EP2010926A2

EP2010926A2 - Method and circuit arrangement for measuring field intensity and integrated circuit

Info

Publication number: EP2010926A2
Application number: EP07724550A
Authority: EP
Inventors: Daniel Moser; Helmut Moser
Original assignee: Atmel Germany GmbH
Current assignee: Atmel Germany GmbH
Priority date: 2006-04-25
Filing date: 2007-04-25
Publication date: 2009-01-07
Also published as: US20070257770A1; DE102006020423B3; WO2007121995A3; CN101473234A; WO2007121995A2; US7796965B2

Abstract

A circuit arrangement for measuring the field intensity of a carrier signal comprises an antenna circuit (214) having an antenna (202) and an output terminal (N1) to which an output variable in the form of an output voltage (UAF) and/or of an output current is applied, said output variable being a function of the field intensity and a function of characteristic parameters of the antenna circuit (214). The arrangement also comprises a reference input variable generation unit (216) for generating a reference input variable in the form of a reference input voltage (UI) and/or of a reference input current having a known reference frequency and reference amplitude; a controlled switching unit (215) which is coupled to the antenna circuit (214) and the reference input variable generation unit (216) and which supplies the antenna circuit (214) with the reference input variable (UI) or which decouples the antenna circuit from the reference input variable (UI), depending on the state of actuation; and an evaluation unit which is adapted to evaluate, for the purpose of measuring field intensity, an output variable (UAF) when the reference input variable (UI) is not applied and an output variable (UAI) when the reference input variable (UI) is applied.

Description

Method and circuit arrangement for field strength determination and integrated circuit

The invention relates to a method and a circuit arrangement for field strength determination and an integrated circuit.

In so-called remote keyless entry systems for motor vehicles unlocking of the motor vehicle is not carried out with a mechanical key, but with an electronic key medium or an electronic key, for example in the form of a molded part, are arranged on the actuators. On or in the key medium one or more integrated circuits are arranged, which realize the function of the electronic key medium or the electronic key.

As a counterpart to the electronic key, a so-called base station is arranged in the motor vehicle, which communicates wirelessly with the electronic key or exchanges data.

For unlocking or locking the motor vehicle, an associated operating element on the electronic key is actuated by a user, whereby a data transmission between the electronic key or its integrated circuits and the base station is initiated. If the information transmitted between the base station and the electronic key corresponds to an agreed protocol and has the expected content, the motor vehicle is unlocked with the aid of the base station.

In the case of so-called passive-entry / passive-go systems or passive-entry-go (PEG) systems, actuation of the key for locking or unlocking the motor vehicle is no longer necessary. The user of the motor vehicle only has to carry with him an electronic key medium, for example in the form of a card. When the user actuates a door handle of the motor vehicle, this is detected in the motor vehicle and reported to the base station. The base station then sends a low-frequency carrier signal, for example with a frequency of 125 kHz, to the electronic key medium. In addition, data can also be transmitted to the electronic key medium by the low-frequency carrier signal.

For receiving the low-frequency carrier signal, the key medium comprises an antenna circuit with an antenna, for example in the form of a coil, the antenna circuit generating an output, 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 the German patent application DE 101 59 551 A1 of the same Applicant.

The output is used to determine the removal of the key medium from the base station or from one or more transmit antennas of the base station. If more than one transmission antenna are provided at different positions, for example in a front region and in a rear region of the motor vehicle, the position of the user or the user can also be determined on the basis of two determined distances to the respective antennas by so-called triangulation. the key medium are determined relative to the motor vehicle.

For determining the field strength independently of an orientation or position of the key medium with respect to the transmitting antenna of the base station tion devices are known in which three mutually perpendicular antenna coils are provided. The respective field strengths of the antenna coils are vectorially superimposed to calculate a resulting field strength.

If the distance between the antenna of the base station and the antenna of the antenna circuit of the key medium has been determined, it is subsequently checked whether the determined distance is 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 only certain locks can be made dependent on which position with respect to the motor vehicle, the user is.

If the user subsequently enters the motor vehicle and actuates a start button for starting the engine, a distance is also determined in the key medium by means of a field strength measurement. Position determination performed. If the position determination indicates that the operator is at the position required to start the engine, the engine is started.

In addition to the access control and the engine start, there are numerous other application scenarios in which a distance measurement is performed, for example, when the operator exits the vehicle and moves away from it.

It is clear from the above that high accuracy requirements are placed on the distance or position measurement on the basis of the field strength determination of the carrier signal in the key medium.

