Sensor element, test device, and method for testing a data carrier having a spin resonance feature

Abstract

The invention relates to a sensor element (30) for testing a flat data carrier (10), in particular a banknote, having a spin resonance feature (12). The sensor element comprises a magnetic core (35) having an air gap (32), which is delimited by two pole surfaces (38) of the magnetic core and into which the flat data carrier (10) can be inserted in order to be tested, a polarization device (34) for generating a static magnetic flux in the air gap (32), and a resonator device (40) for exciting the spin resonance feature (12) of the data carrier to be tested in the air gap. According to the invention, the resonator device (40) comprises a signal source (22) and a plurality of stripline resonators (46) which are simultaneously fed from the signal source (22), and the stripline resonators (46) are formed in a planar manner with a main extension plane (50) which is plan-parallel to at least one of the pole surfaces (38) of the magnetic core.

Description

[0001] Sensor element, test device and method for testing a data carrier with spin resonance feature

[0002] The invention relates to a sensor element for authenticating a flat 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 authenticating using such a sensor element or such a testing device.

[0003] Data storage media, such as valuables or identification documents, as well as other valuable items such as branded goods, are often provided with security elements for security purposes. These elements allow verification of the authenticity of the data storage media and also serve as protection against unauthorized reproduction. It is known that security elements with spin resonance features are used in machine authentication to protect documents and other data storage media. For this purpose, the security elements are provided with substances that exhibit a spin resonance signature. The spin resonance signatures that can be used for authentication include, in particular, nuclear magnetic resonance (NMR), electron spin resonance (ESR), and ferromagnetic resonance (FMR).

[0004] When testing banknotes, three different magnetic fields are usually generated in the measuring area of, for example, a banknote processing machine to detect spin resonance signatures. Specifically, this involves 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 f modin 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. The excitation field oscillates at the resonant frequency of the material, also known as the Larmor frequency, which is proportional to the polarization field B0. To generate the polarization field B0, a magnetic circuit is often used, which directs the magnetic flux from permanent magnets and / or coils to an air gap in which the testing of the flat data carriers takes place.

[0005] A high-frequency resonator, such as a stripline resonator, is used to generate the excitation field B1. This is a conductive structure with a characteristic length 1, arranged on a carrier. If, during the authentication, the wavelength λ of the coupled high-frequency signal matches the dimension 1 of the conductive structure, a standing wave can form in the resonator, and the stripline resonator resonates with the excitation frequency corresponding to the wavelength λ. Since the extension of a stripline resonator is significantly larger in the plane of the carrier than perpendicular to it, it is also referred to as the plane of the stripline resonator, which corresponds to the plane of the carrier.

[0006] A sensor element for verifying a data carrier, such as a banknote, based on a stripline resonator typically has a limited signal-to-noise ratio (SNR) that is proportional to the number of spins of the spin resonance feature in a sensitive region. Since the sensitive region scales with the size of the resonator, the signal-to-noise ratio can generally be improved by enlarging the resonator.

[0007] The length of the resonator determines its resonance frequency, so that for a given resonance frequency tuned to the Larmor frequency of the feature substance to be measured, the resonator length cannot be increased arbitrarily. If, for example, a suitable multiple of the λ / 2 or λ / 4 fundamental mode is selected for the resonator length, the resonator can be operated in a higher spatial mode and thus the sensitive range and thus the signal-to-noise ratio. However, this has the disadvantage that an increase in the mode is often accompanied by a decrease in the quality factor Q of the resonator, which is associated with a reduction in signal strength. Furthermore, only a contiguous area of ​​the data carrier can be measured. Measuring two non-adjacent positions on the data carrier, however, is not possible.

[0008] Widening the stripline resonator keeps the resonance frequency constant to a first approximation. However, for very wide resonators, matching the resonator impedance to the signal source impedance is often not possible. Here, too, there is the additional disadvantage that only a contiguous area of ​​the data carrier can be measured, while measuring two non-adjacent positions on the data carrier is not possible.

