Abstract:
A passive entry and/or passive go system and am associated operating method is provided. According to an embodiment of the invention, the following steps are performed in an electronic key of the passive entry and/or passive go system: generation of a reference input value, supplying the antenna circuit with the reference input value, measurement of the characteristic parameters, while the antenna circuit is supplied with the reference input value, storage of the characteristic parameters, measurement of a first output value of the antenna circuit, and determination of the field strength from the first output value and the characteristic parameters, whereby an effect of the characteristic parameters on the field strength is compensated. Use, for example, in motor vehicles.

Description:
This nonprovisional application claims priority to German Patent Application No. DE 102006020422, which was filed in Germany on Apr. 26, 2006, to U.S. Provisional Application No. 60/801,402, which was filed on May 19, 2006, and which are both herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for operating a passive entry and/or a passive go system and to a passive entry and/or passive go system. 
     2. Description of the Background Art 
     In so-called remote keyless entry systems for motor vehicles, an unlocking of the vehicle occurs not with a mechanical key, but with an electronic key medium or an electronic key, for example, in the shape of a molded article, on which actuation elements are arranged. One or more integrated circuits, which realize the function of the electronic key medium or of the electronic key, are arranged on or in the key medium. 
     A so-called base station, which communicates or exchanges data with the electronic key in a wireless manner, is placed in the motor vehicle as a counterpart to the electronic key. 
     To unlock or lock the motor vehicle, a user actuates an associated control element on the electronic key, as a result of which a data transmission is initiated between the electronic key or its integrated circuit and the base station. 
     If the information transmitted between the base station and the electronic key corresponds to a stipulated protocol and has the expected content, the motor vehicle is unlocked with the aid of the base station. 
     In so-called passive entry/passive go systems or passive entry go (PEG) systems, actuation of the key to lock or unlock the motor vehicle is no longer necessary. The user of the motor vehicle must only keep an electronic key medium with him, for example, such as a card. 
     When the user operates a door handle of the motor vehicle, this is detected in the motor vehicle and reported to the base station. The base station thereupon transmits a low-frequency carrier signal, for example, with a frequency of 125 kHz, to the electronic key medium. In addition, data may also be transmitted to the electronic key medium by the low-frequency carrier signal. 
     To receive the low-frequency carrier signal, the key medium has an antenna circuit with an antenna, for example, in the form of a coil, whereby the antenna circuit generates an output value, for example, a voltage, which is a function of the field strength and a function of characteristic parameters of the antenna circuit. 
     An exemplary circuit arrangement for obtaining field strength information or for determining the field strength is described in German Patent Application DE 101 59 551 A1, which corresponds to U.S. Pat. No. 6,922,553, and which is incorporated herein by reference. 
     The output value is used to determine the distance of the key medium from the base station or from one or more transmitting antennas. If more than one transmitting antenna is provided at different positions, for example, in a front area and in a back area, of the motor vehicle, the position of the user or the key medium relative to the motor vehicle can also be determined by triangulation based on two determined distances from the particular antennas. 
     To determine the field strength independent of an orientation or position of the key medium relative to the transmitting antenna of the base station, devices are known in which three antenna coils, each perpendicular to one another, are provided. The specific field strengths of the antenna coils are vectorially superposed to calculate a resulting field strength. 
     When the distance between the antenna of the base station and the antenna of the key medium antenna circuit has been determined, it is then verified whether the determined distance lies within a permitted tolerance range. If this is the case, access to the motor vehicle is made possible by unlocking all or only certain locks. The unlocking of only certain locks can be made dependent on the position of the user relative to the motor vehicle. 
     If the user then enters the motor vehicle and operates a start button to start the engine, a distance or position determination is performed in the key medium also based on a field strength measurement. If the position determination indicates that the user is in the required position for starting the engine, the engine is started. 
     Apart from the access control and the engine start, there are numerous other application scenarios in which a distance measurement is made, for example, when the user climbs out of the motor vehicle and moves away from it. 
     It becomes clear from the above statements that there are high requirements for the distance or position measurement based on field strength determination of the carrier signal in the key medium. 
