JP2005536911A - Low frequency radio transmitter for automobile passive entry system - Google Patents

Abstract

A wireless transmission device is disclosed that includes a drive circuit (GEN) that generates a drive signal (SC) that excites a serial LC type antenna (EA) and that is suitable for wireless transmission of a paging signal in a vehicle passive entry system. In order to increase the internal resistance of the drive circuit without affecting the size of the drive circuit, the circuit comprises a sine wave or pseudo sine wave constant current source (600) that delivers the drive signal in the form of a current control signal. To do. This arrangement mitigates variations in the range of the dialog area of the system by reducing the Q (quality) factor of the device.

Description

  The present invention relates to a low-frequency radio transmission apparatus incorporating a drive circuit that generates a signal for exciting a serial LC type antenna, a passive entry system incorporating the apparatus, and a drive circuit used in the apparatus.

  The present invention is suitable for passive entry systems (ie, systems that do not require user-side operation), especially for automobiles.

  The wireless transmission device includes a predetermined number N of transmission antennas, where N is an integer generally larger than one unit, and at least one drive circuit that generates a drive signal that excites these antennas. Good. This drive circuit may be included in a centralized or decentralized management device.

  Each of the transmit antennas includes an electromagnetic coil that is optionally tuned. The antenna may be tuned or untuned depending on the case. A tuned antenna is associated with the tuning capacitor. The antenna is a series LC type when the inductor and capacitor are connected in series. On the other hand, when the inductor and the capacitor are connected in parallel, the antenna is a parallel LC type.

  A non-tuned antenna does not have a tuning capacitor. Nevertheless, the electromagnetic coil of the antenna is typically connected in series with at least one capacitor that blocks direct current (DC) and at least in series with stray capacitance. For this reason, in the meaning of the present invention and the following disclosure, the term “series LC” type antenna is also used to denote this configuration.

  In other words, the term “series LC” antenna is used as long as the antenna's electromagnetic coil is not connected in parallel to the tuning capacitor.

  In the intended type of application, the management device may be included in a cabin computer that functions in particular for vehicle entry control and / or vehicle start-up control. The management device includes a control module that generates a data signal including transmission data. Further, the transmitting antenna is arranged inside the cabin and / or around the outside of the vehicle. The excitation signal supplied to the transmission antenna is a signal for calling an identification number (ID number) carried by the user, such as a badge, from a functional viewpoint. If the badge is present within the range of the call signal (previously referred to as the dialog area), the badge receives the call signal and sends an encoded response signal instead. The management device has means for receiving and decoding a response signal to authenticate the user. Upon authentication, the management device performs a specific function such as unlocking the vehicle door and / or deactivating the device that prevents the vehicle from starting.

  Wireless transmission is set to low frequency (LF) when the ringing signal is on the order of hundreds of kilohertz (kHz). Under this condition, the radio signal propagates in the near field mode. Since there is no electromagnetic radiation, there is little difficulty in meeting electromagnetic compatibility (EMC) standards. Such transmission need only conform to the specifications of the IETS 300-330 (or EN 300-330) standard for the spectrum of the transmitted signal.

  In the conventional example in the automotive field, the fundamental frequency of the ringing signal is equal to 125 kHz.

  When the management device has one drive circuit per transmission antenna, it is called “decentralized” and each drive circuit generates a corresponding antenna drive signal. Each of these circuits is arranged in the vicinity of the corresponding transmitting antenna.

  When the management device has a single drive circuit that generates a single antenna drive signal, it is called “centralized” and this drive device is one of the antennas (or other antennas) (or Is associated with switch means that allow for some actuation. The drive signal applies only to the activated antenna.

  FIG. 1 is a schematic diagram showing the principle of how a serial LC type transmitting antenna is driven in the prior art.

  The transmission antenna EA includes an electromagnetic coil L in series with the capacitor C. For convenience in the following, the symbol L is used to indicate both the coil itself and its inductance. Similarly, the symbol C is used to indicate both the capacitor and its capacitance. In this example, capacitance C corresponds to all capacitances present in series with coil L, in particular tuning capacitors (if any), one or more DC blocking capacitors (if any), and the stray capacitance of the coil. To do.