[0009] Furthermore, it can be exploited that the signal strength of a spin resonance measurement is proportional to the square root of the signal power supplied to the resonator. It is therefore sensible to maximize the power of the excitation signal. However, this is only possible up to a certain, feature-dependent limit power P opt possible. If the power supplied to the resonator is increased above P optBeyond this value, no further signal enhancement occurs; instead, a signal decrease or distortion of the spin resonance line is to be expected. Therefore, the signal-to-noise ratio cannot be increased indefinitely, even by increasing the applied power.

[0010] Based on this, the invention is based on the object of specifying a generic sensor element with a high signal-to-noise ratio of the spin resonance measurement, which avoids the aforementioned disadvantages.

[0011] This object is achieved by the features of the independent claims. Further developments of the invention are the subject of the dependent claims.

[0012] The invention provides a sensor element for testing, in particular authenticity testing, a flat data carrier with a spin resonance feature. The flat data carrier can be a banknote, for example. The sensor element contains a magnetic core with an air gap, in particular delimited by two pole faces of the magnetic core, into which the flat data carrier can be inserted for testing, a polarization device for generating a static magnetic flux in the air gap, and a resonator unit for exciting the spin resonance feature of the data carrier to be tested in the air gap. The spin resonance feature is preferably an ESR feature.

[0013] At the pole faces, the magnetic core preferably consists of a ferromagnetic material with a magnetic permeability μ r >>1, so in particular μ r greater than 1+10 2, but the pole faces can also be made of a paramagnetic material with μ r ≈ 1, so in particular μ r at most 1+10 -2 , are formed.

[0014] The resonator unit comprises one, in particular a single, signal source and a resonator device with a plurality of stripline resonators that are simultaneously fed from the signal source. The stripline resonators are flat in design with a main extension plane that is plane-parallel to at least one of the pole faces of the magnetic core. Preferably, the resonator unit comprises precisely one signal source and / or all stripline resonators of the resonator device are simultaneously fed from the same signal source.

[0015] As explained in more detail below, the use of a plurality of stripline resonators aligned in this way and simultaneously fed from the same signal source enables a significant increase in the signal-to-noise ratio compared to the use of a single resonator. Furthermore, not only contiguous spin resonance features but also spin resonance features with non-adjacent sub-regions can be easily tested, since the positions of the multiple stripline resonators can be easily coordinated with the positions of the sub-regions to be detected during the spin resonance measurement. The stripline resonators used are characterized in particular by the fact that their sensitive area is very easily accessible and that they have a very high fill factor for flat samples, such as the banknotes to be tested.The stripline resonators are sometimes referred to simply as resonators in the following.

[0016] Preferably, the plurality of stripline resonators is arranged in the form of a linear array or two-dimensionally on the grid points of a regular grid, for example, in a rectangular, hexagonal, or row-staggered arrangement. Adjacent stripline resonators in different rows and / or columns of the array can be arranged equidistantly, but can also have different distances from each other.

[0017] In an advantageous embodiment, the plurality of stripline resonators forms an Nx1 array with N stripline resonators connected in parallel, where N ≥ 2, in particular N = 2, 3, 4 or 5.

[0018] In another, likewise advantageous embodiment, the plurality of stripline resonators forms a 1xM array with M stripline resonators connected in series, where M ≥ 2, in particular M = 2, 3, 4 or 5.

[0019] In a further advantageous embodiment, the plurality of stripline resonators forms an NxM array with stripline resonators connected partly in series and partly in parallel, where N ≥ 2 and M ≥ 2, in particular N = 2, 3, 4 or 5 and / or M = 2, 3, 4 or 5.

[0020] The stripline resonators of the resonator device advantageously have the same resonant frequency. Preferably, the stripline resonators are also designed and configured to test the spin resonance feature in the same spatial mode of the excitation field. Particularly preferably, the stripline resonators have the same geometric shape, for example, a square, rectangular, or ring shape.

