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 planar data carrier (10), in particular a banknote, that has a spin resonance feature (12). The sensor element comprises: a magnetic core having an air gap (32) into which the planar data carrier (10) can be inserted for testing purposes; a polarisation 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) contains a plurality of stripline resonators (46-1, 46-2) which are designed and configured to operate independently of one another at the same excitation frequency. The polarisation device (34) generates a homogeneous magnetic flux in the air gap (32) so that the static magnetic flux for each pair of stripline resonators (46-1, 46-2) of the resonator device has substantially the same field strength at the position of a first stripline resonator (46-1) as at the position of a second stripline resonator (46-2).

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. 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 Bo, 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 mod in the kHz range. To excite transitions between the split spin energy levels of the spin resonance signature substances, an excitation field Bi is provided, which is polarized perpendicular to the Bo 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 Bo. To generate the polarization field Bo, 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 Bi. This is a conductive structure with a characteristic length l arranged on a carrier. If, during the authentication, the wavelength X of the coupled high-frequency signal matches the dimension l of the conductive structure, a standing wave can form in the resonator, and the stripline resonator resonates at the excitation frequency corresponding to the wavelength X. 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] Using a sensor element with a stripline resonator, a spatial resolution along the scanning direction can be achieved when testing a data carrier with a single scan, which is proportional to the dimensions of the resonator. With a typical edge length of the stripline resonator of approximately 10 mm, this results in a spatial resolution in the range of several millimeters.

[0007] Based on this, the object of the invention is to provide an improved device for testing data carriers with spin resonance features, and in particular to provide a sensor element that also allows testing of data carriers with complex spin resonance features.

[0008] This object is achieved by the features of the independent claims. Further developments of the invention are the subject of the dependent claims. 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 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 device 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.

[0009] The resonator device contains a plurality of stripline resonators designed and configured for independent operation at the same excitation frequency, for example, with a frequency deviation of less than 1%, preferably less than 0.1%. Furthermore, the polarization device generates a homogeneous magnetic flux in the air gap, so that the static magnetic flux for each two stripline resonators of the resonator device has substantially the same field strength at the position of a first stripline resonator as at the position of a second stripline resonator.

[0010] As described in more detail below, such a design of the resonator device and such tuning of the polarization device can meet very extensive requirements for automated verification of data carriers, particularly banknotes. For example, the achieved spatial resolution of a spin resonance feature incorporated into the data carrier over a large area can be used to verify the completeness of the data carrier. Even with a localized spin resonance feature, the inventive design can be used to perform a spatially resolved measurement of the feature, thus verifying a predefined, specified geometry and position of the feature.

[0011] The stripline resonators used are characterized by their highly accessible sensitive area and their very high fill factor for flat samples such as the banknotes being tested. Stripline resonators are sometimes referred to simply as resonators below.

[0012] Advantageously, in a sensor element according to the invention, the static magnetic flux at the location of each two stripline resonators of the resonator device has a maximum deviation of 2%.

[0013] In an advantageous embodiment, the stripline resonators of the resonator device are arranged in the form of a one-dimensional array.

[0014] The resonator device can, in particular, contain two, three, four, five, or six stripline resonators, although a larger number of stripline resonators, for example, a multi-track array with two or three tracks, each with five stripline resonators, can also be advantageous. While a one-dimensional array enables spatially resolved measurements on a moving data carrier, a multi-track array also allows spatially resolved measurements on a stationary data carrier.

[0015] Each of the stripline resonators of the resonator device is advantageously fed by a different signal source. Alternatively, the stripline resonators can also be fed by a single signal source via a multiplexer. A hybrid design is also possible, in which the stripline resonators of the resonator device are divided into several groups, and the resonators of each group are each fed by a single signal source via a multiplexer, while different groups are fed by different signal sources.

