Sensor element, inspection device and method for inspecting data carrier having spin resonance characteristic

Abstract

The invention relates to a sensor element (30) for inspecting a flat data carrier (10) having a spin resonance feature (12), in particular a banknote. The sensor element comprises a magnetic core (35) having an air gap (32) defined by two pole faces (38) of the magnetic core, into which the planar data carrier (10) can be introduced for inspection; a polarization device (34) for generating a static magnetic flux in the air gap (32); and a resonator device (40) for exciting a spin resonance feature (12) of the data carrier to be inspected in the air gap. According to the invention, the resonator arrangement (40) comprises a signal source (22) and a plurality of stripline resonators (46) which are simultaneously supplied by the signal source (22), and the stripline resonators (46) are designed in a planar manner with a main extension plane (50) which is parallel to the plane of the at least one pole face (38) of the magnetic core.

Description

[0001] The invention relates to a sensor element for authenticity checking of planar data carriers, in particular banknotes, having spin resonance properties, a checking device having such a sensor element and a method for authenticity checking by means of such a sensor element or such a checking device.

[0002] Data carriers, such as valuable documents or license documents, and other valuable items, such as branded goods are usually equipped with security elements, so as to achieve protection, and the security element realizes the verification of the authenticity of the data carrier and is used as protection for unallowed imitation at the same time. It is known to use security elements with spin resonance characteristics to protect files and other data carriers in machine-type authenticity checks. For this reason, security elements are provided with materials with spin resonance marks. Spin resonance marks that can be used for authenticity checks especially include nuclear magnetic resonance effect (NMR), electron spin resonance effect (ESR) and ferromagnetic resonance effect (FMR).

[0003] When checking banknotes, three different magnetic fields are usually generated in the measuring range of, for example, a banknote processing machine in order to detect spin resonance marks. Specifically, this is a quasi-static polarization field B0 extending parallel to the axial direction (z direction) of the magnetic circuit air gap. The second magnetic field is composed of a modulation field B mod The modulated field also extends parallel to the z-axis and typically has a frequency f in the kHz range. mod In order to excite the spin resonance marking transitions between the split spin energy levels of the material, an excitation field B1 is provided which is polarized perpendicularly to the direction B0. The excitation field oscillates at the resonance frequency of the material, which is also called the Larmor frequency and is proportional to the polarization field B0.

[0004] To generate the polarization field B0 , a magnetic circuit is usually used which conducts the magnetic flux of a permanent magnet and / or a coil to an air gap in which the flat data carrier is examined.

[0005] A high-frequency resonator, for example a stripline resonator, is used to generate an excitation field B1. This is a conductive structure with a characteristic length l, which is arranged on a carrier. If the wavelength λ of the high-frequency signal coupled in during the plausibility check is adapted to the dimension l of the conductive structure, a standing wave can form in the resonator and the stripline resonator resonates with respect to the excitation frequency associated with the wavelength λ. Since the extension of the stripline resonator in the plane of the carrier is significantly greater than perpendicular to this plane, the plane of the stripline resonator corresponding to the carrier plane is also referred to.

[0006] Sensor elements based on stripline resonators for checking data carriers, such as banknotes, usually have a limited signal-to-noise ratio (SNR), which is proportional to the number of spins of the spin resonance feature in the sensitive area. Since the sensitive area scales with the size of the resonator, the signal-to-noise ratio can be improved substantially by enlarging the resonator.

[0007] The length of the resonator determines its resonant frequency, so that at a predetermined resonant frequency that is tuned to the Larmor frequency of the characteristic material to be measured, the resonator length cannot be increased arbitrarily. For example, if a suitable multiple of the λ / 2 or λ / 4 fundamental harmonic mode (Grundmode) is selected for the resonator length, the resonator can operate in a higher spatial mode and thus increase the sensitive area and thus the signal-to-noise ratio. However, this has the disadvantage that the increase in modes is usually accompanied by a decrease in the quality Q of the resonator, which is associated with a decrease in the signal strength. In addition, only continuous areas of the data carrier can be measured here. On the contrary, two non-adjacent positions on the data carrier cannot be measured.

[0008] Widening the stripline resonator makes the resonance frequency constant in a first approximation, but for very wide resonators, adaptation of the resonator impedance to the impedance of the signal source is generally not possible. There is also the additional disadvantage that only continuous areas of the data carrier can be measured, not two non-adjacent locations on the data carrier.

