TRANSPONDER SYSTEM AND METHOD FOR READOUT A PASSIVE TRANSPONDER

Description

[0001] The invention relates to a transponder system comprising at least one passive transponder having an oscillating circuit with a variable resonant frequency, and a readout device, wherein the readout device is configured to adapt a frequency of the readout signal to the variable resonant frequency of the oscillating circuit. The invention also relates to a method for reading a passive transponder having an oscillating circuit with a variable resonant frequency.

[0002] Passive transponders usually contain an oscillating circuit that can be excited by a reading device generating an alternating electromagnetic field whose frequency is the resonant frequency of the transponder.

[0003] Such passive transponders can be used for monitoring tasks, among other things. In this case, the resonant frequency of the oscillating circuit can be made dependent on an environmental condition to be monitored. Such a dependency can be established, for example, by using sensors that change an inductance or capacitance depending on a variable to be monitored. If such a sensor is used in the oscillating circuit of the passive transponder, its resonant frequency changes depending on the value of the variable to be monitored. If the passive transponder is then exposed to an externally exciting alternating electromagnetic field, the occurrence of resonance depends on whether the resonant frequency, which depends on the value of the environmental condition, is the same as the resonant frequency of the excitation.If the resonance state is defined as the target state, a deviation from the target state can be detected by the transponder's oscillating circuit no longer resonating. This allows the existence of the target state to be monitored.

[0004] Such systems allow a decision to be made as to whether the value measured by the sensor is a target value or not. If the system is in resonance, it can be concluded that the value is at the target value. If the system is out of resonance, it can be concluded that the value deviates from the target value. However, state-of-the-art systems do not allow for continuous measurement of the value. The value is only known if the value is at the target value. If the value to be measured by the sensor deviates from the target value, it is unknown in the state-of-the-art systems.

[0005] WO 2006 / 049796 A2 describes a method for communicating with an implanted wireless sensor. A capacitive sensor is connected to an inductor in the sensor to form an oscillating circuit.

[0006] DE 10 2011 009 695 A1 describes a cylindrical device for fluid flow, which has at least two pressure sensors with which the pressure of the fluid inside the device can be measured. The pressure sensors are capacitive pressure sensors.

[0007] EP 1 990 027 A2 describes a medical implant, in particular a stent, for use in the body lumen. The implant comprises a passive electronic oscillating circuit arrangement with an inductor and a capacitor, whose natural frequency can be detected externally by exciting the oscillating circuit when implanted.

[0008] DE 197 33 360 A1 describes a humidity sensor for detecting rainwater on a motor vehicle windshield. The humidity sensor essentially consists of a coil arrangement comprising a spirally wound coil resting flat on an insulating substrate.

[0009] EP 1 854 406 A1 D5 describes a system for determining position. The system comprises a field generator, a transponder, and a position tracker. The field generator generates a position-variable magnetic field. The transponder comprises an oscillating circuit with a resonant frequency and an induction coil that changes the resonant frequency depending on the position-variable magnetic field. The position tracker detects the resonant frequency and determines the position of the transponder based on the detected resonant frequency.

[0010] The object of the present invention is to provide a transponder system with which the variable resonance frequency can be determined in a passive transponder whose oscillating circuit has a variable resonance frequency. It is also an object to provide a method for reading a passive transponder that allows this resonance frequency to be determined in a passive transponder with a variable resonance frequency.

[0011] The object is achieved by the transponder system according to claim 1 and the method for reading a transponder according to claim 7. The respective dependent claims specify advantageous developments of the transponder system according to the invention and of the method according to the invention for reading a passive transponder.

[0012] According to the invention, a transponder system is provided that has at least one passive transponder. The passive transponder contains at least one resonant circuit that has a variable resonant frequency. The variable resonant frequency can be understood to mean that the value of the resonant frequency changes over time when the transponder is used as intended, i.e., it is temporally variable. Particularly preferably, the resonant frequency changes continuously over time.

