Patent Publication Number: US-2021175605-A1

Title: System of at least two transmitting and/or receiving units connected to a common antenna

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase application of PCT International Application No. PCT/FR2018/052850, filed Nov. 15, 2018, which claims priority to French Patent Application No. 1760845, filed Nov. 17, 2017, the contents of such applications being incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a system of at least two transmitting and/or receiving units connected to a common antenna. A first unit transmits and/or receives a signal at a first frequency and at least a second unit transmits and/or receives a signal at a second frequency. Each of the units is individually connected to the common antenna by a first branch and a second branch respectively for the passage of the signal. 
     The invention is particularly applicable to a transmission and/or reception device having at least two frequencies. Such a transmitting and/or receiving device may form part of a fob for gaining “hands-free” access to a motor vehicle or of a tire pressure sensor in a wheel unit fitted to a wheel of a motor vehicle. 
     BACKGROUND OF THE INVENTION 
     Such a transmission and/or reception device integrated into an access fob communicates by radiofrequency waves with an electronic unit on board a motor vehicle, in order to unlock the doors or the trunk of the vehicle or perform another function such as starting the vehicle without the user having to manually operate the vehicle doors. 
     Such a transmitting and/or receiving device may be integrated in a wheel unit comprising a tire pressure sensor of a wheel of a motor vehicle, and fastened either to the rim of the wheel of the vehicle, or to the tread of a vehicle tire. The pressure sensor communicates by radiofrequency waves with a central electronic unit on board the vehicle in order to send, to the central electronic unit, pressure measurements that are, if necessary, coupled with measurements of other parameters, such as temperature, that are carried out by the sensor in the tire, and thus warn the user of a possible underinflation of the tire. Such a transmission and/or reception device transmits a radiofrequency signal and/or receives in return a radiofrequency signal. 
     It is known to use a system of at least two connected transmitter and/or receiver units. Two solutions are proposed in the prior art. The first is to use a common antenna of complex geometry suitable for each frequency and the second solution is to have two antennas optimized respectively for each frequency. 
     When a single antenna is used for two transmit and/or receive frequencies, it is necessary to find a compromise in terms of antenna efficiency in order mainly to minimize the return loss for each frequency. This means that the efficiency of the antenna for each frequency is not optimal. 
     An active switch is also used in each branch connecting the antenna to a transmitting and/or receiving unit. This requires a control unit for each switch, which increases the cost of the system. Furthermore, the switches may be sources of failure. 
     When two antennas are used, the reception and/or the emission of each frequency is better, but cost is increased and the system made more bulky, which is unacceptable if it is to be used in a wheel unit or in an access fob, the available space being very limited in both cases. 
     SUMMARY OF THE INVENTION 
     The problem underlying the present invention is, for a system with at least two units for transmitting and/or receiving respective signals having two different frequencies, to provide a single signal transmitting and/or receiving antenna which is capable of receiving or transmitting signals optimally without the use of an active switch between each unit and the associated antenna. 
     To this end, an aspect of the present invention relates to a system of at least two transmitting and/or receiving units connected to a common antenna, a first unit that transmits and/or receives a signal at a first frequency and at least one second unit that transmits and/or receives a signal at a second frequency, each of said at least two units being individually connected to the common antenna for the passage of the radiofrequency signal by a first branch or a second branch, respectively, this system being noteworthy in that the first branch or the antenna comprises first passive electronic means preventing passage of the signal at the second frequency to the first unit while the first passive electronic means allow passage of the signal at the first frequency to the antenna, and in that the second branch or the antenna comprises second passive electronic means preventing passage of the signal at the first frequency to the second unit while the second passive electronic means allow passage of the signal at the second frequency to the antenna. 
     By passive electronic means, what is meant is electronic means that do not perform an active circuit-opening or -closing action but that have an electronic structure that selectively prevents the passage of the signal for the frequency not associated with the unit while these passive electronic means allow the pulsed voltage signal to pass for the frequency associated with the unit. It follows that the connection between a unit and the antenna is not completely cut, as it would be by an active switch, but is only for a signal with a predefined frequency that corresponds to the frequency of the voltage signal from the other unit. 
