Abstract:
The present invention relates to the field of electronic devices known as baluns. It concerns an active balun which is broadband and reciprocal. Embodiments of the invention integrate an active splitter balun with an active combiner balun so as to form three transmission lines. A first active coupling is provided between the first and second transmission lines and a second active coupling is provided between the first and third transmission lines. The active couplings are provided by means of amplifier cells distributed along the transmission lines. Embodiments of the invention have configurable means for polarizing the different amplifier cells so as to create a specific coupling direction between the various transmission lines. The device according to the invention can be applied in the field of broadband mixers which are used, notably, in radio transmission and reception circuits.

Description:
The present application claims priority under 35 U.S.C. §119 of French patent application No. 0707855000, which was filed in the French Patent Office on Nov. 9, 2007 and which is incorporated by reference herein in its entirety. 
   FIELD OF THE INVENTION 
   The present invention relates to the field of electronic devices known as baluns, and more specifically to the field of broadband mixers, used notably in radio transmission and reception circuits, and produced with the use of baluns. 
   BACKGROUND OF THE INVENTION 
   There is a significant demand for broadband mixers in present-day industry. These circuits are used in some reception and transmission circuits. It is possible to envisage numerous architectures for the production of mixers. These include, in particular, what are known as “balanced” or “double-balanced” structures which offer the best performance in terms of parasitic frequency rejection and very good channel isolation. There is a known way of producing a double-balanced mixer using a field-effect transistor bridge or a diode bridge. In order to produce such a circuit in MMIC (Monolithic Microwave Integrated Circuit) technology which can operate over a wide frequency range, it is necessary to use functional units known as broadband baluns. 
   The term “balun”, which is a contraction of the expression “balanced-unbalanced”, generally denotes a three-terminal device capable of either splitting an input signal into two output signals which have the same amplitude and have a phase difference of 180° between them, thus forming a power splitter with outputs in phase opposition (a splitter balun), or combining two input signals in such a way that the output signal represents the difference between the input signals, thus forming a combiner circuit with inputs in phase opposition (a combiner balun). 
   There are currently two known methods of producing baluns. 
   In the first method, shown in  FIG. 1 , mid-point transformers  12 ,  13  and  14  are produced and used to drive, for example, a field-effect transistor bridge  11 . This device, which is passive and reciprocal, can advantageously provide both of the aforementioned functions. However, it is very difficult to use planar technology to produce a circuit of this type for operation at hyperfrequency. Moreover, it cannot be used for operation over a very wide frequency band (more than two to three octaves) or at low frequency (f&lt;1 GHz). 
   In the second method, an active balun is used, based on a distributed structure, as described in the French patent application filed by the present applicant on 21 Dec. 2005 and published under the number 2 895 168. This active balun combines two balanced structures, one of which is composed of amplifier cells of the “cascade” or “Darlington” type, while the other is composed of amplifier cells of the “cascode” type. This specific circuit can be used, notably, to resolve the problem of gain unbalance between channels, which generally occurs in conventional circuits in the form of unbalance between the two output channels (in a splitter circuit) or between the two input channels (in a combiner circuit). It also enables a phase difference of 180° to be produced between channels. 
   This second method can be used to produce baluns operating at low frequencies and in much wider frequency bands (on the order of a decade or more) than those at which other existing devices operate. It is therefore a more advantageous solution than the first method. However, the active structures produced in this way are not reciprocal. Unfortunately, therefore, it is necessary to produce two different structures in order to produce a splitter and a combiner. If this type of structure is used in a mixer, the mixer will be unilateral. 
   Thus, regardless of which method is chosen, it is not possible at present to design an active balun, operating over a very wide frequency band (of the order of a decade), which is reciprocal and which can handle low frequencies (f≦1 GHz). 
   Consequently, anyone wishing to produce a reciprocal balun at the present time has to use a passive balun, for example a structure such as that of the Marchand balun. This structure, which is known and is illustrated in  FIG. 2 , is based on line coupling, with a main line  21  having a length of λ/2 and two lines  22  and  23  coupled to the main line and each having a length of λ/4, these two lines being arranged so as to provide signals of the same value and in phase opposition on their respective outputs  25  and  26 . This structure can be used advantageously to produce reciprocal baluns. However, the use of passive couplers gives rise to problems of large overall dimensions, especially when operation at relatively low frequencies (around 1 GHz) is desired, since the passive coupler must always have a length of λ/2 with respect to the central operating frequency of the mixing device in which it is incorporated. This type of structure is therefore difficult to incorporate in MMIC technologies. Moreover, because of its structure, a balun of this type cannot be used for broadband applications (covering several octaves). 