[0013] The transponder system according to the invention also comprises at least one readout device having at least one readout inductance. The readout inductance can be used to apply a readout signal to the at least one resonant circuit of the passive transponder. The readout signal can be an alternating electromagnetic field. According to the invention, the readout device is configured to adapt a frequency of the readout signal to the time-varying resonant frequency of the resonant circuit. As a result, the readout device can always apply a readout signal to the resonant circuit whose frequency is equal to the resonant frequency of the resonant circuit at the time of application. If the resonant frequency of the resonant circuit changes, the readout device can adapt the frequency of the readout signal to the changed resonant frequency.By adjusting the frequency of the readout signal to the resonant frequency of the oscillating circuit, the resonant frequency of the oscillating circuit can be determined with the readout unit. In this way, the value of the resonant frequency can be determined even if the resonant frequency of the transponder's oscillating circuit varies. It is advantageous if the frequency of the readout signal continuously tracks the variable resonant frequency of the oscillating circuit, as this allows for continuous knowledge of the resonant frequency.

[0014] In an advantageous embodiment of the invention, the resonant frequency of the oscillating circuit can be varied by adjusting an inductance or a capacitance of the oscillating circuit using a measured value from a sensor. If the sensor value changes, the capacitance or inductance changes, and thus the resonant frequency of the oscillating circuit. Advantageously, at least one inductance can therefore be connected together with at least one capacitive sensor, or at least one inductive sensor with at least one capacitance, to the at least one oscillating circuit of the transponder. A particularly advantageous embodiment is one in which a capacitive pressure sensor with an inductance is connected to the oscillating circuit. Capacitive pressure sensors can have at least one capacitance or capacitor. The capacitance can then change depending on the pressure applied to the pressure sensor.A simple example of such a capacitive pressure sensor is a capacitor with two electrically conductive surfaces, the distance between which depends on the ambient pressure. For this purpose, the two surfaces can be arranged, for example, on opposite surfaces of a pressure-tight housing.

[0015] The design with at least one capacitive pressure sensor is particularly advantageous when the passive transponder is used to measure the pressure of fluid, for example, in a fluid line. The transponder system according to the invention then allows the pressure in the fluid line to be measured without having to intervene in the fluid line for the measurement. Such a fluid line can be a blood vessel, for example. In this case, it is particularly advantageous that the pressure can be measured without interfering with the blood vessel.

[0016] In an advantageous embodiment of the invention, the readout device can have an excitation unit for generating a readout signal and a feedback unit for detecting a feedback signal emanating from the passive transponder. The feedback unit can then be configured to transmit a frequency adjustment signal to the excitation unit, which is determined based on the feedback signal. The excitation unit can then adjust the frequency of the readout signal based on the frequency adjustment signal. A readout signal can therefore be generated here, which is applied to the passive transponder. A feedback signal thus emanates from the passive transponder, which is received by the readout device. If the feedback signal indicates that the resonant frequency of the resonant circuit has changed, the frequency of the readout signal can be adjusted accordingly.In this way, the frequency of the readout signal can be adjusted to the resonance frequency of the oscillating circuit or tracked to it.

[0017] For this purpose, the excitation unit can preferably have a signal source with which the readout signal can be generated. The readout signal can be generated at a specific frequency. Advantageously, the readout unit can also have a directional coupler, to whose output the signal source is electrically coupled. The signal with the given frequency is thus sent to the output of the signal source. The readout inductance can advantageously be electrically coupled to the input of the directional coupler. In this way, the signal generated by the signal source at the given frequency can be transmitted to the readout inductance and / or the signal reflected by the readout unit can be transmitted back to the input of the feedback unit.

[0018] In an advantageous embodiment of the invention, the feedback unit can have a mixer with which a signal received from the passive transponder through the readout inductance can be mixed or multiplied with the readout signal. In this way, the signal received from the passive transponder through the readout inductance can be filtered to the excitation frequency, so that deviations in the frequency of the signal received from the passive transponder from the readout signal can be determined. Advantageously, the feedback unit can also have a low-pass filter with which a signal output by the mixer can be filtered. This allows high-frequency noise and other interference signals to be filtered out. In particular, the feedback unit can advantageously be designed as a lock-in amplifier.The current value of the resonance frequency can be fed back to the control loop and the frequency of the readout signal can be adjusted so that the deviation of the frequency of the readout signal is reduced or eliminated.