     The technical effect is that the antenna is supplied with a signal at the frequency desired among the first and second frequencies. Passive electronic means do not need to be controlled by a control unit like switches would, simplifying control and allowing miniaturization of the system. Passive electronic means are different from the mechanical or active means of conventional switches and are more efficient. The closure or opening of a branch for a signal of a given frequency occurs automatically with respect to the frequency of the signal. 
     Advantageously, at least the first passive electronic means or the second passive electronic means comprise at least one so-called trap circuit that is connected to the first or the second unit and that is formed of an inductor and a capacitor in parallel, the trap circuit being closed for all frequencies except a resonant frequency of the trap circuit, an inductance of the inductor and a capacitance of the capacitor of the trap circuit being selected so that the trap circuit connected to the first unit resonates at the second frequency and the trap circuit connected to the second unit resonates at the first frequency. 
     In this first embodiment, the resonant properties of a trap circuit are used to open the associated branch at the resonant frequency, which is chosen to be the frequency of the signal to be stopped. It is then possible to select a trap circuit that has a resonance at the frequency to be barred from a branch, so that the trap circuit acts as a passive electronic switch for this branch. 
     Advantageously, an inductor having a parasitic capacitance in parallel, and a resonant frequency being previously assigned to the inductor, the trap circuit associated with the first unit or with the second unit is formed of at least one inductor by itself forming a trap circuit, said inductor being selected to have a preset resonant frequency corresponding to the second frequency when said at least one inductor by itself forming a trap circuit forms the trap circuit associated with the first unit or to the first frequency when said at least one inductor by itself forming a trap circuit forms the trap circuit associated with the second unit, respectively. 
     This embodiment exploits the fact that an inductor behaves as though it had a parasitic capacitor in parallel and therefore in isolation acts as a trap circuit. By “in isolation” what is meant is that the inductor is not associated with a capacitor. In contrast, there may be several inductors in series resonating at different frequencies, in the case of a system with more than two transmitting and/or receiving units. Using the resonant properties of an inductor for which the resonant frequency is known and given by its manufacturer simplifies a trap circuit while keeping the same advantages. 
     Advantageously, at least the first passive electronic means or the second passive electronic means comprise, in the respective first branch or second branch or in the antenna, at least one series circuit formed of an inductor and a capacitor in series, the series circuit being open for all frequencies except a resonant frequency of the series circuit, an inductance of the inductor and a capacitance of the capacitor being selected so that the series circuit resonates at the second frequency towards the first branch or the first unit and at the first frequency towards the second branch or the second unit. 
     In this second embodiment, the resonant properties of a series circuit containing an inductor and capacitor in series are used to close the associated branch for the resonant frequency, which is chosen to be the frequency of the radiofrequency signal to be passed. It is then possible to select a series circuit having a resonance at the frequency to be driven into a branch so that the circuit acts as a passive electronic switch for this branch. 
     Advantageously, the series circuit formed of an inductor and a capacitor in series is mounted at one end of a stub from the first branch or second branch or the antenna, and is connected to ground at its other end. 
     Advantageously, the first passive electronic means take the form of a circuit blocking a specific frequency, said circuit being formed of an inductor and a capacitor in parallel or of a lone inductor, and the second passive electronic means comprise at least one series circuit formed of an inductor and a capacitor in series connected to ground. 
     It is thus possible to use a trap circuit or an inductor that by itself forms a trap circuit in one of the branches while the other branch comprises a series circuit connected to ground. It would also be possible to associate a trap circuit or an inductor that by itself forms a trap circuit with a series circuit connected to ground in the same branch, the isolation of the transmitting and/or receiving units of the circuit being guaranteed in the system by the presence of at least one of the trap circuits. 
     Advantageously, the antenna is a loop antenna connected at each of its ends to the first branch or the second branch, respectively, the trap circuits, associated with the series passive circuits, nonetheless being implemented. This embodiment does not limit the present invention, another form of antenna may also come within the scope of an aspect of the present invention. In contrast, this form of loop antenna is preferred since it may be supplied at each of its two ends by a respective transmitting and/or receiving unit at the first or at the second frequency. 