As stated above, an output variable is generated for the distance measurement by the antenna circuit, for example an output signal. - A -

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 respective antenna circuit form the tolerances, manufacturing variations and other specific properties of the components of the respective antenna circuit.

Since each antenna circuit has its specific, characteristic parameters, which in practice can differ greatly from one another, different antenna circuit voltages result in different output voltages of the antenna circuits in the case of different antenna circuits with the same field strength. If, for example, a non-characteristic table is stored in the respective key medium for the distance calculation, in which an assignment of output voltage to field strength is stored, this can lead to significant errors in the distance calculation.

In order to take this problem into account, a complex measurement of the key media or of the antenna circuits during a production process usually takes place. For this purpose, for example, a known reference field strength from the outside can be predetermined by a calibration station, which serves to generate a calibration variable on the respective key medium. This method is very complex and only takes into account the parameter situation during the calibration process. A change in the characteristic parameters due to long-term influences, temperature and variable operating voltage can not be detected by the calibration or calibration during the manufacturing process, i. The distance measurement deteriorates accordingly in such long-term influences.

Another significant problem is that the antenna circuit or its antenna is more heavily attenuated in response to metallic objects in its environment, such as a keychain, resulting in its characteristic parameters also change significantly depending on the situation. These influences can not be detected by the calibration or calibration during the manufacturing process.

From GB 2 307 050 A a method for measuring characteristic parameters of an antenna system is known, for which purpose a reference input in the form of a carrier signal modulated with a symbol sequence is generated and the antenna system is supplied with the reference input quantity thus generated.

The invention has for its object to provide a method for determining the field strength of a carrier signal, a circuit arrangement for determining the field strength of a carrier signal and an integrated circuit for a key medium available that allow a precise, long-term stable field strength measurement, without a complex calibration during a To require a manufacturing process.

The invention achieves this object by a method for determining the field strength of a carrier signal according to claim 1, a circuit arrangement for determining the field strength of a carrier signal according to claim 11 and an integrated circuit according to claim 16.

In the method for determining the field strength of a carrier signal on an antenna of an electronic circuit antenna circuit, the antenna circuit generates an output which is a function of the field strength and a function of characteristic parameters of the antenna circuit. The characteristic parameters of a respective antenna circuit form the tolerances, manufacturing variations and other specific properties of the components of the respective antenna circuit. According to the invention, the characteristic parameters are determined in the electronic key on the basis of a reference generated in the electronic key itself. measured during the measurement of the characteristic parameters, that is, the reference input is used during the measurement as an input to the antenna circuit. Subsequently, the characteristic parameters are stored. Subsequently, a first, field-related output of the antenna circuit is measured, wherein in the measurement of the first output of the antenna circuit is not acted upon by the reference input. The field strength is determined on the basis of the first output variable and the characteristic parameters, wherein in particular an influence of the characteristic parameters on the first output variable is compensated. The measurement of the characteristic parameters can be cyclical, whereby a continuous calibration of the antenna circuit can take place. The generation of the in-circuit reference input variable allows calibration even without a complicated calibration process during production. Since the calibration can be carried out continuously, a precise, long-term stable field strength measurement is possible, which also takes into account changing environmental conditions, for example metallic objects in the surroundings of the key medium or of the antenna circuit.

In a development of the method, the reference input variable is generated in the form of a reference input voltage and / or a reference input current with a predetermined reference frequency and reference amplitudes, a second output variable is measured when the reference input variable is present and the characteristic parameters are determined from the second output variable. The characteristic parameters may be identical to the second output variable or the second output variable may be a measure of the characteristic parameters. For determining the field strength, the quotient of the first field-strength-related output variable and the second reference input-variable-related output variable is preferably formed. In a development of the method, the reference frequency is set equal to a frequency of the carrier signal. In this way, the characteristic parameters are determined by the reference variable at the relevant operating frequency.

In a development of the method, the reference frequency is derived from a frequency of the carrier signal. In this way it can be ensured that the calibration by the reference input quantity also takes place at the actual operating frequency of the antenna circuit.

In a development of the method, a distance between the antenna and a transmitting antenna of a transmitter of the carrier signal is determined from the determined field strength. In the context of distance determination, the field strength is preferably additionally determined on a second antenna and on a third antenna of the antenna circuit, wherein the antennas are perpendicular to one another and a distance between the antennas and a transmitting antenna of a transmitter of the carrier signal is determined by superposition from the determined field strengths , By vectoring the field strengths calculated per antenna, it is possible to calculate the distance independently of an orientation of the antennas or the key medium with respect to the transmitting antenna.