[0021] Advantageously, the polarization device is designed to generate a static magnetic flux in the air gap that is essentially the same strength for all stripline resonators. Specifically, it is advantageous that the static magnetic flux at the location of the stripline resonators has a maximum deviation of 2% or less.

[0022] The main extension plane of the stripline resonators is expediently oriented perpendicular to the direction of the static magnetic flux generated by the polarization device. For the purposes of this description, the direction of the static magnetic flux is also referred to as the z-direction. The main extension plane of the stripline resonator then extends in the xy-plane perpendicular to the z-direction. In particular, it is advantageously provided that the main extension plane of the stripline resonators is plane-parallel to both pole faces of the magnetic core that define the air gap.

[0023] Advantageously, the stripline resonators of the resonator device are all arranged in the same plane, in particular the stripline resonators are all arranged in one plane of a common carrier, for example a printed circuit board.

[0024] The resonator unit is preferably advantageously designed and configured for operation of the, in particular, single, signal source at high output power, where signal saturation occurs when testing the spin resonance feature with a single stripline resonator. Since the supplied signal power is distributed across the majority of stripline resonators, the resonator unit can be operated with significantly higher excitation power than a single resonator without saturation effects occurring. If the resonator device contains, for example, K = NxM similar stripline resonators, the excitation power of the signal source can be evenly distributed among the K stripline resonators through suitable wiring, and thus the resonator device as a whole can be fed with essentially K times the excitation power of a single resonator.The signal-to-noise ratio can therefore be increased by a factor of √K compared to a single resonator without saturation effects occurring.

[0025] The arrangement of stripline resonators expediently has a smaller surface area than the data carrier to be checked, in particular a banknote to be checked.

[0026] In an advantageous embodiment, the stripline resonators are rectangular with an aspect ratio of less than 3:1.

[0027] The stripline resonators advantageously form independent electromagnetic modes of the excitation field, whereby the entire excitation field preferably has local minima between the stripline resonators.

[0028] 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. This allows a particularly strong polarization field, i.e., a strong static magnetic flux, to be generated in the air gap.

[0029] In an advantageous embodiment, the sensor element further comprises a modulation device for generating a time-varying magnetic modulation field in the air gap, wherein the modulation frequency is preferably the same for all stripline resonators of the resonator device. For example, the modulation frequency at the location of any two stripline resonators differs by a maximum of 2%. The modulation device is advantageously formed by a single modulation coil arranged in the air gap, in particular a single planar coil. In a particularly advantageous embodiment, the sensor element further comprises a ramp coil for generating a ramp function of the static magnetic flux.

[0030] The resonator device is advantageously designed for the excitation of spin resonance signals with a frequency above 1 GHz, particularly between 1 GHz and 10 GHz. Compared to lower frequencies, this enables higher spectral resolution and a stronger measurement signal.

[0031] The resonator device is also designed, in particular, to detect spin resonance signals of the spin resonance feature. The stripline resonators of the resonator device can, in particular, record a response signal of the spin resonance feature and output it to a detector. The spin resonances can be determined, for example, using a continuous wave (CW) method, a pulsed method, or a rapid scan method.

[0032] The stripline resonators can be operated in both reflection and transmission modes when testing the data carrier. The latter has the advantage that no element, such as a circulator, is required in the signal path to separate the signals flowing forward and back to the resonator.

[0033] The resonator device advantageously comprises a flat carrier on which the stripline resonators are mounted. The carrier is expediently formed by a printed circuit board, which allows for reproducible and cost-effective production. However, it is also advantageous to use carriers based on ceramic, Teflon, or hydrocarbons, particularly to reduce dielectric losses in the carrier material.

[0034] The invention also includes a testing device for testing a flat data carrier, in particular a banknote, with a spin resonance feature, with a sensor element of the type described and with a transport device that guides the flat data carriers to be tested along a transport path through the air gap of the magnetic core.

[0035] The transport device is designed and configured in particular for high-speed transport, for example between 1 m / s and 12 m / s, of the flat data carriers to be tested along the transport path.

