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 having an air gap into which a planar data carrier (10) can be introduced for inspection; a polarization device (34) for generating a static magnetic flux in the air gap; and a resonator device (40) for exciting a spin resonance characteristic of the data carrier to be inspected in the air gap. According to the invention, the resonator arrangement (40) comprises at least two stripline resonators (44, 46), which are designed and arranged to operate at different excitation frequencies.

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, the quasi-static polarization field B extends parallel to the axial direction (z direction) of the magnetic circuit air gap. 0 The second magnetic field is composed of the 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 stimulate the transition between the split spin energy levels of the spin resonance marker material, a 0 Directionally polarized excitation field B 1 The excitation field oscillates at the resonant frequency of the material, which is also called the Larmor frequency and is related to the polarization field B 0 Proportional.

[0004] In order to generate the polarization field B 0 , 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 checked.

[0005] A high frequency resonator, such as a stripline resonator, is used to generate the excitation field B 1 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] When checking a data carrier, for example within the scope of an authenticity check, a spin resonance spectrum of a spin resonance characteristic is usually determined and compared with an expected spectrum based on characteristic markers. The spin resonance spectrum is usually obtained in a time-consuming B 0 The slope method (also called B 0 Scanning method) record. In this static polarization field B 0 In the excitation field B 1 The field strength changes slowly around the resonance at a fixed frequency, and thus passes through the polarization field B 0 Since the Larmor frequency of the spin resonance feature to be examined is related to the polarization field strength B 0 is proportional to , so the excitation frequency is effectively shifted relative to the Larmor frequency, which allows recording of a spectrum of spin resonance features. 0 Polarization field B in the ramp method 0 The temporal variation of the field strength of the modulated field B mod and the excitation field B 1 The change in time is much slower, so in the scope of this application, even if there is a ramp field, B 0 Also called static magnetic field or static flux.

[0007] However, especially in fast-running banknote processing machines, the short measuring times required for the operation of the sensor are not sufficient to measure the complete spectrum of the spin resonance feature using a ramp (often also called a sweep). The spectrum can then only be recorded with a small number of measuring points, i.e. with a low resolution or over a narrow frequency band. However, for many applications, high-resolution, broadband measurements of the spectrum are desired, for example in order to be able to distinguish between characteristic materials with different Larmor frequencies. Spin resonance features with spectral codes can also be used with high spectral resolution, for example for different currencies or different denominations.

[0008] On this basis, the technical problem to be solved by the present invention is to provide a better device for checking data carriers with spin resonance characteristics, in particular to provide a sensor element which allows high-resolution and / or broadband measurement of the spin resonance spectrum of the data carrier to be checked in a short time.

[0009] This technical problem is solved by the features of the independent claim. Improvements of the invention are the subject matter of the dependent claims.

[0010] 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 with an air gap, 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 device 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.

[0011] The resonator arrangement comprises at least two stripline resonators which are designed and arranged to be operated at different excitation frequencies. For example, the excitation frequencies of the at least two stripline resonators differ by at least 1%, preferably by at least 2%.

[0012] As explained in more detail below, the arrangement of multiple stripline resonators operated at different excitation frequencies enables simultaneous measurement of spin resonances at multiple different frequencies and thus allows higher spectral resolution and / or shorter measurement times. Furthermore, the requirements on the field slope for measuring the spectral lines are significantly reduced.

[0013] 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.

[0014] In an advantageous embodiment, the stripline resonators of the resonator arrangement are arranged in the form of a one-dimensional array. In the test device, the one-dimensional array is in particular parallel to the transport path of the data carrier, so that the stripline resonators are successively swept past by the moving data carrier.

[0015] In another, likewise preferred, design, the stripline resonators of the resonator arrangement form a multi-line layout with a plurality of parallel lines, wherein each line is formed by a one-dimensional array of stripline resonators.

[0016] The resonator arrangement can in particular comprise two, three, four, five or six stripline resonators, wherein a greater number of stripline resonators, for example a multiline arrangement with two or three lines each with five stripline resonators, can also be advantageous. Increasing the number of stripline resonators has the advantage of better spectral resolution or shorter required measurement times.

