SENSOR ELEMENT, TESTING DEVICE AND METHOD FOR TESTING DATA CARRS WITH SPIN RESONANCE CHARACTERISTIC

Description

[0001] The invention relates to a sensor element for verifying the authenticity of a planar data carrier, in particular a banknote, with a spin resonance feature. The invention also relates to a testing device with such a sensor element and a method for verifying authenticity with such a sensor element or such a testing device.

[0002] Data carriers, such as valuables or identification documents, but also other valuables like branded goods, are often equipped with security features to ensure their authenticity and protect against unauthorized reproduction. It is known to use security features with spin resonance characteristics for automated authentication of documents and other data carriers. These security features are coated with substances that exhibit a spin resonance signature. Spin resonance signatures suitable for authentication include, in particular, nuclear magnetic resonance (NMR), electron spin resonance (ESR), and ferromagnetic resonance (FMR) effects.

[0003] When examining banknotes, three different magnetic fields are typically generated within the measuring range of, for example, a banknote processing machine to detect spin resonance signatures. Specifically, these include a quasi-static polarization field B0, which runs parallel to the axial direction (z-direction) of the air gap of a magnetic circuit. A second magnetic field is formed by a modulation field Bmod, which also runs parallel to the z-axis and typically has a frequency fmod in the kHz range. To excite transitions between the split spin energy levels of the spin resonance signature substances, an excitation field B1 is provided, which is polarized perpendicular to the B0 direction. This excitation field oscillates at the resonance frequency of the material, also known as the Larmor frequency, which is proportional to the polarization field B0.

[0004] To generate the polarization field B 0, a magnetic circuit is often used, which directs the magnetic flux of permanent magnets and / or coils to an air gap in which the testing of the planar data carriers takes place.

[0005] A high-frequency resonator, such as a stripline resonator, is used to generate the excitation field B1. The RF power reflected by the resonator is measured using a detector diode. If a device under test (DUT) is resonant at the coupled frequency, the resonator's quality factor changes, and thus the power reflected by the resonator changes. Due to the field modulation B mod, the exact value of the Larmor frequency of the DUT oscillates, and the measurement signal is amplitude-modulated by f mod. The spectral power distribution at the input of the detector diode then shows, in addition to the central microwave carrier frequency, modulation peaks offset by ± f mod, which carry the desired spin resonance information.

[0006] When verifying the authenticity of a data carrier, there is often a need to operate several independent resonators at the same frequency, for example, to capture spin resonance information with spatial resolution. Traditionally, this requires an independent microwave circuit for each resonator to detect and evaluate the measurement signal. These microwave circuits are typically implemented on circuit boards using SMD components and striplines. With multiple independent circuits, a considerable amount of installation space is required. If this space is not available, crosstalk between the different circuits can quickly occur, leading to signal distortion.

[0007] Furthermore, the functionality of many elements in a microwave circuit is defined by the geometry of the components involved. Especially at high frequencies, i.e., short wavelengths, manufacturing tolerances have a significant impact on the circuit's functionality. For example, if several identical microwave circuits are to be built in parallel, such as for the spatially resolved measurement of spin resonances mentioned above, the nominally identical microwave circuits can differ considerably in their functionality in practice, making reliable spatially resolved measurements difficult.

[0008] US 3 638 104 A discloses a device for measuring NMR properties of objects and US 5 149 946 A shows a device for testing spin resonance features on planar data carriers.

[0009] Based on this, the invention aims to avoid the disadvantages of the prior art and, in particular, to provide a sensor element of the type mentioned at the outset that allows improved detection of the spin resonance characteristic of a planar data carrier.

[0010] This problem is solved by the features of the independent claims. Further developments of the invention are the subject of the dependent claims.

[0011] The invention provides a sensor element for testing, in particular for authentication, a planar data carrier with a spin resonance feature. The planar data carrier can, for example, be a banknote. The sensor element comprises a magnetic core with an air gap into which the planar data carrier can be inserted for testing, a polarization device for generating a static magnetic flux in the air gap, and a resonator device for exciting the spin resonance feature of the data carrier to be tested in the air gap, with at least one stripline resonator fed by a signal source. The spin resonance feature is preferably an ESR feature.

[0012] The sensor element further includes a modulation device for generating a time-varying magnetic modulation field in the air gap parallel to the static magnetic field. The modulation device comprises a plurality of modulation coils designed and configured to generate different modulation frequencies, such that the modulated magnetic field generated by the modulation device together with the polarization device exhibits different modulation frequencies at different locations within the air gap.

