Abstract:
The present invention relates to a power coupler for hyperfrequency signals. The single-section coupler with microstrip lines comprises a dielectric substrate, a main line and a secondary line comprising a coupling section, the lines being deposited on the substrate, the main line being substantially rectilinear and uniform over its entire length, the coupling section comprising a protuberance at each of its ends, the protuberances being interlinked by a portion of conductive line of which the section, the shape and the disposition are adapted to minimize the coupling between said portion and the main line relative to the coupling made between the protuberances and the main line. The invention applies notably to the measurement of the power of a signal passing through a transmission line.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage of International Application No. PCT/EP2008/055327, filed Apr. 30, 2008, which claims priority to foreign French Application No. FR 07 03381, filed May 11, 2007, the disclosure of each application is hereby incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a microstrip technology hyperfrequency signal coupler. It applies notably to the measurement of the power of a signal passing through a transmission line. In the telecommunications field, such couplers are, for example, integrated in amplifiers to measure the power of a signal delivered to an antenna. 
     BACKGROUND OF THE INVENTION 
     A proximity coupler, hereinafter simply referred to as “coupler”, comprises a main transmission line making it possible to route a hyperfrequency signal, and a secondary line of which a section is placed in proximity to the main line. By electromagnetic radiation, the secondary line is thus coupled to the main line. The microstrip technology signal couplers are very widely used because they are inexpensive to make and easy to integrate. However, this technology limits their performance. In particular, a satisfactory coupling directivity, that is to say a good separation of the incoming and outgoing power measurements in the coupler, is difficult to obtain. This difficulty is mainly due to the asymmetries of the even and odd transmission modes that appear with the use of this technology. Finally, in general, the insertion losses and the signal reflections—which are reflected in a non-zero standing wave ratio—are parameters to be taken into account when designing a coupler. 
     By comparison, the coaxial technology or triplate technology couplers provide for high level performance thanks to the shielding surrounding the propagation lines. However, these technologies increase the bulk and, above all, the fabrication cost of a coupler. 
     In order to improve the performance level of the microstrip technology couplers toward that of the coaxial or triplate technology couplers, a number of adaptations have already been proposed. Thus, it is known to add one or more capacitive components linking the main transmission line with the coupled secondary line. However, this solution presents a number of drawbacks. On the one hand, components that theoretically have the same capacitive values in reality exhibit capacitance values that are scattered around a mean value. It is therefore difficult to fabricate couplers in series that offer reproducible performance. On the other hand, the implanting of capacitive elements increases the production complexity of the coupler, consequently increasing its fabrication cost. Another known solution is to design transmission lines in singular shapes, in order to optimize the coupling between the main transmission line and the coupled line. However, singularities introduced in the main transmission line often cause the transmission of the signal to be disturbed and therefore the insertion losses to be increased. 
     SUMMARY OF THE INVENTION 
     One aim of the invention is to increase the coupling directivity without affecting the fabrication reproducibility of the coupler, while keeping the insertion losses at low levels, for a fabrication cost that is not very high. To this end, the subject of the invention is a single-section coupler with microstrip lines comprising a dielectric substrate, a main line and a secondary line comprising a coupling section, the lines being deposited on the substrate, characterized in that the main line is substantially rectilinear and uniform over its entire length, and in that the coupling section comprises a protuberance at each of its ends, the protuberances being interlinked by a portion of conductive line of which the section, the shape and the disposition are adapted to minimize the coupling between said portion and the main line relative to the coupling made between the protuberances and the main line, the coupling being mostly made between each of the protuberances and the main line. 
     According to one embodiment, the coupler according to the invention is asymmetrical. 
     A resistive balancing element can be connected between one end of the coupling section and the electrical ground. This resistive element makes it possible to optimize the directivity characteristic of the coupler and, to this end, can have capacitive or resistive characteristics that make it possible to improve performance. This resistive element does not replace the terminal loads conventionally connected to each of the access ports of the coupler. 
     According to one embodiment, the coupler according to the invention comprises at least one first resistive balancing element connected to the first protuberance, at least one second resistive element being connected to the second protuberance, the first and second resistive elements having different impedance values. 