In a development of the method, the antenna and a transmitting antenna of a transmitter of the carrier signal are transformer-coupled. For the transformational or inductive coupling, reference is made to the textbook by Klaus Finkenzeller, RFID Handbook, 3rd ed., HANSER, 2002, see in particular chapter 3.2.1 "inductive coupling", pages 42 to 45.

In a development of the method, a parallel resonant circuit or a series resonant circuit is formed by the antenna circuit. In a development of the method, the frequency of the carrier signal is in a range from 50 kHz to 150 kHz or in a range from 5 MHz to 25 MHz.

The circuit arrangement according to the invention for determining the field strength of a carrier signal has an antenna circuit with an antenna and an output terminal at which an output quantity in the form of an output voltage and / or an output current is present, which is a function of the field strength and a function of characteristic parameters of the antenna circuit is. According to the invention, a reference input quantity generating unit is provided for generating a reference input quantity in the form of a reference input voltage and / or a reference input current with a known reference frequency and reference amplitude and a controllable switching unit coupled to the antenna circuit and the reference input variable generating unit, which supplies the antenna circuit with the reference input variable depending on the control state or the antenna circuit of FIG decoupled from the reference input quantity.

The circuit arrangement further comprises an evaluation unit, which is designed such that it evaluates an output variable when the reference input quantity is not applied and an output variable when the reference input quantity is applied to determine the field strength. The evaluation unit may be, for example, a microcontroller with low power consumption.

In a development of the circuit arrangement, the reference input quantity generating unit is an oscillator. The oscillator may include, for example, a PLL, crystals, voltage controlled oscillators, etc. In a development of the circuit arrangement, the antenna circuit has an antenna coil and a capacitor, which together form a parallel resonant circuit.

In a development of the circuit arrangement, the switching unit has a first switch, which is looped in between a terminal of the capacitor and a reference potential, in particular ground, and a second switch, which is looped in between the terminal of the capacitor and a terminal of the reference input quantity generating unit to which the reference input is pending. The switches are driven so that the reference input serves as an input to the antenna circuit when the characteristic parameters are determined, and that the antenna circuit is decoupled from the reference input and the reference input when the field strength is measured.

In a development of the circuit arrangement, the antenna circuit is designed for a transformer coupling with a transmitting antenna of a transmitter of the carrier signal.

The integrated circuit according to the invention for a key medium of a Passive E ntry and / or a Passive-Go system is designed for coupling with a circuit arrangement according to the invention.

The invention will be described in detail below with reference to the drawings. Here are shown schematically:

1 is a block diagram of a passive entry / passive go system for automatic, distance-dependent unlocking and / or locking and keyless starting a motor vehicle, FIG. 2 is a detailed block diagram of a key medium and a base station of FIG. 1 and FIG

3 is a detailed block diagram of an antenna circuit of an LF transmitter / receiver of FIG. 2.

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 arranged in the motor vehicle 100, and at least one card-shaped electronic key medium 200 assigned to the base station 110.

When a user of the key medium 200, not shown, actuates a door handle 120 of the motor vehicle 100, this is detected in the motor vehicle 100 and reported to the base station 110, for example via a motor vehicle bus system (not shown). The base station 110 then sends a low frequency (LF) carrier signal at a frequency of 125kHz via an LF antenna of the base station 110 in the form of a coil 114 to the electronic key medium 200. The key medium 200 transmits after receiving the LF carrier signal and a Distance determination based on a field strength of the LF carrier signal calculated in the key medium 200, a signal with unlocking information in a UHF frequency range to the base station 110 when the determined or calculated distance is in an allowable range. The UHF signal is received at a UHF antenna 115 of the base station 110, and when the information transmitted from the key medium 200 to the base station 110 is protocol compliant, the motor vehicle 100 is unlocked and the user can be seated, for example, on a driver's seat of the motor vehicle, not shown Take 100 seats. To start the motor vehicle 100, the user presses a start button, whereupon in turn the low-frequency LF carrier signal is sent to the key medium 200. After a new distance or position calculation in the key medium 200, in which it is checked whether the user is on a driver's seat, not shown, a start release is sent to the base station 110 from the key medium 200, again via the UHF channel.

The UHF transmission is based on a so-called far field coupling and the LF transmission on an inductive or transformer coupling in the near field. If more than one antenna 114 is arranged at different positions of the motor vehicle 100, in addition to a distance measurement, a position measurement relative to the motor vehicle 100 can also be carried out by determining the respective antenna field strength, calculating the distance to the respective antenna from the field strength and subsequent triangulation.