[0019] The transponder system according to the invention can be used particularly advantageously for measuring pressure in fluid lines. By measuring pressure in a fluid line, in particular, a speed of pulse waves in the fluid within the fluid line can be determined. For this purpose, the passive transponder can advantageously have at least two oscillating circuits, which can be configured as described. In particular, the at least two oscillating circuits can each have at least one capacitive pressure sensor as a capacitance. The passive transponder can therefore have a first oscillating circuit with a first capacitive pressure sensor as a capacitance and, in addition, a second oscillating circuit with a second capacitive pressure sensor as a capacitance. In this configuration, the transponder system can then have a cylindrical device for the flow of fluid in the direction of a cylinder axis of the cylindrical device.In most cases, the cylinder shape can be a circular cylinder, but it can also be any other cylindrical shape. The fluid line can, in particular, also be a liquid line or a gas line.

[0020] A fluid can be understood here as a substance that offers no resistance to arbitrarily slow shear, i.e., one that has a finite viscosity. Gases and liquids, in particular, are fluids in this sense.

[0021] The capacitive pressure sensors of the at least two oscillating circuits can then be arranged at different locations on the cylindrical device along the cylinder axis. Thus, the capacitive pressure sensor of the first oscillating circuit can be arranged at a first location on the cylindrical device, and the capacitance of the second oscillating circuit can be arranged at a second location on the cylindrical device spaced apart from the first location along the cylinder axis. In this way, the at least two capacitive pressure sensors can measure the pressure of the fluid at different locations on the cylindrical device along the cylinder axis. The cylindrical device can be, for example, a stent, an implant, a catheter, and / or a fluid line, which can also be flexible.

[0022] Such a transponder design enables pressure measurement in the cylindrical device at different locations. This allows, in particular, the speed of pulse waves passing through the cylindrical device to be determined. A pulse wave leads to a temporal change in pressure at a given location along the cylinder axis. If the pressure is measured at different locations, the pressure wave leads to a sequential change in pressure at the different locations. A time difference of the pressure change can be determined using the two oscillating circuits, and the pressure wave speed can then be calculated from a known distance between the pressure sensors.

[0023] It has proven advantageous if, in the case of a passive transponder having multiple resonant circuits, these circuits have different resonant frequencies. This allows the signals from the two resonant circuits to be differentiated from one another during reading. It is particularly advantageous if the resonant frequencies differ at least by such an amount that they do not overlap in a floating manner. If, as described above, the resonant frequencies depend on measured values ​​from one or more sensors, the advantageous condition that the resonant frequencies do not overlap in a floating manner is preferably met for all expected or occurring measured values ​​and associated resonant frequencies. For example, if the resonant circuits have capacitive pressure sensors as described above, the resonant circuits are advantageously configured so that their resonant frequencies do not overlap in a floating manner for all pressures to be measured.

[0024] The pressure sensors can advantageously be arranged at the same distance from the cylinder axis. It is possible, but not essential, that the pressure sensors are arranged in the same direction from the cylinder axis.

[0025] The invention also relates to a method for reading a passive transponder, wherein the passive transponder has at least one resonant circuit with a variable resonant frequency. A readout signal is applied to the at least one resonant circuit, and a frequency of the readout signal is adjusted to the variable resonant frequency of the resonant circuit. The statements made above regarding the passive transponder system apply accordingly to the method.

[0026] Advantageously, the frequency of the readout signal is continuously adjusted to the variable resonant frequency of the oscillating circuit. Thus, the frequency preferably does not jump. Preferably, the frequency of the readout signal is adjusted to the resonant frequency of the passive transponder using a control loop so that the frequency of the readout signal is precisely the current resonant frequency of the oscillating circuit.