     As stated above, for the distance measurement by the antenna circuit, an output value is generated, for example, an output voltage, which is a function of the field strength and a function of the characteristic parameters of the antenna circuit. The characteristic parameters of a particular antenna circuit represent the tolerances, production variations, and other specific properties of the components of the particular antenna circuit. 
     Because each antenna circuit has its specific characteristic parameters, which can deviate considerably from one another in practice, different antenna circuit output voltages result in different antenna circuits at identical field strength. If, for example, an atypical table is provided in the particular key medium for distance calculation, in which an assignment of the output voltage to field strength is stored, this can lead to obvious errors in the distance calculation. 
     In order to take this problem into account, a laborious calibration of the key media or the antenna circuits usually takes place during a manufacturing process. For this purpose, for example, a known reference field strength can be predefined from outside by a calibration station, which is used to generate a calibration value in the particular key medium. This method is very laborious and takes into account only the parameter situation during the calibration process. A change in the characteristic parameters by long-term effects, temperature, and variable operating voltage cannot be detected by calibration during the manufacturing process; i.e., the distance measurement becomes accordingly poorer with such long-term effects. 
     Another major problem is that the antenna circuit or its antenna is more greatly dampened depending on metallic objects in its environment, for example, a bunch of keys. As a result, its characteristic parameters are also substantially changed depending on the situation. These effects as well cannot be detected by calibration during the manufacturing process. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method for operating a passive entry and/or a passive go system, as well as a passive entry and/or passive go system, which enable a precise, long-term stable field strength measurement, without requiring a laborious calibration during the manufacturing process. 
     In the method for operating a passive entry and/or a passive go system, the system has a base station for placement in a motor vehicle and at least one electronic key assigned to the base station, whereby a carrier signal is generated by the base station and the carrier signal is received at least at one antenna of an antenna circuit of the electronic key. The antenna circuit generates an output value, which is a function of the field strength of the carrier signal and a function of characteristic parameters of the antenna circuit. In the electronic key, a distance between the base station and the electronic key is determined from the field strength and system functions are carried out depending on the determined distance. According to the invention, the antenna circuit of the electronic key generates an output value, which is a function of the field strength and a function of characteristic parameters of the antenna circuit. The characteristic parameters of a particular antenna circuit represent the tolerances, production variations, and other specific properties of the components of the particular antenna circuit. According to the invention, the characteristic parameters are measured in the electronic key based on a reference input value generated within the electronic key itself; here, the antenna circuit during the measurement of the characteristic parameters is supplied with the reference input value, i.e., the reference input value serves as the input value of the antenna circuit during the measurement. Then, the characteristic parameters are saved. After this, a first, field-induced output value of the antenna circuit is measured, the antenna circuit not being supplied with the reference input value during the measurement of the first output value. The field strength is determined from the first output value and the characteristic parameters, an effect of the characteristic parameters on the field strength being compensated. The measurement of the characteristic parameters can occur in a cyclic manner, as a result of which a continuous calibration of the antenna circuit can occur. The generation of the in-circuit reference input value enables a calibration also without a laborious calibration process during manufacture. Because the calibration can take place continuously, a precise, long-time stable field strength measurement is possible, which also takes into account changed conditions in the surroundings, for example, metallic objects in the vicinity of the key medium or of the antenna circuit. 
     In an embodiment, the functions of the system comprise an unlocking, a locking, and/or starting of the motor vehicle as a function of the determined distance. 
     In an embodiment, the reference input value is generated in the form of a reference input voltage and/or a reference input current with a predefined reference frequency and reference amplitude. A second output value is measured with an applied reference input value, and the characteristic parameters are determined from the second output value. The characteristic parameters can be identical to the second output value or the second output value can be a measure for the characteristic parameters. Preferably, to determine the field strength, the quotient is formed from the first, field-strength-determined output value and the second, reference input value-determined output value. 