FIG. 2 shows a detailed block diagram of the key medium 200 and the base station 110 of FIG. 1.

The base station 110 includes an LF transmitter / receiver 111 and the LF antenna 114 connected to the LF transmitter / receiver 111 in the form of a coil, a UHF transmitter / receiver 113, and the UHF connected to the UHF transmitter / receiver 113 Antenna 115 and a microprocessor 112 which is coupled to the LF transceiver 111 and the UHF transceiver 113 and exchanges bidirectionally with data to be transmitted and received.

The key medium 200 comprises an LF transmitter / receiver 201 for so-called 3D reception, to which antennas 202, 203 and 204 are connected in the form of coils. The antenna coils or axes of symmetry in the winding direction of the antenna coils 202, 203 and 204 are each perpendicular to each other. The field strengths calculated per antenna may be construed as components of a 3-dimensional field strength vector the magnitude of which depends on the distance of the key medium 200 from the transmitting antenna 114 of the base station 110 but which is independent of an orientation of the key medium 200 relative to the transmitting antenna 114 is.

For UHF transmission, the key medium 200 comprises a UHF transmitter / receiver 207 and a UHF antenna 208 connected to the UHF transmitter / receiver 207.

Furthermore, the key medium 200 has a microprocessor 205 coupled to and communicating with the LF transceiver 201 and the UHF transceiver 207 and exchanging bidirectional data to be transmitted and received, and a battery 206 for powering , The LF transmitter / receiver 201 additionally outputs a respective field strength signal associated with the antennas 202, 203 and 204 to the microprocessor 205.

In the simplest case, LF data transmission takes place unidirectionally from the base station 110 to the key medium 200, in which case the unit 111 is merely a transmitter and the unit 201 is merely a receiver. Accordingly, the UHF data transmission may be unidirectional from the key medium 200 to the base station 110, in which case the unit 207 is merely a transmitter and the unit 113 is merely a receiver. In the exemplary embodiment shown, both the LF data transmission and the UHF data transmission are bidirectional.

FIG. 3 shows a detailed block diagram of an antenna circuit 214 of the LF transceiver 201 of FIG. 2. For the sake of clarity, FIG. Visibility is shown in Fig. 3, only one antenna circuit, which is associated with the antenna 202. The antennas 203 and 204 are associated with respective antenna circuits, not shown.

The antenna circuit 214 comprises the antenna or antenna coil 202, a resistor 212 which images a copper parasitic resistance of the antenna coil 202, and a capacitor 213. The antenna coil 202 and the capacitor 213 form a parallel resonant circuit. At an output terminal N1 is an output large in the form of an output voltage UAF or UAI, which is a function of the field strength of the LF carrier signal and a function of characteristic parameters of the antenna circuit 214 in a normal operation. The output voltage UAF or UAI serves as an analog input variable for an A / D converter (not shown) of the microprocessor 205 and is processed further in this digitized form.

Hereinafter, the operation of the arrangement shown in Fig. 3 will be described in detail. In a transmission unit 217 of the base station 110, which is shown only in sections, a signal with a frequency fθ is provided via a driver stage 209. The signal is supplied to a series resonant circuit with the transmitting antenna coil 114, a resistor 210 and a capacitor 211. A magnetic carrier field generated in the transmitting antenna coil 114 induces a voltage UQ in the antenna coil 202. The following formulas mathematically describe the coupling between the antenna coils 114 and 202. They are taken from the textbook Klaus Finkenzeller, RFID Handbook, 3rd ed., HANSER, 2002, see in particular pages 72, 73 and 77.

UQ = ω _o * k ^ jL \ * L2 * i _ι (1)

In Equation (1), UQ denotes a voltage induced in the coil 202, ω ₀ is the angular frequency associated with the transmission frequency fθ, k a coupling factor, L1 is an inductance of the antenna coil 114, L2 is an inductance of the antenna coil 202, and /, is a current through the transmitting antenna coil 114.

The voltage UQ induced in the antenna coil 202 produces the following output voltage UAF:

UAF = UQ

(2)

_Λ l (ω ₀ * R2 * C2) ² + (l - ω ₀ ² * L2 * C2γ

Equation (2), compared to the formula in Finkenzeller, simplifies the assumption that RL = oo, which eliminates a term with RL in the denominator. R2 denotes a resistance value of the resistor 212, and C2 denotes a capacitance of the capacitor 213.