[0027] The method can advantageously be carried out using a lock-in amplifier. A feedback signal originating from the passive transponder can be received by the readout unit. The received feedback signal can then be fed to an input of a lock-in amplifier. A reference signal of the lock-in amplifier can be the readout signal. The phase and amplitude of the feedback signal are output from the lock-in amplifier. These can be fed to the control loop so that the frequency of the readout signal can be corrected based on the magnitude or amplitude and the phase of the feedback signal. If the frequency change is small compared to the bandwidth of the sensor resonance, the phase signal can be used as an input variable for a setpoint-actual value comparison in the control loop.Optionally, a variable can be formed from the phase and amplitude signals, i.e., the entire complex signal, which is used for control. This also makes it possible to determine whether the deviation of the feedback signal from the readout signal is positive or negative. Using the phase is also possible; in this case, a new function can be advantageously combined from the phase and its derivative, which allows the assessment of how strongly the frequency deviates positively or negatively (see ). Fig. 5 ). The use of amplitude is also possible, whereby the derivative of amplitude with respect to frequency is preferably also considered in order to determine whether the deviation of the feedback signal from the readout signal is positive or negative. Alternatively, phase and amplitude can be used in combination, so that the phase provides information about the sign of the deviation.

[0028] Advantageously, a relative pressure applied to the capacitive pressure sensor of the oscillating circuit is determined from the frequency of the readout signal. According to the invention, it is not necessary to determine an absolute pressure value; rather, it is sufficient to determine a relative pressure value related to a reference value. Thus, pressure differences, in particular, can be determined according to the invention.

[0029] Advantageously, the method according to the invention is carried out with a transponder system according to the invention as described above.

[0030] The invention will be explained below using a few figures as examples. Identical or corresponding features are identified by the same reference numerals. The features described in the examples can also be implemented independently of the specific example and combined among the examples.

[0031] It shows: Figure 1 shows a schematic block diagram of an exemplary transponder system according to the invention, Figure 2 shows an exemplary implementation of a transponder system according to the invention as a block diagram, Figure 3 shows an exemplary implementation of a generation of a readout signal and an evaluation of the feedback signal, as shown in Figure 2 used in a control loop, Figure 4 shows an exemplary implementation of the Figure 3 shown control loop, Figure 5 a representation of phase and amplitude as they are produced by a lock-in amplifier as in Figure 3 and 4 shown, as well as a control signal derived from the phase and its derivative.

[0032] Figure 1shows an example of a transponder system according to the invention as a schematic block diagram. The transponder system comprises a passive transponder 1 containing an oscillating circuit 2 with a variable resonant frequency. In the example shown, the oscillating circuit 2 has an inductance 3 and a capacitance 4. A resistor 5 is also shown for the oscillating circuit 2.

[0033] The Figure 1The transponder system shown also has a readout device 7 containing a readout inductance 6. The readout inductance is connected here to an optional balun transformer 111, which can convert the non-ground-referenced signal of the readout coil 6 (a symmetrical signal) into a ground-referenced signal. The signal generated by the balun transformer 111 is then transmitted to a secondary coil 112, which generates a signal that is fed into a ground-referenced input of a directional coupler 8. In the example shown, the readout coil has a parasitic capacitance 111a. A resistor 111d is drawn between the readout coil 6 and the parallel circuit comprising the parasitic capacitance 111a. It represents the parasitic resistance of the readout coil. The inductances 111b and 111c are connected in parallel to the readout coil 6.