     In another embodiment, the reference frequency can be set equal to a frequency of the carrier signal. In this way, the characteristic parameters are determined by the reference value at the appropriate operating frequency. 
     In an embodiment, the reference frequency can be derived from a frequency of the carrier signal. It can be assured in this way that the calibration takes place by means of the reference input value also at the actual operating frequency of the antenna circuit. 
     In an embodiment, a distance between the antenna and a transmitting antenna of a transmitter of the carrier signal can be determined from the determined field strength. Preferably, within the context of the distance determination, the field strength is determined in addition at a second antenna and at a third antenna of the antenna circuit, whereby the antennas are each perpendicular to one another and a distance between the antennas and a transmitting antenna of a transmitter of the carrier signal is determined from the determined field strengths by superposition. By vectorial superposition of the field strengths calculated per antenna, it is possible to calculate the distance independent of an orientation of the antennas or of the key medium relative to the transmitting antenna. 
     In an embodiment, the antenna and a transmitting antenna of a transmitter of the carrier signal are mutually coupled. For mutual or inductive coupling, reference is made to the manual of Klaus Finkenzeller, RFID-Handbuch [RFID Manual], 3rd ed., HANSER, 2002; see in particular Chapter 3.2.1 “Inductive Coupling”, pages 42 to 45. 
     In an embodiment, a parallel resonant circuit or a series resonant circuit is formed by the antenna circuit. 
     Also, the frequency of the carrier signal can be within a range of 50 KHz to 150 KHz or within a range of 5 MHz to 25 MHz. 
     The passive entry and/or passive go system of the invention comprises a base station for placement in a motor vehicle, at least one electronic key, which is assigned to the base station, and at least one antenna circuit, assigned to the key medium, with at least one antenna and an output terminal, at which an output value is applied in the form of an output voltage and/or an output current, which is a function of the field strength and a function of characteristic parameters of the antenna circuit. According to the invention, the electronic key has: a reference input value generating unit for generating a reference input value in the form of a reference input voltage and/or a reference input current with a known reference frequency and reference amplitude and an activatable switching unit, coupled to the antenna circuit and the reference input value generating unit, is provided, which supplies the antenna circuit with the reference input value as a function of the drive state or decouples the antenna circuit from the reference input value. 
     In an embodiment, the reference input value generating unit can be an oscillator. The oscillator may comprise, for example, a PLL, quartzes, voltage-controlled oscillators, etc. 
     In an embodiment, the system may further comprise an evaluation unit, which is determined in such a way that it evaluates an output value when a reference input value is not applied and an output value when a reference input value is applied for determining the field strength. The evaluation unit may be, for example, a microcontroller with a low power requirement. 
     In another embodiment, the antenna circuit can have an antenna coil and a capacitor, which together form a parallel resonant circuit. 
     In an embodiment, the switching unit can have a first switch, which is looped between a terminal of the capacitor and a reference potential, particularly ground, and a second switch, which is looped between the terminal of the capacitor and a terminal of the reference input value generating unit, at which the reference input value is applied. The switches are activated in such a way that the reference input value serves as an input value of the antenna circuit, when the characteristic parameters are determined, and that the antenna circuit is decoupled from the reference input value or the reference input value generating unit, when the field strength is measured. 
     In another embodiment, the antenna circuit can be designed for a mutual coupling with a transmitting antenna of a transmitter of the carrier signal. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
         FIG. 1  illustrates a block diagram of a passive entry/passive go system for automatic, distance-dependent unlocking and/or locking and for keyless starting of a motor vehicle; 
         FIG. 2  illustrates a detailed block diagram of a key medium and a base station of  FIG. 1 ; and 
         FIG. 3  illustrates a detailed block diagram of an antenna circuit of an LF transmitter/receiver of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a block diagram of a passive entry/passive go (PEG) system for automatic, distance-dependent unlocking and/or locking and for keyless starting of a motor vehicle  100 . 
     The PEG system comprises a base station  110 , which is placed in motor vehicle  100 , and at least one card-shaped, electronic key medium  200  assigned to base station  110 . 