[0036] The invention also includes a method for testing a flat data carrier, in particular a banknote, with a spin resonance feature by means of a sensor element of the type described or a testing device of the type described, wherein in the method a flat data carrier to be tested is guided along a transport path through the air gap of the magnetic core of said sensor element, a static magnetic flux is generated in the air gap using the polarization device and preferably a time-varying magnetic modulation field is generated using a modulation device, and the signal source of the resonator unit simultaneously feeds the majority of the stripline resonators of the resonator device, and the resonator device excites the spin resonance feature of the data carrier to be tested.

[0037] In an advantageous method, the resonator device is fed from the signal source with a high output power, at which signal saturation already occurs when testing the spin resonance feature with a single stripline resonator. Further embodiments and advantages of the invention are explained below with reference to the figures, which are not drawn to scale or proportions for the sake of clarity.

[0038] They show:

[0039] Fig. 1 shows schematically a testing device of a banknote processing system for measuring spin resonances of a banknote test specimen,

[0040] Fig. 2 shows a schematic plan view of a resonator device according to an embodiment of the invention,

[0041] Fig. 3 shows a resonator device according to the invention designed for operation in transmission with a serial 1x5 array of five rectangular stripline resonators, and

[0042] Fig. 4 shows a resonator device designed for operation in reflection with a serial 1x3 array of three square stripline resonators.

[0043] The invention will now be explained using the example of banknote authentication. Figure 1 schematically shows a testing device 20 of a banknote processing system for measuring spin resonances of a banknote test item 10.

[0044] The banknote test specimen 10 has a spin resonance feature 12, the characteristic properties of which serve to verify the authenticity of the banknote. The testing device 20 contains a sensor element 30 with a magnetic core 35, which has an air gap 32 defined by two pole faces 38, through which the banknote test specimen 10 is guided along a transport path 14 during the authenticity test. To detect spin resonance signatures of the spin resonance feature 12, the sensor element 30 generates three different magnetic fields in a measuring range of the air gap 32.

[0045] First, a polarization device 34 generates a static magnetic flux parallel to the z-axis in the measuring area. To generate a strong polarization field, the height of the air gap in the z-direction is advantageously less than 10 mm, in particular even less than 5 mm.

[0046] 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 a modulation frequency f Mod in the range between 1 kHz and 1 MHz. Finally, a resonator device 40 with stripline resonators in the air gap 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.

[0047] 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 authenticity testing.

[0048] For this purpose, a resonator unit of the sensor element contains, in addition to the resonator device 40, a signal source 22 whose excitation frequency f MWcorresponds to the expected Larmor frequency of the spin resonance feature 12. The excitation signal from the signal source 22 is simultaneously fed to the stripline resonators of the resonator device 40 and generates there an alternating magnetic field of frequency f MW .

[0049] In addition to the aforementioned elements, the test device 20 typically contains a detector diode for measuring the reflected high-frequency power and an evaluation unit for evaluating and, if necessary, displaying the measurement result. If the spin resonance feature 12 resonates at a coupled frequency, the resonator quality factor changes and thus the power reflected by the stripline resonators. Due to the modulation of the static polarization field by the modulation device 36, the exact value of the Larmor frequency of the sample oscillates, so that the resulting measurement signal is amplitude-modulated at the modulation frequency.

[0050] As a special feature, to achieve a high signal-to-noise ratio, the resonator device 40 contains, instead of a conventional single resonator, an NxM array of stripline resonators with N and / or M greater than 1, wherein the stripline resonators, in particular all of them, are fed simultaneously by the same signal source 22. In the context of this description, N denotes the number of signal branches electrically connected in parallel, and M denotes the number of resonators connected in series per signal branch.