[0034] A signal source 9 for generating an alternating voltage signal with a predetermined frequency is connected to one output of the directional coupler 8. A mixer 10 is arranged at a second output of the directional coupler. This mixer mixes a signal entering the directional coupler 8 from the readout inductance 6 or the balun transformer 111, which exits through the second output, with the alternating voltage signal generated by the signal source 9. The signal output by the mixer 10 is fed to a low-pass filter 11, which filters out noise and interference components, for example. The signal output by the low-pass filter can then be fed to a sensor 12, with which the amplitude and / or phase of the signal received by the readout coil 6 can be determined from the signal output by the low-pass filter 11. The sensor 12 can optionally also convert a complex signal output by the low-pass filter 11 into a real signal, which is then used for control.The signal output by sensor 12 can then be compared in a comparator 13 with a signal generated by a guide filter 15. If the frequency change of the resonant circuits 2a, 2b is small compared to the bandwidth of the resonance of the resonant circuits 2a, 2b, the phase signal is advantageously suitable as an input variable for the comparator 13. The signal output by the comparator 13 can then be fed to a controller 14, which outputs a frequency adjustment signal to the signal source 9, so that the frequency of the signal generated by the signal source 9 can be adjusted based on the frequency adjustment signal output by the controller 14. In this way, the circuit shown in . Figure 1The control circuit shown adjusts the frequency of the excitation signal generated by the signal source 9 to a changed resonance frequency of the oscillating circuit 2. In the example shown, the signal source 9 is a voltage-controlled oscillator which generates a signal with a frequency that depends on the applied voltage, here generated by the controller 14.

[0035] If the passive transponder 1 has only one resonant circuit 2, the signal source 9 can generate a single sinusoidal signal with a specific frequency for reading the resonant circuits. If the passive transponder 1 optionally has multiple resonant circuits 2, these can advantageously have different frequencies from one another. In this case, the signal source 9 can advantageously generate a signal in which a number of sinusoidal functions corresponding to the number of different resonant circuits are superimposed with the different frequencies of the multiple resonant circuits. The different frequencies can be read out using the readout coil 6. In this case, the signal output from the directional coupler 8 to the mixer 10 is preferably mixed separately with the sinusoidal signals with the different resonant frequencies. A corresponding number of separate mixers 10 can advantageously be provided for this purpose.The separate signals generated in this way can then each be fed to a low-pass filter 11 and a sensor 12, which determines the amplitude and / or phase for all components. Separate feedback can then be provided to the signal source 9 for all frequency components. In the case that the passive transponder 1 has several resonant circuits 2 with different resonant frequencies, a separate mixer 10, a separate low-pass filter 11, a separate sensor 12, a separate comparator 13, a separate controller 14, a separate guide filter 15, and a separate actuator 9 are preferably provided for each of the resonant frequencies. Additionally, an adder can be provided, which superimposes the signals generated by the actuators and outputs them jointly to the directional coupler 8. A configuration with two resonant circuits 2 is particularly advantageous, as this allows pressure wave velocities to be measured.

[0036] Figure 2 shows an example block diagram of an implementation of the control loop in Figure 1 for reading a transponder 1 with an oscillating circuit 2. The oscillating circuit 2, the balun transformer 111, the readout coil 6 and the directional coupler 8 are as in Figure 1 shown. Please refer to the description of the Figure 1 should therefore be referred to here. The mixer 10, the low-pass filter 11, the sensor 12, the comparator 13, the guide filter 15, the controller 14 and the actuator 9 are implemented in this example in a computer 21, which is connected to the directional coupler 8 via a Universal Software Radio Peripheral (USRP) 22. However, the above-mentioned elements can also be implemented as individual digital components or as analog circuits, which leads to the Figure 1The USRP 22 transmits a reference signal Tx to one output of the directional coupler 8 and receives a feedback signal Rx from the other output of the directional coupler 8.

[0037] Figure 3shows a schematic implementation of a circuit 31 for evaluating the signal output by the USRP 22. The circuit 31 can be connected to the USRP 22, for example, via a USB connection. The feedback signal Rx is fed to the mixer 10 via a USRP source (USRP Source). The actuator 9 feeds the readout signal Tx to the USRP 22 via a USRP sink (USRP Sink). The mixer 10, the low-pass filter 11, the sensor 12, the comparator 13, the guide filter 15, the controller 14, and the actuator 9 are interconnected as shown in Figure 9, so reference should be made to the description there. These elements are implemented here as software objects, but they can also be implemented as individual digital components or as analog components.