  The drive circuit GEN includes a square wave power supply 100. Using Thevenin's theorem, the power supply 100 includes an ideal power supply 101 in series with an internal resistance Rs. For simplicity, in the following, it is assumed that the resistor Rs has not only an internal resistance to the power supply but also an intrinsic resistance to the electromagnetic coil L and the capacitor (if any). The positive electrode of power supply 101 is coupled to the first terminal of antenna EA via resistor Rs. Both the negative electrode of power supply 100 and the second terminal of antenna EA are coupled to a reference potential, eg, ground. The power source 101 is a power source having a low internal impedance. The amplitude of the power supply and the amplitude of the current supplied by the power supply 101 are described as U and i, respectively.

Since the antenna is an LC circuit, resonance starts when excited at a frequency F such that L × C × (2π × F) ² = 1. A resonance frequency satisfying this relationship is described as Fo. The quality factor Q of the apparatus is given by Q = (L × 2π × Fo) / Rs. The strength of the magnetic field transmitted by the coil is proportional to M × i, where M indicates the number of rotations of the coil L. Thus, due to the constant shape and current i, M is increased to increase the field strength and thereby extend the range of the dialog area. Nevertheless, since the inductance L is proportional to M ² , the quality factor Q increases in proportion to M ² as well.

  Thus, as shown in the graph in FIG. 2, how the current i varies as a function of the frequency F of the drive signal generated by the drive circuit GEN, the transmission device conventionally has a high Q factor. Met. The current i has a maximum value when the frequency F is equal to the resonance frequency Fo. The curve shown here is of a very steep “bell curve” function. Thus, a small amount of deviation ΔF at the frequency F close to the resonance frequency Fo results in a large current deviation Δi in the antenna, and also a large deviation in the range of the dialog area.

  Unfortunately, due to dispersion and temperature drift in component values, the values L and C can fluctuate in an uncontrollable manner. This changes the situation of the frequency F of the drive signal generated by the drive circuit GEN in relation to the resonance frequency Fo of the device. As a result, the current i and further the range of the dialog area can vary significantly. For example, for a Q factor greater than 30 (Q> 30), if the factor varies from 1 to 2, the size of the dialog area may vary by 20%.

  This is a disadvantage especially for an internal antenna, that is, an antenna that is arranged inside a vehicle and transmits a calling signal having a function of controlling vehicle starting. It is appropriate to avoid radio signals originating from these antennas that exceed a certain threshold strength. Otherwise, the dialog area will spread to the outside of the vehicle, resulting in poor quality system operation and starting the vehicle while the batch is still outside the vehicle.

  In order to eliminate this drawback, for example, it has been proposed to include a device that detects the current flow of each antenna and controls a dynamically tuned antenna based on an operation by switching a parallel capacitor. Nevertheless, this method is complex and its cost is prohibitive for application in the automotive industry.

  It is also conceivable to reduce the Q factor of the apparatus. For this purpose, it is conceivable to increase the resistance Rs by adding a resistance in series with the power supply 100. In such a situation, the graph of FIG. 3 shows that a very low Q factor is shown as the variation of the current i is plotted as a function of the frequency F of the drive signal generated by the drive circuit 100. Brought about. As is apparent from this figure, a relatively large deviation ΔF at a frequency F in the vicinity of the resonance frequency Fo results in only a small deviation Δi in the antenna current flow i and therefore little variation in the range of the dialog area. .

  Nevertheless, this also reduces the current i available to excite the antenna, leaving the other points equivalent. This reduction is illustrated by the fact that the maximum current in the graph of FIG. 3 (when F equals Fo) is less than that of the graph of FIG. This reduction in current can be compensated by increasing the voltage U. Theoretically, the required voltage U may exceed the available power supply voltage, ie the voltage obtained by the vehicle battery. Nevertheless, in practice, the voltage U is limited to the battery voltage unless a transformer or power converter is used, and using them increases costs.

  The present invention has been made to eliminate the above-mentioned drawbacks in the prior art.

  According to a first aspect of the present invention, there is provided a low frequency radio transmitter for a vehicle passive entry system, the device comprising at least one transmit antenna of series LC type and at least one for supplying a drive signal to the antenna. Drive circuit. According to the present invention, the drive circuit includes a constant current source that supplies a drive signal as a current drive signal.

  In other words, even if the antenna is a series LC type, the antenna is excited with current. This is contrary to all preconceptions of those skilled in the art to which the present invention pertains. This preconception is that parallel LC type antennas are excited with current, whereas series LC type antennas are excited with voltage. That's it. However, this makes it possible to increase the resistance of the device in order to reduce its Q factor, while this does not impair the current driving the antenna.