From equation (2) it follows immediately that the output voltage UAF caused by the field of the carrier signal is determined by the value R2 of the resistor 212, the inductance L2 of the receiver coil 202 and the capacitance C2 of the capacitor 213. These values thus form the characteristic parameters of the antenna circuit 214.

In order to determine the characteristic parameters or a measure of the characteristic parameters or of a parameter for the characteristic parameters which maps their output voltage-relevant properties, the antenna circuit 214 is charged with a reference input variable. The reference input is generated in the form of a reference input voltage Ul by a reference input generating unit in the form of an oscillator 216, which is part of the LF transceiver 201 of FIG. The frequency of the reference input voltage Ul is equal to the frequency fθ of the carrier signal. The amplitude of the reference input voltage U1 is precisely generated with a known value. For measuring the characteristic parameters, a switching unit 215 controlled by the microprocessor 205 with a first switch 218 and a second switch 219 is activated such that the switch 218 is opened and the switch 219 is closed. This causes the antenna circuit 214 to be supplied with the reference input Ul as a simulated input voltage. The signal generated by the transmitting unit 217 is turned off during the measurement of the characteristic parameters, ie, UQ = 0. The switching unit 215 is part of the LF transceiver 201 of FIG. 2.

An output voltage UAF which is established at the output terminal N1 of the antenna circuit 214 can be calculated using the following equation:

UAF = ^UI = (3)

_Λ j (ω ₀ * Rl * C2) ² + (1 - ω _Q ² * Ll * C2) ²

If a quotient of the first output variable UAF and the second output variable UAI is formed, the following equation results:

UQ

UAF ₌ Λ / K * Rl * C2f + (1 - ω _Q ² * L2 * C2) ² ₌ UQ UAI ^~ UI ^~ UI ^{ '

■ yj (ω ₀ * R2 * C2) ² + (1 - ω ² * Ll * Cl) ²

If equation (4) is solved after UQ, the result is:

UAF

UQ = UI * (5) UAI

On the basis of the calculated voltage UQ, with the following equation (6), a distance x of the transmitting antenna or of the transmitting coil 114 are calculated by the receiving antenna 202:

where r _L i is a radius of the transmit antenna coil 114 and r _L 2 is a radius of the receive 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 adapted accordingly. For this, the coupling factor dependent on the distance x (by transformation of equation (1))

in equation (6) by a coupling factor valid for a coil type used. For this purpose, again reference is made, for example, to Finkenzeller, see in particular page 108, or the data book: ATMEL, data book 2001, ICs for Wireless Control Systems, page 326 ff.

In summary, the field strength or the distance is determined as follows:

In a first step, the switching unit 215 is controlled by the micro-processor 205 such that the antenna circuit 214 is supplied with the reference input quantity Ul. The reference input value U1 can be permanently active or can only be activated for the measuring process. It should be known or ensured that the carrier signal is not active. Subsequently, the resulting output voltage UAI is measured and stored.

After storing the output voltage UAI generated by switching on the reference voltage source 216, the switching unit 215 is controlled by the microprocessor 205 such that the antenna circuit 214 is decoupled from the reference input quantity U L. The now adjusting output voltage UAF is effected by the field of the carrier signal at the antenna coil 202.

The actual field strength, i. that field strength at which an influence of the characteristic parameters is compensated is calculated by forming the ratio of UAF and UAI and multiplying by the known voltage Ul.

For the final distance measurement, the respective field strengths, determined as described above, of the antennas 202, 203 and 204 are superimposed for calculating a total field strength which is independent of the orientation. From the total field strength calculated by conventional vector calculation, the distance is finally calculated using equation (6).

The measurement of the voltage UAI can be measured cyclically or triggered by specific events, whereby 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 instead of the reference input in the form of the voltage Ul, a current can be used. For this purpose, switch 218 must remain closed in switching unit 215 during the measurement of the characteristic parameters, and a reference current source feeds its current into a connection node between resistor 212 and capacitor 213. In the LF transmitter / receiver 201 of Fig. 2, in addition to the antenna circuit 214 further circuit parts may be provided. For example, an integrated circuit may be provided which is designed to be coupled to the antenna circuit 214. The integrated circuit can then take over, for example, the evaluation of the output voltage UAF or UAI instead of the microprocessor 205. In other words, the complete evaluation of the output voltages UAI and UAF takes place in the integrated circuit, whereby the evaluation in the microprocessor 205 is simplified, as there no specific knowledge is required. Furthermore, the switching unit 215 and the reference input generating unit 216 may also be part of the integrated circuit.