[0038] Figure 4 shows the Figure 3The embodiment of the invention shown here includes a detailed representation of the controller 14 and a bandpass filter 41 downstream of the USPR source 42. Here, the signal output from the USRP source 42 is first fed to the bandpass filter 41, which, for example, only allows frequencies around a specific center frequency to pass and removes higher and / or lower frequencies as an upper or lower cutoff frequency. The signal output from the bandpass filter 41 is then fed to the mixer 10.

[0039] In Figure 4 In the example shown, the controller 14 has an integrator 14a that integrates the signal output by the comparator 13 over time. The controller 14 also has a differentiator 14b that calculates a time derivative of the signal output by the comparator.

[0040] The signal output by the comparator is fed to the integrator 14a and the differentiator 14b in the controller 14. In an adder 14c, the signal output by the comparator is added to the signal output by the integrator 14a and the differentiator 14b, resulting in a voltage that controls the voltage-controlled oscillator 9. The signal output by the comparator 13 can optionally be multiplied by a constant value before being fed to the integrator 14a, the differentiator 14b, and the adder 14c. Figure 4 the signal introduced into the integrator 14a can be multiplied by a value Ki, the signal introduced into the differentiator by a value Kd and the signal fed directly to the adder 14c by a value Kp.

[0041] The constants Ki, Kd, ​​and Kp can be adapted to the system to be controlled. They can be used to adjust the controller so that the controller and the system as a whole behave in a stable manner. The goal was to find an optimum between control speed and accuracy. For this purpose, the step response of the system was determined in an open-loop configuration. For this purpose, the overall system, which is Fig. 3 As shown, a step function is applied to the input of actuator 9. Controller 14 has been removed, thus interrupting the circuit at this point. The step response is then measured at the output of comparator 13. Based on the step response, a suitable parameter combination for Ki, Kd, ​​and Kp can then be found, for example, using the method of Ziegler and Nichols (Ziegler, JG; Nichols, NB: Optimum settings for automatic controllers, Trans. ASME, 64 (1942), pp. 759-768).

[0042] Since the controller 14 follows the signal and always supplies the signal source 9 or the actuator 9 with the currently set frequency in the form of a voltage value, this can also be done there, i.e. between the controller 14 and the actuator 9 in Fig. 4 ) can be tapped.

[0043] Figure 5 shows an example of a course of a phase 51, an amplitude 52 and a control signal 53, as they are received from the sensor 12 in the Figures 1 to 4can be generated. The center of the diagram is the injected excitation frequency in the frequency direction (horizontal direction) and a zero point of a normalized phase, amplitude or control signal in the vertical direction. The phase signal has a linear range around the zero point in the center of the diagram, in which the deviation of the feedback signal from the readout signal can be directly determined from the phase. The amplitude is symmetrical in terms of the frequency of the readout signal, so that additional information should be used for control that provides information about the direction in which the feedback signal deviates from the readout signal. This can be, for example, the phase or a derivative of the amplitude with respect to the frequency, the sign of which depends on the sign of the deviation.While the phase signal is particularly suitable for small frequency deviations, the deviation in the control signal can be determined over a wide range of frequency deviations.

[0044] The control signal can be generated, for example, as described below. Using the control signal is advantageous because it clearly defines whether the actual resonance frequency lies below or above the excitation frequency across the entire range of frequency deviations. This is particularly advantageous for resonance frequency changes of more than f(min(z_phase)) to f(max(z_phase)).

[0045] Since this involves real-time signal processing with a signal that repeats at a pulse frequency (e.g., approximately 70 Hz), the parameters (e.g., phase minimum z_phase_min and phase maximum z_phase_max) that can be used to generate the controlled variable control_out can be determined in a first step over one or more periods. In the second step, real-time processing can then be performed using the previously acquired parameters. Adaptive acquisition of the parameters in real time is also possible.

[0046] In an exemplary embodiment in which the controlled variable is determined from the phase, the following sequence can be used, which is reproduced here as pseudo-code:

[0047] Where z_phase denotes the phase, z_phase_d the derivative of the phase and control_out the value of the control signal.

