Abstract:
The present invention relates to a bidirectional frequency mixer, as well as a radiofrequency transceiver system including at least such a mixer. The mixer includes two ports separated in intermediate frequency FI, one for the reception, the other for the emission and a common port in frequency RF both for reception and for emission. It also includes at least fours mixing cells and three phase shifting means of signals used to remove the undesirable frequencies generated by the mixing cells. The mixer enables a rejection of the frequencies produced by a local oscillator in transmitting phase and a rejection of the image phase in receiving phase to be preformed. The invention is in particular applicable to designing microwave integrated circuits, in particular in millimetric frequency band.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a bidirectional frequency mixer, as well as a radiofrequency transceiver system including such a mixer. In particular, it is applicable to designing microwave integrated circuits, in particular in millimetric frequency band. 
         [0002]    The radiofrequency communication systems generally include receiving and transmitting devices such as antennas, a processing unit as well as an interface enabling signals to be exchanged between the transceiver devices and the processing unit. 
         [0003]    On one hand, to make processable by a processing unit a signal received by the receiving/transmitting device, and on the other hand, to allow to transmit a signal produced by the processing unit, the receiving and transmitting functions generally comprise amplification, filtering, mixing and modulation/demodulation steps. The mixers are, among other things, used to translate high frequency signals to lower frequency bands in order to make the processing easier. In particular, the use of some filters, calculators or demodulators sometimes requires operating at less high frequencies, called intermediate frequencies. 
         [0004]    One of the problems encountered with the communication systems is the simultaneous transmission/reception, or at least the transmission/reception in half-duplex mode. In particular, a further difficulty arises with using mixers. Indeed, there occurs undesirable frequency signals produced upon mixing, whether in receiving phase or in transmitting phase. 
         [0005]    In order to address these problems, it is known to use a signal transmitting string separated from a receiving string, as shown in  FIG. 1 . Each of these processing strings then includes its own components and can operate without disturbing the attendant string significantly. Such a structure has in particular the drawback to raise the cost and the size of the circuit. In particular, at least two mixers  5 ,  7  are required, one for the receiving phase that converts the high frequency signals to an intermediate frequency and the other for the transmitting phase that converts the signals from the processing unit in a higher frequency. In a millimetric frequency band, these elements can in particular be integrated on a microwave integrated circuit further called MMIC according to the Anglo-Saxon acronym “Monolithic Microwave Integrated Circuit”. However, the number and the size of the components to be integrated on this type of circuits is a crucial criterion to be taken into account in the designing phase. 
         [0006]      FIG. 1  presents a radiofrequency transceiver system according to the prior art. The system  1  includes an antenna  2 , a switching device  3 , a processing unit  4 , a local isolator  5  and two mixers  6  and  7 . 
         [0007]    In receiving phase, a signal S RF  of frequency RF picked up by the antenna  2  is transmitted by the switching device  3  towards a first input  6   a  of the first mixer  6 . By combining a signal S RF  with a signal of frequency F OL  provided on a second input  6   b  by the local oscillator  5 , the first mixer  6  produces a signal S FI  on an output  6   c  at an intermediate frequency compatible with the operation of the processing unit  4 . 
         [0008]    In transmitting phase, the processing unit  4  provides on a first input  7   c  of the second mixer  7  a signal S FI  of frequency FI. By combining the signal S FI  with a signal of frequency F OL  provided by the local oscillator  5  on the second input  7   b , the second mixer  7  produces on an output  7   c  a signal S RF ′ of frequency RF. The signal S RF ′ is then transmitted by the switching device  3  to the antenna  2  that can transmit it. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of the invention to use in particular a single mixer common to the transmitting and receiving strings performing a rejection of the undesirable frequencies in receiving phase and in transmitting phase. To that end, it is an object to the invention to provide a bidirectional signal mixer adapted to operate according to two modes:
       a reception mode, combining a radioelectric signal S RF+IM  comprising a frequency component RF and an image component frequency IM with a signal S FOL  of frequency F OL  to produce a signal S FI  of intermediate frequency FI,   an emission mode, combining a signal S FI  of intermediate frequency FI with a signal S FOL  of frequency F OL  to produce a radioelectric signal S RF ,       
 
         [0012]    the mixer comprising at least:
       four mixing cells, each being adapted to combine two signals to produce an output signal converted in frequencies with respect to the first of the two signals,   a first phase shifting means adapted to dispatch the signal S FOL  on each mixing cell,   a second phase shifting means adapted, in the reception mode, to dispatch the power of the signal S RF+IM  on each mixing cell,   a third phase shifting means adapted, in the reception mode, to combine the signals from said mixing cells to produce a signal S FI  free from a frequency component IM, the combined signals comprising image frequency components IM in opposite phase and frequency components FI in phase,
 
the third shifting means being also adapted, in the emission mode, to dispatch the power of the signal S FI  on each mixing cell and the second phase shifting means being also adapted, in the emission mode, to combine the signals from said mixing cells to produce a radioelectric signal S RF  free from the frequency component F OL , the combined signals comprising frequency components F OL  in opposite phase and frequency components RF in phase. Preferably, the shift means work in analogue mode.
