Abstract:
A mode-switching transformer having a first conductive line formed of two primary windings electrically in series, and the length of which is a function of a central operating frequency of the transformer; and a second conductive line formed of two secondary windings electrically in series, coupled two-by-two with the primary windings, the length of which is a function of said central frequency. The conductive lines exhibit different widths selected according to the desired impedance ratio of the transformer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to mode-switching transformers which are used to convert a voltage from the common mode to a differential mode and conversely. Such transformers are generally called “balun” (balanced-unbalanced) transformers. 
   2. Discussion of the Related Art 
   A mode-switching transformer is, for example, used in radio frequency transceiver chains, for example, of mobile phones. This type of application currently uses devices of balun type, the antenna side being most often associated with a single-ended device. 
   Two categories of mode-switching transformers are coupled-line or distributed baluns, and baluns with discrete components. Baluns with coupled lines are formed of pairs of coupled conductive tracks, the transformer&#39;s operating frequency being conditioned by the line length. Baluns with discrete components are formed of capacitive and inductive components forming LC cells. 
   The present invention more specifically relates to baluns with coupled lines. Among these, the present invention more specifically relates to a Marchand-type balun, that is, a symmetrical balun having its coupled lines calculated from a λ/4 length, where λ represents the wavelength corresponding to the central frequency of the desired passband of the balun. 
     FIG. 1  very schematically shows the function of a balun  1 . On the common-mode side, the device includes a so-called common-mode port  2  (for example, intended to be connected to the end of an antenna). On the differential mode side, the balun includes two ports  3  and  4  symmetrical with respect to a reference M, generally the ground on the side of the equipment for processing the received signals. On the common mode side, the received signal is most often referenced to earth T. The grounds on the common mode and on the differential mode sides may (but not necessarily) exhibit potentials different from each other. For a perfectly symmetrical balun, a voltage V on the common mode side is converted into two voltages V/2 on the differential mode side. 
     FIG. 2  very schematically shows the equivalent electric diagram of a conventional Marchand type balun. Four conductive sections  5  to  8 , each having a quarter wavelength (λ/4) are coupled two by two. Two first sections  5  and  6  are in series between common-mode port  2  and a port  9  generally open and left floating. The other two sections  7  and  8  are in series between the two differential mode ports  3  and  4  and have their midpoint  10  connected to ground M forming the reference on the differential side. The signals present on differential mode input-output terminals  3  and  4  are phase-shifted by 180° with respect to each other. 
     FIG. 3  very schematically shows in the form of blocks an example of an assembly using a balun  1  of the type to which the present invention applies. In this example, the common-mode port  2  is connected to the end of a radio frequency transceiver antenna  11  and the two differential ports are intended to be connected to circuits  12  of exploitation of the received signals and of preparation of the signals to be transmitted (APPLI). Between balun  1  and circuit(s)  12  is provided an impedance matching circuit  13  (ZMATCH), necessary in most applications. 
   A balun such as illustrated in  FIG. 2  in which all the conductive sections have identical structures and same lengths provides a unity impedance ratio between the primary (common mode) and the secondary (differential mode). 
   According to the application, the circuits on the primary and secondary side may or may not have the same impedance (for example, 50 Ω). A non-unity impedance ratio may be required (for example, 50/100 between the antenna (50 Ω) and surface acoustic wave filters (100 Ω), 50/200, 25/50, 100/25, etc.). 
   Conventionally, to modify the impedance ratio of a balun with coupled lines, the spirals on one of the balun sides are connected in parallel.  FIG. 4  shows the equivalent electric diagram of an example of such a balun with a non-unity impedance ratio. In  FIG. 4 , coupled lines  5  to  8  have been shown in the form of inductances since they most often are coupled spirals. On the primary side, the two spirals  5  and  6  are in series. On the secondary side, spirals  7  and  8  are (50 Ω) connected in parallel. Thus, for a voltage V present on the common-mode port  2  generating a current I in spirals  5  and  6  in series, a voltage V/2 is obtained across each spiral  7  and  8  but said spirals conduct a current  2 I. Accordingly, the impedance ratio of such a balun here is 4 (for example, 200/50). 
   A problem to solve is the parallel connection of spirals  7  and  8 . Indeed, baluns with coupled lines are most often made in the form of conductive tracks wound to gain space with respect to rectilinear tracks. The coupling between sections  5  and  7 , respectively  6  and  8 , is then obtained by forming these sections on the same level or in different conductive levels in interdigited and/or superposed fashion. Windings  5  and  6 , respectively  7  and  8 , are for example laterally spaced apart. In such a structure, the parallel connection of the two spirals requires additional vias and bridges. Further, for impedance ratios other than four or one quarter, the connections of multiple spirals in parallel make the forming of such baluns all the more complex. 
   SUMMARY OF THE INVENTION 
   The present invention aims at providing a Marchand-type distributed balun with a non-unity impedance ratio which is simple to form and which overcomes the disadvantages of known solutions. 
   The present invention especially aims at providing a solution which does not increase the need for vias and bridges of connection of conductive windings for a balun formed by means of conductive tracks wound in different conductive levels. 
   The present invention also aims at providing a solution which is compatible with the forming of a balun by means of superposed conductive windings. 
   To achieve these and other objects, the present invention provides a mode-switching transformer with coupled lines, in which the lines coupled two-by-two have different widths. 