     Advantageously, the loop antenna has a stub branch internal to the antenna connecting it to ground, the internal stub branch comprising passive electronic stub means that open the stub branch for the signal at the lowest frequency of the first and second frequencies and that close the stub branch for the voltage signal at the highest frequency of the first and second frequencies. 
     If the antenna length is considered to have been optimized for the lowest frequency, for the highest frequency it is not necessary and even disadvantageous to use the entire antenna length. Shortening the antenna for the highest frequency allows antenna efficiency to be improved and the signal at the highest frequency to be better transmitted or received. 
     Advantageously, the internal stub branch is connected to the loop antenna at a distance, which is measured from one of the ends of the loop antenna, which is connected to the highest of the first and second frequencies, the distance being dependent on the highest frequency. The higher the frequency, the more it is possible to decrease the used length of the antenna. The antenna length is thus dependent on frequency, this improving antenna efficiency and therefore transmission. 
     Advantageously, the passive electronic stub means take the form of a series circuit of the respective internal stub branch, said circuit being formed of an inductor and a capacitor in series, the series circuit of the internal stub branch being open for all frequencies except a resonant frequency of the series circuit corresponding to the highest frequency of the first and second frequencies. 
     This makes it possible to use only a portion of the loop antenna for the highest frequency while the lowest frequency passes through the entire loop antenna. The transmission and/or reception of signals are/is improved. 
     Advantageously, the system comprises at least a third unit that transmits and/or receives a radiofrequency signal at a third frequency, said unit being connected to the antenna by a third branch, the third branch comprising third passive electronic means that open the third branch for a signal at the first frequency and at the second frequency while the third passive electronic means close the third branch for a signal at the third frequency. 
     There may then be, in the same branch, several passive electronic means that open the branch for a respective frequency. This may, for example, be achieved with two single inductors, which are said to be “in isolation” since they are not associated with a respective capacitor, and which are resonant at a respective frequency corresponding to the frequencies of the system that are not emitted or received by the unit associated with the branch housing the passive electronic means in question. 
     Advantageously, the antenna is a loop antenna, the third branch being a stub branch connected between the two ends of the loop antenna at a distance, measured from one of the ends of the loop antenna, depending on the third frequency (with a predetermined margin of error). 
     For one loop antenna, there may be one stub branch connected to ground and another stub branch connected to a third unit. There may be more than two stub branches for one loop antenna in the case of a system comprising more than three transmitting and/or receiving units. 
     The stub branch connected to the third unit may be at least partially merged with the branch connected to the ground, so as to end up stubbed out of the branch connected to the ground, provided that passive electronic means are also provided in the end part stubbed out of the branch connected to ground. This works when the third frequency is close to the highest frequency or corresponds to the portion of the loop antenna not traveled by the signal at the highest frequency, the length of the antenna being distributed between the signal at the highest frequency and the third frequency. 