[0017] In an advantageous embodiment, the stripline resonators of the resonator arrangement all have different resonance frequencies. The resonance frequencies can in particular be distributed equidistantly, so that in the case of N resonators the resonance frequency for the i-th resonator is determined by the following relation f i =f 0 +(i-1)*Δf, where i=1,…,N, where f 0 is the minimum frequency, and Δf is a constant frequency interval. The resonator with the highest frequency has f N =f 0 +(N-1)*Δf. For example, the resonant frequencies of two stripline resonators differ by at least 1%, preferably at least 2%. In order to increase the signal-to-noise ratio, it is also possible to i A plurality of stripline resonators are arranged at a location.

[0018] The stripline resonators are advantageously designed geometrically similarly, ie have the same shape but different dimensions. In particular, the stripline resonators can have different edge lengths, for example square stripline resonators with different edge lengths l can be provided, or square stripline resonators with different edge lengths l can be provided. x、 l y But with the same aspect ratio l x / l y or rectangular stripline resonators with different lengths but the same width.

[0019] The air gap is advantageously defined by two plane-parallel pole faces of the magnetic core. On the pole faces, the magnetic core is preferably formed with µ r >>1 Magnetic permeability, 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 The polarization device advantageously generates a static magnetic flux in the air gap, which has substantially the same strength at the location of each stripline resonator. In particular, it is provided that the static magnetic flux at the location of the stripline resonators has a maximum deviation of 2%.

[0020] Advantageously, the stripline resonator is designed in a planar manner with a main extension plane, which is parallel to at least one pole plane of the magnetic core that defines an air gap. The main extension plane is also advantageously perpendicular 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.

[0021] 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.

[0022] 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.

[0023] In an advantageous development of the invention, in order to increase the signal-to-noise ratio, the arrangement and design are such that at least a portion of the stripline resonators operating at different resonance frequencies are replaced by an NxM array of stripline resonators, wherein N and M are natural numbers and at least one of the values ​​of N and M is greater than 1, wherein the stripline resonators of the NxM array are respectively all supplied by the same signal source and are electrically connected in parallel and / or in series.

[0024] In a particularly advantageous embodiment, the sensor element also has a ramp coil for generating a ramp function of the static magnetic flux.

[0025] 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.

[0026] The resonator device is also particularly designed to detect a spin resonance signal characteristic of a spin resonance. The resonator device can particularly receive a response signal characteristic of a spin resonance and output it to a detector. Spin resonance can be determined, for example, by a continuous wave (CW) method, a pulse method or a fast scanning method.

[0027] 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.

[0028] 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.

[0029] The invention also includes an inspection device for inspecting planar data carriers, in particular banknotes, having a spin resonance characteristic, the inspection device having a sensor element of the above-described type. In addition, the inspection device comprises a plurality of signal sources with different excitation frequencies, from which the stripline resonators of the resonator device are supplied, or the inspection device comprises a single signal source with an excitation signal with a plurality of different frequency components, from which the stripline resonators are supplied. In the latter case, the resonator device advantageously has stripline resonators with different resonance frequencies, the resonance frequencies respectively corresponding to one of the frequency components of the excitation signal, so that each stripline resonator is excited by the excitation signal via a suitable frequency component at its resonance frequency. This is explained in detail with reference to the exemplary embodiment in conjunction with FIG. 5 .

[0030] Advantageously, the inspection device further comprises a transport device which guides the planar data carrier to be inspected along a transport path through the air gap of the magnetic core. The transport device is particularly designed and arranged to transport the planar data carrier to be inspected along the transport path quickly, for example between 1 m / s and 12 m / s.

[0031] 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

[0032] - guiding the flat data carrier to be checked along a transport path through the air gap of the magnetic core of the sensor element, wherein a plurality of stripline resonators of the resonator arrangement are arranged one behind the other parallel to the transport path,

[0033] - 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,

[0034] - Exciting a spin resonance feature of the data carrier to be checked by means of a resonator device.