[0013] Stripline resonators are generally characterized by their highly accessible sensitive area and their very high fill factor for planar samples such as banknotes. Stripline resonators will sometimes be referred to simply as resonators in the following text.

[0014] The resonator is specifically designed to detect spin resonance signals of the spin resonance feature. In particular, the resonator can receive a response signal from the spin resonance feature and output it to a detector. The spin resonances can be determined, for example, using a continuous wave (CW) technique, a pulsed technique, or a rapid scan technique.

[0015] In an advantageous embodiment, the modulation coils of the modulation device are arranged offset from one another. Each modulation coil then generates a spatial area of ​​the modulation field with its respective modulation frequency.

[0016] Advantageously, at least one, or preferably all, of the modulation coils of the modulation device are formed by planar coils that have one or more turns around an axial direction of the air gap in a plane. This plane with the one or more turns is advantageously perpendicular to the axial direction (z-direction) of the air gap, i.e., the direction between the pole faces of the magnetic core adjacent to the air gap. This advantageously aligns the modulation field parallel to the polarization field.

[0017] The modulation coils are advantageously all arranged in the same plane on a common coil carrier, in particular a common printed circuit board.

[0018] In an advantageous embodiment of the invention, the modulation coils are designed and configured to generate modulation frequencies that are not in a simple integer ratio to each other, in particular not in a ratio of 1:n, with a natural number n less than 6. In other words, the generated modulation frequencies of any two modulation coils are then not in a simple ratio of 1:2, 1:3, 1:4, 1:5, or a reciprocal ratio thereto.

[0019] The modulation coils are preferably designed and configured to generate modulation frequencies that differ by more than their linewidth and by more than the linewidth of the high-frequency signal from the resonators. For example, the modulation frequencies of the majority of modulation coils differ by more than 5%, preferably by more than 20%.

[0020] In an advantageous embodiment, the modulation coils of the modulation device are arranged in the form of a one-dimensional, in particular linear, array. The one-dimensional array extends, in particular, transversely to a transport direction of the data carrier to be tested and enables a two-dimensional scan of a data carrier moving in the transport direction.

[0021] It is also possible to arrange several modulation coils in series along the transport direction, particularly in combination with a spatially inhomogeneous polarization field, and this can be used to generate spectral resolution. In a further advantageous embodiment, the modulation coils of the modulation device are arranged in the form of a two-dimensional array, for example on the grid points of a regular grid, perhaps in a rectangular, hexagonal, or row-offset arrangement, thus enabling two-dimensional spatial resolution even on a stationary data carrier.

[0022] The arrangement of the modulation coils advantageously extends over the entire width of the data carrier to be tested, in particular a banknote, in order to enable a check for completeness.

[0023] According to an advantageous embodiment, the resonator device has a plurality of stripline resonators; in particular, it is preferably provided that the number of stripline resonators in the resonator device is equal to the number of modulation coils in the modulation device, or that the number of stripline resonators in the resonator device is an integer multiple of the number of modulation coils in the modulation device.

[0024] The stripline resonators of the resonator assembly advantageously have the same resonant frequency, for example with a frequency deviation of less than 1%, preferably less than 0.1%. Preferably, the stripline resonators of the resonator assembly are even designed and configured for operation in the same spatial mode. Furthermore, alternatively or additionally, it is advantageously provided that the stripline resonators of the resonator assembly have the same geometric shape, for example, a square, a rectangular, or a ring shape. Preferably, the stripline resonators are designed and configured for operation at the same excitation frequency, for example with a frequency deviation of less than 1%, preferably less than 0.1%.

[0025] The stripline resonators of the resonator assembly are preferably arranged in the same plane, advantageously on a common resonator support, in particular a common printed circuit board. This plane is expediently perpendicular to the direction of the static magnetic flux. Since the planar stripline resonators generate a B1 field with field components primarily in the plane of the resonators, the generated field is then perpendicular to the polarization field B0, as required for spin resonance excitation.

[0026] In an advantageous embodiment of the invention, the polarization device generates essentially the same static magnetic flux at each of the stripline resonators. The maximum deviation of the static magnetic flux at the location of different stripline resonators is advantageously less than 2%.

[0027] In another, equally advantageous variant of the invention, the polarization device generates a spatially inhomogeneous static magnetic flux in the air gap, for example to achieve spectral resolution during measurement.