[0048] The invention can, for example, measure the time-varying pressure in blood vessels. For this purpose, the oscillating circuit 2 with a capacitive pressure sensor 4 and a coil 3, which is also used for coupling to the readout coil 6, can be introduced into the blood vessel so that it is exposed to the time-varying pressure there. An excitation frequency, which is the resonant frequency of the sensor at a specific pressure, can then be fed in via an extracorporeally inductively coupled readout coil 6, and the feedback can be observed. By means of the invention, all occurring pressures can advantageously be measured over time, thus enabling continuous pressure measurement. This also makes it possible to determine quantities that can be determined from one or more pressures, such as pulse wave velocity, with greater accuracy.Since a larger part of the pressure curve can be measured, more data points can be determined, so that the pulse wave velocity can be determined with greater accuracy than if only a trigger pressure is used.

[0049] The invention can be implemented as an analog circuit or by means of digital signal processing, for example, as an SDR with frequency conversion or as an analog-to-digital converter with a DA converter without frequency conversion. The invention can be advantageously used in all applications in which inductive coupling is used between sensors, in particular implanted sensors, and a readout unit. Periodic or non-periodic signals can be detected particularly advantageously in this way.

Claims

1. A transponder system for measuring pressure in fluid lines, said system comprising at least one passive transponder (1), wherein the passive transponder (1) comprises at least one resonant circuit (2) with a variable resonance frequency, and at least one readout device (7), wherein the readout device (7) comprises at least one readout inductor (6), wherein a readout signal may be applied to the at least one resonant circuit (2) using the readout inductor (6), wherein the readout device (7) is configured to adapt a frequency of the readout signal to the variable resonance frequency of the resonant circuit (2), characterized in that at least one inductive sensor (3) and at least one capacitor (4) are connected to form the at least one resonant circuit (2).

2. The transponder system according to the preceding claim, wherein the readout device (7) is configured to continuously track the frequency of the readout signal of the variable resonance frequency of the resonant circuit (2).

3. The transponder system according to any one of the preceding claims, wherein the readout device comprises an excitation unit for generating the readout signal and a feedback unit for detecting a feedback signal originating from the passive transponder, wherein the feedback unit is configured to transfer a frequency matching signal to the excitation unit, which frequency matching signal is determined on the basis of the feedback signal, and wherein the excitation unit is configured to match the frequency of the readout signal on the basis of the frequency matching signal.

4. The transponder system according to the preceding claim, wherein the excitation unit comprises a signal source (9), by means of which the readout signal may be generated, wherein the readout unit additionally comprises a directional coupler (8), to the output of which the signal source (9) is electrically coupled and to the input of which the readout inductor (6) is electrically coupled.

5. The transponder system according to any one of the two preceding claims, wherein the feedback unit comprises a mixer (10), wherein a signal received from the passive transponder by the readout inductor (6) may be mixed or may be multiplied with the readout signal by means of the mixer, wherein preferably the feedback unit additionally comprises a low-pass filter (11), by means of which a signal output by the mixer (10) may be filtered.

6. The transponder system according to any one of claims 3 to 5, wherein the feedback unit together with the excitation unit forms a control loop.

7. A method for reading out a passive transponder, wherein the passive transponder has at least one resonant circuit with a variable resonance frequency, wherein a readout signal is applied to the at least one resonant circuit, wherein a frequency of the readout signal is matched to the variable resonance frequency of the resonant circuit, wherein the method is carried out with a transponder system according to any one of claims 1 to 6.

8. The method according to the preceding claim, wherein the frequency of the readout signal of the variable resonance frequency of the resonant circuit is continuously tracked.

9. The method according to any one of the two preceding claims, wherein the frequency of the readout signal is tracked by means of a control loop such that the frequency of the readout signal is the respective current resonance frequency of the resonant circuit.

10. The method according to any one of the three preceding claims, wherein a feedback signal originating from the passive transponder is received, and the feedback signal is fed to an input of a lock-in amplifier, wherein a reference signal of the lock-in amplifier is the readout signal.

11. The method according to any one of claims 7 to 10, wherein a relative pressure is determined from the frequency of the readout signal and is applied to a pressure sensor acting as capacitor of the resonant circuit.