     According to one embodiment, the distance D 1  between the first protuberance and the main line, on the one hand, and the distance D 2  between the second protuberance and the main line, on the other hand, are unequal. 
     According to one embodiment, the dimensions of the first protuberance, on the one hand, and the dimensions of the second protuberance, on the other hand, are different. 
     Another subject of the invention is a power amplifier comprising a coupler as claimed as described above. 
     Other features and benefits will become apparent from reading the following detailed description given as a nonlimiting example, in light of the appended drawings which represent: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 , a plan view of a first embodiment of the coupler according to the invention, 
         FIG. 2 , a plan view of a second embodiment of the coupler according to the invention, 
         FIG. 3 , a variant embodiment of the coupler according to the invention, 
         FIG. 4 , an example of use of a coupler according to the invention in a power amplifier. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a plan view of a first embodiment of the coupler according to the invention. A coupler  1  comprises a metal plate  2 , placed on the underside of the coupler and acting as electrical ground. The metal plate  2  has a layer of dielectric substrate  3  applied to it, with microstrips of conductive material deposited thereupon. A first conductive microstrip forms a main transmission line  10  routing a signal  10  from which a fraction of the power is to be taken. The main line  10  has an access port  11 ,  12  at each of its ends. The first access port  11  receives the signal S, of power P, incoming into the coupler  1 , whereas the second access port  12  is linked to a load, not represented in the figure, for example an antenna. Depending on the impedance of the load, a more or less significant power P ref  of the signal S is reflected into the main line  10 . The coupler  1  also comprises a secondary line  20  comprising, at each of its ends, a third and a fourth access port  21 ,  22 . 
     The secondary line  20  comprises a central portion of conductive line  23  that is relatively thin, conductive protuberances  24 ,  25 , and conductive microstrips  26 ,  27  connecting to the access ports  21 ,  22 . The whole consisting of the protuberances  24 ,  25  and the central portion  23  forms a coupling section with the main line  10 . The coupling section is produced so that the third access port  21  receives a fraction P′ of the power P of the signal S and the fourth access port  22  receives a fraction P ref ′ of the power P ref  reflected into the main line  10 . 
     The main line  10  is substantially rectilinear and its width, selected according to the desired characteristic impedance, remains virtually constant over its entire length. This design simplicity makes it possible to retain a characteristic line impedance close to the terminal impedances at the access ports  11 ,  12 , so reducing the standing wave ratio present in the line  10 . 
     Moreover, in the example, a metallized layer, in contact with the metal plate  2 , is applied to the top of the coupler  1  and around the lines  10 ,  20  to perfect the electromagnetic shielding of the coupler. 
     The first conductive protuberance  24  is placed at a first end  23   a  of the central portion  23  and the second protuberance  25  is placed at its opposite end  23   b . The protuberances  24 ,  25  are, in the example, quasi-rectangular in shape, but can have different shapes and dimensions. The barycenters of the protuberances  24 ,  25  are separated by a distance L of the order of a quarter of the median value of the wavelengths corresponding to the operating band of the coupler  1 . The distance D 1  separating the first protuberance  24  from the main line  10  can be different from the distance D 2  separating the second protuberance  25  from the main line  10 , but both protuberances  24 ,  25  must be sufficiently close to the main line  10  for an electromagnetic coupling to exist with the secondary line  20 . Similarly, the shapes (length and/or width) of each of the protuberances can be different. In practice, most of the coupling between the two lines  10 ,  20  is made via the conductive protuberances  24 ,  25 . The distances D 1  and D 2  separating the protuberances  24 ,  25  from the main line  10  and the dimensions of the protuberances  24 ,  25  are selected notably according to the dielectric characteristics (notably the permittivity) of the substrate  3 , the thickness of the substrate layer and the desired coupling level, that is to say, the power ratio P/P′. 