[0028] In advantageous embodiments, several, and in particular an equal number, stripline resonators are located within the modulation field of each modulation coil. In the case of identical, jointly connected stripline resonators, this enables an improvement in the signal-to-noise ratio. In particular, each modulation coil can be assigned an NxM array of stripline resonators, where N and M are natural numbers and at least one of the values ​​of N and M is greater than 1, wherein the stripline resonators of the NxM array are all fed by the same signal source and are electrically connected in parallel and / or in series.

[0029] In a further advantageous embodiment, several, and in particular an equal number, stripline resonators with different resonant frequencies are located within the modulation field of each modulation coil. Specifically, the stripline resonators are excited with different excitation frequencies, each corresponding to its resonant frequency. In this way, spectral resolution can be achieved in addition to spatial resolution.

[0030] In another equally advantageous embodiment, the sensor element comprises only one or a few stripline resonators with an extended field distribution, each covering the area of ​​several modulation coils. The extended field distribution advantageously includes several localized field maxima, for example, by operating the individual resonators in a higher spatial mode.

[0031] In advantageous embodiments, the modulated magnetic field in the measuring range of each stripline resonator i µ essentially has only a single modulation frequency f Mod,i. However, the same modulation frequency f Mod,i can be present in the measuring range of several stripline resonators i 1 , ..., i µ , ..., in where n ≥ 1. In addition, further stripline resonators j 1 , ..., jn are provided, in whose measuring range the modulated magnetic field has a different modulation frequency f Mod,j. The crosstalk of this modulation field B Mod,j to the considered, for example, adjacent, stripline resonator i µ is controlled by a contamination factor. χ i μ j = ∫ B Mod , j dV i μ ∫ B Mod , i dV i μ described, where the integrals extend over the volume V iµ, which encompasses the sensitive region of the resonator i µ. The sum S iµ of the contamination factors at the location of the resonator i µ then describes the contribution of all modulation field components of other frequencies: S i μ = ∑ j ≠ i χ i μ j

[0032] This sum S iµ is advantageously less than 2% for all stripline resonators i µ, and in particular less than 0.5%. This allows for a clean separation of the resonators based on their respective assigned modulation frequencies.

[0033] The air gap advantageously has a height, i.e. a dimension in the z-direction, of less than 10 mm, preferably less than 5 mm.

[0034] This allows a particularly strong polarization field, i.e. a strong static magnetic flux, to be generated in the air gap.

[0035] The invention also includes a test device for testing a planar data carrier, in particular a banknote, with a spin resonance feature, with a sensor element of the type described above and exactly one signal source from which all stripline resonators of the resonator device are fed.

[0036] Advantageously, the test device further comprises a transport device that introduces the planar data carriers to be tested into the air gap of the magnetic core along a transport path or guides them through the air gap of the magnetic core, wherein the modulation coils of the modulation device are arranged in the form of a one-dimensional array extending transversely to the direction of the transport path. Preferably, the transport device is configured for high-speed transport of the planar data carriers, for example, between 1 m / s and 12 m / s.

[0037] The invention also includes a method for testing a planar data carrier, in particular a banknote, with a spin resonance feature by means of a sensor element of the described type or a testing device of the described type, wherein in the method A planar data carrier to be tested is placed in the air gap of the magnetic core of the aforementioned sensor element, a static magnetic flux is generated in the air gap by the polarization device and a time-varying magnetic modulation field is generated by the modulation device, so that the modulated magnetic field generated by the modulation device together with the polarization device has different modulation frequencies at different locations within the air gap, and the spin resonance feature of the data carrier to be tested is excited by the resonator device.

[0038] Advantageously, the resonator setup also records the response signal generated by the excitation of the spin resonance feature and outputs it to a detector. The excitation of the spin resonance feature and / or the recording of the response signal can be performed using a continuous wave (CW) method, a pulsed method, or a rapid scan method.

[0039] Further embodiments and advantages of the invention are explained below with reference to the figures, in the representation of which a scale and proportion-accurate reproduction has been omitted in order to increase clarity.

[0040] They show: Fig. 1 schematically shows a test device of a banknote processing system for measuring spin resonances of a banknote test piece, Fig. 2 schematically shows the design of the resonator device and the modulation device of a sensor element according to the invention in a first embodiment, Fig. 3 shows the spectral power distribution of the signal emitted by the sensor element. Fig. 2 reflected RF signal for different modulation frequencies, and Fig. 4 schematically shows the circuitry of a sensor element used in the described simulation with an array of two modulation coils and two square λ / 2 stripline resonators.