   According to an embodiment of the present invention, the widths of the lines are selected according to the impedance ratio desired between the common-mode and differential-mode ports. 
   The present invention also provides a mode-switching transformer comprising a first conductive line formed of two primary windings electrically in series, and the length of which is a function of a central operating frequency of the transformer; and a second conductive line formed of two secondary windings electrically in series, coupled two-by-two with the primary windings, and the length of which is a function of said central frequency; the conductive lines having different widths selected according to the impedance ratio desired for the transformer. 
   According to an embodiment of the present invention, first and second windings of each of the lines are formed, laterally spaced apart, in a same conductive level, with the primary or secondary of the transformer having the smallest impedance having the largest line width. 
   According to an embodiment of the present invention, the two windings of each of the conductive lines are superposed in two different metal levels, with the primary or secondary of the transformer having the smallest impedance having the narrowest width. 
   According to an embodiment of the present invention, the intertrack between two spirals of a same winding is sized according to the desired impedance. 
   The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 to 4 , previously described, are intended to show the state of the art and the problem to solve; and 
       FIG. 5  shows in a top view an embodiment of a mode-switching transformer according to the present invention. 
   

   DETAILED DESCRIPTION 
   The same elements have been referred to with the same reference numerals in the different drawings. For clarity, only those elements which are necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the upstream and downstream circuits of a balun have not been described in detail. The present invention applies whatever the circuits or equipment connected on the differential mode side and on the common mode side. Further, the practical forming of conductive windings on a multiple-level substrate has not been described in detail. The present invention may be implemented using conventional techniques of balun forming by means of conductive tracks. 
   A feature of the present invention is to provide conductive tracks having different widths between the primary (common mode) and the secondary (differential mode). 
   It could have been thought to modify the impedance ratio by modifying the number of spirals of the conductive tracks on one side with respect to the other to modify the values of the respective inductances and, accordingly, the impedance ratio. However, such a solution is incompatible with the need to have λ/4 line lengths for the entire balun. 
     FIG. 5  is a top view of an embodiment of a mode-switching transformer according to the present invention. This balun may be formed on an insulating substrate  20  (for example, a glass substrate) on which are formed several conductive levels electrically isolated from one another. In the example of  FIG. 5 , at least three metal levels are necessary. In a first level  50  is generally formed a ground plane M corresponding to the differential mode ground. All the windings are here formed in a same conductive level  51  and a third level  52  is used to form bridges to have the central contacts of the windings come out. 
   For example, sections  5  and  6  are formed of two windings laterally spaced apart from each other and sections  7  and  8  are formed of windings interlaced in windings  5  and  6 . A via  21  is used at the center of winding  5  to connect a bridge  22  in a different metallization level for connection of port  2 . At the center of winding  7  are provided vias  23  and  24  for connection to ground plane M by means of a bridge  25  and a via  26 . The same structure is reproduced on the side of windings  6  and  8  with vias  27  and  28  at the center of winding  8  to connect the ground plane with a bridge  29  and a via  30 . In  FIG. 5 , a via  31  and a bridge  32  have been shown for connection to port  9 . It should however be noted that this bridge and this via are generally omitted since port  9  is open. On the free end side of windings  7  and  8  are provided vias  33  and  34  for respective connections to differential-mode ports  3  and  4 . 
   As illustrated in  FIG. 5 , the primary ( 5  and  6 ) and secondary ( 7  and  8 ) windings do not have the same width. In one example, the primary winding is narrower than the secondary winding, which results in a primary-to-secondary impedance ratio greater than one. 
   In a structure such as illustrated in  FIG. 5  where the windings coupled two-by-two are laterally spaced apart, a decrease in the width of one of the conductive tracks with respect to the other increases its inductance and, accordingly, its impedance. For an impedance ratio greater than one, primary windings with tracks narrower than those of the secondary windings are provided, while for an impedance ratio smaller than one, secondary winding tracks narrower than those of the primary are provided. 
   Another parameter which conditions the impedance value is the intertrack, that is, the space between two spirals of the same inductance (of the same winding). The wider this intertrack, the more the impedance decreases since the mutual inductance decreases. 
     FIG. 6  is a schematic cross-sectional view of a balun according to a second embodiment of the present invention. The bridges and ground plan are similar to those of the first embodiment of  FIG. 5 . According to the second embodiment, the present invention is applied to a balun formed by means of stacked windings ( 5 ′,  6 ′,  7 ′ and  8 ′), which amounts, in a way, to folding back the structure of  FIG. 5  on itself. Such an embodiment requires at least one additional metal level  51 ′ to be implemented. 
   According to this second embodiment, a decrease in the width of one of the windings decreases its inductance and accordingly its impedance because of the coupling between the two windings in series on the same side of the transformer. In the example of  FIG. 6 , the primary winding is narrower than the secondary winding, which results in a primary-to-secondary impedance ratio smaller than one. As for the interspiral  39 , a decrease in the interspiral increases the inductance and thus increases the impedance. 
   Based on the indications given hereabove, it will be within the abilities of those skilled in the art to size the transformer windings on each mode switching according to its configuration (stacked or non-stacked) to obtain the desired impedance ratio. 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the sizing and the practical forming of a mode-switching transformer according to the present invention is within the abilities of those skilled in the art based on the functional indications given hereabove. 
   Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.