     An aspect of the invention also relates to a wheel unit for a motor-vehicle wheel or a fob for gaining hands-free access to a motor vehicle, characterized in that it comprises such a system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features, aspects and advantages of the present invention will become apparent on reading the following detailed description and on examining the appended drawings given by way of non-limiting examples, and in which: 
         FIGS. 1A and 1B  are schematic representations of a system of two transmitting and/or receiving units connected to a common antenna, a first transmitting and/or receiving unit transmitting and/or receiving a radiofrequency signal at a first frequency and a second transmitting and/or receiving unit transmitting and/or receiving a radiofrequency signal at a second frequency according to an aspect of the present invention, the signal at the lowest frequency reaching the antenna in  FIG. 1A  and the signal at the highest frequency reaching the antenna in  FIG. 1B  by traversing only part of the antenna, 
         FIG. 2  is a schematic representation of a system of two transmitting and/or receiving units connected to a common antenna according to an embodiment of the present invention, the first passive electronic means taking the form of an inductor by itself forming a trap circuit that opens the first branch for a signal at the second frequency and that closes the same branch for the first frequency and the second passive electronic means taking the form of a series circuit connected to ground, which closes the second branch at the first frequency and opens the second branch for a signal at the second frequency, the antenna formed at the first frequency thus being of the loop short-circuited to ground type, the antenna formed at the second frequency being of the monopole type because it is open at its end, 
         FIG. 3  shows two curves of the loss in the first and second branches of the system shown in  FIG. 2 , the losses being highest at the resonance of the first and second passive electronic means, 
         FIGS. 4A and 4B  show a trap circuit with a capacitor mounted so as to bypass an inductor and a curve of the loss in the trap circuit, the maximum loss being at the resonance of the trap circuit, respectively, 
         FIGS. 5A and 5B  graphically show the attenuation of transmission or reception level at the resonant frequencies of coil inductors used as trap circuits by themselves and the resonant frequency of each coil as a function of the inductance of the coil, respectively, 
         FIGS. 6A-6C  respectively show a series circuit with a capacitor connected in series with an inductor, the series circuit being mounted stubbed out of the first or second branch and connected to ground in  FIG. 6B ,  FIG. 6C  showing a curve of the loss in the series circuit, the maximum loss between the two ports being due to an optimal short-circuit to ground at the resonance of the series circuit, and 
         FIG. 7  is a schematic representation of a system comprising three transmitting and/or receiving units connected to a common antenna according to one embodiment of the present invention, the third unit being connected at a point on a length of the antenna. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the figures, the antenna associated with the two transmitting and/or receiving units is a loop antenna. This is non-limiting and the antenna may take another form. 
     Below, reference is made to all the figures taken in combination, though the figures most relevant to the features being described will be mentioned. 
     With particular reference to  FIGS. 1A, 1B, 2 and 7 , an aspect of the present invention relates to a system of at least two transmitting and/or receiving units  1 ,  2  connected to a common antenna  4 . In  FIGS. 1A, 1B and 2 , two transmitting and/or receiving units  1 ,  2  are shown and in  FIG. 7 , three transmitting and/or receiving units  1 ,  2 ,  3  are shown. 
     A first unit  1  transmits and/or receives a signal at a first frequency and at least a second unit  2  transmits and/or receives a signal at a second frequency. To pass the respective signal, each of said at least two units  1 ,  2  is individually connected to the common antenna  4 , the first unit  1  being connected by a first branch  10  and the second unit being connected by a second branch  20 , respectively. 
     The first branch  10  or the antenna  4  comprises first passive electronic means  51   a ,  61   a  (which are therefore not controlled mechanical switches) preventing passage of the signal at the second frequency to the first unit  1  while the first passive electronic means  51   a ,  61   a  allow passage of the voltage signal pulsed at the first frequency to the antenna  4 . 
     The passive electronic means of a branch or of the antenna  4  are therefore configured not to let pass one or more signals with a frequency associated with another transmitting and/or receiving unit of the system. 
     Likewise, the second branch  20  or the antenna  4  comprises second passive electronic means  60   a ,  62   a  preventing passage of the voltage signal pulsed at the first frequency to the second unit  2  while the second passive electronic means  60   a ,  62   a  allow passage of the voltage signal pulsed at the second frequency to the antenna  4 . 
     It follows that the passive electronic means may be in a branch or incorporated into the antenna  4 . For example, in  FIG. 1A , first passive electronic means  61   a  are incorporated into the first branch  10  to prevent passage of the voltage signal pulsed at the second frequency to the first unit  1  while second passive electronic means  60   a  preventing passage of the voltage signal pulsed at the first frequency to the second unit  2  are incorporated into the antenna  4 . 
     In  FIG. 2 , passive electronic means  51   a ,  62   a  are incorporated into the first branch  10  and into the second branch  20 , respectively. In  FIG. 7 , which shows a system comprising three transmitting and/or receiving units, with a third unit  3  connected to the antenna  4  by a third branch  30 , first passive electronic means  61   a  are incorporated into the first branch  10  to prevent passage of the voltage signals pulsed at the second frequency from the second unit  2  and at a third frequency from the third unit  3  to the first unit  1 . 