[0041] The invention will now be explained using the example of banknote authentication. Figure 1 schematically shows a test device 20 of a banknote processing system for measuring spin resonances of a banknote test specimen 10.

[0042] The banknote test specimen 10 has a spin resonance feature 12 to be tested, the characteristic properties of which serve to verify the authenticity of the banknote. For the authenticity check, the banknote test specimen 10 is guided along a transport path 14 through a sensor element 30 of the testing device 20 according to the invention. To detect spin resonance signatures of the spin resonance feature 12, the sensor element 30 generates three different magnetic fields in the measuring area.

[0043] Firstly, a static magnetic flux parallel to the z-axis is generated in the measuring area by a polarization device 34. Secondly, a modulation device 36 generates a time-varying magnetic modulation field in the air gap, which also runs parallel to the z-axis and has modulation frequencies fMod in the range between 1 kHz and 1 MHz. While conventional modulation devices generally generate only a single modulation frequency, the modulation device 36 according to the invention, as described in more detail below, contains several modulation coils designed and configured to generate different modulation frequencies.

[0044] Finally, a resonator device 32 generates an excitation field that induces the energy transitions between the spin energy levels in the spin resonance feature 12. The excitation field typically has frequencies above 1 GHz and is polarized perpendicular to the z-direction.

[0045] The frequency of the excitation field is tuned to the Larmor frequency of the spin resonance feature 12 to be detected, in order to measure its spin resonance signature and use it for authentication. For this purpose, the test device 20 contains a signal source 22 whose excitation frequency f MW corresponds to the expected Larmor frequency of the spin resonance feature 12. The excitation signal from signal source 22 is fed to the resonator device 32 via a duplexer 24, where it generates an alternating magnetic field of frequency f MW.

[0046] In the present invention, the resonator device 32 for generating the excitation field comprises one or more stripline resonators, as described in more detail below. A stripline resonator is a conductive structure with a characteristic length 1, which is mounted on a substrate, such as a printed circuit board or a ceramic. If the wavelength λ of the coupled high-frequency signal on the printed circuit board matches the dimensions of the conductor structure, a standing wave can form, and the resonator is then in resonance at the frequency corresponding to λ. Stripline resonators are particularly characterized by the fact that their sensitive area is very easily accessible and they exhibit a very high fill factor for planar samples, such as the banknotes to be tested.

[0047] In addition to the aforementioned elements, the test device 20 includes a detector diode 26 for measuring the high-frequency power reflected by the resonator assembly 32 and an evaluation unit 28 for evaluating and, if necessary, displaying the measurement result. If the spin resonance feature 12 is in resonance at a coupled frequency f MW, the resonator quality factor changes, and thus the power reflected by the stripline resonators changes. Due to the modulation of the static polarization field by the modulation unit 36, the exact value of the Larmor frequency of the sample oscillates, so that the obtained measurement signal is amplitude-modulated by the modulation frequencies.

[0048] Figure 2Figure 1 schematically shows the configuration of the resonator assembly 32 and the modulation assembly 36 in a sensor element 30 according to a first embodiment of the invention. In this embodiment, the sensor element 30 comprises a resonator assembly 32 consisting of an array of jointly connected stripline resonators 32-1, 32-2 in combination with a modulation assembly 36 consisting of an array of modulation coils 36-1, 36-2.

[0049] The figure illustrates the basic principle by showing a configuration with only two resonators 32-1, 32-2 and only two modulation coils 36-1, 36-2. However, it is understood that a larger number of resonators 32-j, with j = 1, ..., m, and of modulation coils 36-i, with i = 1, ..., n, with natural numbers n and m (m = number of stripline resonators, n = number of modulation coils), can also be provided. While in the design of the Fig. 2Since the case n = m = 2 has been shown, in general m ≥ 1 and n ≥ 2 hold, where n can be equal to m, but can also be not equal to m.

[0050] In the exemplary embodiment, the modulation coils 36-i of the modulation coil array 36 generate a local modulation field with its own modulation frequency f Mod,i for each resonator of the resonator assembly 32. This results in specific modulation peaks 44 and 46 in the spectral power distribution 40 of the reflected microwave signal for each resonator 32-j, as shown in Fig. 3 illustrated. These modulation peaks 44, 46 can be digitally separated from each other via frequency using a lock-in method, thus saving installation space.