  Thus, in the embodiment of the present invention, the drive circuit further includes means for determining the internal resistance of the constant current source. This makes it possible to reduce the Q factor of the device. The resistance is preferably high enough so that the Q factor is less than 10. In this application, a Q factor of about 5 is preferred, resulting in a tolerance variation in the range of the dialog area.

  According to a second aspect of the present invention, the present invention is an automobile passive entry system, an identification badge carried by a user, and a low-frequency wireless transmission device according to the first aspect of the present invention. And a device for transmitting a call signal to a badge.

  According to a third aspect of the present invention, the present invention provides a drive circuit for use in a device according to the first aspect of the present invention. Other features and advantages of the invention will be apparent from the following disclosure. The following disclosure is merely exemplary and should be understood with reference to the accompanying drawings.

  FIG. 4 is a plan view of the vehicle 10.

  Such a passive entry system for a vehicle has a transmitting antenna arranged inside the cabin and / or outside the vehicle 10. The function of these antennas is to send a calling signal suitable for being received by an ID member of the system, such as a badge 17 carried by the user, in which case the badge 17 is transmitted by one of the antennas. It is within the range of the signal to be transmitted (dialog area). Once the message is recognized as valid, the badge responds by sending back a response message received by the access system installed in the vehicle. If the response message is recognized as valid, the door is unlocked (“passive entry” function) and / or the device that prevents the vehicle from moving or starting to unlock (“passive progression”) "Function)".

  Conventionally, the ringing signal is, for example, a 125 kHz LF signal, while the response signal is, for example, a radio frequency (RF) signal whose frequency is equal to 433 megahertz (MHz). A passive entry system that fits this example is commonly referred to in the art as an LF / RF system. It should be noted that the response signal (RF signal) transmitted by the badge must comply with the specifications of the IETS 300-330 (or EN 300-330) standard.

  In the example of FIG. 4, the passive entry system comprises five external transmit antennas arranged around the exterior of the vehicle, each of which is associated with a corresponding one of the five dialog areas 11-15. . These dialog areas extend from the left front door, left rear door, luggage compartment, right rear door, and right front door, respectively. The external transmit antenna is dedicated to the passive entry function. Also in this example, the system is further associated with a dialog area 16 (indicated by a dashed line in the figure) that is a single internal transmit antenna located within the vehicle cabin, corresponding to the internal volume of the vehicle cabin. An antenna is provided. The internal transmit antenna is dedicated to the passive travel function.

  FIG. 5 shows an example of a passive entry system according to the present invention. This system comprises a cabin computer CU located at an advantageous position of the vehicle 10. The computer CU is powered by the vehicle battery 20. The system further comprises a predetermined number P of external transmit antennas EA1, EA2,..., EAp, where p is an integer. These antennas are arranged around the outside of the vehicle 10, such as a door and a luggage compartment. Each of these has first and second terminals connected to the computer CU. The system also includes one or more switches LSW, such as unlock buttons, connected to the computer CU. This switch is operated from the outside of the vehicle 10 by the user.

  The system also comprises a predetermined number N−P of internal transmit antennas EAp + 1, EAp + 2,..., EAn, where N is an integer greater than P, and these antennas are located in the vehicle cabin. Each of these antennas has first and second terminals connected to the computer CU. In the example of FIG. 4, P is equal to 5 and N is equal to 6 (thus NP equals 1 unit). The system also includes a switch SSW, such as a start button, which is activated only once by the user from within the vehicle 10.

  The external transmission antennas EA1 to EAp are electromagnetic coils (inductance L) that are wound around a ferrite bar in series with a capacitor (capacitance C) that is composed of stray capacitance or a capacitor added intentionally. Are serial LC type antennas. The internal transmission antennas EAp + 1 to EAn are, for example, series LC type air coils (frame antennas), and each antenna is in series with a capacitor (capacitance C) composed of a stray capacitance or an intentionally added capacitor. It comprises an electromagnetic coil (inductance L) constructed by a certain number of rotations arranged around a suitable support.

  In some applications, the antenna may also include corresponding tuning capacitors connected in series with their electromagnetic coils. Further, a DC blocking capacitor may be connected in series with the antenna. For each antenna, the capacitance of capacitor C in the figure corresponds to the total capacitance from all sources.

  The function of the external transmission antennas EA1 to EAp is to transmit the calling signal 41 to the outside of the vehicle under the control of the computer CU when the switch LSW is activated. Similarly, the internal transmission antennas EAp + 1 to EAn serve to transmit the calling signal 41 to the inside of the vehicle under the control of the computer CU when the switch SSW is activated.

