shaped cut-out portion  236 ′. Again, consistent with the previous embodiment, the lower triangular-shaped end portion  234 ′ of the second end  224   b ′ of the third lower conductive trace  224 ′ is spaced from the first and second lower short-circuit trace end portions  226 ′,  228 ′ at the lower cut-out portion  236 ′. Accordingly, the third lower conductive trace portion  224 ′ does not contact either of the first or second lower short-circuit trace end portions  226 ′,  228 ′.  
         [0045]    The second end  226   b ′ of the first lower short-circuit trace end portion  226 ′ includes a plurality of coplanar first fingers  238   a  extending in parallel towards the lower triangular-shaped end portion  234 ′. The lower triangular-shaped end portion  234 ′ also includes a plurality of coplanar second fingers  238   b  extending in parallel towards the second end  226   b ′ of the first lower short-circuit trace end portion  226   b ′. The first fingers  238   a  are interlaced with the second fingers  238   b  but do not contact the second fingers  238   b.    
         [0046]    Similarly, the second end  228   b ′ of the second lower short-circuit trace end portion  228 ′ includes a plurality of coplanar third fingers  238   a ′ extending in parallel towards the lower triangular-shaped end portion  234 ′. The lower triangular-shaped end portion  234 ′ also includes a plurality of coplanar fourth fingers  238   b ′ extending in parallel towards the second end  228   b ′ of the second lower short-circuit trace end portion  228 ′. The third fingers  238   a ′ are interlaced with the fourth fingers  238   b ′ but do not contact the third fingers  238   b′.    
         [0047]    The transformer  204  includes a plurality of first transmission line links  240   a , fabricated as through-holes, extending transversely through the substrate (circuit board  14 ) between the first lower short-circuit trace end portion  226 ′ and the upper triangular-shaped end portion  234  for coupling together the first lower conductive trace portion  220 ′ with the third upper conductive trace portion  224 . The transformer  204  also includes a plurality of second transmission line links  240   b , fabricated as through-holes, extending transversely through the substrate  14  between the lower triangular-shaped end portion  234 ′ and the first upper short-circuit trace end portion  226  for coupling together the third lower conductive trace portion  224 ′ with the first upper conductive trace portion  220 .  
         [0048]    The transformer  204  also includes a plurality of third transmission line links  240   c , fabricated as through-holes, extending transversely through the substrate  14  between the second lower short-circuit trace end portion  228 ′ and the upper triangular-shaped end portion  234  for coupling together the second lower conductive trace portion  222 ′ with the third upper conductive trace portion  224 . A plurality of fourth transmission line links  240   d  is also included, fabricated as through-holes, extending transversely through the substrate  14  between the lower triangular-shaped end portion  234 ′ and the second upper short-circuit trace end portion  228  for coupling together the third lower conductive trace portion  224 ′ with the second upper conductive trace portion  222 . Preferably, the through holes are equidistantly spaced for maintaining the links  240  substantially parallel to each other.  
         [0049]    The foregoing arrangement couples the third upper conductive trace portion  224  with the first lower conductive trace portion  220 ′ and the second lower conductive trace portion  222 ′, and also couples the third lower conductive trace portion  224 ′ with the first upper conductive trace portion  220  and the second lower conductive trace portion  222 . As will be appreciated, the resulting transformer  204  mimics the operation of the 9:1 transmission line transformer shown in FIG. 8. For convenience, the constituent elements of the 9:1 transformer shown in FIG. 8 are denoted, in brackets, with the reference numerals of the corresponding elements of the transformer-configured stripline  204 .  
         [0050]    The foregoing description is intended to be illustrative of the preferred embodiments of the present invention. Those of ordinary skill many envisage certain additions, deletions and/or modifications to the describe embodiments which, although not explicitly described herein, do not depart from the spirit or scope of the present invention, as defined by the claims appended hereto.