     If a user (not shown) of key medium  200  operates a door handle  120  of motor vehicle  100 , this is detected in motor vehicle  100  and reported to base station  110 , for example, via a motor vehicle bus system (not shown). Base station  110  thereupon transmits a low-frequency (LF) carrier signal with a frequency of 125 kHz over an LF antenna of base station  110  in the form of a coil  114  to electronic key medium  200 . Key medium  200 , after receiving the LF carrier signal and a distance determination using the LF carrier signal field strength calculated in the key medium  200 , transmits a signal with unlocking information in a UHF frequency range to base station  110 , when the determined or calculated distance is within a permissible range. The UHF signal is received by a UHF antenna  115  of base station  110 , and when the information transmitted from key medium  200  to base station  110  conforms with the protocol, motor vehicle  100  is unlocked, and the user can sit, for example, on a driver&#39;s seat (not shown) of motor vehicle  100 . 
     To start motor vehicle  100 , the user presses a start button, whereupon the low-frequency LF carrier signal is again transmitted to key medium  200 . After a repeated distance or position calculation in key medium  200 , during which it is verified whether the user is sitting in a driver&#39;s seat (not shown), a start release is transmitted by key medium  200 , again via the UHF channel, to base station  110 . 
     The UHF transmission is based on a far-field coupling and the LF transmission on an inductive or mutual coupling in the near field. If more than one antenna  114  is placed at different positions in motor vehicle  100 , apart from a distance measurement, a position measurement relative to motor vehicle  100  can also be made by determining the respective antenna field strength, calculation of the distance to the respective antenna from the field strength, and subsequent triangulation. 
       FIG. 2  shows a detailed block diagram of key medium  200  and base station  110  of  FIG. 1 . 
     Base station  110  comprises an LF transmitter/receiver  111  and LF antenna  114  in the form of a coil, connected to LF transmitter/receiver  111 , a UHF transmitter/receiver  113 , a UHF antenna  115 , connected to UHF transmitter/receiver  113 , and a microprocessor  112 , which is coupled to LF transmitter/receiver  111  and UHF transmitter/receiver  113  and exchanges data, to be transmitted and received bidirectionally, with the transmitter/receiver. 
     Key medium  200  comprises an LF transmitter/receiver  201  for a 3D reception, to which antennas  202 ,  203 , and  204  are connected in the form of coils. The antenna coils or symmetry axes in the winding direction of antenna coils  202 ,  203 , and  204  are each perpendicular to one another. The field strengths calculated per antenna can be interpreted as components of a three-dimensional field strength vector, whose contribution has a value dependent on the distance of key medium  200  from transmitting antenna  114  of base station  110 , but the value is independent of an orientation of key medium  200  relative to transmitting antenna  114 . 
     For UHF transmission, key medium  200  has a UHF transmitter/receiver  207  and a UHF antenna  208  connected to UHF transmitter/receiver  207 . 
     Furthermore, key medium  200  has a microprocessor  205 , which is coupled to LF transmitter/receiver  201  and UHF transmitter/receiver  207  and exchanges data, to be transmitted and received bidirectionally, with the transmitters/receivers, and a battery or an accumulator  206  for supplying power. LF transmitter/receiver  201  in addition outputs a field strength signal, associated with each of antennas  202 ,  203 , and  204 , to microprocessor  205 . 
     In the simplest case, an LF data transmission occurs unidirectionally from base station  110  to key medium  200 , whereby in this case, unit  111  is only a transmitter and unit  201  only a receiver. Accordingly, the UHF data transmission can occur unidirectionally from key medium  200  to base station  110 , whereby in this case, unit  207  is only a transmitter and unit  113  only a receiver. In the shown exemplary embodiment, both the LF data transmission and the UHF data transmission occur bidirectionally. 
       FIG. 3  shows a detailed block diagram of an antenna circuit  214  of LF transmitter/receiver  201  of  FIG. 2 . For reasons of clarity, only the antenna circuit which is assigned to antenna  202  is shown in  FIG. 3 . Antennas  203  and  204  are assigned corresponding antenna circuits (not shown). 
     Antenna circuit  214  comprises antenna or antenna coil  202 , a resistor  212 , which represents a parasitic copper resistor of antenna coil  202 , and a capacitor  213 . Antenna coil  202  and capacitor  213  form a parallel resonant circuit. An output value, which during normal operation is a function of the field strength of the LF carrier signal and a function of characteristic parameters of antenna circuit  214 , is applied at the output terminal N 1  in the form of an output voltage UAF or UAI. The output voltage UAF or UAI is used as an analog input value for an A/D converter (not shown) of microprocessor  205  and is processed further digitalized in said microprocessor. 
     The operation of the arrangement shown in  FIG. 3  will be described in detail next. In a transmission unit  217  of base station  110 , which is shown only as a detail, a signal with a frequency f 0  is provided via a driver stage  209 . The signal is supplied to a series resonant circuit with transmitting antenna coil  114 , a resistor  210 , and a capacitor  211 . A voltage UQ is induced in antenna coil  202  by a magnetic carrier field generated in transmitting antenna coil  114 . The following formulas describe in mathematic terms the coupling between antenna coils  114  and  202 . They are derived from the manual Klaus Finkenzeller, RFID-Handbuch [RFID Manual], 3rd ed., HANSER, 2002; see particularly pages 72, 73, and 77.
 