       
 
         [0017]    According to one embodiment, the first phase shifting means comprises at least one coupler adapted to dispatch the power of an input signal onto two output signals, the first output signal S FOL   90°  being phase shifting with respect to the second output signal S FOL   0° . 
         [0018]    The first phase shifting means can comprise a power divider and two couplers, said divider dispatching the power of the signal S FOL  on one input of each coupler, each coupler dispatching the power of its incoming signal onto two output signals, the first output signal S FOL   90°  being phase shifting with respect to the second output signal S FOL   0° . 
         [0019]    The second phase shifting means can comprise at least three couplers a first input-output of the first coupler being connected to one input-output of the second coupler, and a second input-output of the first coupler being connected to one input-output of the third coupler, the second and third couplers being connected to the mixing cells, in order:
       to dispatch, at the output of the second and third couplers, the power of the signal S RF+IM  received by an input-output of the first coupler,   and/or to combine the signals received by the second and third couplers to produce a signal S RF  on a input-output of the first coupler.       
 
         [0022]    The third phase shifting means can comprise at least three couplers a first input-output of the first coupler being connected to one input-output of the second coupler, and a second input-output of the first coupler being connected to one input-output of the third coupler, the second and third couplers being connected to the mixing cells, in order:
       to dispatch, at the output of the second and third couplers, the power of the signal S RF+IM  received by an input-output of the first coupler,   and/or to combine the signals received by the second and third couplers to produce a signal S RF  on a input-output of the first coupler.       
 
         [0025]    The invention also relates to a method for implementing a bidirectional signal mixer as described above, comprising the following steps of:
       combining, for a period of time Δt 1 , an input signal of frequency RF with an input signal of frequency F OL  in order to produce at the output a signal of frequency FI,   combining, for a period of time Δt 2 , time offset with respect to Δt 1 , an input signal of frequency FI with an input signal of frequency F OL  in order to produce at the output a signal of frequency RF.       
 
         [0028]    The invention also relates to a radiofrequency transceiver system comprising at least one bidirectional mixer comprising the features described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    Other features and advantages will become apparent to the reader from the illustrative unrestricted detailed description that follows with respect to the appended drawings in which: 
           [0030]      FIG. 1  shows a radiofrequency transceiver system according to the prior art, the figure having already being presented, 
           [0031]      FIG. 2  shows a radiofrequency transceiver system including a bidirectional frequency mixer according to the invention, 
           [0032]      FIG. 3  shows an exemplary embodiment of a mixer according to the invention with arrowed marks on the propagation directions of the signals corresponding to the receiving phase, 
           [0033]      FIG. 4  shows an exemplary embodiment of a mixer according to the invention with arrowed marks on the propagation directions of the signals corresponding to the transmitting phase, 
           [0034]      FIG. 5  shows an exemplary use of the mixer according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]      FIG. 2  shows a transceiver system of a bidirectional mixer  9 , according to the invention, enabling signals in receiving phase and in transmitting phase to be converted. 
         [0036]    A system  8 , whose detailed operation is described below, operates according to a half-duplex mode. The system  8  includes the bidirectional mixer  9 , a signal processor device  10 , an antenna  11  and a local oscillator  12  providing a translation frequency. The mixer  9  includes a first input  9   b , a second input  9   d , an input-output  9   c  and an output  9   c . The system  9  operates as described below for example. 
