EP1037300A1 - Radio frequency transformer and its use - Google Patents

Abstract

The radio frequency transformer has two lines (1,9) with parallel paths, and four ports (5,13,19,20) at the line ends. The line units are on four levels, with the two lines being interlaced.

Description

The subject of the invention is a symmetrical radiofrequency transformer asymmetric says balun and its use. Its field of application is that processing of radio frequency signals, and more particularly that of mobile telephony. The object of the invention is to reduce the size of a such a transformer, produced in lines printed on a main board, so to reduce the cost price.

These transformers by their nature have characteristics of dimension frozen. These transformers are seen as octopoles (four access) reciprocal liabilities. Each access is connected respectively to an output by a line. One of the accesses is connected to a ground, while on a second access is applied a high frequency signal of amplitude A. We recover at the output two signals, each on an output, having an amplitude A / k, and has degrees out of phase. The factors k and  are determined respectively by the distance between each of the lines making up the transformer and by the length and width of each of these lines. Conversely, we apply as output two amplitude signals A / k and phase shifted by  degrees for recover, on the second access, a signal of amplitude 2A / k. The dimensions elements of the transformer components are imposed by the phase shift . Therefore such a transformer has a minimum size.

At present radio frequency transformers contain a substrate plate made of insulating material. Each of the lines making up the transformer is placed on one side of the substrate plate. These lines have parallel paths which are either a straight line or a loop, in order to save space. The lines are thus made on two levels, on either side of the substrate. These transformers are also sometimes made from coaxial cable. The two coupled lines are then formed by the core and the shield of the coaxial cable. To the problem space congestion then graft a problem space requirement.

The realization of a mobile phone may require the use of several radio frequency transformers. If these are done using coaxial cables for example, they take up space in thickness and surface, which can affect the design and cost of the mobile phone. Yes other solutions are chosen, thickness problems can be resolved but the area used by the transformers cannot be used for other functions, the size of the mobile phone will increase.

The invention solves these problems by making a high transformer frequency on at least four levels. The four levels are for example metallization levels of a multilayer printed circuit. In one example the printed circuit has six levels. The fact of making the transformer on four levels is not annoying for printed circuits used, since the majority of mobile phones already use printed circuits with six-level technologies. The surface dimensions of the transformer according to the invention on printed circuits is then strongly reduced. Using the internal layers of a multilayer circuit frees up the surface where surface components are implantable.

• deux lignes principales ayant des cheminements parallèles,
• quatre ports constitués par les extrémités des lignes principales,
  caractérisé en ce que
• les lignes principales s'étendent sur au moins quatre niveaux différents matérialisés par quatre plans parallèles,
• deux niveaux correspondent à une première ligne et deux niveaux correspondent à une deuxième ligne.

The subject of the invention is therefore a radiofrequency transformer comprising:

• two main lines with parallel paths,
• four ports formed by the ends of the main lines,
  characterized in that
• the main lines extend over at least four different levels materialized by four parallel planes,
• two levels correspond to a first line and two levels correspond to a second line.

The invention also relates to the use of such a transformer in a mobile phone modulator or demodulator.

• Figure 1 : une illustration d'un transformateur selon l'invention ;
• Figure 2 : une illustration d'un transformateur selon l'invention dont tous les trous métallisés sont débouchants ;
• Figure 3 : un exemple d'utilisation d'un transformateur radiofréquence selon l'invention dans un modulateur d'un téléphone mobile.
• Figure 4 : un exemple d'utilisation d'un transformateur selon l'invention dans un modulateur-démodulateur.

The invention will be better understood on reading the description which follows and on examining the figures which accompany it. These are presented for information only and in no way limit the invention. The figures show:

• Figure 1: an illustration of a transformer according to the invention;
• Figure 2: an illustration of a transformer according to the invention in which all the metallized holes are through;
• Figure 3: an example of use of a radio frequency transformer according to the invention in a modulator of a mobile phone.
• Figure 4: an example of the use of a transformer according to the invention in a modulator-demodulator.

Figure 1 shows a transformer according to the invention. The transformer includes a first main line 1 composed of two sections 2 and 4 of lines connected between them by a metallized hole 3. Hole 3 is represented by a coil because of its inductive effect. This inductive effect is also present for the other holes. Section 2 of line at an origin 5 and an end of line 6. Section 4 of line has an origin 7 and an end 8 of line. The hole 3 connects the sections 2 and 4 by their respective ends 6 and 7. The origin 5 of section 2 of the line is considered to be the origin of line 1. The end 8 of line section 4 is considered to be the exit port of the line 1. Figure 1 also shows a line 9 comprising elements numbered from 10 to 16 similar to the elements of line 1 respectively numbered from 2 to 8.