[0009] Furthermore, it can be used 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 expedient to maximize the power of the excitation signal. However, this is only possible for a power not exceeding a certain, characteristic-related limiting power P. opt If the power delivered to the resonator increases by more than P opt , then there is no longer a signal increase, but rather a signal drop or distortion of the spin resonance line can even be expected. Therefore, even by increasing the delivered power, the signal-to-noise ratio cannot be increased arbitrarily.

[0010] The object of the present invention is therefore to provide such a sensor element with a high signal-to-noise ratio for spin resonance measurements which avoids the above-mentioned disadvantages.

[0011] This technical problem is solved by the features of the independent claim. Improvements of the invention are the subject matter of the dependent claims.

[0012] The invention provides a sensor element for checking, in particular authenticity checking, a planar data carrier having a spin resonance characteristic. The planar data carrier can be, for example, a banknote. The sensor element comprises a magnetic core having an air gap, in particular defined by two pole faces of the magnetic core, into which the planar data carrier can be introduced for checking, a polarization device for generating a static magnetic flux in the air gap, and a resonator unit for exciting the spin resonance characteristic of the data carrier to be checked in the air gap. The spin resonance characteristic is preferably an ESR characteristic.

[0013] On the pole faces, the magnetic core preferably consists of a r >>1, especially µ r Greater than 1×10 2 The pole faces can also be made of ferromagnetic material with µ r ≈1, that is, the maximum value is 1+10 -2 µ r Made of paramagnetic materials.

[0014] The resonator unit comprises a signal source, in particular a single signal source, and a resonator arrangement having a plurality of stripline resonators, which are supplied simultaneously by the signal source. The stripline resonators are designed in a planar manner with a main extension plane, which is parallel to at least one pole plane of the magnetic core. Preferably, the resonator unit comprises exactly one signal source and / or all stripline resonators of the resonator arrangement are supplied simultaneously by the same signal source.

[0015] As explained in more detail below, the use of a plurality of stripline resonators oriented in this way and supplied simultaneously by the same signal source results in a significant increase in the signal-to-noise ratio compared to the use of a single resonator. Furthermore, not only correlated spin resonance features can be examined in a simple manner, but also spin resonance features with non-adjacent sub-regions, since the position of the plurality of stripline resonators can be easily matched to the position of the sub-region to be detected in the spin resonance measurement.

[0016] The stripline resonators used are characterized in particular by their very accessible sensitive region and by their very high filling factor for flat samples, such as banknotes to be checked. Stripline resonators are also sometimes referred to simply as resonators below.

[0017] Preferably, a plurality of stripline resonators are arranged in a linear array or two-dimensionally at grid points of a regular grid, for example in a rectangular, hexagonal or row-by-row offset 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.

[0018] In an advantageous embodiment, the plurality of stripline resonators forms an N×1 array with N parallel-connected stripline resonators, where N≧2, in particular N=2, 3, 4 or 5.

[0019] In another likewise advantageous embodiment, the plurality of stripline resonators forms a 1×M array having M stripline resonators connected in series, wherein M≧2, in particular M=2, 3, 4 or 5.

[0020] In a further advantageous embodiment, the plurality of stripline resonators forms an NxM array with stripline resonators partially connected in series and partially connected in parallel, wherein N≧2 and M≧2, in particular N=2, 3, 4 or 5 and / or M=2, 3, 4 or 5.

[0021] The stripline resonators of the resonator arrangement advantageously have the same resonance frequency. The stripline resonators are also preferably designed and arranged to detect spin resonance features in the same spatial mode of the excitation field, and particularly preferably the stripline resonators have the same geometry, for example square, rectangular or ring.

[0022] Advantageously, the polarization device is designed in such a way that it generates a static magnetic flux in the air gap which is substantially equally strong in all stripline resonators. In particular, it is advantageously provided that the static magnetic flux at the position of the stripline resonators has a maximum deviation of 2% or less.

[0023] The main extension plane of the stripline resonator is advantageously oriented perpendicularly to the direction of the static magnetic flux generated by the polarization device. In the context 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 extends in the xy plane perpendicular to the z-direction. It is particularly advantageously provided that the main extension plane of the stripline resonator is parallel to the two pole face planes of the magnetic core that define the air gap.

[0024] Advantageously, the stripline resonators of the resonator arrangement are all arranged in the same plane, in particular the stripline resonators are all arranged in the plane of a common carrier, for example a printed circuit board.

