Abstract:
A radio frequency transformer has the two lines which constitute it on at least four levels of a printed circuit. Two of the four levels correspond to each line. In one variant the lines are wrapped onto a cube. The levels corresponding to one line are interleaved with the levels corresponding to the other one. The transformer has two input ports, one of which is connected to ground, and two output ports. This reduces the area and therefore the cost of the circuit or provides the facility to mount other components and thus other functions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a balanced-unbalanced radio frequency transformer (balun) and its use. Its field of application is that of processing radio frequency signals and more particularly that of mobile telephony. The object of the invention is to reduce the size of a transformer of this kind implemented in the form of lines printed on a main card in order to reduce the unit cost. 
     2. Description of the Prior Art 
     These transformers have fixed dimensions because of their nature. They are seen as reciprocal passive octopoles (four ports). Each port is connected to a respective output by a line. One port is connected to ground and a radio frequency signal of amplitude A is applied to a second port. Two output signals are obtained, each on one output, having an amplitude A/k and a relative phase of θ°. The factors k and θ are respectively determined by the distance between the lines constituting the transformer and by the length and the width of each line. Reciprocally, two signals of amplitude A/k and with a relative phase of θ° are applied at the output to recover a signal of amplitude 2A/k at the second port. The physical dimensions of the components constituting the transformer are imposed by the relative phase θ. A transformer of this kind therefore has minimum overall dimensions. 
     At the present time radio frequency transformers include an insulative material substrate plate. Each of the lines constituting the transformer is placed on one face of the substrate plate. With the aim of economizing on area the lines follow parallel paths which are either a straight line or a loop. The lines are therefore on two levels, on respective opposite sides of the substrate. These transformers are also sometimes implemented in coaxial cable. The two coupled lines are then the core and the shield of the coaxial cable. The problem of area congestion is then associated with a problem of volume congestion. 
     A mobile telephone may require several radio frequency transformers. If they are implemented in coaxial cable, for example, they take up room in terms of thickness and area, which can be prejudicial to the design and to the cost of the mobile telephone. If other solutions are adopted the thickness problems can be solved but the area used by the transformers cannot be used for other functions and the size of the mobile telephone will be increased. 
     The invention solves the above problem by providing a radio frequency transformer implemented on at least four levels. The four levels are layers of metallization of a multilayer printed circuit, for example. In one example the printed circuit has six layers. Implementing the transformer on four levels is not a problem with regard to the printed circuits used, since most mobile telephones already use printed circuits with six-level technologies. The area congestion of the transformer of the invention on printed circuits is then greatly reduced. Using the inner layers of a multilayer circuit frees up the surface on which surface-mount components can be mounted. 
     SUMMARY OF THE INVENTION 
     The invention therefore provides a radio frequency transformer including two main lines which have parallel routes and which lie on at least four different levels materialized by four parallel planes, two of which levels correspond to a first of the two main lines and two other of which levels correspond to the second of the two main lines, and four ports consisting of the ends of the main lines. 
     The invention also encompasses the use of a transformer of the above kind in a mobile telephone modulator or demodulator. 
     The invention will be understood better after reading the following description and examining the accompanying drawings. The drawings are provided by way of non-limiting example of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a transformer in accordance with the invention. 
     FIG. 2 shows a transformer in accordance with the invention in which all the plated-through holes are open at the ends. 
     FIG. 3 shows one example of the use of a radio frequency transformer in accordance with the invention in a mobile telephone modulator. 
     FIG. 4 shows one example of the use of a transformer in accordance with the invention in a modulator-demodulator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a transformer in accordance with the invention. The transformer includes a first main line  1  made up of two line sections  2  and  4  interconnected by a plated-through hole  3 . The hole  3  is represented as a coil because of its inductive effect. The other holes are also subject to this inductive effect. The line section  2  has an origin  5  and an end of line  6 . The line section  4  has an origin  7  and an end of line  8 . The hole  3  connects the respective ends  6  and  7  of the sections  2  and  4 . The origin  5  of the line section  2  is regarded as the origin of the line  1 . The end  8  of the line section  4  is considered as the output port of the line  1 . FIG. 1 also shows a line  9  including elements numbered  10  through  16  similar to the elements of the line  1  respectively numbered  2  through  8 . 