[0016] Advantageously, the plurality of stripline resonators covers an area encompassing the entire width of the data carrier to be tested, in particular a banknote. A linear scan along the length of the data carrier can then be used to check for completeness, since each position on the data carrier is detected by a stripline resonator during the scan. If testing is planned on a stationary device under test, the plurality of stripline resonators advantageously covers an area encompassing the entire surface of the data carrier to be tested, in particular a banknote.

[0017] The stripline resonators of the resonator device advantageously have the same resonant frequency; for example, the resonant frequencies differ from each other by less than 1%, preferably by less than 0.1%. 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.

[0018] The air gap is advantageously limited by two plane-parallel pole faces of the magnetic core. The magnetic core preferably consists of a ferromagnetic material with a magnetic permeability |i r »1, so in particular gi greater than IxlO 2 , the pole faces can also be made of a paramagnetic material with gr « 1, i.e. in particular g, at most 1+KF2 , are formed.

[0019] The stripline resonators are advantageously designed to be flat, with a main extension plane that is plane-parallel to at least one of the pole faces of the magnetic core that define the air gap. Furthermore, the main extension plane is advantageously 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 resonators then extends in the xy-plane perpendicular to the z-direction. 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 two stripline resonators differs by a maximum of 2%. The modulation device is advantageously formed by a single modulation coil, in particular a single planar coil, arranged in the air gap.

[0020] 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.

[0021] The resonator device is advantageously arranged in the air gap such that a flat data carrier introduced for testing is located in the near field of the excitation field generated by the stripline resonators.

[0022] In an advantageous development of the invention, in order to increase the signal-to-noise ratio, at least some of the stripline resonators mentioned are each replaced by 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, where 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.

[0023] In a particularly advantageous embodiment, the sensor element further comprises a ramp coil for generating a ramp function of the static magnetic flux.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] The invention also includes a testing device for testing a flat data carrier, in particular a banknote, with a spin resonance feature using a sensor element of the type described above. Furthermore, the testing device contains either a plurality of signal sources with the same excitation frequency, from which the stripline resonators of the resonator device are fed, or a single signal source from which the stripline resonators are fed via a multiplexer. Advantageously, the testing device further includes a transport device that transports the flat data carriers to be tested along a transport path into a test position in the air gap or through a test position in the air gap of the magnetic core. The resonator device is arranged in the air gap such that the test position is in the near field of the excitation field generated by the stripline resonators.

[0029] 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.

[0030] 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 introduced into the air gap of the magnetic core of said sensor element, a static magnetic flux is generated in the air gap with the polarization device and preferably a time-varying magnetic modulation field is generated with a modulation device, and the spin resonance feature of the data carrier to be tested is excited with the resonator device.

[0031] Advantageously, the method involves spatially resolving the flat data carrier to be tested by exciting the spin resonance feature, particularly checking for completeness. In an advantageous embodiment of the method, the flat data carrier to be tested is guided along a transport path through the air gap of the magnetic core of the sensor element, and a single-track or multi-track scan of the data carrier is performed using the stripline resonators of the resonator device.

[0032] Further embodiments and advantages of the invention are explained below with reference to the figures, in which a true-to-scale and true-to-proportion reproduction has been omitted in order to increase clarity.

[0033] They show:

[0034] Fig. 1 shows schematically a testing device of a banknote processing system for measuring spin resonances of a banknote test specimen,

[0035] Fig. 2 shows a schematic plan view of a resonator device of a sensor element according to the invention,

[0036] Fig. 3 shows schematically the setup for a measurement of a paper sample with the resonator device of Fig. 2,

[0037] Fig. 4 Signal curves when measuring the spin resonance feature of the paper sample of Fig. 3, and

[0038] Fig. 5 shows a circuit for connecting the resonator device of a sensor element according to the invention with only a single signal branch.

[0039] 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 specimen 10. 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 spin resonance feature can be present only in a partial area of ​​the banknote, as in the illustrated embodiment, or it can extend over the entire surface of the banknote test specimen.

[0040] The testing device 20 contains a sensor element 30 with a magnetic core 35, which has an air gap 32 delimited by two pole faces 38, through which the banknote test piece 10 is guided along a transport path 14 during the authenticity test.