[0051] For further explanation, Figure 2 schematically shows a plan view of the stripline resonator arrangement of a resonator device 40 according to an exemplary embodiment of the invention. In the resonator device 40, a 2x1 array 44 of stripline resonators 46 is constructed on a flat carrier, for example, a printed circuit board 42. The arrangement of Figure 2 forms a 2x1 array 44 with N=2 and M=1 in the sense defined above, since the array contains two resonators connected in parallel. The two stripline resonators 46 of the 2x1 array 44 are fed by the same signal branch 48 through the same signal source 22 of the resonator unit.

[0052] The resonator device 40 with the stripline resonators 46 is flat with a main extension plane 50, which is plane-parallel to the two pole faces 38 of the magnetic core 35. The stripline resonators 46 are all located in the same plane and are located in a homogeneous polarization field B0. According to Figure 2, the two resonators 46 are arranged on a line transverse to the transport direction 14 of the data carriers to be tested; however, they can alternatively be arranged one behind the other in the transport direction or obliquely to the transport direction. An array of N > 2 resonators connected electrically in parallel does not have to be geometrically linear; two-dimensional arrangements are also possible, for example, at the grid points of a rectangular or hexagonal grid, or in other arrangements tailored to the authenticity feature to be tested, such as along a circular circumference.

[0053] Compared to a single resonator with the area of ​​one of the two stripline resonators 46, the number of spins in the sensitive region of the 2x1 array 44 is doubled. If each of the two stripline resonators 46 of the array 44 is supplied with the same power as the comparison single resonator, the signal-to-noise ratio is doubled compared to the comparison single resonator.

[0054] In the circuit 48 illustrated in Fig. 2, the signal power supplied to the array 44 from the signal source 22 is divided equally between the two stripline resonators 46. The 2x1 array 44 can therefore be operated with twice the excitation power of a comparison single resonator without saturation effects occurring.

[0055] In addition to the 2x1 array shown for illustration in Fig. 2, larger single-column arrays, generally Nx1 arrays with N > 2, for example N = 3, 4 or 5, are of course also possible.

[0056] In addition, 1xM arrays of stripline resonators are also possible, i.e., arrays consisting of only one row of several (M ≥ 2) resonators connected in series. Figure 3 shows, as an exemplary embodiment, a resonator device 60 designed for operation in transmission with a serial 1x5 array 64 of five rectangular stripline resonators 66, which are fed by the same signal branch 68 through the same signal source (not shown). In the example, the stripline resonators 66 are arranged on a line in the transport direction 14 and each have a length 1 = λ / 2 and a distance d = λ in the transport direction 14, where λ is the wavelength of the coupled radio-frequency signal f MWEven with a series-connected array, other geometric arrangements of the resonators are possible, for example, on a line transverse to the transport direction or two-dimensional arrangements, for example on the grid points of a rectangular or hexagonal grid.

[0057] As a further embodiment of the invention, Fig. 4 shows a resonator device 70 designed for operation in reflection with a serial 1x3 array 74 of three square stripline resonators 76 with a side length 1 = λ / 2, which are fed by the same signal branch 78 through the same signal source.

[0058] The embodiments of Figures 3 and 4 also have the advantage that the sensitive area of ​​the resonator device is enlarged compared to a comparison single resonator and the maximum possible excitation power is increased.

[0059] In addition to the one-dimensional Nx1 arrays and 1xM arrays shown, circuits in which the stripline resonators form an NxM array with N ≥ 2 and M ≥ 2, and the stripline resonators are connected partly in series and partly in parallel, are also advantageous. In this case, the resonators are preferably arranged geometrically in a grid with NxM grid points, for example, in the form of a rectangular grid or with a row-by-row staggered arrangement.

[0060] In order to demonstrate the advantages of sensor elements according to the invention with an array of stripline resonators, the behavior of a sensor element with a 2x1 array in parallel configuration according to Fig. 2 was simulated and compared with a single resonator.

[0061] The stripline resonator array according to the invention and the comparison single resonator are each constructed on a 1.5 mm thick circuit board with a dielectric constant of 3.66. The basic element in each case is a square λ / 2 stripline resonator with an edge length of 7.1 mm, corresponding to a resonance frequency of 9.8 GHz.