In a preferred embodiment of the transformer, this transformer includes a winding of lines on a cylinder. In the invention a turn of this winding is carried out on a level. To go to another turn, we change the level along the cylinder. The towers of the two cylindrical transformer lines are intertwined. Passing a level to another for a line is done at a place other than the passage of a level to another for another line. In this way, the lines are not never shorted.

In an example, we want to obtain a 180 ° phase shift between the output ports 8 and 16. Line theory requires that each lines have a length of λ / 4, where λ is the wavelength of the frequency, in the dielectric, at which the radiofrequency transformer must work. In the example, the cylinder has a square section. From its origin 5 line 1 extends in a direction D over a length of λ / 4/15. Then line 1 rotates an angle of 90 ° counterclockwise and extends over a length of λ / 4 / 7.5. It turns again by an angle of 90 ° counterclockwise to extend again by a distance of λ / 4 / 7.5. These four extensions extend from end 5 to end 6 on a level. Calling an extension in L the fact, for the line, to turn at an angle of 90 ° in a counterclockwise direction and then to extend over a distance L. The line makes an extension in L of length λ / 4 / 7.5 for get to the end 6 of the line section. The ends 6 and 7 are at the vertical to each other and connected by hole 3. From end 7 the line 1 extends on a second level in direction D over a length λ / 4 / 7.5. Then she makes two extensions in λ / 4 / 7.5 followed by two extensions in λ / 4/15. Line 1 therefore extends in total over a length of λ / 4 and on two levels.

Origin 13 of line 9 is located on a third level at vertical and below the origin 5 of the line 1. From its origin 13 line 9 extends over a distance λ / 4/15 in a direction D. Then it performs two extensions in λ / 4 / 7.5. Then an extension on a distance slightly less than λ / 4 / 7.5, to reach the end 14. It is necessary indeed prevent line 9 from meeting hole 3 belonging to line 1, which would create a short circuit. The end 15 is located on a fourth level vertically and below, in the example chosen, the end 14. From from end 15 line 9 extends in a straight line until it is at the vertical lines 6 and 7 of line 1. From here, line 9 extends in direction D over a length λ / 4 / 7.5. Then do two extensions of λ / 4 / 7.5 followed by two extensions of λ / 4/15. Line 9 also has a length of λ / 4 and also extends over two levels.

The ends 8 and 16 of lines 1 and 9 should be slightly offset from each other. They are close to each other in front of λ / 4. They are located inside the loops drawn by the path of the lines 1 and 9. The recovery of the signals on these lines cannot therefore be done only thanks to holes 17 and 18 metallized, and drilled from the ends 8 and 16. These ends 8 and 16 must not be vertical or from each other, nor of the line to which they do not belong because holes 17 and 18 do not must meet no line in their path. The other ends of the holes 17 and 18 are ports 19 and 20 which allow signals to be recovered present respectively on the ends 8 and 16. In the example chosen, the line 1 extends in planes P1 and P3, line 9 extends in planes P2 and P4. Then ports 19 and 20 are preferably in a P5 plane located in below plans P1 to P4.

In the example chosen the plans stack in order, from the highest down P1, P2, P3, P4 then P5. In these plans lines 1 and 9 are wrapped around a cube.

In a variant of the invention, the lines could be wound on a circular cylinder or any other geometric element of constant section and having a vertical axis.

In another variant of the invention, ports 19 and 20 could be locate in any plane. It would suffice for this that the last extension of lines 1 and 9 is not done with a 90 ° angle in the trigonometric, but with an angle of 90 ° clockwise of a watch. This would allow the positioning of the ports 19 and 20 in any other plane, from P1 to P5, but that would have consequence an increase of the surface necessary for the implantation of the high frequency transformer.

FIG. 1 also shows a ground plane 21 of triangular shape and contained in the P2 plan. One end of the ground plane 21 is connected to the input port 13 of line 9. By its large surface, the plane 21 allows to bring the mass to port 13 by limiting parasitic effects.

The distance between the planes P1, P2, P3 and P4 is determined by the coupling factor k that one wishes to obtain, it also varies according to the dielectric between the planes. Generally it is weak by compared to λ / 4. Preferably the planes are equidistant.