     A preferred embodiment of the transformer includes a wrapping of lines on a cylinder. In accordance with the invention, one turn of this wrapping is on one level. The transition to another turn is effected by a change of level along the cylinder. The turns of the two cylindrical lines of the transformer are interleaved. The transition from one level to another for one line is effected at a location other than the transition from one level to another for another line. Thus the lines are never short-circuited. 
     In one example of the invention a relative phase of 180° is required between the output ports  8  and  16 . Theory indicates that each line must have a length of λ/4 where λ is the wavelength in the dielectric corresponding to the frequency at which the radio frequency transformer is required to operate. In this example the cylinder has a square cross section. From its origin  5 , the line  1  extends a distance λ/4/15 in a direction D. The line  1  then turns 90° counterclockwise and extends a distance λ/4/7.5. It again turns 90° counterclockwise and again extends a distance λ/4/7.5. These four extensions run from the end  5  to the end  6  on one level. The expression “extension of L” refers to the line turning through an angle of 90° in the counterclockwise direction and then extending a distance L. The line effects an extension of λ/4/7.5 to reach the end  6  of the line section. The ends  6  and  7  are vertically aligned and connected by the hole  3 . From the end  7  the line  1  extends in the direction D on a second level a distance λ/4/7.5. It then effects two extensions of λ/4/7.5 followed by two extensions of λ/4/15. The line  1  therefore extends a total distance of λ/4 on two levels. 
     The origin  13  of the line  9  is on a third level and vertically below the origin  5  of the line  1 . From its origin  13  the line  9  extends a distance λ/4/15 in a direction D. It then effects two extensions of λ/4/7.5. It then reaches the end  14  via an extension of slightly less than λ/4/7.5. This is because the line  9  must not impinge on the hole  3  which is part of the line  1 , as this would cause a short circuit. The end  15  is on a fourth level and, in this example, vertically below the end  14 . From the end  15  the line  9  extends in a straight line to a point vertically in line with the ends  6  and  7  of the line  1 . From here, the line  9  extends a distance λ/4/7.5 in the direction D. It then effects two extensions of λ/4/7.5 followed by two extensions of λ/4/15. The line  9  also has a length of λ/4 and also lies on two levels. 
     The ends  8  and  16  of the lines  1  and  9  must be slightly offset from each other. They are close together compared to the distance λ/4. This is because they are inside loops traced out by the route of the lines  1  and  9 . Signals can be recovered from these lines only by means of plated-through holes  17  and  18  at the ends  8  and  16 . The ends  8  and  16  must not be vertically in line with each other or with the line of which they are not part, because the holes  17  and  18  must not encounter any line on their route. The other ends of the holes  17  and  18  are ports  19  and  20  for recovering the signals at the ends  8  and  16 , respectively. In this example, the line  1  lies in planes P 1  and P 3  and the line  9  in planes P 2  and P 4 . The ports  19  and  20  are then preferably in a plane P 5  below the planes P 1  to P 4 . 
     In this example the planes are stacked up in the order P 1 , P 2 , P 3 , P 4  and P 5 , from the highest to the lowest. In these planes the lines  1  and  9  wrap around a cube. 
     In a variant of the invention the lines could wrap around a circular cylinder or any other geometrical element of constant cross section with a vertical axis. 
     In another variant of the invention the ports  19  and  20  could be in any plane. For this it would be sufficient for the last extension of the lines  1  and  9  to be at an angle of 90° clockwise, rather than 90° counterclockwise. This would enable the ports  19  and  20  to be placed in any other plane, from plane P 1  through P 5 , but would increase the area required for the radio frequency transformer. 
     FIG. 1 also shows a triangular ground plane  21  in the plane P 2 . One end of the ground plane  21  is connected to the input port  13  of the line  9 . Because of its large area, the plane  21  extends the ground to the port  13 , limiting interference effects. 