     Likewise, second passive electronic means  62   a  are incorporated into the second branch  20  to prevent passage of voltage signals pulsed at the first frequency from the first unit  1  and at the third frequency from the third unit  3  to the second unit  2 . Finally, third passive electronic means  63   a  are incorporated into the third branch  30  to prevent passage of voltage signals pulsed at the first frequency from the first unit  1  and at the second frequency from the second unit  2  to the third unit  3 . 
     In the following description of the trap circuits and of the series circuits, reference will only be made to the first unit  1  and to the second unit  2 . However, what is stated remains valid for a third unit  3  and even for a higher number of units. 
     A trap circuit made up of an inductor that by itself forms a trap circuit  51   a  is shown in  FIG. 2  in the first branch  10  associated with the first transmitting and/or receiving unit  1 , and a serial circuit connected to ground  62   a  is shown stubbed out from the second branch  20  associated with the second transmitting and/or receiving unit  2 . 
     In a first optional embodiment, at least the first passive electronic means  51   a ,  61   a  or the second passive electronic means  60   a ,  62   a  may take the form of at least one so-called trap circuit, referenced  5  in  FIG. 4A , which is associated with the first unit  1  or second unit  2 , and which is formed of an inductor and a capacitor in parallel. The trap circuit is closed for all frequencies except a resonant frequency of the trap circuit. 
     An inductance of the inductor and a capacitance of the capacitor of the trap circuit are then preselected so that the trap circuit associated with the first unit  1  resonates at the second frequency and so that the trap circuit associated with the second unit  2  resonates at the first frequency. 
     An example of a trap circuit  5  is shown in  FIG. 4A .  FIG. 4B  shows a curve of the losses between the ports of the trap circuit  5  as a function of frequency. Energy and therefore the signal circulates between the ports of the trap circuit  5  for all frequencies, except the resonant frequency. At the resonant frequency, the losses are substantial and the energy no longer circulates, hence the name trap circuit  5 . 
     As any inductor has a parasitic capacitance in parallel with the inductor, and as it is possible to determine and assign a resonant frequency to an inductor during its manufacture, the trap circuit associated with the first unit  1  or with the second unit  2  or an additional unit  3  may be formed of at least one inductor that by itself forms a trap circuit  51   a , this being shown in  FIG. 2 . 
     This inductor is selected to have a previously assigned resonant frequency corresponding respectively to the second frequency, when said at least one inductor by itself forms the trap circuit  51   a  associated with the first unit  1 , or to the first frequency, when said at least one inductor by itself forms the trap circuit associated with the second unit  2 . 
     There may be several inductors forming a trap circuit  5  for a respective frequency when there are more than two transmitting and/or receiving units  1 ,  2 . For example, for a three-unit system, there may be, in the first branch  10  or the antenna  4 , an inductor forming a trap circuit  5  associated with the first unit  1 , this circuit trapping the second frequency associated with the second unit  2 , and an additional inductor also forming a trap circuit  5  associated with the first unit  1 , this circuit trapping the third frequency associated with the third unit  3 , and so on for additional transmitting and/or receiving units. 
       FIG. 5A  shows a decrease in amplitude in decibels dB (%) as a function of frequency f(Hz) in hertz for three inductors L 2 , L 3  and L 4  having different resonant frequencies fRL 2 , fRL 3  and fRL 4 . Inductors integrated into a branch associated with a transmitting and/or receiving unit to form by themselves a respective trap circuit may be chosen so that their own resonant frequency fRL 2 , fRL 3 , fRL 4  is substantially equal to one or more of the frequencies associated with the other transmitting and/or receiving units of the system. 
       FIG. 5B  illustrates the curve of the resonant frequency f(Hz) of the inductors L 2 , L 3  as a function of their inductance. If these inductors L 2  and L 3  are intended to form by themselves a respective trap circuit associated with a transmitting and/or receiving unit, their respective inductances may be chosen in such a way that the resonant frequency fRL 2  or fRL 3  of the inductor L 2  or L 3  corresponds to one of the frequencies, coming from other transmitting and/or receiving units, to be blocked from said transmitting and/or receiving unit. 