[0051] Specifically, in the exemplary embodiment of the Figures 2 and 3The resonators 32-1, 32-2 receive an excitation signal of the same frequency f MW from the signal source 22 via the common circuit 38. In the modulation device 36, the first modulation coil 36-1 generates a modulation field with the frequency f Mod,1 , and the modulation coil 36-2 generates a modulation field with a different frequency f Mod,2 ≠ f Mod,1 .

[0052] The two modulation frequencies are chosen such that they do not have a simple integer ratio and their difference is chosen to be so large that the modulation frequencies f Mod,1 , f Mod,2 differ by more than their linewidth and by more than the linewidth of the high-frequency signal of the resonators 32-1, 32-2.

[0053] In particular, the modulation frequencies differ by more than twice the largest of the mentioned line widths.

[0054] With reference to the in Fig. 3The power spectrum 40 of the reflected high-frequency signal shown is the carrier frequency f MW (reference symbol 42) flanked by first modulation peaks 44, which are offset by ±f Mod,1 from the carrier frequency 42, and is flanked by two modulation peaks 46, which are offset by ±f Mod,2 from the carrier frequency 42. The modulation peaks 44 and 46 can be easily separated from each other by frequency using the lock-in method.

[0055] Within the scope of the invention, resonator devices 32 and modulation devices 36 with a larger number of stripline resonators or modulation coils are also possible, as long as it is ensured that the different frequency components in the power spectrum can still be cleanly separated.

[0056] The exemplary embodiment of the Fig. 2Figure 1 shows an advantageous embodiment in which the number m of resonators 32-j is equal to the number n of modulation coils 36-i and in which the resonators 32-j and modulation coils 36-i are paired. In this embodiment, the modulation coils 36-i are arranged in the form of a one-dimensional array 36 extending transversely to the transport direction 14 of the banknote 10. In conjunction with the movement of the banknote sample, this enables a two-dimensional scan of the transported banknote sample 10.

[0057] Besides the in Fig. 2In addition to the parallel circuit shown, a series circuit or a combination of series and parallel circuits for the resonators 32-i is also possible. In another embodiment, a single resonator with an extended field distribution, in particular with several localized field maxima, can cover the range of several modulation coils by operating in a higher spatial mode.

[0058] For the modulation coil array to function correctly, it is advantageous if the modulation coils exhibit as little "cross-modulation" as possible, i.e., contaminating the modulation channels of neighboring modulation coils. Such contamination can occur, for example, if modulation coil 36-1 of the Fig. 2a modulation field B Mod,1 is generated which still has a substantial amplitude in the sensitive area of ​​the resonator 32-2, or, conversely, if the modulation coil 36-2 generates a modulation field B Mod,2 which still has a substantial amplitude in the sensitive area of ​​the resonator 32-1.

[0059] If there is an equal number k = n = m of resonators and modulation coils, a contamination factor χ ij can be defined by χ ij = ∫ B Mod , j dV i ∫ B Mod , i dV i with j = 1, ..., k, and i = 1, ..., k.

[0060] The contamination factor χij indicates the magnitude of the parasitic signal components of other modulation frequencies that arise from transverse modulation in resonator 32-i. The volume Vi used for integration describes the sensitive region of resonator 32-i.

[0061] For i = j, the quantities χ ii are also formally defined and, by construction, equal to 1; however, they do not describe a contamination, but rather the desired signal generated by the modulation coil 36-i in the associated resonator 32-i, and are therefore disregarded in the summation.

[0062] For the functionality of the modulation coil array 36, it is now advantageous that for each modulation channel 36-i the sum over all contamination factors, i.e. ∑ j j ≠ i χ ij less than 2%. The sum for each modulation channel is particularly advantageous if it is even less than 0.5%.

[0063] An analogous definition of the contamination factors and the sum of the contamination factors can be used if the number m of resonators is not equal to the number n of modulation coils.

[0064] To demonstrate the superior performance of sensor elements according to the invention, the behavior of a sensor element was measured according to Fig. 2simulated with an array of two modulation coils 36-1, 36-2 and two square λ / 2 stripline resonators 32-1, 32-2.

[0065] The circuit 50 underlying the simulation is in Fig. 4 schematically represented. It contains a digital part 52 and an analog part 54.