     The distance between the planes P 1 , P 2 , P 3  and P 4  is determined by the coupling factor k required and also varies as a function of the dielectric between the planes. As a general rule it is small compared to λ/4. The planes are preferably equidistant. 
     In practice a transformer in accordance with the invention can be mounted on a printed circuit as a discrete component. It is preferably formed directly in the printed circuit, however. The principle is the same in both cases. A multilayer circuit is used, i.e. a circuit which can be regarded as a stack of several plates of the same substrate or different substrates. Lines can be traced between each plate and the next. Thus with five stacked substrate plates a circuit with six layers is obtained. The various substrate plates can be pierced with holes and each hole can be metal-plated. It is therefore possible to incorporate a high-frequency transformer in accordance with the invention in a circuit of this kind. To make it into a discrete component all that is required is to cut out the circuit of interest and to place it in a package with leads connected to the four ports of the transformer by tracks. This enables the resulting component to be mounted on a circuit. 
     The difficulty in the technology just described lies in making holes through only some plates of the substrate. The holes  3  and  11  in FIG. 1 are examples of holes which do not pass completely through the structure. The hole  3  is vertically in line with line section  12  and the hole  11  is vertically in line with line section  10 . FIG. 2 shows how to make these holes open-ended holes, i.e. holes passing through all the substrate plates.  5   
     As an alternative to the above, the transitions from one turn to another are effected by choosing a cylinder for one line different to that chosen for another line. If required the two cylinders differ from each other only in a slight offset. 
     Thus FIG. 2 shows a line  21  including a line section  22 , a hole  23  and a line section  24  similar to the line  1  shown in FIG.  1 . FIG. 2 also shows a line  25  including a line section  26 , a hole  27  and a line section  28 . The line  21  has an origin  29  and the line  25  has an origin  30  vertically in line with the origin  29 . The line  25  extends a distance λ/4/15 from its origin  30  in a direction D and then effects two extensions of λ/4/7.5 followed by an extension of slightly less than λ/4/7.5. At this point the line  25  is offset by a distance that is very small compared to λ/4/7.5 in order to move it away from the vertical line through the line section  22  at the end  31  of the section  26  of the line  25 . One end  32  of the line section  28  is vertically in line with the end  31 . From the end  32 , the line  25  extends in a direction perpendicular to the line section  24  in contact with the hole  23  of the line  21  until it is vertically in line therewith. The line  25  then extends a distance slightly less than λ/4/7.5 in a direction D and then effects two extensions of λ/4/7.5 and then two extensions of λ/4/15 to reach an end  33  of the line  25 . The line  21  has an end  34 . The signals are recovered at the ports  33  and  34  in exactly the same way as described with reference to FIG. 1 for the ports  8  and  16 . The manner in which the hole  27  is offset from the line  21  means that the holes  23  and  27  can be open-ended holes, which represents a saving in the final cost of a circuit containing one or more transformers in accordance with the invention. 
     FIG. 3 shows one example of the use of transformers in accordance with the invention. FIG. 3 shows a local oscillator  301  connected to a phase-shifter  302 . The phase-shifter  302  provides at its output two signals corresponding to the signal from the local oscillator but with a relative phase of 90°. One output of the phase-shifter  302  is connected to an input  304  of the first transformer  303  in accordance with the invention. A second input  305  of the transformer  303  is connected to ground. The transformer  303  provides at an output  306  a signal corresponding to that from the oscillator  301  and at an output  307  a signal corresponding to that from the oscillator  301  but with a relative phase of 180°. A second output of the phase-shifter  302  provides a signal corresponding to that from the oscillator  301  with a relative phase of 90°. This output is connected to a first input  309  of a second transformer  308  in accordance with the invention. The second input  310  of the transformer  308  is connected to ground. The first output  311  of the transformer  308  provides a signal corresponding to that from the oscillator  301  with a relative phase of 90°. A second output  312  of the transformer  308  provides a signal corresponding to that from the oscillator  301  with a relative phase of 270°. The outputs  306 ,  307 ,  311  and  312  are connected to a modulator  313 . The modulator  313  also receives an I signal  314  and a Q signal  315 . The I and Q signals are obtained in a manner that is well known in the mobile telephone art. From all the signals applied to it the modulator  313  produces a radio frequency signal  316  in a manner well known in the art. The radio frequency signal is then transmitted by the mobile telephone. 