     With particular reference to  FIGS. 1A, 1B, 2, 6   a  and  6   b , in a second optional embodiment, at least the first passive electronic means  61   a  or the second passive electronic means  60   a ,  62   a  may take the form, in the respective first or second branch  10 ,  20  or in the antenna  4 , of at least one series circuit  6 ,  6   a  formed by an inductor and a capacitor in series. 
     Such a series circuit  6 ,  6   a  is open for all frequencies except for a resonant frequency of the series circuit  6 ,  6   a . An inductance of the inductor and a capacitance of the capacitor are selected so that the series circuit  6 ,  6   a  resonates at the second frequency towards the first branch  10  or the first unit  1  and at the first frequency towards the second branch  20  or the second unit  2 . 
     Again for a number of transmitting and/or receiving units greater than two, a signal at a frequency associated with a transmitting and/or receiving unit may be blocked by a series circuit  6 ,  6   a  from another transmitting and/or receiving unit. 
     A series circuit  6  that may be used as passive electronic means is shown in  FIG. 6A . The series circuit  6 , which is formed of an inductor and a capacitor in series, may be connected at one end as a stub to the first branch, the second branch  20  or the xth branch or, alternatively, to the antenna  4 . 
     It is possible to connect this circuit in series to ground at its other end. Such a stubbed-out, ground-connected series circuit  6   a  is shown in  FIG. 6B . In  FIGS. 6A and 6B , the capacitance of the capacitor may be 10 picofarads and the inductance of the inductor 10 nanohenry, this in no way being limiting, the capacitance of the capacitor and the inductance of the inductor being selected so that in-circuit the circuit resonates at the frequency of the signal to be blocked. 
       FIG. 6C  shows the loss in decibels of the signal at the resonant frequency of the series circuit  6  shown in  FIG. 6B . Since the series resonance causes passage of the signal to ground, it is no longer transmitted between the two ports. 
     In the case of the series circuit  6  shown in  FIG. 6A , at the series resonant frequency the losses of the circuit are minimum and the circuit closes. The losses are substantial for all the other frequencies, and hence the circuit may be said to be open. 
     As shown in an example in  FIG. 2 , the first passive electronic means  51   a  may take the form of a trap circuit formed of an inductor and a capacitor in parallel or of a single inductor, the latter case being illustrated in  FIG. 2 . The second passive electronic means  62   a  may take the form of at least one series circuit formed of an inductor and a capacitor in series, the series circuit being in  FIG. 2  stubbed out of the second branch  20  and connected to ground. 
     The reverse is also possible and similar arrangements may be envisaged for a system with more than two transmitting and/or receiving units  1 ,  2 . The first passive electronic means  51   a  may be associated with the first transmitting and/or receiving unit  1 , which may transmit/receive at the highest but also at the lowest frequency of the two transmitting and/or receiving units  1 ,  2 , and vice versa for the second passive electronic means  62   a.    
       FIG. 3  shows the percentage losses Dec (%) in the first and second branches  10 ,  20  of the system illustrated in  FIG. 2 , respectively, as a function of the frequency f(Hz) in millihertz. 
     The curve with the triangles shows the loss in the second branch  20  caused by the series circuit employed as the second passive electronic means  62   a , for a signal of low frequency coming from the first unit  1 , and the curve with the diamonds shows the loss in the first branch  10  caused by the lone inductor employed as a trap circuit  5  that plays the role of the first passive electronic means  51   a ,  61   a , for a signal of high frequency coming from the second unit  2 . The low frequency is in  FIG. 3  about 300 MHz and the high frequency about 2400 MHz, this being non-limiting. 
     In all the figures showing the system, the antenna  4  may be a loop antenna  4  connected at each of its ends to the first branch  10  or the second branch  20  respectively, this being preferred but non-limiting. 