  The system also includes Q actuators A1, A2,..., Aq, such as an electric motor, etc., which in some cases is associated with each electromechanical and / or electropneumatic mechanism, where Q is a predetermined value. Is an integer. These actuators are driven by the computer CU and perform various functions such as unlocking the door and / or stopping the operation of the device that prevents the vehicle 10 from starting.

  Finally, the system comprises a receiving antenna 31 located inside the vehicle and coupled to the computer 10 via the receiver means 30. The function of this antenna 31 is to receive a response signal 42 transmitted from the badge 17 when a call signal 41 transmitted from one of the external transmission antennas EA to EAp or from one of the internal transmission antennas EAp + 1 to EAn is received. To receive.

  The badge 17 includes a control device 176 such as a microcontroller. This also includes a receiving antenna 172 for receiving the call signal 41, which is coupled to the control device 176 via the receiving means 171. This further includes a transmitting antenna 174 coupled to the controller 176 by the transmitting means 173. The control device 176, the reception unit 171 and the transmission unit 173 are powered by the battery 175. The transmission antenna 174 functions to transmit the response signal 42 under the control of the control device 176 when the valid call signal 41 is received by the reception antenna 172 and decoded by the control device 176.

  The schematic diagram of FIG. 6 shows an example of a management device that manages the transmission of a paging signal by a device according to the present invention, for example a predetermined number N of antennas configured in the system shown in FIG.

  In this example, the management device 500 surrounded by a surrounding broken line is a centralization device. It comprises a single drive circuit GEN for generating a single antenna drive signal SC, and further comprises switch means for actuating only one (or more than two) of the antennas except for the other antennas.

  The management device 500 includes a control module CTRL, a drive circuit GEN, and switch means DEMUX. The circuit GEN receives the data signal Sd supplied by the control module CTRL and generates a drive signal SC.

  The switch means DEMUX is constituted by a demultiplexer having, for example, one input IN and N outputs each being OUT1 to OUTn.

  The excitation signal SC is supplied to the respective first terminals of the antennas EA1 to EAn via respective first connections, which may be several meters long. Furthermore, the switch means DEMUX is arranged between the respective second terminals of the antennas EA1 to EAn and a reference potential, for example ground. In other words, the input IN of the demultiplexer DEMUX is connected to the ground. Furthermore, the outputs OUT1 to OUTn of the demultiplexer DEMUX are respectively connected from EA1 via a second connection which may be substantially the same as the corresponding first connection length, i.e. may also be several meters long. Connected to the second terminal of the antenna, which is EAn.

  The demultiplexer DEMUX has a predetermined number N of switches, SW1 to SWn, respectively, and each switch is arranged between a corresponding one of the input IN and the outputs OUT1 to OUTn. The control module CTRL generates a selection signal SEL that is supplied to the demultiplexer DEMUX. The function of this signal is to close only one of the switches SW1 to SWn, that is, select only one of the outputs OUT1 to OUTn of the demultiplexer DEMUX and couple it to the input IN through one of the switches. That is. In other words, the selection signal SEL functions to operate only one of the antennas EA1 to EAn and deactivate the other antennas. It should be noted that in some applications, the manner in which multiple transmit antennas are activated simultaneously is not excluded.

  Such an arrangement of the switch means serves to switch the transmitting antenna through its “cold” point. The electrical connection that connects the drive circuit GEN to the inactive antenna continuously receives the drive signal SC, even though they are open circuit. These connections may be more or less tightly coupled to ground via stray capacitance, so that one or more inactive antennas carry the drive signal current, thereby causing the fundamental frequency of the excitation signal There is a danger of sending a call signal at a frequency offset from

Since these antennas are LC circuits, they resonate when excited at a frequency F where L × C × (2π × F) ² = 1, where L and C are the inductance and capacitance of the antenna, respectively. In general, the fundamental frequency of the drive signal SC is selected to be equal to the frequency Fo satisfying this relationship (for the resonance frequency), that is, the frequency Fo at which L × C × (2π × Fo) ² = 1. In an open circuit connection between the control circuit GEN and the switch means DEMUX for the non-actuating antenna, the stray capacitance is taken into account and the antenna capacitance C is replaced by a capacitance C ′ where C ′ <C. For this reason, it is necessary to avoid the spectrum of the drive signal including the predetermined frequency F ′ where L × C ′ × (2π × F ′) ² = 1. Nevertheless, if the excitation signal exhibits harmonic content as in the case of a square wave signal in the prior art and this frequency F ′ oscillates harmonically to the frequency Fo, an unexpected resonance of the antenna can occur. . The spectrum of such a signal includes not only the fundamental frequency Fo but also all of its harmonics.