[0025] The resonator unit is preferably advantageously designed and arranged as a signal source, in particular as a single signal source, operating at a high output power, wherein signal saturation occurs when examining the spin resonance characteristics with a single stripline resonator. Since the input signal power is distributed over a plurality of stripline resonators, the resonator unit can be operated with a significantly higher excitation power than a single resonator without saturation effects occurring. For example, if the resonator arrangement comprises K=NxM stripline resonators of the same type, then by appropriate wiring, the excitation power of the signal source can be evenly distributed over the K stripline resonators, whereby the resonator arrangement as a whole is supplied with essentially K times the excitation power of a single resonator. The signal-to-noise ratio can thus be increased by √K relative to a single resonator without saturation effects occurring.

[0026] The arrangement of stripline resonators expediently has a smaller surface extension than the data carrier to be checked, in particular a banknote to be checked.

[0027] In an advantageous embodiment, the stripline resonator is designed as a rectangular strip with a length-to-width ratio of less than 3:1.

[0028] The stripline resonators advantageously form independent electromagnetic modes of the excitation field, wherein the entire excitation field preferably has a local minimum between the stripline resonators.

[0029] The air gap advantageously has a height, ie a dimension in the z direction, of less than 10 mm, preferably less than 5 mm. As a result, a particularly strong polarization field, ie a strong static magnetic flux, can be generated in the air gap.

[0030] 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 equally high in all stripline resonators of the resonator device. For example, the modulation frequencies at the location of two stripline resonators in each case differ 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.

[0031] In a particularly advantageous embodiment, the sensor element also has a ramp coil for generating a ramp function of the static magnetic flux.

[0032] The resonator arrangement is advantageously designed to excite a spin resonance signal with a frequency above 1 GHz, in particular between 1 GHz and 10 GHz. This enables a higher spectral resolution and a stronger measurement signal compared to lower frequencies.

[0033] The resonator arrangement is also designed in particular to detect a spin resonance signal characteristic of a spin resonance. The stripline resonator can in particular receive a response signal characteristic of a spin resonance and output it to a detector. The spin resonance can be determined, for example, by a continuous wave (CW) method, a pulse method or a fast scanning method.

[0034] Stripline resonators can be operated both in reflection and in transmission when checking data carriers. The latter has the advantage that no elements such as circulators are required in the signal path, which separate the forward and return signals relative to the resonator.

[0035] Advantageously, the resonator arrangement comprises a planar carrier on which the stripline resonator is applied. The carrier is expediently formed by a printed circuit board, which allows reproducible and cost-effective production. However, it is also advantageous to use a carrier based on ceramic, Teflon or hydrocarbon, in particular in order to reduce the dielectric losses in the carrier material.

[0036] The invention also includes a checking device for checking planar data carriers, in particular banknotes, having a spin resonance characteristic, comprising a sensor element of the type described above and a transport device which guides the planar data carrier to be checked along a transport path through an air gap of a magnetic core.

[0037] The transport device is designed and arranged in particular for fast transport of the flat data carrier to be checked along the transport path, for example between 1 m / s and 12 m / s.

[0038] The invention also includes a method for checking a planar data carrier, in particular a banknote, having a spin resonance characteristic by means of a sensor element of the type described or a checking device of the type described, wherein in the method

[0039] - guiding the flat data carrier to be checked along a transport path through the air gap of the magnetic core of the sensor element,

[0040] - generating a static magnetic flux in the air gap by means of the polarization means and preferably generating a time-varying magnetic modulation field in the air gap by means of the modulation means,

[0041] - simultaneously supplying a plurality of stripline resonators of the resonator arrangement by a signal source of the resonator unit, and

[0042] - Exciting a spin resonance feature of the data carrier to be checked by means of a resonator device.

[0043] In an advantageous method, the resonator arrangement is supplied by a signal source with a high output power, wherein signal saturation already occurs when examining the spin resonance signature with a single stripline resonator.

[0044] Further embodiments and advantages of the present invention are described below with reference to the accompanying drawings, in which scales and ratios are not shown in order to improve readability.

[0045] In the attached figure:

[0046] FIG1 schematically shows an inspection device of a banknote processing system for measuring the spin resonance of a banknote inspection sample,

[0047] FIG. 2 schematically shows a top view of a resonator device according to an embodiment of the present invention,

[0048] FIG. 3 shows a resonator arrangement according to the invention designed for operation in transmission with a 1×5 array of five rectangular stripline resonators connected in series,

[0049] FIG. 4 shows a resonator arrangement designed for operation in reflection with a 1x3 array of three square stripline resonators connected in series,

[0050] The invention will now be explained using the example of an authenticity check of a banknote. FIG1 schematically shows a check device 20 of a banknote processing system for measuring the spin resonance of a banknote check sample 10 .