[0066] The two λ / 2 stripline resonators 32-1, 32-2 are mounted on a 1.5 mm thick printed circuit board with a dielectric constant of 3.66. The resonators 32-1, 32-2 are spaced 15 mm apart, and each has an edge length of 7.1 mm, corresponding to a resonant frequency of 9.8 GHz.

[0067] The impedance of each resonator 32-1, 32-2 is transformed to 100 Ω using a λ / 4 impedance transformer. Connecting the two basic elements in parallel then yields a total impedance of 50 Ω. The resonator array 32 formed from the two resonators is fed via a circulator 56 by the amplified output signal of a signal source 22. The signal source 22 operates at 9.8 GHz in continuous wave (CW) mode.

[0068] A planar modulation coil 36-1 or 36-2 is positioned opposite each resonator 32-1 or 32-2 at a distance of 2 mm. The modulation coils 36-1 and 36-2 are spirally wound, have 15 turns, and a diameter of 5 mm. The modulation coil 36-1 of the first resonator 32-1 operates at a frequency f Mod,1 = 20 kHz, and the modulation coil 36-2 of the second resonator 32-2 operates at a frequency f Mod,2 = 30 kHz. The respective modulation signals are digitally generated in an FPGA, then subjected to D / A conversion and amplified so that the same current flows through both modulation coils 36-1 and 36-2.

[0069] The resonator array 32 and the modulation coil array 36 were placed in the air gap of a magnetic circuit and loaded there with a paper sample featuring a spin resonance feature. The Larmor frequency of the spin resonance feature corresponds precisely to the 9.8 GHz excitation frequency in the given polarization field.

[0070] In the next step, the signal reflected by the resonator array 32 was amplified with a low-noise receiver amplifier 58 and down-mixed with the 9.8 GHz excitation signal (reference 60). For clarity, the phase shifter and the filter banks are shown in Fig. 4 Not shown. After downmixing, the signal is digitized and fed to the FPGA.

[0071] In the FPGA, the signal is split into two channels. Both channels are bandpass-filtered (reference 62), with the first channel having a center frequency of 20 kHz and the second channel a center frequency of 30 kHz. Both filters have a bandwidth of 5 kHz. The first channel is then demodulated with the 20 kHz modulation signal, and the second channel with the 30 kHz modulation signal. The demodulated output signals are fed to an evaluation unit 64-1 for channel 1 and an evaluation unit 64-2 for channel 2. The demodulation is quadrature amplitude demodulation. The phase shifters used are not shown in the figure. The corresponding filter banks, which are also not shown in the figure, have a bandwidth of 2.5 kHz.

[0072] Finally, the polarization field of the magnetic circuit was swept, and the output signals of the two channels were recorded by the expansion units 64-1 and 64-2. Both channels show the spectrum of the spin resonance feature used for doping and correspond to different measurement points on the banknote. Reference symbol list

[0073] 10 Banknote test piece 12 Spin resonance feature 14 Transport path 20 Test device 22 Signal source 24 Duplexer 26 Detector diode 28 Evaluation unit 30 Sensor element 32 Resonator assembly 32-1, 32-2, 32-j Resonators 34 Polarization unit 36 ​​Modulation unit 36-1, 36-2, 36-i Modulation coils 40 Spectral power distribution 42 Carrier frequency 44, 46 Modulation peaks 50 Circuit 52 Digital circuit section 54 Analog circuit section 56 Circulator 58 Receiver amplifier 60 Down-conversion 62 Bandpass filtering 64-1, 64-2 Evaluation units

Claims

1. Sensor element (30) for testing a flat data carrier (10), in particular a banknote, with a spin resonance feature (12), with - a magnetic core with an air gap into which the flat data carrier (10) can be inserted for testing, - a polarization device (34) for generating a static magnetic flux in the air gap, - a resonator device (32) for exciting the spin resonance characteristic of the data carrier to be tested in the air gap, with at least one strip line resonator (32-1, 32-2) fed by a signal source (22), and - a modulation device (36) for generating a temporally varying magnetic modulation field in the air gap parallel to the static magnetic field, wherein the modulation device (36) comprises a plurality of modulation coils (36-1, 36-2) that are designed and set up to generate different modulation frequencies, so that the modulated magnetic field generated by the modulation device (36) together with the polarization device (34) has different modulation frequencies at different locations within the air gap.

2. Sensor element (30) according to claim 1, characterized in that the modulation coils (36-1, 36-2) of the modulation device (36) are arranged offset next to each other.