 UQ=ω   0   *k √{square root over ( L 1 *L 2)}* i   1   (1)
 
     In Equation (1), UQ designates a voltage induced in coil  202 ; ω 0  is the angular frequency assigned to the transmission frequency f 0 , k is a coupling factor, L 1  is an inductance of antenna coil  114 , L 2  is an inductance of antenna coil  202 , and i 1  is a current through transmitting antenna coil  114 . 
     The voltage UQ induced in antenna coil  202  generates the following output voltage UAF: 
     
       
         
           
             
               
                 
                   UAF 
                   = 
                   
                     UQ 
                     
                       
                         
                           
                             ( 
                             
                               
                                 ω 
                                 0 
                               
                               * 
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                               * 
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                             
                             ) 
                           
                           2 
                         
                         + 
                         
                           
                             ( 
                             
                               1 
                               - 
                               
                                 
                                   ω 
                                   0 
                                   2 
                                 
                                 * 
                                 L 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                                 * 
                                 C 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Equation (2), in comparison with the formula in Finkenzeller, contains the simplified assumption that RL=∞, as a result of which a term with RL is eliminated in the denominator. R 2  designates a resistance value of resistor  212  and C 2  designates a capacitance of capacitor  213 . 
     Equation (2) shows directly that the output voltage UAF, produced by the field of the carrier signal, is determined by the value R 2  of resistor  212 , the inductance L 2  of receiving coil  202 , and the capacitance C 2  of capacitor  213 . These values therefore form the characteristic parameters of antenna circuit  214 . 
     To determine the characteristic parameters or a measure for the characteristic parameters or a characteristic quantity for the characteristic parameters, which represents their output voltage-relevant properties, antenna circuit  214  is supplied with a reference input value. The reference input value is generated in the form of a reference input voltage UI by a reference input value generating unit in the form of an oscillator  216 , which is part of LF transmitter/receiver  201  of  FIG. 2 . The frequency of the reference input voltage UI is the same as frequency f 0  of the carrier signal. The amplitude of the reference input voltage UI is generated precisely with a previously known value. 
     For measuring the characteristic parameters, a switching unit  215 , activated by microprocessor  205 , with a first switch  218  and a second switch  219 , is activated in such a way that switch  218  is opened and switch  219  is closed. This has the result that antenna circuit  214  is supplied with the reference input value UI as a simulated input voltage. The signal generated by transmitting unit  217  is turned off during the measurement of the characteristic parameters, i.e., UQ=0. Switching unit  215  is part of LF transmitter/receiver  201  of  FIG. 2 . 
     An output voltage UAF arising at output terminal N 1  of antenna circuit  214  can be calculated using the following equation: 
     
       
         
           
             
               
                 
                   UAF 
                   = 
                   
                     UI 
                     
                       
                         
                           
                             ( 
                             
                               
                                 ω 
                                 0 
                               
                               * 
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                               * 
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                             
                             ) 
                           
                           2 
                         
                         + 
                         
                           
                             ( 
                             
                               1 
                               - 
                               
                                 
                                   ω 
                                   0 
                                   2 
                                 
                                 * 
                                 L 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                                 * 
                                 C 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     If a quotient is formed from the first output value UAF and the second output value UAI, the following equation results: 
     
       
         
           
             
               
                 
                   
                     UAF 
                     UAI 
                   
                   = 
                   
                     
                       
                         UQ 
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     ω 
                                     0 
                                   
                                   * 
                                   R 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                   * 
                                   C 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                 
                                 ) 
                               
                               2 
                             
                             + 
                             
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     
                                       ω 
                                       0 
                                       2 
                                     
                                     * 
                                     L 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                     * 
                                     C 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                       
                       
                         UI 
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     ω 
                                     0 
                                   
                                   * 
                                   R 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                   * 
                                   C 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                 
                                 ) 
                               
                               2 
                             
                             + 
                             
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     
                                       ω 
                                       0 
                                       2 
                                     
                                     * 
                                     L 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                     * 
                                     C 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                       
                     
                     = 
                     
                       UQ 
                       UI 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     If Equation (4) is solved for UQ, we obtain: 
     
       
         
           
             
               
                 