[0051] The banknote inspection sample 10 has a spin resonance feature 12, whose characteristic properties are used to verify the authenticity of the banknote. The inspection device 20 comprises a sensor element 30 with a magnetic core 35 having an air gap 32 defined by two pole surfaces 38, through which the banknote inspection sample 10 is guided along the transport path 14 during the authenticity check.

[0052] In order to detect the spin resonance signature of the spin resonance feature 12 , the sensor element 30 generates three different magnetic fields within the measuring range of the air gap 32 .

[0053] On the one hand, a static magnetic flux parallel to the z-axis is generated in the measuring range by the polarization device 34. In order 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.

[0054] Secondly, the modulation device 36 generates a time-varying magnetic modulation field in the air gap, which also extends parallel to the z-axis and has a modulation frequency f in the range of 1 kHz to 1 MHz. Mod Finally, the resonator arrangement 40 with the stripline resonators generates an excitation field in the air gap which induces an energy transition between the spin energy levels in the spin resonance feature 12. The excitation field typically has a frequency above 1 GHz and is polarized perpendicular to the z direction.

[0055] The frequency of the excitation field is tuned to the Larmor frequency of the spin resonance feature 12 to be detected, in order to be able to measure its spin resonance signature and use it for a plausibility check.

[0056] For this purpose, the resonator unit of the sensor element comprises, in addition to the resonator device 40, a signal source 22 whose excitation frequency f MW The excitation signal of the signal source 22 is simultaneously supplied to the stripline resonator of the resonator arrangement 40 and generates there a desired Larmor frequency of the spin resonance feature 12. MW of alternating magnetic field.

[0057] In addition to the above-mentioned elements, the inspection device 20 generally comprises a detector diode for measuring the reflected high-frequency power and an evaluation unit for evaluating and, if necessary, displaying the measurement results. If the spin resonance feature 12 resonates at the coupling frequency, the resonator quality changes and thus also the power reflected by the stripline resonator. 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 obtained measurement signal is amplitude modulated by the modulation frequency.

[0058] As a special feature, the resonator device 40 comprises an NxM array of stripline resonators, N and / or M being greater than 1, instead of a conventional single resonator, in order to achieve a high signal-to-noise ratio, wherein the stripline resonators are supplied, in particular, all at the same time, by the same signal source 22. Within the scope of the present description, N here denotes the number of signal branches electrically connected in parallel, and M denotes the number of resonators connected in series per signal branch.

[0059] For a more detailed explanation, FIG2 schematically shows a top view of a stripline resonator layout of a resonator arrangement 40 according to an embodiment of the invention. In the resonator arrangement 40, a 2x1 array 44 of stripline resonators 46 is mounted on a planar carrier, for example a printed circuit board 42. In the sense of the above definition, the layout of FIG2 forms a 2x1 array 44 with N=2 and M=1, since the array contains two resonators connected in parallel. The two stripline resonators 46 of the 2x1 array 44 are supplied from the same signal branch 48 by the same signal source 22 of the resonator unit.

[0060] The resonator arrangement 40 with the stripline resonators 46 is of planar design 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 in the uniform polarization field B0. According to FIG. 2 , the two resonators 46 are arranged on a line transverse to the transport direction 14 of the data carrier to be checked, but alternatively they can also be arranged one after the other in the transport direction or arranged obliquely with respect to the transport direction. The array consisting of N>2 electrically parallel resonators does not have to be geometrically linear either, but can also be arranged two-dimensionally, for example on grid points of a rectangular or hexagonal grid, or in another arrangement that matches the authenticity feature to be checked, for example along a circle.

[0061] The number of spins in the sensitive area of ​​the 2x1 array 44 is doubled compared to a single resonator of the area of ​​one of the two stripline resonators 46. If each of the two stripline resonators 46 of the array 44 is fed with the same power as the comparative single resonator, twice the signal-to-noise ratio is obtained compared to the comparative single resonator.

[0062] 2, the signal power supplied from the signal source 22 to the array 44 is distributed equally to the two stripline resonators 46. Thus, the 2x1 array 44 can be operated at twice the excitation power of a comparable single resonator without saturation effects.

[0063] In addition to the 2x1 array shown in FIG. 2 , of course larger single-column arrays, typically Nx1 arrays, where N>2, such as N=3, 4 or 5, are also possible.