     In the case of a particularly small mobile telephone, the use of the invention is more beneficial when two transformers are needed. Being able to incorporate them into a printed circuit of the mobile telephone helps to improve the compactness and reduce the size of the mobile telephone. 
     FIG. 4 shows a first transformer  401  in accordance with the invention. A radio frequency input signal RFE is applied to a first port  402  of the transformer  401 . A second port  403  is connected to ground. In this configuration an output  405  of the transformer  401  delivers a signal of amplitude A 1  equal to half the amplitude of the input signal and in phase therewith. An output  404  of the transformer  401  delivers a signal of amplitude Al with a relative phase of 180° to the input signal. By analogy with FIG. 1 the ports  402  through  404  respectively correspond to the ports  5 ,  13 ,  20  and  19 . The outputs  404  and  405  are simultaneously connected to mixers  406  and  407 . 
     FIG. 4 also shows a second transformer  408  in accordance with the invention. A signal delivered by a local oscillator  413  is applied to a first port  409  of the transformer  408 . A second port  410  of the transformer  408  is connected to ground. In this configuration an output  411  of the transformer  408  delivers a signal with a phase of 180° relative to the signal from the local oscillator  413  and of amplitude A 2  equal to half the amplitude of the signal delivered by the oscillator  413 . An output  412  of the transformer  408  delivers a signal of amplitude A 2  in phase with the signal delivered by the oscillator  413 . By analogy with FIG. 1 the ports  409  through  412  respectively correspond to the ports  5 ,  13 ,  20  and  18 . The outputs  411  and  412  are connected to a quadratic generator  414 . 
     The function of the generator  414  is to shift the phase of the signals applied to it 90°. The generator  414  delivers at separate ports respective signals S 0 , S 90 , S 180  and S 270  whose amplitude is a fraction or a multiple of the amplitude of the signal delivered by the oscillator  413  and having phases of 0°, 90°, 180° and 270° relative to the signal from the oscillator  413 . The ports of the generator  414  delivering the signals S 90  and S 270  are connected to the mixer  406 . The ports of the generator  414  delivering the signals S 0  and S 180  are connected to the mixer  407 . The mixer  406  delivers signals +I and −I. The mixer  407  delivers signals +Q and −Q. These signals are demodulated signals available for subsequent processing, for example in a mobile telephone. 
     The signals S 90  and S 270  delivered by the generator  414  are also applied to inputs of a mixer  415 . Other inputs of the mixer  415  receive the signals +I and −I obtained in a manner known in the art. The mixer  415  then delivers two radio frequency signals with a relative phase of 180°. One of the two signals is in phase with the signal delivered by the oscillator  413 . That signal is applied to a port  420  of a third transformer  417  in accordance with the invention. The other signal is applied to an input  421  of the transformer  417 . The signals S 0  and S 180  delivered by the generator  414  are applied to inputs of the mixer  416 . Other inputs of the mixer  416  receive the signals +Q and −Q obtained in a manner known in the art. The mixer  416  then delivers two radio frequency signals with a relative phase of 180°. One of the two signals is in phase with the signal delivered by the oscillator  413 . That signal is applied to a port  420  of the transformer  417 . The other signal is applied to an input  421  of the transformer  417 . 
     An output  419  of the transformer  417  is connected to ground. By analogy with FIG. 1, the ports  418  through  421  respectively correspond to the ports  5 ,  13 ,  19  and  20 . In this configuration the transformer  417  delivers a radio frequency signal RFS at an output  418 . 
     A device like that shown in FIG. 4 can be used in a mobile telephone, for example.