3. Sensor element (30) according to claim 1 or 2, characterized in that at least one, preferably all, modulation coils (36-1, 36-2) of the modulation device (36) are formed by planar coils ( ) which have one or more turns in a plane around an axial direction of the air gap.

4. Sensor element (30) according to at least one of claims 1 to 3, characterized in that the modulation coils (36-1, 36-2) are arranged in the same plane on a common coil carrier ( ), in particular a common printed circuit board.

5. Sensor element (30) according to at least one of claims 1 to 4, characterized in that the modulation coils (36-1, 36-2) are designed and configured to generate modulation frequencies that are not in a simple integer ratio to each other, in particular not in a ratio of 1:n, with a natural number n less than 6; and / or characterized in that the modulation coils (36-1, 36-2) are designed and configured to generate modulation frequencies that differ by more than their line width and by more than the line width of the high-frequency signal of the resonators.

6. Sensor element (30) according to at least one of claims 1 to 5, characterized in that the modulation coils (36-1, 36-2) of the modulation device (36) are arranged in the form of a one-dimensional array; and / or characterized in that the arrangement of the modulation coils (36-1, 36-2) extends over the entire width of the data carrier to be tested, in particular a banknote.

7. Sensor element (30) according to at least one of claims 1 to 6, characterized in that the resonator device (32) has a plurality of strip line resonators (32-1, 32-2), in particular in that the number of strip line resonators (32-1, 32-2) in the resonator device (32) is equal to the number of modulation coils (36-1, 36-2) in the modulation device (36), or that the number of strip line resonators in the resonator device is an integer multiple of the number of modulation coils in the modulation device.

8. Sensor element (30) according to at least one of claims 1 to 7, characterized in that the stripline resonators (32-1, 32-2) of the resonator device (32) have the same resonant frequency, preferably in that the stripline resonators (32-1, 32-2) of the resonator device (32) are designed and configured for operation in the same spatial mode, and further preferably that the stripline resonators (32-1, 32-2) of the resonator device (32) have the same geometric shape.

9. Sensor element (30) according to at least one of claims 1 to 8, characterized in that the strip line resonators (32-1, 32-2) of the resonator device (32) are arranged in the same plane, advantageously on a common resonator carrier, in particular a common printed circuit board, wherein said plane is in particular perpendicular to the direction of the static magnetic flux.

10. Sensor element (30) according to at least one of claims 1 to 9, characterized in that the polarization device (34) generates essentially the same static magnetic flux at the location of each of the strip line resonators (32-1, 32-2); or characterized in that the polarization device (34) generates a spatially inhomogeneous static magnetic flux in the air gap.

11. Sensor element (30) according to at least one of claims 1 to 10, characterized in that the modulated magnetic field in the measuring range of each strip line resonator has essentially only a single modulation frequency.

12. Sensor element (30) according to at least one of claims 1 to 11, characterized in that the air gap has a height of less than 10 mm, in particular less than 5 mm.

13. Test device (20) for testing a flat data carrier, in particular a banknote, with a spin resonance feature, with - a sensor element (30) according to one of claims 1 to 12, - exactly one signal source (22) from which all strip line resonators of the resonator device (32) are fed, - a detector for measuring the high-frequency power reflected by the resonator device (32), and - an evaluation unit (28) for evaluating and, if necessary, displaying the measurement result.

14. Test device (20) according to claim 13, with a transport device that transports the flat data carriers (10) to be tested along a transport path (14) into the air gap of the magnetic core or guides them through the air gap of the magnetic core, wherein the modulation coils of the modulation device (36) are arranged in the form of a one-dimensional array extending transversely to the direction of the transport path (14).

15. Method for testing a flat data carrier (10), in particular a banknote, with a spin resonance feature (12) ( ) using a sensor element (30) according to one of claims 1 to 12 or a testing device (20) according to claim 13 or 14, wherein in the method - a flat data carrier (10) to be tested is introduced into the air gap of the magnetic core of the said sensor element (30), - a static magnetic flux is generated in the air gap using the polarization device (34) and a temporally varying magnetic modulation field is generated in the air gap using the modulation device (36), so that the modulated magnetic field generated by the modulation device (36) together with the polarization device (34) has different modulation frequencies at different locations within the air gap, and - the resonator device (32) excites the spin resonance feature (12) of the data carrier (10) to be tested.