   SUMMARY OF THE INVENTION 
   In view of the limitations associated with the design of traditional balun structures, embodiments of the present invention provide balun type structures which are both reciprocal and broadband (having a bandwidth of more than an octave). 
   In one embodiment, the invention provides a reciprocal active balun, including:
         an active splitter balun element including a first transmission line coupled to a second and a third transmission line, a first directive active coupling of the first transmission line to the second transmission line being provided by means of distributed cascode amplifier cells connected to the two transmission lines, and a second directive active coupling of the first transmission line to the third transmission line being provided by means of distributed Darlington amplifier cells connected between the two transmission lines,   an active combiner balun element including a first and a second transmission line, each coupled to a third transmission line, a first directive active coupling of the first transmission line to the third transmission line being provided by means of distributed cascode amplifier cells connected to the two transmission lines, and a second directive active coupling of the second transmission line to the third transmission line being provided by means of distributed Darlington amplifier cells connected to the two transmission lines,       

   The device according to this embodiment also being characterized in that the splitter balun and combiner balun elements are integrated so as to form three transmission lines, a first active coupling being provided between the first and second transmission lines by means of cascode amplifier cells of the splitter and combiner balun elements, and a second active coupling being provided between the first and third transmission lines by means of the Darlington amplifier cells of the splitter and combiner balun elements. 
   The device according to this embodiment is also characterized in that it has configurable means for polarizing the different amplifier cells so as to create a specific coupling direction between the first and second transmission lines on the one hand, and between the first and third transmission lines on the other hand. 
   In some embodiments, the device has an operating mode in which the device acts as a splitter balun. In these embodiments, when operating as a splitter balun, the means for polarizing the amplifier cells are configured and controlled in such a way that they activate the amplifier cells which provide directive coupling in the splitter balun element and deactivate the amplifier cells which provide the directive coupling in the combiner balun element. 
   In some embodiments, the device has an operating mode in which the device acts as a combiner balun. In these embodiments, when operating as a combiner balun, the means for polarizing the amplifier cells are configured and controlled in such a way that they deactivate the amplifier cells which provide directive coupling in the splitter balun element and activate the amplifier cells which provide the directive coupling in the combiner balun element. 
   The device according to the invention has the advantage of being a reciprocal active element which can provide one or other of the functions of a splitter balun or combiner balun on command. 
   The reciprocal nature of the device is an advantageous addition to the features of large bandwidth and wide operating range of the non-reciprocal active baluns known from the prior art. 
   The features and advantages of the invention will be made more evident by the following description which relates to a specific embodiment of the invention, to be considered as a non-limiting example, and which refers to the appended drawings, in which 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing the structure of a double-balanced mixer based on a field-effect transistor bridge, the mixer using transformer-based passive baluns according to the prior art; 
       FIG. 2  is a diagram showing the structure of the Marchand balun; 
       FIG. 3  is a diagram showing the known structure of a non-reciprocal broadband active balun, based on a distributed structure forming actively coupled lines, in a splitter balun configuration; 
       FIG. 4  is a diagram showing the known structure of a non-reciprocal broadband active balun, based on a distributed structure forming actively coupled lines, in a combiner balun configuration; 
       FIG. 5  shows the structure of a reciprocal broadband active balun according to one embodiment of the present invention; 
       FIG. 6  illustrates the use of a reciprocal active balun according to one embodiment of the present invention in splitter balun mode; 
       FIG. 7  illustrates the use of a reciprocal active balun according to one embodiment of the present invention in combiner balun mode. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Let us start by considering  FIGS. 3 and 4  which are schematic illustrations of the two active distributed structures from which the device according to the invention was developed. 
   These two basic structures are very fully described in the French patent application filed by the present applicant on 21 Dec. 2005 and published under the number 2 895 168. Consequently, their operation is not described in detail here, but the important elements will be indicated.  FIG. 3  shows a first structure arranged to form a non-reciprocal active balun acting as a splitter device, while  FIG. 4  shows a second structure arranged to form a non-reciprocal active balun acting as a combiner device. 