                   UQ 
                   = 
                   
                     UI 
                     * 
                     
                       UAF 
                       UAI 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     A distance x of the transmitting antenna or transmitting coil  114  of receiving antenna or receiving coil  202  can be calculated from the calculated voltage UQ using the following Equation (6): 
     
       
         
           
             
               
                 
                   x 
                   = 
                   
                     
                       
                         
                           ( 
                           
                             
                               
                                 
                                   r 
                                   
                                     L 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                   2 
                                 
                                 * 
                                 
                                   r 
                                   
                                     L 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                   2 
                                 
                               
                               
                                 2 
                                 * 
                                 
                                   
                                     
                                       r 
                                       
                                         L 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                       
                                     
                                     * 
                                     
                                       r 
                                       
                                         L 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                 
                                 * 
                                 
                                   UQ 
                                   
                                     
                                       ω 
                                       0 
                                     
                                     * 
                                     
                                       
                                         L 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                         * 
                                         L 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                       
                                     
                                     * 
                                     
                                       i 
                                       1 
                                     
                                   
                                 
                               
                             
                             3 
                           
                           ) 
                         
                         2 
                       
                       - 
                       
                         r 
                         
                           L 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     where r L1  is a radius of transmitting antenna coil  114  and r L2  a radius of receiving antenna coil  202 . Equation (6) applies to air coils as transmitting antenna  114  and receiving antenna  202 . If no air coils are used, Equation (6) can be modified accordingly. For this purpose, the coupling factor dependent on the distance x (by transformation of Equation (1)) 
     
       
         
           
             
               k 
               ⁡ 
               
                 ( 
                 x 
                 ) 
               
             
             = 
             
               UQ 
               
                 
                   ω 
                   0 
                 
                 * 
                 
                   
                     L 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                     * 
                     L 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                 
                 * 
                 
                   i 
                   1 
                 
               
             
           
         
       
     
     in Equation (6) is to be replaced by a coupling factor valid for an employed coil type. For this purpose, reference is again made, for example, to Finkenzeller, see particularly page 108, or the data book: ATMEL, Data Book 2001, ICs for wireless control systems, pages 326ff. 
     In summary, the field strength or the distance is determined as follows: 
     In a first step, switching unit  215  is activated by microprocessor  205  such that antenna circuit  214  is supplied with the reference input value UI. The reference input value UI can be permanently active or activated solely for the measuring process. Here, it should be known or made certain that the carrier signal is not active. 
     Next, the arising output voltage UAI is measured and stored. 
     After storage of output voltage UAI generated by turning on reference voltage source  216 , switching unit  215  is activated by microprocessor  205  in such a way that antenna circuit  214  is decoupled from the reference input value UI. The now arising output voltage UAF is produced by the field of the carrier signal at antenna coil  202 . 
     The actual field strength, i.e., the field strength at which an effect of the characteristic parameters is compensated, is calculated by forming the ratio of UAF and UAI and multiplying by the known voltage UI. 
     For the final distance measurement, the specific field strengths, determined as described above, of antennas  202 ,  203 , and  204  are superposed for calculating a total field strength, which is independent of the orientation. The distance is finally calculated using Equation (6) from the total field strength calculated by conventional vector calculus. 
     The measurement of the voltage UAI can be measured cyclically or triggered by certain events, as a result of which a change in the characteristic parameters of the antenna circuit, for example, due to a temperature drift, is taken into account. 
     It is understood that a current may also be used instead of the reference input value in the form of the voltage UI. For this purpose, switch  218  must remain closed in switching unit  215  during the measurement of the characteristic parameters and a reference current source supplies its current to a connection node between resistor  212  and capacitor  213 . 
     In LF transmitter/receiver  201  of  FIG. 2 , other circuit parts can be provided in addition to antenna circuit  214 . For example, an integrated circuit can be provided, which is designed for coupling to antenna circuit  214 . The integrated circuit can then take over, for example, the evaluation of the output voltage UAF or UAI instead of microprocessor  205 . In other words, the entire evaluation of the output voltage UAI and UAF occurs in the integrated circuit, as a result of which the evaluation in microprocessor  205  is simplified, because specific information is no longer necessary there. Furthermore, switching unit  215  and reference input value generating unit  216  can also be part of the integrated circuit. 
     The shown embodiments enable a precise, long-term stable field strength or distance measurement, without a laborious calibration being necessary during a manufacturing process. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.