         [0037]    During a period of time Δt 1 , the system  8  operates in receiving mode, that is the antenna  11  picks up outside signals. These signals are transmitted to the mixer  9 , and then to the processing unit  10 . When the period Δt 1  is completed, the antenna  11  switches in transmitting mode for a period Δt 2 , for example. The mixer  9  does not necessary include a switching device to move from one mode to the other. The presence or the absence of signals on the input-output  9   a  is sufficient to define the operating mode for example. The signals from the processing unit  10  are then transmitted to the mixer  9  and then to the antenna  11  that transmits. When the period Δt 2  is completed, the antenna  11  switches again into the receiving mode for a period Δt 2  and the cycle starts again. 
         [0038]    The mixer  9  is said bidirectional as it can proceed to a frequency translation in both directions, that is it converts signals from one frequency A to a frequency B and from the frequency B to the frequency A. 
         [0039]    In receiving phase, the bidirectional mixer  9  receives on the input-output  9   a  a signal S RF  of frequency RF from the device of the antenna  11 , and on the first input  9   b , a signal F OL  of frequency Fol produced by the local oscillator  12 . The role of the mixer  9  is then to convert the signal S RF  to a signal S FI  delivered by the output  9   c , of reduced intermediate frequency FI equal to |RF−Fol|. For example, a mixer can be used in a radiofrequency transmitter/receiver, which receives a carrier wave at a frequency RF equal to 40 Ghz and should translate this signal at an intermediate frequency FI equal to 5 Ghz. In this case, the local oscillator delivers a signal frequency RF−FI equal to 35 GHz of a signal frequency RF+FI equal to 45 Ghz. In the exemplary embodiment presented in this description, the translation frequency Fol is selected to be lower than RF and consequently Fol=RF−FI. 
         [0040]    However, the input signal S RF  can be noisy. In particular, it can contain a parasitic frequency component at the frequency IM equal to Fol−Fi, commonly called the image frequency by those skilled in art. As the mixer  9  acts on a wide band of the frequency spectrum, this noise S IM  at the frequency IM can be translated at the frequency |IM−Fol| equal to FI and disturb the signal from the output  9   c  by adding to the wanted signal. A mixer  9  according to the invention performs a rejection of the image frequency IM and the signal from the output  9   c  is then composed of two main frequency components: FI and Fol. The undesirable frequency Fol can be readily filtered because it is generally much higher than FI. 
         [0041]    In transmitting phase, the mixer  9  receives on the second input  9   d  a signal S FIT  of frequency FI and on the first input  9   b  a signal S FOL  Of frequency Fol that can be produced by the same local oscillator  12  as for the receiving phase. The role of the mixer  9  is then to convert the signal S FIT  to a signal of higher frequency RF outgoing on the input-output  9   a . The antenna  11  then emits this signal. 
         [0042]    The mixer  9  uses a signal S Fol  that has generally a strong power in order to perform a frequency translation for the emission. As described hereinafter, a mixer can transmit at the output a portion of the power at the input frequencies. However, it is to be avoided to transmit the component Fol by the input-output  9   a  because Fol is relatively close to RF, and the closer it is to the RF smaller FI is, it would be then difficult to remove this frequency Fol by filtering. According to a method described hereinafter in relation to  FIG. 3 , the mixer  9  proceeds consequently to a rejection of the frequency Fol and the signal from the input-output  9   a  is then composed of the single frequency component RF. 
         [0043]    According to another embodiment, the system  8  includes two antennas, a first for the emission and a second for the reception of signals. In that case, the link  13  can be split, for example with a power divider, an input of the power divider being connected to the first antenna and another input being connected to the second antenna. This embodiment is not shown in the figures. 
         [0044]    Filtering or amplifying means, not represented herein, can also be placed on the ways connecting the mixer  9  to other elements  10 ,  11  and  12  of the system  8 . 
         [0045]      FIG. 3  shows an exemplary embodiment of the invention with arrowed marks representing the propagation directions of the signals corresponding to the receiving phase. 
         [0046]    The bidirectional mixer  9  can be made by using passive elements such as a first coupler  14 , a second coupler  15 , a third coupler  16 , a fourth coupler  17 , a fifth coupler  18 , a sixth coupler  25 , a seventh coupler  26  and a eighth coupler  27  but also with active elements such as a first mixing cell  19 , a second mixing cell  20 , a third mixing cell  21  and a fourth mixing cell  22 . 