[0041] To detect spin resonance signatures of the spin resonance feature 12, the sensor element 30 generates three different magnetic fields in a measuring area of ​​the air gap 32.

[0042] First, a polarization device 34 generates a homogeneous, 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.

[0043] Second, 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 fMod in the range between 1 kHz and 1 MHz. Finally, a resonator device 40 generates an excitation field in the air gap that induces 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.

[0044] 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. For this purpose, the test device 20 contains a signal source 22 whose excitation frequency fiaw corresponds to the expected Larmor frequency of the spin resonance feature 12. The excitation signal from the signal source 22 is fed via a duplexer 24 to a resonator device 40, where it generates an alternating magnetic field of frequency fMw.

[0045] In addition to the aforementioned elements, the test device 20 contains a detector diode 26 for measuring the high-frequency power reflected by the resonator device 40 and an evaluation unit 28 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.

[0046] Figure 2 schematically shows the design of the resonator device 40 of a sensor element 30 according to a first embodiment of the invention. The resonator device 40 comprises a flat carrier, for example, a printed circuit board 42, on which a plurality of stripline resonators 46 are arranged for a spatially resolved measurement. These stripline resonators 46 are operated independently of one another at the same excitation frequency.

[0047] Specifically, in the embodiment of Fig. 2, the resonator device 40 contains a 2x1 array of two stripline resonators 46-1, 46-2, which form a one-dimensional array extending perpendicular to the transport direction 14. As explained in more detail below, this allows the spin resonance intensity of the banknote test object 10 to be measured with spatial resolution, namely along two spaced-apart tracks parallel to the transport direction 14. Figure 2 shows a small array with only 2x1 stripline resonators merely to explain the functional principle; however, it is understood that in practice, one-dimensional arrays with more than two, for example, 3, 4, 5, 6, or 10, resonators are also possible in order to achieve higher spatial resolution. Two-dimensional arrays, for example with 2x2, 2x4 or 2x10 resonators, are also possible and enable spatially resolved measurements even on a stationary banknote test piece 10.Preferably, the plurality of stripline resonators are 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.

[0048] The polarization device 34 generates a homogeneous static magnetic flux in the air gap 32, so that the static magnetic flux for each two stripline resonators 46-1, 46-2 of the resonator device 40 has essentially the same field strength at the position of a first stripline resonator 46-1 as at the position of a second stripline resonator 46-2. "Essentially" the same field strength means that the field strengths at the positions of the stripline resonators 46-1, 46-2 differ by at most 2%.

[0049] In order to demonstrate the functioning of the invention, the behavior of a sensor element according to the invention with a resonator device 40 according to Fig. 2 in a setup according to Fig. 3 was simulated during a spin resonance measurement.

[0050] The resonator device 40 contains two square Å / 2 stripline resonators 46-1, 46-2, which are mounted on a circuit board 42 with a thickness of 1.5 mm and a dielectric constant of 3.66. The resonators 46-1, 46-2 have an edge length of 7.1 mm, corresponding to a resonant frequency of 9.8 GHz, and are arranged on the circuit board 42 in the y-direction perpendicular to the transport direction 14 at a distance of 50 mm from one another.

[0051] For coupling to the signal source 22 via a circulator, the impedance of the resonators 46-1, 46-2 is transformed to 50 Ω using a 4-wave transformer. The polarization field Bo and the modulation field B mod are identical at the location of the two resonators 46-1, 46-2.

[0052] With reference to Fig. 3, a measurement on a suitably prepared paper sample 60 was simulated using such a resonator device 40. For this purpose, a paper sample with a height of 100 mm was first loaded over its entire surface with a spin resonance feature 62. After the feature loading, a 35 mm x 35 mm section was cut out in the upper area of ​​the sample and replaced with a feature-free paper section 64.