   For this purpose, the structure of  FIG. 3  has three transmission lines  33 ,  34  and  35 , coupled by means of active amplifier cells  31  and  32 , shown schematically in the drawing, each amplifier cell being formed from transistors (such as field-effect transistors) assembled to form a circuit of the cascode type (cells  32 ) or Darlington type (cells  31 ). These two types of circuit are known to provide a phase difference of about 180° with respect to each other in transmission. 
   The cells  31  and  32  are also polarized so as to provide directive active coupling between the main line  35  (the gate line) and the secondary lines  33  and  34  (the drain lines). Thus a signal on the line  35  can be propagated by active coupling to the lines  33  and  34 , but the reverse operation is not possible. The signal applied to the input  36  of the device is thus propagated onto the two secondary lines  33  and  34  coupled to the line  35  in the form of two signals in phase opposition, these signals being delivered on the outputs  37  and  38  respectively of the lines. 
   Similarly, the structure of  FIG. 4  also has three transmission lines  43 ,  44  and  45  for this purpose, these lines being coupled by means of active amplifier cells  41  and  42 , shown schematically in the drawing, each amplifier cell being formed from transistors (such as field-effect transistors) assembled to form a circuit of the cascode type (cells  42 ) or Darlington type (cells  41 ). The cells  41  and  42  are also polarized so as to provide active coupling between the secondary lines  43  and  44  (the gate lines) and the main line  45  (the drain line). Thus signals on the lines  43  and  44  can be propagated by coupling onto the line  45 , but the reverse operation is not possible. The signals applied to the inputs  46  and  47  of the device are thus propagated by coupling onto the main line  45  and form a combined signal corresponding to the difference between the two input signals, because of the intrinsic 180° difference between the phases of the cascode and Darlington circuits, which is delivered to the output  48 . 
   In these two structures, active coupling is thus provided by means of cascode cells  32 ,  42 , and Darlington cells  31 ,  41 , distributed between a gate line and a drain line. The number of cells is also determined, in particular, by the desired coupling and gain characteristics. The illustrations in  FIGS. 3 and 4  show non-limiting examples in which the lines have two cells. The propagation lines formed in this way are also terminated at their unused ends by terminal loads  39 ,  49 . 
   As stated previously, these two active structures have the advantage of making it possible to produce baluns having a large bandwidth and capable of operating at low frequencies, typically from about a hundred megahertz. However, baluns made in this way have the disadvantage of not being reciprocal in their operation, making it necessary to use different devices according to whether it is desired to split one signal into two signals or to combine two signals into one signal. Since said devices are produced in the form of integrated circuits, it is therefore necessary to use two different types of circuit. 
   We will now consider  FIG. 5  which shows a schematic diagram of the reciprocal active balun according to the invention. 
   As shown in  FIG. 5 , the architecture of the reciprocal balun according to one embodiment of the present invention is based on the integration of the architecture of an active balun  30  of the splitter type with that of an active balun  40  of the combiner type. These two architectures are initially integrated by connecting the outputs  37  and  38  of the propagation lines  33  and  34  of the splitter balun  30  to the inputs  46  and  47  of the propagation lines  43  and  44  of the combiner balun  40 . The integration is then continued by the connection of the common lines of each of the structures, in other words by the connection of the gate line  35  of the splitter balun to the drain line  45  of the combiner balun. 
   It should be noted that each link between these lines is made via a capacitor  51  in order to achieve correct operation of the structure, and, in particular, to enable the cells of the splitter and combiner baluns to be controlled independently. The integration of these two architectures is completed by the removal of the matching loads  39  and  49  connected to the initially unused ends  311  and  411  of the main lines  35  and  45  and of the secondary lines  33 ,  34  and  43 ,  44 . In the device according to this embodiment of the present invention, these ends are used to form the interconnection of the two common lines  35  and  45 , and to provide two connection points to the ports  52  and  53  for connecting the device to an external circuit. Finally, the integration is completed by the removal of the output  48  of the initial combiner circuit  40  and by the addition of a matching load  54 . 
   This produces a dual structure  50 , having a splitter element connected in series with a combiner element, and having three ports  36 ,  52  and  53 . This structure provides three propagation lines  55 ,  56  and  57 , the lines  55  and  56  being coupled to the line  57  by active transistor cells. Advantageously, this dual structure makes it possible to produce an active splitter balun and an active combiner balun with equal facility, the ports  36 ,  52  and  53  being used alternatively as inputs or outputs. The rest of the description explains how the structure according to this embodiment of the present invention can be used to provide one or other function. 