         [0047]    The fifth coupler  18  includes a first input-output  18   a , a second input-output  18   b , a third input-output  18   c , and a fourth input-output  18   d . The signal incoming from the first input-output  18   a  is power dispatched on the inputs-outputs  18   b  and  18   c . On the third input-output  18   c , the signal is outputted with a phase shifting of 90° whereas when it is transmitted directly and thus without phase shifting to the second input-output  18   b . The fourth input-output  18   d  is simply connected to a resistive load  23 . The other couplers  14 ,  15 ,  16 ,  17 ,  25  and  26 , except for the eighth coupler  27 , operate the same way by performing a phase shifting of 90°. The eighth coupler  27  includes four inputs-outputs, a first input-output  27   a , a second input-output  27   b , a third input-output  9   c  and a fourth input-output  9   d . When two signals enter by the first input-output  27   a  and the second input-output  27   b , respectively, their powers are combined to be outputted on the third input-output  9   c . When the signal is received by the fourth input-output  9   c , it is power dispatched on the first and second inputs-outputs  27   a ,  27   b . The signal coming then from the first input-output  27   a  is phase shifted of 180° whereas the signal from the second input-output  27   b  is not phase shifted. 
         [0048]    A first mixing cell  19  includes a first port  19   a , a second port  19   b  and a third port  19   c . Its function is in particular to combine two input signals of respective frequencies A and B to produce a signal at the output including the following frequency component A, B, A+B and |A-B|. These are the frequency components A+B and |A-B| that are useful, because these are what enables an input frequency to be translated to the lower frequency, for example for reception, or higher, for example for emission. The undesirable components can for example be filtered at the output. Within the operation described herein, the first port  19   a  remains an input whereas the two other ports  19   b  and  19   c  operate in opposition and become alternatively an input or an output of the cell  19  depending on whether the mixer  9  is in receiving or transmitting mode. The other mixing cells  20 ,  21  and  22  described below operate the same way. 
         [0049]    In receiving phase, the input signal S RF  goes through the fifth coupler  18  that generates a signal S RF   90° , of the same frequency, phase shifted of 90° on its third input-output  18   c , whereas its second input-output  18   b  produces a signal S RF   0°  of the same frequency and of the same phase as S RF . On the other hand, the input signal S RF  can contain noises about the image frequency IM=Fol−FI. A noise S IM  on this frequency is potentially troublesome because it is translated by a mixing cell to the intermediate frequency FI, causing then interference to the wanted signal. A noisy signal S RF +S IM  is consequently transmitted into the first coupler  18  and the signals S RF   0° +S IM   0° and S RF   90° +S IM   90° , exit from this coupler by its second  18   b  and its third input-output, respectively. According to the same principle, the signals S RF   0° +S IM   0°  and S RF   90° +S IM   90° , go through the third and fourth coupler  16  and  17  respectively via the first inputs-outputs  16   a  and  17   a  to undergo a phase shifting of 90°. Thus, the second and third inputs-outputs  16   c ,  16   b ,  17   b  and  17   c  of the third and fourth couplers  16  and  17  all produce a signal of the noisy frequency RF, but each with their own phase shifting. The signals S RF   90° +S IM   90°  from the third input-output  16   c  of the third coupler  16  and of the second input-output  17   b  of the fourth coupler  17  are phase shifted of 90°, the signals S RF   90° +S IM   90°  from the second input-output  16   c  of the third coupler  16  is not phase shifted and the signal S RF   180° +S IM   180°  from the third input-output  17   c  of the fourth coupler  17  is phase shifted of 180°. 
         [0050]    Likewise, the signal S FOL  produced by the local oscillator  12  is transmitted to the first and second couplers  14  and  15  after going through a power divider  24  to dispatch the signal to their first two inputs-outputs  14   a  and  15   a . According to the same principle as for the third and fourth couplers  16  and  17 , the signals exiting from the first and second couplers  14  and  15  are of the same frequency Fol but of different phases. The signals S FOL   90° , from the third inputs-outputs  14   c  and  15   c  of the first and second couplers  14  and  15  are phase shifted of 90° whereas the signals S FOL   90°  from the second inputs-outputs  14   b  and  15   b  of the first and second couplers  14  and  15  are not phase shifted. 