The embodiments shown enable a precise, long-term stable field strength or distance measurement, without requiring a complex calibration during a manufacturing process.

Claims

claims

A method of determining the field strength of a carrier signal at an antenna (202) of an antenna circuit (214) of an electronic key (200), wherein the antenna circuit (214) generates an output indicative of a field strength function and a function of characteristic parameters of the antenna circuit (214), characterized by the steps carried out in the electronic key (200):

Generating a reference input (Ul) ₁ Applying the antenna circuit (214) to the reference input (Ul),

Measuring the characteristic parameters while the antenna circuit is being supplied with the reference input (Ul),

Storing the characteristic parameters, measuring a first output (UAF) of the antenna circuit (214) while not subjecting the antenna circuit to the reference input (Ul), and determining the field strength from the first output (UAF) and the characteristic parameters.

2. The method according to claim 1, characterized in that the reference input quantity is generated in the form of a reference input voltage (U1) and / or a reference input current with a predetermined reference frequency and reference amplitude, a second output variable (UAI) is measured when the reference input variable (U1) is present and the characteristic Parameters from the second output variable (UAI) are determined.

3. The method according to claim 2, characterized in that for determining the field strength of the quotient of the first output variable (UAF) and the second output variable (UAI) is formed.

4. The method according to claim 2 or 3, characterized in that the reference frequency is set equal to a frequency (fθ) of the carrier signal.

5. The method according to any one of claims 2 to 4, characterized in that the reference frequency is derived from a frequency (fθ) of the carrier signal.

6. The method according to any one of the preceding claims, characterized in that from the determined field strength, a distance between the antenna (202) and a transmitting antenna (114) of a transmitter of the carrier signal is determined.

7. The method according to claim 6, characterized in that the field strength at a second antenna (203) and at a third antenna (204) is determined, wherein the antennas (202, 203, 204) each perpendicular to each other and from the determined field strengths a distance between the antennas (202, 203, 204) and a transmitting antenna (114) of a transmitter of the carrier signal is determined.

8. The method according to any one of the preceding claims, characterized in that the antenna (202) and a transmitting antenna (114) of a transmitter of the carrier signal are transformer coupled.

9. The method according to any one of the preceding claims, characterized in that a parallel resonant circuit or a series resonant circuit is formed by the antenna circuit (214).

10. The method according to any one of the preceding claims, characterized in that the frequency (fθ) of the carrier signal is in a range of 50KHz to 150KHz or in a range of 5MHz to 25MHz.

11. Circuit arrangement for determining the field strength of a carrier signal with an antenna circuit (214) having an antenna (202) and an output terminal (N1) to which an output in the form of an output voltage (UAF) and / or an output current is present, which is a function of Field strength and a function of characteristic parameters of the antenna circuit (214), characterized by a reference input quantity generating unit (216) for generating a reference input in the form of a reference input voltage (Ul) and / or a reference input current of known reference frequency and reference amplitude, one connected to the antenna circuit (214). and the reference input generating unit (216) coupled, controllable switching unit (215) which the antenna circuit (214) depending on the control state dependent on the reference input (Ul) or decouples the antenna circuit from the reference input (Ul), and a Evaluation unit (205) which is designed such that it has an output variable (UAF) in the case of non-applied reference Ren input (Ul) and an output variable (UAI) with applied reference input value (Ul) for determining the field strength evaluates.

12. Circuit arrangement according to claim 11, characterized in that the reference input quantity generating unit is an oscillator (216).

13. Circuit arrangement according to one of claims 11 or 12, characterized in that the antenna circuit (214) comprises an antenna coil (202) and a capacitor (213), which together form a parallel resonant circuit.

14. Circuit arrangement according to claim 13, characterized in that the switching unit has a first switch (218), which is looped between a terminal of the capacitor (213) and a reference potential, and a second switch (219) which is connected between the terminal of Capacitor (213) and a terminal of the reference input quantity generating unit (216) is looped to which the reference input quantity (Ul) is present.

15. Circuit arrangement according to one of claims 11 to 14, characterized in that the antenna circuit is designed for a transformer coupling with a transmitting antenna (114) of a transmitter of the carrier signal.

16. An integrated circuit for a key medium of a passive entry and / or a passive go system, characterized in that the integrated circuit for coupling with a circuit arrangement according to one of claims 11 to 15 is formed.