   In the device according to this embodiment of the present invention, which is a reciprocal active balun, the coupling of the line  55  to the line  57  is provided by active Darlington cells  31  and  41 . The amplifier cells  31  are used for the directive coupling of the line  57  to the line  55 , and the amplifier cells  41  are used for the directive coupling of the line  55  to the line  57 . Similarly, the coupling of the line  56  to the line  57  is provided by active amplifier cells of the cascode type,  32  and  42 . The cells  32  are used for the directive coupling of the line  57  to the line  56 , and the cells  42  are used for the directive coupling of the line  56  to the line  57 . However, this arrangement of active cells is not the only possible one, as the device operates equally well if the cells  41  and  42  are interchanged. 
   To enable this dual architecture to operate, the device according to this embodiment of the present invention also includes means, not shown in  FIG. 5 , for providing, according to the desired operating mode (splitter balun or combiner balun mode), the corresponding polarization of the different active cells  31 ,  32 ,  41  and  42 . According to this embodiment of the present invention, it is possible to use any means for polarizing the different cells providing the active coupling, to bring them into either an active (or “passing”) state or an inactive state. This means can, for example, be a control device which polarizes the different cells, as a function of the value of a control voltage applied to an input of the device according to the invention, so as to establish an appropriate coupling direction for the operating mode concerned. 
   It is important to note that, when a line (a gate line or a drain line) or a portion of line is connected to cells polarized to the “inactive” state, this line becomes purely passive and decoupled from any other line parallel to it, and that a signal which may be propagated along this line, in one or other direction, undergoes very little attenuation.  FIGS. 6 and 7  illustrate the description of the two operating modes which characterize this embodiment of the present invention. 
     FIG. 6  is a schematic illustration of the operation of this embodiment of the device according to the present invention when it is configured to operate as a splitter balun. In this first configuration, the amplifier cells  31  and  32  are polarized in such a way that they provide active coupling of the line  57  to the lines  55  and  56 , while the amplifier cells  41  and  42  are polarized in such a way that they are inactive. As the drawing also shows, if a signal is applied to the input  36  of the line  57  it is propagated by active coupling onto the lines  55  and  56  and to the outputs  52  and  53  of the device, as shown by the arrows  71 . In this first configuration, the active cells of the splitter element are therefore polarized in such a way that they amplify the signal between the input line  57  (the gate line in this case) and the two output lines  55  and  56  (the drain lines in this case). Since the splitter element is composed of cascode cells  32  and Darlington cells  31 , the signal arriving on the gate line via the input port  36  is split into two signals having the same amplitude and in phase opposition, which are available at the output ports  52  and  53 . 
   Conversely, the active cells of the combiner element are polarized in such a way that the amplifier cells which form them isolate the lines  55  and  56  from the line  57 , thus preventing the signals on these lines from being recombined in any way on the line  57 . 
     FIG. 7  is a schematic illustration of the operation of the device according to this embodiment of the present invention when it is configured to operate as a combiner balun. In this first configuration, the cells  41  and  42  are polarized in such a way that they provide active coupling of the lines  55  and  56  to the line  57 , while the cells  31  and  32  are polarized in such a way that they are inactive. Thus, as shown in the drawing, if signals are applied to the inputs  52  and  53  of the lines  55  and  56 , they are combined by active coupling on the line  57 , the signal resulting from this combination being propagated to the output  36  of the device, as shown by the arrows  81 . In this second configuration, the active cells of the combiner element are therefore polarized in such a way that they amplify the signal between the two lines  55  and  56  (the gate lines in this case) and the line  57  (the drain line in this case). Since the combiner element is composed of cascode amplifier cells  42  and Darlington amplifier cells  41 , two signals arriving on the two gate lines  55  and  56 , with identical amplitudes and opposite phases, are added on the line  57 , the addition signal being delivered at the output port  36  of the device. 
   Conversely, the active amplifier cells of the splitter element are polarized in such a way that the amplifier stages which form them isolate the line  57  from the lines  55  and  56 , thus preventing any propagation on the lines  55  and  56  of the signal combined by coupling. 
   This embodiment of the device according to the present invention therefore acts as a reciprocal active balun.