         [0051]    According to another embodiment, the signals S FOL   0°  and S FOL   90° , produced at the output of the first and second couplers  14  and  15  are produced by using a single coupler receiving the signals S FOL  produced by the local oscillator  12 . A power divider is then placed at each of both outputs of said coupler to dispatch the power of the two signals S FOL   0°  and S FOL   90° , into four signals of substantially equal powers. These four signals are then dispatched at the output so as to present successively the same phase shifting as those produced by the previous embodiment, implying two couplers. 
         [0052]    The signals entering the first and third ports ( 19   a  and  19   c ) ( 20   a  and  20   c ), ( 21   a  and  21   c ), ( 22   a  and  22   c ) of each mixing cell  19 ,  20 ,  21  and  22  produce the following combinations: 
         [0053]    S FOL   90° , with S RF   90° +S IM   90° , on the first mixing cell  19 , 
         [0054]    S FOL   0°  with S RF   0° +S IM   0°  on the second mixing cell  20 , 
         [0055]    S FOL   0°  with S RF   90° +S IM   90° , on the third mixing cell  21 , 
         [0056]    S FOL   90°  with S RF   180° +S IM   180°  on the fourth mixing cell  22 . 
         [0057]    According to the operation of the mixing cell described above, the frequency component at the output of the mixing cell are: Fol, RF, IM, RF+Fol, IM+Fol, and FI=RF−Fol=Fol−IM. The components Fol, RF, IM, and RF+Fol are undesirable but can be readily filtered subsequently because these frequencies and FI deviate strongly. To be more clear, these components although potentially present in the frequency spectrum, will thus be ignored in the following of the description. The frequency component FI comes from both RF-Fol and Fol-IM, accordingly the frequency spectrum at the output is subjected to interference by a signal created by the translation of the frequency IM. This parasitic signal must then be removed. 
         [0058]    A signal S FI   90° , of frequency FI from the second port  21   b  of the third mixing cell  21  is the product of the translation of the signal S RF   90° , phase shifted of 90° and of frequency RF by the signal S FOL   0°  of frequency Fol. The noise S IM   90°  of image frequency IM is also translated into the signal S FI/IM   −90°  of frequency FI. However, this signal S FI/IM   −90°  is in opposite phase with respect to the signal S FI   90°  from the component RF. Let us remind that IM=Fol−FI and thus that IM−Fol=−FI whereas RF−Fol=FI. At the output of the second port  21   b  of the third mixing cell  21 , there is thus a signal of frequency FI S FI   90° +S FI/IM   −90° . In a similar mode, the fourth cell  22  processes input signals S FOL   90° , and S RF   180° +S IM   180° mutually phase shifted of 90°, accordingly the signal from the second port  22   b  of the fourth mixing cell  22  is the same as that from  21   b . For the first and second cells  19  and  20 , the input signals S FOL   90° , and S RF   90° +S IM   90° , on the one hand, and S RF   0° +S IM   0°  on the other hand, are not phase shifted between each other. At the outputs of the second ports  19   b  and  20   b  of the first and second mixing cells, there are consequently the non phase shifted signals S FI   0° +S FI/IM   0° . 
         [0059]    The signals from the second ports  19   b  and  22   b  of the first and fourth mixing cells  19  and  22  are combined again and the sixth coupler  25  and the signals from the second ports  20   b  and  21   b  of the second and third mixing cells  20  and  21  are combined again in the seventh coupler  26 . 
         [0060]    Thus, the sixth coupler  25  receives the signal S FI   0° +S FI/IM   0°  from the second port  19   b  of the first mixing cell by its third input-output  25   c  and it receives the second signal S FI   90° +S FI/IM   −90°  from the second port  22   b  of the fourth mixing cell by its second input-output  25   b . Likewise, the signal S FI   0° +S FI/IM   0°  from the second port  20   b  of the second mixing cell  20  enters the seventh coupler  26  through its third input-output  26   c  and the signal S FI   90° +S FI/IM   −90°  from the second port  21   b  of the third mixing cell enters by its second input-output  26   b . The signals entering by the third input-output  25   c  and  26   c  of the sixth and seventh couplers are phase shifted of 90°. Thus, the following combinations are performed:
       For the sixth coupler  25 , S FI   0°+90° +S FI/IM   0°+90°  (third input-output  25   c  phase shifted) with S FI   90° +S FI/IM   −90°  (second input-output  25   b  non phase shifted),   For the seventh coupler  26 , S FI   0°+90° +S FI/IM   0°+90°  (third input-output  26   c  phase shifted) with S FI   90° +S FI/IM   −90°  (second input-output  26   b  non phase shifted).       