  For this reason, according to an advantageous feature of the invention, the antenna drive signal SC is an LF signal, preferably a sine wave or a pseudo sine wave. Such a low harmonic content of the excitation signal serves to avoid the risk of unexpected antenna resonance as described above. This avoids any risk of unstable operation of the passive entry system.

  If the wireless transmission device of FIG. 6 is used in a passive entry system for a vehicle of the type shown in FIG. 5, the management device 500 is advantageously included in the cabin computer CU.

  The principle by which a series LC type antenna is driven according to the invention is shown in a simplified manner in the schematic diagram of FIG. 7, in which the same elements as in FIG. 1 are denoted by the same reference numerals.

  The antenna EA is a series LC type antenna, i.e. an antenna comprising an electromagnetic coil in series with a capacitor C, a tuning capacitor, and / or a plurality of DC blocking capacitors. In a preferred embodiment, the electromagnetic coil L of the antenna EA is not tuned, i.e. not connected in series with a tuning capacitor.

  The drive circuit GEN supplies a drive signal SC to drive the antenna EA, and this signal is a current drive signal. The drive circuit includes a constant current source including an ideal current source 601 that is in parallel with the internal resistance Ri using the Norton theorem. The positive electrode of current source 601 is coupled to the first terminal of antenna EA. Both the negative electrode of current source 601 and the second terminal of antenna EA are coupled to ground. The constant current source supplies a current Io.

  In particular, when the drive circuit GEN is included in the centralized management device of the passive entry system, the drive signal SC is preferably a sine wave or pseudo sine wave signal. As noted above, this feature can limit the system operational effects of stray coupling to the connection ground. An example of such a centralized management device has been described above with reference to the schematic diagram of FIG.

  In order to reduce the Q factor of the device, the drive circuit GEN also comprises means arranged to increase the internal resistance Ri of the constant current source 600. The schematic diagram of FIG. 8 shows such a variant of the drive circuit GEN. In this figure, the additional resistance Rp is symbolically indicated as being connected in series with the internal resistance Ri of the constant current source 600. Actually, the resistor Rp functions to determine the internal resistance Ri. Nevertheless, it will be apparent to those skilled in the art to which this invention pertains that it is not essential that such additional resistance correspond to the physical presence of the resistive component. A high value of the internal resistance of the constant current source 600 is obtained by an appropriate structure of the constant current source. Inevitably, the strength of the constant current Io is selected in consideration of the internal resistance of the constant current source 600, thereby obtaining the desired maximum driving current of the antenna EA.

  In an embodiment of the invention, the Q factor of the device is less than 10. Preferably, it is set to about 5.

  The schematic diagram of FIG. 9 shows another variation of the present invention, where the drive circuit GEN corresponds to a voltage tracking stage 602 that is arranged at the output of the constant current source 600 and has a predetermined current gain Gi. The current gain Gi is greater than one unit (Gi> 1). This variant is recommended when high excitation current is required, ie in applications that require high transmission power. Such a stage limits the intensity of the constant current Io, thereby making it possible to limit the heat loss of the constant current source 600. The current source 601 and its internal resistance Ri are expected as a current source Gi × Io and a resistance Ri / Gi, respectively, depending on the load (antenna EA).

  The voltage following stage 602 is disposed between a terminal 603 serving as a power supply voltage Vbatt supplied by the ground and the battery. This may be constituted by a class B push-pull type power stage, for example. Nevertheless, this is merely a preferred example and is only a non-limiting example.

  Naturally, the variants of FIGS. 8 and 9 can be combined.

  As an example, the constant current source 600 may comprise a voltage source followed by a transductance amplifier.

  FIG. 10 shows a first embodiment of such a constant current source 60.

  The constant current source 600 includes a low-frequency sine wave oscillator 71 that supplies a sine wave signal Sm that is an LF signal having a frequency Fo. The current source further comprises a variable gain amplifier 74 having one input for receiving the signal Sm and having an output for providing the excitation signal SC. The amplifier 74 is a transconductance amplifier. In other words, the signal Sm received by the amplifier is a voltage signal, and the signal supplied by the amplifier is a current signal, that is, a current referred to as Io in FIGS. This current excites the electromagnetic coil of the activated antenna.