     As shown more particularly in  FIGS. 1A and 1B , it is possible for the loop antenna  4  to have a stub branch  21  internal to the antenna  4  connecting it to ground. The internal stub branch  21  may thus comprise passive electronic stub means  60   a  that open the stub branch  21  for the signal at the lowest frequency of the first and second frequencies and that close the stub branch  21  for the signal at the highest frequency of the first and second frequencies. 
     As shown in  FIGS. 1A, 1B , considering the second unit  2  to be the transmitting and/or receiving unit associated with the lowest frequency, this being non-limiting, the passive electronic stub means  60   a  are equivalent to the second passive means that block the passage of the signal at the highest frequency from the second transmitting and/or transmitting unit  2  associated with the lowest frequency. 
     The stub branch  21  internal to the antenna  4  may be connected to the loop antenna  4  at a distance measured from one of the ends of the loop antenna  4 , which is connected to the highest of the first and second frequencies, the distance being dependent on the highest frequency. Thus for a signal at the highest frequency, only one portion of the loop antenna  4  is used. 
     The passive electronic stub means  60   a  may take the form of a series circuit of the respective internal stub branch  21 , said circuit being formed of an inductor and a capacitor in series, the series circuit of the internal stub branch  21  being open for all frequencies except a resonant frequency of the series circuit corresponding to the highest frequency of the first and second frequencies. For the signal at the highest frequency, the series circuit of the internal stub branch  21  is closed. 
     Alternatively, the passive electronic stub means  60   a  may take, in the respective internal stub branch  21 , the form of a trap circuit formed of an inductor and a capacitor mounted in parallel with the inductor, the trap circuit of the internal stub branch  21  being closed for all frequencies (and in particular at the highest frequency) except for a resonant frequency of the trap circuit corresponding to the lowest frequency of the first and second frequencies, for which frequency the circuit is open. 
     As shown in  FIG. 7 , the system may comprise at least a third unit  3  that transmits and/or receives a voltage signal pulsed at a third frequency, which is connected to the antenna  4  by a third branch  30 . The third branch  30  may comprise third passive electronic means  63   a  that open the third branch  30  for a voltage signal pulsed at the first frequency and at the second frequency, while the third passive electronic means  63   a  close the third branch  30  for a voltage signal pulsed at the third frequency, so as to allow it to be sent to the antenna  4 . 
     In a particularly advantageous preferred embodiment of the present invention, which embodiment is shown in  FIG. 7 , the antenna  4  may be a loop antenna  4 . The third branch  30  may be a stub branch  21  connected between the two ends of the loop antenna  4  at a distance measured from one of the ends of the loop antenna  4  that is dependent on the third frequency (with a predetermined margin of error). 
     With particular reference to  FIGS. 1A, 1B and 7 , the third branch  30  may be at least partially common to the stub branch  21  of the loop antenna  4  for the passage of the signal at the highest frequency. In this case, the stub branch  21  of the loop antenna  4  for the passage of the signal at the highest frequency extends from the third branch  30  to be connected by an end stub branch to ground, and comprises passive electronic means that prevent passage of signals at a frequency other than the highest frequency, i.e. signals from the second and third transmitting and/or receiving units  2 ,  3 , when the first transmitting and/or receiving unit  1  is the unit associated with the highest frequency. 
     Considering the case where a first transmitting and/or receiving unit  1  is the unit associated with the highest frequency and where a third transmitting and/or receiving unit  3  associated with a frequency intermediate between the highest frequency and the lowest frequency is that of the second transmitting and/or receiving unit  2 , the signal of the third unit  3  may pass through the portion of the antenna  4  complementary to the portion serving for the passage of the signal of highest frequency coming from the first unit  1 . 
     A particularly advantageous but nonlimiting application of the system according to an aspect of the invention, i.e. the system described above, is to a motor-vehicle tire wheel unit or to a fob for gaining hands-free access to a motor vehicle, the bulk of such a wheel unit or of such an access fob needing necessarily to be small, and this meaning the implementation of a system according to an aspect of the invention may be required.