 
         [0063]    The powers between the inputs are substantially equally dispatched, consequently it is the signal S FI   0°+90° +S FI/IM   0°+90° +S FI   90° +S FI/IM   −90° =S FI   90°  that exits from the first input-output  25   a  of the sixth coupler with a power output twice as that of the signal S FI   90° , entered by the second input-output  25   b  of the sixth coupler, this because of the combination of two signals entered by the second and third inputs-outputs  25   b  and  25   c . As the seventh coupler  26  applies the same operation as the coupler  25  on signals of identical input, a signal S FI   90° , also exits from the first input-output  26   a  of the sixth coupler. The signals S FI/IM  when removed with the recombination of signals of substantially the same power S FI/IM   90°  and S FI/IM   −90°  in opposite phase. The mixer  9  thus proceeds to a rejection of the image frequency during the receiving phase. 
         [0064]    Finally, the signals S FI   90°  from the first inputs-outputs  25   a  and  26   a  of the sixth and seventh couplers are combined again in the eighth coupler  27 . These two signals are received by the first and second inputs-outputs  27   a  and  27   b  of the eighth coupler and combine to exit on the output  9   c  of the mixer  9 . 
         [0065]    To sum up, the signal S RF +S IM  entering on the input-output  9   a  of the mixer  9  combined with the signal S FOL  entering on the first input-output  9   b  of the mixer  9  produces a signal S FI   90°  exiting on the output  9   c  of the mixer  9 . frequency components Fol, RF and RF+Fol from mixing cells  19 ,  20   21 ,  22 , then transmitted at the output  9   c , can then be readily filtered because these frequencies and FI deviate strongly. 
         [0066]    The phase shifts are applied with three groups of couplers. A first group consisting in the first and the second couplers (14, 15) is the first means enabling the signals from the local oscillator  12  to be phase shifted. A second group, consisting in the third, the fourth and the fifth couplers (16, 17, 18), is a second means enabling the signals of frequency RF to be phase shifted. A third group consisting in the sixth, the seventh and the eighth couplers (25, 26, 27) is a third means enabling the second frequency FI to be phase shifted. 
         [0067]      FIG. 4  presents an exemplary embodiment of the invention with arrowed marks on the propagation directions of the signals corresponding to the transmitting phase. 
         [0068]    During this phase, a signal S FI  is addressed to the mixer by its second input  9   d . The eighth coupler  27  then dispatches the signal S FI  on its first and second inputs-outputs  27   a  and  27   b . The signal S FI   180°  from the first input-output  27   a  of the eighth coupler  27  is phase shifted of 180° with respect to the input signal S FI , whereas the signal S FI   0°  from the second input-output  27   b  of the eighth coupler  27  is not phase shifted. The signal S FI   180°  goes then through the sixth coupler  25  via its first input-output  25   a  and the signal S FI   0°  enters the seventh coupler  26  by its first input-output  26   a . When the signals S FI   180°  and S FI   0°  have gone through these two couplers  25  and  26 , each of them is divided again into two signals. The signals S FI   180°  and S FI   0°  coming from the second inputs-outputs  25   b  and  26   b  of the sixth and the seventh couplers  25  and  26  are not phase shifted whereas the signals S FI   −90°  and S FI   90° , coming from the third inputs-outputs  25   c  and  26   c  of the sixth and seventh couplers respectively are phase shifted of 90°. As previously described in the receiving phase, the sixth and seventh couplers  25  and  26  are connected to the mixing cells  19 ,  20 ,  21 ,  22 . The initial signal S FI  switching the second input-output  9   d  of the mixer  9  is thus dispatched on the second ports  19   b ,  22   b ,  21   b ,  20   b  of the mixing cells  19 ,  22 ,  21 ,  20  with a phase difference of 90° in between each successive way:
       on the second port  19   b  of the first mixing cell  19 , a signal S FI   −90°  of frequency FI phase shifted of −90°,   on the second port  22   b  of the fourth mixing cell  22 , a signal S FI   180°  of frequency FI phase shifted of 180°,   on the second port  21   b  of the third mixing cell  21 , a signal S FI   0°  of frequency FI not phase shifted of,   on the second port  20   b  of the second mixing cell  20 , a signal S FI   90° , of frequency FI phase shifted of 90°.       