[0053] As shown in Fig. 3, the thus prepared sample 60 is moved along the transport direction 14 centrally over the resonator device 40 with the two stripline resonators 46-1, 46-2, and the resulting spin resonance signal is recorded. At the given polarization field strength, the Larmor frequency of the spin resonance feature corresponds precisely to the resonance frequency of 9.8 GHz of the two resonators 46-1, 46-2. Diagram 70 of Fig. 4 shows the simulated signal intensities obtained in this way for the upper resonator 46-1 as signal curve 72-1 and for the lower resonator 46-2 as signal curve 72-2 as a function of location x.

[0054] The signal curves were normalized to the mean signal intensity of the undisturbed signal curve 72-2 and displayed slightly offset for better clarity.

[0055] Fig. 4 clearly shows that the feature-free region 64 of the sample 60 can be easily detected using the signal dip 74 of the upper resonator 46-1. This would not be possible with a single resonator, over which the paper sample 60 moves, for example, centrally.

[0056] In this example, the ordinate is labeled I and indicates the normalized intensity. The abscissa is labeled x and indicates the position in mm.

[0057] The majority of stripline resonators in the resonator device can advantageously be operated using independent signal sources. However, this also requires that the stripline resonators be connected to independent signal branches, which requires a significant amount of space for circuit implementation, especially with a large number of resonators.

[0058] Figure 5 shows an alternative circuit 80 for connecting a resonator device 40 of a sensor element according to the invention with only a single signal branch 82. All three stripline resonators 46-1, 46-2, 46-3 of the resonator device 40 are connected to the circulator 86 of this signal branch via a multiplexer 84. Since only one signal branch 82 is used with only one signal source 22, the required installation space is small. It is also ensured, without further measures, that the stripline resonators 46-1, 46-2, 46-3 are operated at the same excitation frequency. For the sake of simplicity, three resonators are shown in the circuit 80 of Fig. 5; however, with a multiplexer, a different, in particular larger, number of resonators can also be fed from a single signal source 22.

[0059] When using a circuit 80 according to Fig. 5, however, only a single resonator 46-1, 46-2, or 46-3 can be used at any given time. This creates gaps in the transport direction between the locations on the test piece where the spin resonance is measured, and therefore there are locations where no measurement takes place. Measurement points in adjacent tracks are offset from each other in the transport direction.

[0060] Depending on the application's requirements regarding the required installation space and the spatial resolution of the measurement, a hybrid solution can also be chosen, in which the stripline resonators of the resonator device are divided into several groups, each of which is connected to a single signal branch. For example, a resonator device with nine stripline resonators can be divided into three groups of three resonators each, each connected to a single signal branch via a multiplexer. In this way, the required installation space can be reduced to approximately one-third while maintaining high measurement coverage of the device under test.

[0061] 10 banknote test specimen

[0062] 12 Spin resonance feature

[0063] 14 Transport path

[0064] 20 Test device

[0065] 22 Signal source

[0066] 24 duplexers

[0067] 26 detector diode

[0068] 28 Evaluation unit

[0069] 30 sensor element

[0070] 32 air gap

[0071] 34 Polarization device

[0072] 35 magnetic core

[0073] 36 Modulation device

[0074] 38 pole faces

[0075] 40 Resonator device

[0076] 42 carriers

[0077] 46-1, 46-2, 46-3 stripline resonators

[0078] 60 paper samples

[0079] 62 Spin resonance feature

[0080] 64 feature-free paper part

[0081] 70 Diagram

[0082] 72-1, 72-2 signal curves

[0083] 74 Signal drop

[0084] 80 circuit

[0085] 82 Signal branch

[0086] 84 multiplexers

[0087] 86 Circulator

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 with an air gap (32) 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 device (40) for exciting the spin resonance feature (12) of the data carrier to be tested in the air gap, characterized in that the resonator device (40) comprises a plurality of stripline resonators (46-1, 46-2) which are designed and configured for independent operation at the same excitation frequency, and the polarization device (34) generates a homogeneous magnetic flux in the air gap (32), such that the static magnetic flux for each pair of stripline resonators (46-1,46-2) of the resonator arrangement (40) at the position of a first stripline resonator (46-1) has essentially the same field strength as at the position of a second stripline resonator (46-2)., 2. Sensor element (30) according to claim 1, characterized in that the static magnetic flux at the location of each pair of stripline resonators (46-1, 46-2) of the resonator device (40) has a maximum deviation of 2%.