[0064] Furthermore, a 1xM array of stripline resonators is also possible, i.e. an array consisting of only one row of a plurality (M≥2) of resonators connected in series. As an example, FIG. 3 shows a resonator device 60 designed for transmission operation with a 1x5 array 64 of five rectangular stripline resonators 66 connected in series, which are supplied by the same signal branch 68 via the same signal source (not shown). In this example, the stripline resonators 66 are arranged in a line along the transport direction 14 and have a length l=λ / 2 and a distance d=λ in the transport direction 14, respectively, where λ represents the coupled high-frequency signal f MW Other alternative geometrical arrangements of the resonators in the series array are also possible, for example in lines transverse to the transport direction, or even in two dimensions, for example at grid points of a rectangular or hexagonal grid.

[0065] As another embodiment of the present invention, FIG. 4 shows a resonator device 70 designed for operation in reflection, having a series 1x3 array 74 of three square stripline resonators 76 with a side length l=λ / 2, which are supplied by the same signal branch 78 through the same signal source.

[0066] The designs of FIGS. 3 and 4 also have the advantage that the sensitive range of the resonator arrangement is extended and the maximum possible excitation power is increased compared to a comparable single resonator.

[0067] In addition to the one-dimensional Nx1 array and 1xM array shown, advantageous connections also include that the stripline resonators form an NxM array, where N≥2 and M≥2, and the stripline resonators are connected to each other partly in series and partly in parallel. 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 in a layout offset row by row.

[0068] In order to demonstrate the advantages of a sensor element according to the invention with an array of stripline resonators, the behavior of a sensor element with a 2x1 array in a parallel configuration according to FIG. 2 was simulated and compared with a single resonator.

[0069] The stripline resonator array according to the invention and the comparative single resonator were each constructed on a printed circuit board with a thickness of 1.5 mm and a dielectric constant of 3.66. As basic elements, square λ / 2 stripline resonators were used in each case with an edge length of 7.1 mm, corresponding to a resonance frequency of 9.8 GHz.

[0070] In the case of the comparative individual resonators, there is only a single such basic element. The impedance of the basic element is directly adapted to 50Ω via a planar λ / 4 impedance transformer, and the individual resonators are supplied by a 50Ω source via a circulator.

[0071] The stripline resonator array 44 according to the invention comprises two elementary elements 46 connected in parallel in the manner of FIG. 2 . The impedance of each elementary element 46 is converted to 100Ω by a λ / 4 impedance converter. A total impedance of 50Ω is obtained by subsequently connecting the two elementary elements in parallel. The parallel array 44 is supplied by a 50Ω source by means of a circulator, so that the two elementary elements 46 are supplied by the same signal source at the same time.

[0072] By comparing single resonators and 2x1 arrays 44 thus constructed, spin resonance measurements were simulated on planar samples uniformly doped with spin resonance features, respectively.

[0073] The 2x1 array 44 according to the invention operates here with twice the power of a comparable single resonator. Mod is the same in both measurements.

[0074] The signal-to-noise ratio is determined for both configurations and the ratio of the two signal-to-noise ratios η=SNR is determined as a measure for improving the invention. 阵列 / SNR 单个共振器 For the improvement factor a value of η=1.93 is obtained here, so that the signal-to-noise ratio of the stripline resonator array 44 according to the invention is almost twice that of the comparable single resonator.

[0075] and the maximum expected improvement η 理想 The deviation of = 2.0 can be explained by the additional losses in the coupling network of the array.