     In order to optimize the performance of the coupler according to the invention, the width, the shape and the placement of the central portion  23  linking the two protuberances  24 ,  25  are selected so that said central portion  23  is not involved or is almost uninvolved in the coupling between the main line  10  and the secondary line  20 . Thus, in the example of FIG.  1 , the width of the central portion  23  is selected to be thin (in the example, said portion  23  is much thinner than the main line  10 ) in order to minimize the interaction between said central portion  23  and the main line  10 . The central portion  23  is moreover neither necessarily parallel to the main line  10 , nor even rectilinear, thus making its length adjustable. 
     For example, in another embodiment illustrated in  FIG. 2 , this central portion  23  forms a U between the two protuberances  24 ,  25 , in order to guarantee a distancing of said portion  23  from the main line  10  making it possible to minimize the interaction with said main line  10 . In practice, the bottom  29  of the duly formed U is at a distance selected so that, when a signal is transmitted, in the main line  10 , there is virtually no coupling between the central portion  23  and the main line  10 . Moreover, when the distance between the central portion  23  and the main line  10  is increased, the section of the central portion  23  can also be increased. 
     The connecting microstrips  26 ,  27  make it possible to transmit the powers P′ and P ref ′ taken at the access ports  21 ,  22  of the coupler  1 . The first connecting microstrip  26  links the third access port  21  to the end of the central portion  23  closest to the first access port  11 , and the second connecting microstrip  27  links the fourth access port  22  to the end of the central portion  23  closest to the second access port  12 . These connecting microstrips  26 ,  27  are, in the example, connected at the ends  23   a ,  23   b  of the central portion  23 . They can, furthermore, form any angle with the central portion  23 , so offering enhanced possibilities of integration in complex circuits. 
     According to a variant embodiment shown in  FIG. 3 , a resistive balancing element  30  can be connected to one of the protuberances  24 ,  25 . In the example, the resistive element  30  is connected to the protuberance  24  closest to the first access port  11 . This asymmetry of the coupler  1  makes it possible to compensate for the asymmetries of the even and odd transmission modes that appear with the use of the microstrip technology. Optimizing the value of this lateral resistive element  30  makes it possible to improve the performance of the coupler directivity-wise. The resistive element  30  is placed at a distance D 3  from the main line  10  so as not to disturb the propagation of the signal S and is linked to the electrical ground, formed in the example by the metal ground  2 . This resistive element  30  can, for example, consist of a number of sub-elements placed in series and/or in parallel (not shown in the interests of simplification) and having certain inductive or capacitive properties, the operation of which makes it possible to improve the directivity of the coupler  1 . Connecting this resistive element  30  to a protuberance  24 ,  25  (that is to say, a wide metallized land) makes it possible to avoid having its precise positioning affect the performance of the coupler  1 , so facilitating the reproducibility of the performance in a series coupler fabrication context. According to another embodiment, the asymmetry of the coupler can, for example, be obtained by integrating two resistive elements of different characteristics into the coupler, a first resistive element being connected to the first protuberance  24 , a second resistive element being connected to the second protuberance  25 . Finally, since the resistive element  30  has an effect on the impedance of the secondary line  20 , the microstrips  26  and  27  can, in order to improve the adaptation of the third and fourth ports  21  and  22  of the coupler, comprise impedance transforming elements. 
       FIG. 4  shows an example of use of a coupler according to the invention in a power amplifier. An amplifier  40  receives a signal S and delivers an amplified signal S AMP . It comprises an amplification cell  41 , a coupler  1  according to the invention, a measurement module  42  and a resistive load  43 . The measurement module  42  is linked to the third access port  21  of the coupler  1 , and the resistive load  43  is linked to its fourth access port  22 . The amplification cell  41  receives the signal S and supplies a first amplified signal S INT  to the first access port  11  of the coupler  1 . The coupler  1  takes a fraction of the power of the signal S INT , a power fraction that it transmits to the measurement module  42  via its third access port  21 . The coupler  1  also produces a signal S AMP  obtained from its second port  12 , then directed to the output of the amplifier  40 . The association of the coupler  1  with the measurement module  42  therefore makes it possible to know the power of the signal S AMP  delivered at the output of the amplifier  40 . 
     One benefit of the coupler according to the invention is the simplicity with which it can be produced, allowing it to be easily and inexpensively integrated in equipment while benefitting from good performance with excellent reproducibility.