3. Sensor element (30) according to claim 1 or 2, characterized in that the stripline resonators (46-1, 46-2) of the resonator device (40) are arranged in the form of a one-dimensional array.

4. Sensor element (30) according to at least one of claims 1 to 3, characterized in that each of the stripline resonators (46-1, 46-2) of the resonator device (40) is fed by a different signal source.

5. Sensor element (30) according to at least one of claims 1 to 4, characterized in that the plurality of stripline resonators (46-1, 46-2) cover an area which covers the entire width of the data carrier (10) to be tested, in particular a banknote.

6. Sensor element (30) according to at least one of claims 1 to 5, characterized in that the stripline resonators (46-1, 46-2) of the resonator device (40) have the same resonance frequency, preferably that the stripline resonators are also designed and configured for testing the spin resonance feature in the same spatial mode of the excitation field, particularly preferably that the stripline resonators have the same geometric shape.

7. Sensor element (30) according to at least one of claims 1 to 6, characterized in that the air gap (32) is limited by two plane-parallel pole surfaces (38) of the magnetic core (35).

8. Sensor element (30) according to at least one of claims 1 to 7, characterized in that the stripline resonators (46-1, 46-2) are formed planarly with a principal extent plane which is plane-parallel to at least one of the pole faces of the magnetic core that delimit the air gap.

9. Sensor element (30) according to at least one of claims 1 to 8, 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.

10. Sensor element (30) according to claim 9, characterized in that the modulation device is formed by a single modulation coil arranged in the air gap, in particular a single planar coil.

11. Sensor element (30) according to at least one of claims 1 to 10, characterized in that the stripline resonators (46-1, 46-2) are designed planar with a principal extent plane which is perpendicular to the direction of the static magnetic flux generated by the polarization device (34).

12. Sensor element (30) according to at least one of claims 1 to 11, characterized in that the air gap (32) has a height of less than 10 mm, preferably less than 5 mm.

13. Sensor element (30) according to at least one of claims 1 to 12, characterized in that the resonator device (40) is arranged in the air gap such that a planar data carrier (10) introduced for testing is located in the near field of the excitation field generated by the stripline resonators.

14. 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 13, and either a plurality of signal sources with the same excitation frequency from which the stripline resonators of the resonator device are fed, or a single signal source from which the stripline resonators are fed via a multiplexer.

15. Test device (20) according to claim 14, comprising a transport device which places the planar data carriers (10) to be tested along a transport path (14) into a introduces a test position into the air gap or passes through a test position in the air gap of the magnetic core, wherein the resonator device is arranged in the air gap such that the test position is located in the near field of the excitation field generated by the stripline resonators.

16. Test device (20) according to claim 15, characterized in that the transport device is designed and set up for a high-speed transport of the planar data carriers to be tested along the transport path (14).

17. 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 13 or a test device (20) according to one of claims 14 to 16, wherein in the method a planar data carrier (10) to be tested is placed in the air gap of the magnetic core of said sensor element (30), a static magnetic flux is generated in the air gap with the polarization device (34) and preferably a time-varying magnetic modulation field with a modulation device (36), and the spin resonance feature (12) of the data carrier (10) to be tested is excited with the resonator device.

18. Method according to claim 17, characterized in that the planar data carrier (10) to be tested is measured with spatial resolution by excitation of the spin resonance feature (12), in particular checked for completeness.

19. Method according to claim 17 or 18, characterized in that the 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) and a single-track scan or a multi-track scan of the data carrier is performed with the stripline resonators of the resonator device.

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