[0076] Reference numerals list

[0077] 10 banknote inspection samples

[0078] 12 Spin resonance characteristics

[0079] 14Conveying Path

[0080] 20 Inspection device

[0081] 22 Signal Source

[0082] 30 sensor elements

[0083] 32 Air Gap

[0084] 34 Polarization device

[0085] 35 core

[0086] 36 Modulation device

[0087] 38 polar surface

[0088] 40 Resonator Device

[0089] 42 Printed Circuit Board

[0090] 442x1 array

[0091] 46 Stripline Resonator

[0092] 48 connection structure

[0093] 50 Main extension plane

[0094] 60 Resonator Device

[0095] 641x5 array

[0096] 66 Rectangular Stripline Resonator

[0097] 68 signal branch

[0098] 70 Resonator Device

[0099] 741x3 array

[0100] 76 square stripline resonator

[0101] 78 signal branch

Claims

1. A sensor element (30) for checking a planar data carrier (10), in particular a banknote, having a spin resonance characteristic (12), a sensor element having a magnetic core with an air gap (32) defined by two pole faces (38) of the magnetic core, into which the flat data carrier (10) can be introduced for inspection, - the sensor element has polarization means (34) for generating a static magnetic flux in the air gap (32), a sensor element having a resonator unit for exciting a spin resonance feature (12) of a data carrier to be checked in the air gap, It is characterized in that the resonator unit comprises, in particular, exactly one signal source (22) and / or a resonator arrangement (40) with a plurality of stripline resonators (46) which are simultaneously supplied by the signal source (22), and The stripline resonator (46) is designed in a planar manner with a main extension plane (50) which is plane-parallel to at least one pole face (38) of the magnetic core.

2. The 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 arrangement (40) are supplied simultaneously by the one signal source (22).

3. The sensor element (30) according to claim 1, characterized in that The plurality of stripline resonators (44, 46) form an NxM array consisting of N electrically parallel signal branches and M serially connected stripline resonators for each 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. The sensor element (30) according to claim 3, characterized in that The plurality of stripline resonators (44, 46) form an Nx1 array having N parallel-connected stripline resonators, wherein N≥2, in particular N=2, 3, 4 or 5.

5. The sensor element (30) according to claim 3, characterized in that The plurality of stripline resonators forms a 1×M array having M stripline resonators connected in series, wherein M≧2, in particular M=2, 3, 4 or 5.

6. The sensor element (30) according to claim 3, characterized in that The plurality of stripline resonators forms an NxM array with stripline resonators partially connected in series and partially connected in parallel, wherein N≧2 and M≧2, in particular N=2, 3, 4 or 5 and / or M=2, 3, 4 or 5.

7. The sensor element (30) according to at least one of claims 1 to 6, characterized in that The stripline resonators (44, 46) of the resonator arrangement (40) have the same resonance frequency. The stripline resonators are preferably also designed and arranged to detect spin resonance characteristics in the same spatial mode of the excitation field. The stripline resonators particularly preferably have the same geometry.

8. The sensor element (30) according to at least one of claims 1 to 7, characterized in that The polarizing device (34) generates a static magnetic flux in the air gap (32), the static magnetic flux having substantially the same strength for all stripline resonators (44, 46), and in particular having a maximum deviation of 2% or less at the position of the stripline resonator.

9. The 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, and in particular the main extension plane (50) of the stripline resonator is parallel to the planes of two pole faces (38) of the magnetic core (35) defining the air gap (32).

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

11. The sensor element (30) according to at least one of claims 1 to 10, characterized in that The resonator unit is designed and arranged such that the signal source (22) operates at a high output power, wherein signal saturation occurs when examining the spin resonance characteristics with a single stripline resonator.

12. The sensor element (30) according to at least one of claims 1 to 11, characterized in that The arrangement (44, 46) of stripline resonators has a smaller surface extension than a data carrier (10) to be checked, in particular a banknote to be checked.

13. The sensor element (30) according to at least one of claims 1 to 12, characterized in that The stripline resonator is designed in a rectangular shape with a length-to-width ratio of less than 3:

1.

14. The 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 a local minimum between the stripline resonators.

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

17. An inspection device (20) for inspecting planar data carriers, in particular banknotes, having spin resonance characteristics, - a testing device having a sensor element (30) according to one of claims 1 to 16, and The inspection device has a transport device which guides the flat data carrier (10) to be inspected along a transport path (14) through the air gap of the magnetic core, wherein: The transport device is advantageously designed and arranged to rapidly transport the flat data carrier to be checked along the transport path.

18. A method for checking a planar data carrier (10), in particular a banknote, having a spin resonance characteristic (12) by means of a sensor element (30) according to one of claims 1 to 16 or a checking device (20) according to claim 17, wherein: In the method - guiding the flat data carrier (10) to be checked along a transport path through the air gap of the magnetic core of the sensor element (30), - generating a static magnetic flux in the air gap by means of the polarization means (34) and preferably generating a time-varying magnetic modulation field in the air gap by means of the modulation means (36), and - simultaneously supplying a plurality of stripline resonators (46) of the resonator device (40) by a signal source (22), and - Exciting a spin resonance feature (12) of the data carrier (10) to be checked by means of a resonator device (40).

19. The method according to claim 18, characterized in that The resonator arrangement (40) is supplied by a signal source with a high output power, wherein signal saturation occurs when examining the spin resonance characteristics with a single stripline resonator.

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