 
         [0073]    In parallel, the local oscillator delivers, as for the receiving phase, a signal S FOL  of frequency Fol that is transmitted and phase shifted by the first and second couplers  14  and  15 . Each mixing cell then receives two signals of frequencies FI and Fol, but each with a different combination of phases. Indeed,
       the first mixing cell  19  combines S FI   −90°  on the second port  19   b  with S FOL   90° , on the first port  19   a,      the second mixing cell  20  combines S FI   90° , on the second port  20   b  with S FOL   0°  on the first port  20   a,      the third mixing cell  21  combines S FI   0°  on the second port  21   b  with S FOL   0°  on the first port  21   a,      the fourth mixing cell  22  combines S FI   180°  on the second port  22   b  with S FOL   90° , on the first port  22   a.          
 
         [0078]    According to the operation of a mixing cell described above, the frequency components at the output of the mixing cells are FI, Fol, IM=Fol−FI, and RF=FI+Fol. The components FI, Fol, and IM are undesirable. The component FI can be readily filtered subsequently because FI and RF deviate strongly, it will thus be ignored in the following of the description. On the other hand, the components Fol and IM are potentially close enough to RF to both in particular a pass-band filtering about RF. Therefore, they must be removed. 
         [0079]    Thus, at the output of the mixing cell, there is obtained:
       on the third port  19   c  of the first mixing cell  19 , S FI+Fol   −90°+90° +S Fol−FI   90°−(−90)° +S Fol   90° =S RF   0° +S IM   180° +S Fol   90° ,   on the third port  20   c  of the second mixing cell  20 , S FI+Fol   90°+0° +S Fol−FI   0°−90° +S Fol   0° =S RF   90° +S IM   −90° +S Fol   0° ,   on the third port  21   c  of the third mixing cell  21 , S FI+Fol   0°+0° +S Fol−FI   0°−0° +S Fol   0° =S RF   0° +S IM   0° +S Fol   0° ,   on the third port  22   c  of the fourth mixing cell  22 , S FI+Fol   180°+90° +S Fol−FI   90°−180° +S Fol   90° =S RF   −90° S IM   −90° +S Fol   90° .       
 
         [0084]    According to the same principle as in the receiving phase, the signals are combined again in the third, the fourth and the fifth couplers  16 ,  17  and  18 . 
         [0085]    The signal S RF   0° +S IM   180° +S Fol   90° , exiting the third port  19   c  of the first mixing cell  19  is transmitted to the third coupler  16  via its third input-output  16   c  and is phase shifted of 90°. The signal S RF   90° +S IM   −90° +S Fol   0°  exiting the third port  20   c  of the second mixing cell  20  is not phase shifted. The third coupler  16  thus proceeds to the following combination: (S RF   0°+90° +S IM   180°+90° +S Fol   90°+90° )+(S RF   90° +S IM   −90° +S Fol   0° ) and the signal from the first input-output  16   a  of the third coupler  16  is thus S RF   90° +S IM   90° , with the power substantially equal to the sum of the input powers. The component Fol has been removed with the combination of signals in opposite phase of substantially equal powers S Fol   180°  and S Fol   0° . The same principle proceeds in the fourth coupler  17  with the following combination: (S RF   0° +S IM   0° +S Fol   0° )+(S RF   −90°+90° +S IM   −90°+90° +S Fol   90°+90° ). The signal from the first input-output  17   a  of the third coupler  17  is thus S RF   0° +S IM   0° , with the power substantially equal to the sum of the input powers. Once more, the combination of input signals results in a rejection of the frequency Fol at the output. 