In practice a transformer according to the invention can be installed as a discrete component on a printed circuit. In the invention of preferably it is made directly in the printed circuit. In both cases the principle of realization is the same. We use a multilayer circuit, that is to say a circuit which can be seen as a stack of several plates of the same substrate, or of a different substrate. Between each plate can be drawn from the tracks. So with five stacked substrate plates, we obtains a six-layer circuit. The different substrate plates can be drilled with holes, each of these holes being able to be metallized. It is therefore possible to install a high frequency transformer according to the invention in such a circuit. To make it a discrete component just cut the circuit interesting and put it in a case by highlighting the four ports of the transformer by tracks connected to the legs of the housing. This allows the component obtained to be installed on a circuit.

The difficulty in the technology which has just been described resides in the making holes which only pass through certain plates of the substrate. We finds such holes which do not completely cross in Figure 1 with the holes 3 and 11. The hole 3 is in the vertical of the section 12 of line, the hole 11 is in the vertical of the section 10 of line. Figure 2 shows how make these holes through holes, that is to say through all the substrate plates.

As a variant, the passages from one turn to another are carried out in choosing a different cylinder for one line than the one chosen for another line. If necessary, the two cylinders differ from each other only by a little stall.

FIG. 2 shows for this purpose a line 21 comprising a section of line 22, a hole 23 and a section of line 24 similar to line 1 of the Figure 1. Figure 2 also shows a line 25 comprising a section of line 26, a hole 27 and a section of line 28. Line 21 has an origin 29, and line 25 has an origin 30 located vertically from origin 29. Line 25 extends from its origin 30 in a direction D of length λ / 4/15 then two extensions of length λ / 4 / 7.5 followed by an extension slightly less than λ / 4 / 7.5 in length. At this location line 25 performs a deviation of a very small length in front of λ / 4 / 7.5, in order to deviate from the vertical of the line segment 22 at the end 31 of the segment 26 of line 25. One end 32 of section 28 of line is situated vertically the end 31. From the end 32, the line 25 extends in a direction perpendicular to the section of the section 24 in contact with the hole 23 from line 21 until it is vertical to the latter. Then line 25 extends in a direction D over a length slightly less than λ / 4 / 7.5 then performs two extensions in λ / 4 / 7.5, then two extensions in λ / 4/15. We thus arrive at one end 33 of line 25. Line 21 has a end 34. The signals are recovered on ports 33 and 34 in a way identical to that described for figure 1, for ports 8 and 16. The way hole 27 is offset from line 21 allows holes 23 to be made and 27 in a through way, therefore to save on the cost end of the circuit containing one or more transformers according to the invention.

Figure 3 shows an example of the use of transformers according to the invention. Figure 3 shows a local oscillator 301 connected to a phase shifter 302. Phase shifter 302 outputs two signals corresponding to the local oscillator but out of phase with each other by 90 °. An output of the phase shifter 302 is connected to an input 304 of a first transformer 303 according to the invention. A second entrance 305 from transformer 303 is connected to ground. Transformer 303 provides on an output 306 a signal corresponding to that of the oscillator 301, and on an output 307 a signal corresponding to that of the oscillator 301 but 180 ° out of phase with it. A second output of the phase shifter 302 provides a 90 ° phase shifted signal corresponding to that of oscillator 301. This output is connected to a first input 309 of a second transformer 308 according to the invention. A second entry 310 from transformer 308 is connected to ground. A first exit 311 from transformer 308 provides a signal corresponding to that of the oscillator 301 90 ° out of phase with respect to it. A second exit 312 from transformer 308 provides a signal corresponding to that of the oscillator 301 but 270 ° out of phase with respect to it. Exits 306, 307, 311 and 312 are connected to a modulator 313. The modulator 313 also receives a signal I 314 and a signal Q 315. These signals I and Q are obtained from known manner in a mobile phone. From all the signals that the modulator 313 is supplied in a known manner producing a signal radio frequency 316. This radio frequency signal is then transmitted by the mobile phone.

In the case of a particularly small mobile phone the use of the invention is all the more interesting as two transformers are required. The fact of being able to incorporate them into a printed circuit of this mobile phone helps to increase compactness and reduce the size of this mobile phone.

Figure 4 shows a first transformer 401 according to the invention. On a first access 402 of the transformer 401 a signal is applied radio frequency input while a second access 403 is connected to a mass. In this configuration an output 405 of the transformer 401 delivers a signal of amplitude A1, equal to half the amplitude of the signal input, and in phase with this signal. An output 404, from transformer 401, delivers an amplitude signal A1, 180 ° out of phase with the signal entry. In an analogy with figure 1 the accesses 402 to 404 correspond respectively to accesses 5, 13, 20 and 19. The outputs 404 and 405 are connected simultaneously to mixers 406 and 407.