[0062] The reference single resonator contains only one such basic element. The impedance of the basic element is directly adjusted to 50 Ω using a planar λ / 4 impedance transformer, and the single resonator is fed from a 50 Ω source using a circulator.

[0063] The stripline resonator array 44 according to the invention contains two basic elements 46 connected in parallel as shown in Fig. 2. The impedance of each basic element 46 is transformed to 100 Ω using a λ / 4 impedance transformer. By subsequently connecting both basic elements in parallel, a total impedance of 50 Ω is obtained. The parallel array 44 is fed from a 50 Ω source using a circulator, so that both basic elements 46 are fed simultaneously from the same signal source.

[0064] With the comparison single resonator constructed in this way and the 2x1 array 44, a spin resonance measurement was simulated on a planar sample that is homogeneously doped with a spin resonance feature.

[0065] The 2x1 array 44 according to the invention is operated at twice the power of the reference single resonator. The polarization field B0 and the modulation field BMod are identical in both measurements.

[0066] The signal-to-noise ratio was determined for both configurations and the ratio of the two signal-to-noise ratios was used as a measure of the improvement of the present invention. determined. A value of η = 1.93 was obtained for the improvement factor; the signal-to-noise ratio of the stripline resonator array 44 according to the invention is therefore almost twice as high as the signal-to-noise ratio of the comparison single resonator.

[0067] The deviation from the maximum expected improvement η ideal = 2.0 is explained by additional losses in the coupling network of the array.

[0068] List of reference symbols

[0069] 10 banknote test specimen

[0070] 12 Spin resonance feature 14 Transport path

[0071] 20 Test device

[0072] 22 Signal source

[0073] 30 sensor element

[0074] 32 Air gap 34 Polarization device

[0075] 35 magnetic core

[0076] 36 Modulation device

[0077] 38 pole faces

[0078] 40 Resonator device 42 Circuit board

[0079] 44 2x1 arrays

[0080] 46 stripline resonators

[0081] 48 Wiring

[0082] 50 Main extension plane 60 Resonator device

[0083] 64 1x5 arrays

[0084] 66 rectangular stripline resonators

[0085] 68 signal branch

[0086] 70 resonator device 74 1x3 array

[0087] 76 square stripline resonators

[0088] 78 Signal branch

Claims

P a t e n t a n s p r ü c h e 1. Sensor element (30) for testing a planar data carrier (10), in particular a banknote, with a spin resonance feature (12), with a magnetic core (35) with an air gap (32) bounded by two pole faces (38) of the magnetic core, into which the planar data carrier (10) can be inserted for testing, a polarization device (34) for generating a static magnetic flux in the air gap (32) and a resonator unit for exciting the spin resonance feature (12) of the data carrier to be tested in the air gap, characterized in that the resonator unit comprises one, in particular exactly one, signal source (22) and a resonator device (40) with a plurality of stripline resonators (46) which are simultaneously fed from the signal source (22), and that the stripline resonators (46) are planar with a principal extent plane (50) are formed which are plane-parallel to at least one of the pole faces (38) of the magnetic core.

2. Sensor element (30) according to claim 1, characterized in that the resonator unit has exactly one signal source (22) and all stripline resonators (46) of the resonator device (40) are simultaneously fed from the one signal source (22).

3. Sensor element (30) according to claim 1, characterized in that the plurality of stripline resonators (44, 46) form an NxM array of N electrically parallel connected signal branches with M series connected stripline resonators per signal branch, wherein N and M are natural numbers and at least one of the values ​​of N and M is greater than 1.

4. Sensor element (30) according to claim 3, characterized in that the plurality of stripline resonators (44, 46) forms an Nx1 array with N stripline resonators connected in parallel, wherein N ≥ 2, in particular N = 2, 3, 4 or 5.

5. Sensor element (30) according to claim 3, characterized in that the plurality of stripline resonators forms a 1xM array with M stripline resonators connected in series, wherein M ≥ 2, in particular M = 2, 3, 4 or 5.