  The control input of the amplifier 74 receives a data signal Sd which is a square wave LF signal. In one example, the signal Sd includes data transmitted in the form of a Manchester code. The gain of amplifier 74 is equal to zero when signal Sd is low and non-zero when signal Sd is high. The signal Io thus corresponds to the signal Sm modulated by the signal Sd.

  In FIG. 11, elements similar to those of FIG. 10 are referred to by like numerals, and a second embodiment of a constant current source 600 is shown.

  In this example, the constant current source 600 includes a low frequency oscillator 72 that generates a square wave signal Sck that is an LF signal having a frequency equal to the fundamental frequency Fo, that is, 125 kHz. In a variant, the signal Sck is a clock signal supplied by the output of a microcontroller included in the control circuit CTRL. The signal Sm is a pseudo sine wave signal, that is, an LF signal having a fundamental frequency Fo. For this purpose, the filter 73 is at least cubic. This cutoff frequency may be equal to 80 kHz or 100 kHz, for example.

  The remaining part of the constant current source 600 is the same as the circuit according to the first embodiment, as shown in the schematic diagram of FIG.

  The signal Sck may be pulse width modulated, thereby controlling the antenna drive current, i.e. the current flowing through the activated antenna.

It is a circuit diagram which shows the principle in which a serial LC type antenna is driven in a prior art. FIG. 6 shows a graph plotting antenna current as a function of drive signal frequency at a high Q factor. FIG. 6 shows a graph plotting antenna current as a function of drive signal frequency at a low Q factor. 1 is a plan view of an automobile showing respective interface areas or ranges of transmitting antennas in an example of a passive entry system. FIG. It is a circuit diagram showing an embodiment of a passive entry system according to the present invention. FIG. 2 shows an embodiment of the device of the present invention. 1 is a schematic diagram illustrating the principle of driving a serial LC type antenna according to the present invention. It is the schematic which shows the 1st modification which drives the serial LC type antenna which concerns on this invention. It is the schematic which shows the 2nd modification which drives the serial LC type antenna which concerns on this invention. It is the schematic which shows 1st Embodiment of the constant current source in the drive circuit which concerns on this invention. It is the schematic which shows 2nd Embodiment of the constant current source in the drive circuit which concerns on this invention.

Claims (13)

A low frequency radio transmitter for a vehicle passive entry system, the device comprising:
At least one transmit antenna of the serial LC type (EA, EA1-EAn);
And at least one drive circuit (GEN) for supplying an antenna drive signal (SC),
The drive circuit includes a constant current source (600) that supplies the drive signal as a current drive signal. The device according to claim 1, characterized in that the serial LC type transmitting antenna comprises an untuned electromagnetic coil (L). The device according to claim 1, wherein the drive signal (SC) is a sine wave or pseudo sine wave signal. The device according to any one of claims 1 to 3, wherein the driving circuit includes means (Rp) for determining an internal resistance (Ri) of the constant current source. The device according to claim 4, wherein a Q factor of the device is less than 10. 5. 6. A device according to any of the preceding claims, wherein the constant current source comprises a voltage source (71; 72) followed by a transconductance amplifier (74). The device according to claim 6, characterized in that the voltage source comprises a power supply (71) for supplying a sinusoidal signal (Sm). The voltage source comprises a power supply (72) that supplies a square wave signal (Sck), followed by a low pass filter (73), and supplies a voltage that is both a pseudo sine wave (Sm). Item 7. The apparatus according to Item 6. The apparatus of claim 8, wherein the square wave signal (Sck) is pulse width modulated to control the current intensity of the antenna. The drive circuit includes a voltage tracking stage (602) disposed at the output of the constant current source and having a predetermined current gain (Gi) greater than one unit. The device described. The apparatus according to claim 10, wherein the power tracking stage is a class B push-pull power stage. A predetermined number N of transmission antennas (EA1-EAn), where N is an integer larger than a unit,
Furthermore, a device comprising a centralized management device (500),
The management device (500)
A drive circuit (GEN) for generating a single antenna drive signal (SC) supplied to each first terminal of the antenna;
A predetermined number N of switches (SW1-SWn), each switch associated with one of the antennas and disposed between a second terminal of the corresponding antenna and a reference potential, each of the switches 12. A device according to any one of the preceding claims, comprising a switch controlled by a control device which activates or deactivates the associated antenna. A passive entry system for an automobile comprising an ID batch (17) carried by a user,
A system comprising the low-frequency radio transmission device (500) according to any one of claims 1 to 12, further comprising a badge call signal (41).

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