Figure 4 also shows a second transformer 408 according to the invention. On a first access 409 of the transformer 408 we apply a signal delivered by a local oscillator 413. A second access 410 of the transformer 408 is connected to ground. In this configuration a output 411 of transformer 408 delivers a 180 ° phase shifted signal by relation to the signal from the local oscillator 413 and of amplitude A2 equal to the half the amplitude of the signal delivered by the oscillator 413. An output 412 from transformer 408 delivers an amplitude signal A2, in phase with the signal delivered by the oscillator 413. In an analogy with FIG. 1 the accesses 409 to 412 correspond respectively to accesses 5,13, 20 and 18. The outputs 411 and 412 are connected to a quadratic generator 414.

The role of generator 414 is to shift the signals which it receives by 90 ° are subject. The generator 414 delivers, by separate ports, signals whose amplitude is a fraction, or a multiple, of the amplitude of the signal delivered by oscillator 413, and having phases of 0 °, 90 °, 180 ° and 270 ° by compared to the signal from oscillator 413, i.e. S0, S90, S180 and S270 these signals. The ports of the generator 414 delivering the signals S90 and S270 are connected to the mixer 406, the generator ports 414 delivering signals S0 and S180 are connected to mixer 407. Mixer 406 delivers + I and -I signals, the mixer 407 delivers + Q and -Q signals. These signals are demodulated signals available for processing. later, in a mobile phone for example.

The signals S90 and S270 delivered by the generator 414 are also applied to inputs of a 415 mixer. By other inputs the mixer 415 receives signals + I and -I, obtained in a known manner. The mixer 415 then delivers two phase-shifted radio frequency signals one by relative to the other by 180 ° and one of the two signals having a zero phase by report to the signal delivered by the oscillator 413. The signal having the null phase is applied to an access 420 of a third transformer 417 according to the invention. The other signal is applied to an input 421 of the transformer 417. The signals S0 and S180 delivered by the generator 414 are applied to inputs of a mixer 416. By other inputs, the mixer 416 receives + Q and -Q signals, obtained in a known manner. The mixer 416 then delivers two phase-shifted radio frequency signals relative to each other 180 ° and one of the two signals having a zero phase with respect to the signal delivered by oscillator 413. The signal having zero phase is applied to a access 420 of transformer 417. The other signal is applied to an input 421 of transformer 417.

An output 419 of the transformer 417 is connected to ground. In an analogy with FIG. 1, the ports 418 to 421 correspond accesses 5, 13, 19 and 20 respectively. In this configuration the transformer 417 delivers via an output 418 an RFS radio frequency signal.

A device such as that of FIG. 4 can be used in a mobile phone for example.

Claims (10)

A high frequency transformer comprising: two main lines (1, 9; 21, 29) having parallel paths, four ports (5, 13, 19, 20; 29, 30, 33, 34) formed by the ends of the main lines,
characterized in that the main lines extend over at least four different levels materialized by four parallel planes, two levels correspond to a first of said two main lines and two levels correspond to the second of said two main lines. Transformer according to claim 1, characterized in that the main lines have the same length λ / 4, where λ is the wavelength in the dielectric of a frequency at which the transformer. Transformer according to one of claims 1 or 2, characterized in that each line has an origin materialized by one of its ports, and the origins are close in front of λ / 4. Transformer according to one of claims 1 to 3, characterized in what the levels corresponding to the first line are interlaced with the levels corresponding to the second. Transformer according to one of claims 1 to 4, characterized in what the lines wrap around a cube. Transformer according to one of claims 1 to 5, comprising a multilayer printed circuit having at least four layers characterized by what the four levels containing the main lines of the transformer are four layers of the multilayer circuit. Transformer according to claim 6, characterized in that the continuity of a main line between two layers is provided by holes metallized (3, 17; 11, 18) inter-layers. Transformer according to claim 7 characterized in that all the inter-layer metallized holes are through, a main line having then a slight stall compared to the other. Transformer according to one of claims 1 to 3, characterized in that one (13) of the original ports is connected to a ground, by a ground plane (21) having a triangular shape, one of the vertices of the triangle being connected with the original port. Use of a transformer according to one of claims 1 to 9, in a modulator or demodulator of a mobile telephone.

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