         [0086]    The two signals form the first two inputs-outputs  16   a ,  17   a  of the third and fourth couplers  16  and  17  are then combined again in the fifth coupler  18  by going through the second and third inputs-outputs  18   b  and  18   c  respectively. The following combination is then performed: (S RF   90° +S IM   −90° )+(S RF   0°+90° +S IM   0°+90° ). As the powers of the second and third inputs-outputs  18   b  and  18   c  of the fifth coupler  18  are substantially equivalent, the fifth coupler  18  proceeds to a rejection of the image frequency by combining the two component S IM   −90°  and S IM   90°  in opposite phase. The signal from its first input-output  18   a  is thus S RF   90° . 
         [0087]    To sum up, the signal S IF  entering the second input  9   d  of the mixer  9  combined with the signal S FOL  entering on the first input  9   b  of the mixer  9  produces the signal S RF   90°  exiting on the input-output  9   a  of the mixer  9 . The frequency components Fol and IM coming from the mixing cells are thus removed with judicious combinations of signals in opposite phase. The frequency FI coming from mixing cells and transmitted in the input-output  9   a  can be readily filtered because RF and FI deviate strongly. 
         [0088]    According to another embodiment, the local oscillator  12  can deliver a frequency Fol equal to RF+FI. In that case, the frequency translation performed by the mixer enables, once more, a signal of frequency FI to be obtain at the output, but the image frequency to remove is equal to Fol+Fi and not to FI anymore. The architecture of the mixer remains valid in this alternative. Indeed, for the signal RF to go through the first input-output  18   d , the couplers should simply be adapted to the frequency Fol and the connections to the first and the fourth inputs-outputs  18   d  and  18   a  of the fifth coupler should be switched. Preferably, a frequency Fol equal to RF−FI is chosen because it is generally easier to deal with small frequencies. 
         [0089]    It is an advantage of the invention to directly proceed to the rejection of images directly in the mixer. This avoids adding a filter, always to the detriment of the size of the circuit and sometimes even impossible to integrate on an MMIC. 
         [0090]    It is another advantage of the invention to give to the mixer a larger linearity of power. Indeed, each mixing cell is susceptible to the saturation phenomenon of the output power when the input power becomes too large and, therefore, loses its linear feature. Each mixing cell is in particular characterized by its output power at the 1 dB compression point. Let us remind that the 1 dB compression point is, on a curve representing the output power as a function of the input power, the point for which the deviation between the output power and its linear extrapolation is 1 dB. A mixer including four mixing cells  19 ,  20 ,  21  and  22  of the same 1 dB compression point in parallel has the 1 dB compression point higher than that of each of the cells taken separately, thanks to the recombination of the powers in the output couplers  25 ,  26  and  27  downstream—in the reception mode—and in the couplers  16 ,  17  and  18  upstream—in the emission mode. 
         [0091]      FIG. 5  represents an exemplary circuit incorporating a single mixer for a millimetric range communication application. 
         [0092]    The circuit  50  includes a bidirectional frequency mixer  9 , a frequency multiplier  51 , a low noise amplifier  52 , a first voltage controlled amplifier  53 , a coupling component  54  and a second voltage controlled amplifier  55 . 
         [0093]    The circuit  50  is composed of two main portions. The first portion  50   a  contains the mixer  9 , the low noise amplifier  52 , the first voltage controlled amplifier  53  and the frequency multiplier  51 . The second portion  50   b  includes the coupling component  54  and the second voltage controlled amplifier  55 . 
         [0094]    The first portion  50   a  includes elements that operate at millimetric band frequencies, which enables this portion of the circuit to be integrated in the MMIC. The mixer  9  is connected to the frequency multiplier  51  receiving a signal produced by a local oscillator, connected to the low noise amplifier  52  receiving an antenna signal, connected to the first voltage controlled amplifier  53  enabling the signals to be transmitted to be amplified, and connected to the coupling component  54 . On the other hand, the coupling component  54  receives signals at the intermediate frequency FI through the second voltage controlled amplifier  55 , and it also transmits signals at the frequency FI. 
         [0095]    The coupling component  54  comprises the sixth, seventh and eighth couplers  25 ,  26 ,  27  operating at the intermediate frequency FI. In the described example, the frequency FI is equal to 5 Ghz. This frequency is too low to be able to position the too bulky, second portion  50   b  into an integrated circuit, for this reason, the constituting elements of the second portion  50   b  will be made, by way of example, from discrete components or lines divided on a printed circuit.