6. Sensor element (30) according to claim 3, characterized in that the plurality of stripline resonators forms an NxM array with stripline resonators connected partly in series and partly in parallel to each other, wherein N ≥ 2 and M ≥ 2, in particular N = 2, 3, 4 or 5 and / or M = 2, 3, 4 or 5.

7. Sensor element (30) according to at least one of claims 1 to 6, characterized in that the stripline resonators (44, 46) of the resonator device (40) have the same resonance frequency, preferably that the stripline resonators are also designed and configured to test the spin resonance feature in the same spatial mode of the excitation field, particularly preferably that the stripline resonators have the same geometric shape.

8. Sensor element (30) according to at least one of claims 1 to 7, characterized in that the polarization device (34) generates a static magnetic flux in the air gap (32) which is essentially the same strength for all stripline resonators (44, 46), in particular that the static magnetic flux at the location of the stripline resonators has a maximum deviation of 2% or less.

9. Sensor element (30) according to at least one of claims 1 to 8, characterized in that the main extension plane (50) is perpendicular to the direction of the static magnetic flux generated by the polarization device, in particular that the main extension plane (50) of the stripline resonators is plane-parallel to both pole surfaces (38) of the magnetic core (35) that limit the air gap (32).

10. Sensor element (30) according to at least one of claims 1 to 9, characterized in that the stripline resonators (44, 46) of the resonator device (40) are all arranged in the same plane, in particular that the stripline resonators are all arranged in one plane of a common support, for example a printed circuit board.

11. Sensor element (30) according to at least one of claims 1 to 10, characterized in that the resonator unit is designed and configured for operation of the signal source (22) at a high output power, at which signal saturation occurs when testing the spin resonance feature with a single one of the stripline resonators.

12. Sensor element (30) according to at least one of claims 1 to 11, characterized in that the arrangement of strip line resonators (44, 46) has a smaller area than the data carrier (10) to be tested, in particular a banknote to be tested.

13. Sensor element (30) according to at least one of claims 1 to 12, characterized in that the stripline resonators are rectangular with an aspect ratio of less than 3:

1.

14. Sensor element (30) according to at least one of claims 1 to 13, characterized in that the stripline resonators (44, 46) form independent electromagnetic modes of the excitation field, wherein the entire excitation field preferably has local minima between the stripline resonators.

15. Sensor element (30) according to at least one of claims 1 to 14, characterized in that the air gap (32) has a height of less than 10 mm, preferably less than 5 mm.

16. Sensor element (30) according to at least one of claims 1 to 15, characterized in that the sensor element has a modulation device (36) for generating a time-varying magnetic modulation field in the air gap, wherein preferably the modulation frequency is the same for all stripline resonators of the resonator device.

17. Test device (20) for testing a planar data carrier, in particular a banknote, with a spin resonance feature, with a sensor element (30) according to one of claims 1 to 16, and a transport device which guides the planar data carriers (10) to be tested along a transport path (14) through the air gap of the magnetic core, wherein the transport device is advantageously designed and configured for high-speed transport of the planar data carriers to be tested along the transport path.

18. Method for testing a planar data carrier (10), in particular a banknote, with a spin resonance feature (12) by means of a sensor element (30) according to one of claims 1 to 16 or a test device (20) according to claim 17, wherein in the method a planar data carrier (10) to be tested is guided along a transport path through the air gap of the magnetic core of said sensor element (30), a static magnetic flux is generated in the air gap by the polarization device (34) and preferably a time-varying magnetic modulation field is generated in the air gap by a modulation device (36), and the majority of the stripline resonators (46) of the resonator device (40) are simultaneously fed by the signal source (22) and The spin resonance feature (12) of the data carrier (10) to be tested is excited by the resonator device (40).

19. Method according to claim 18, characterized in that the resonator device (40) is fed by the signal source with a high output power, at which signal saturation occurs when testing the spin resonance feature with a single one of the strip line resonators.

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