[0035] In an advantageous method design it is provided that

[0036] - guiding the data carrier to be checked past the stripline resonators arranged in succession and recording from each of the stripline resonators a temporal measurement sequence of a response signal of a spin resonance characteristic generated after excitation,

[0037] - respectively identifying measurement data belonging to the same measurement position from the temporal measurement sequence of the stripline resonator,

[0038] - deriving spectral information about the spin resonance features from the identified measurement data

[0039] - evaluating the data carrier based on the derived spectral information, in particular with regard to authenticity and / or attribution to a class of data carrier.

[0040] The measured data are advantageously spatially resolved or spatially averaged.

[0041] According to the favorable improved design provisions of this method,

[0042] – superimpose a spatially uniform ramp field on the static flux so that the total static flux in the air gap varies in time between a minimum and a maximum value,

[0043] - deriving spectral information from the identified measurement data taking into account the field strength of the static magnetic flux at the respective measurement time, and

[0044] - determining the authenticity of the checked data carrier and / or the affiliation of the checked data carrier to one of a plurality of data carrier categories having different spectral signatures based on the derived spectral information.

[0045] As mentioned above, in a preferred design, the stripline resonators are arranged one after the other in the transport direction of the data carrier. This has the advantage that all resonators measure the same line on the data carrier, i.e. the same measuring point with a certain time offset. This makes the evaluation and inspection of the data carrier easier.

[0046] A multi-line structure is also advantageous for generating spatial resolution transversely to the transport direction. For this purpose, a plurality of lines are each formed by a one-dimensional array of resonators for spectral resolution.

[0047] 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.

[0048] In the attached figure:

[0049] FIG1 schematically shows an inspection device of a banknote processing system for measuring the spin resonance of a banknote inspection sample,

[0050] FIG. 2 schematically shows in top view the resonator device of the inspection device of FIG. 1 and the transported banknote inspection sample, and schematically shows in the bottom the course of the uniform polarization field,

[0051] FIG3 shows a simplified spectrum of a spin resonance line as a function of the excitation frequency for fixed values ​​of the polarization field,

[0052] FIG. 4 shows diagrams representing recordings of the spin resonance spectrum of the spin resonance line of FIG. 3 , in (a) by a conventional sensor element and in (b) by a sensor element according to the invention,

[0053] FIG. 5 shows a circuit for connecting a resonator arrangement of a sensor element according to the invention to only a single signal branch,

[0054] FIG6 shows the spectrum of the spin resonance characteristics of the paper sample,

[0055] FIG. 7 shows a signal curve when the spin resonance feature of FIG. 6 is measured by means of a sensor element according to the invention.

[0056] 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 .

[0057] The banknote inspection specimen 10 has a spin resonance feature 12 whose characteristic properties are used to verify the authenticity of the banknote. The spin resonance feature can be present only in a partial area of ​​the banknote or, as in the illustrated embodiment, can extend over the entire surface of the banknote inspection specimen.

[0058] The checking device 20 comprises a sensor element 30 with a magnetic core 35 having an air gap 32 delimited by two pole surfaces 38 , through which the banknote check specimen 10 is guided along the transport path 14 during the authenticity check.

[0059] 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 .

[0060] On the one hand, a uniform 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.

[0061] 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 device 40 arranged in the air gap 32 generates an excitation field B 1 , which induces an energy transition between the spin levels in the spin resonance feature 12. The resonator arrangement 40 here comprises at least two stripline resonators which are operated at different excitation frequencies.

[0062] For this purpose, the inspection device 20 comprises one or more signal sources 22, whose excitation signals are supplied, for example, via a duplexer 24 to the resonator device 40 and generate alternating magnetic fields with two or more different frequencies therein for simultaneous measurement of the spin resonance characteristics 12 at different frequencies. For this purpose, the inspection device can comprise a plurality of signal sources with different excitation frequencies, or even only a single signal source 22 with an excitation signal with a plurality of different frequency components, which, due to the special design of the stripline resonator, generates alternating magnetic fields with different frequencies therein. The excitation field usually has a frequency of more than 1 GHz and is polarized perpendicular to the z direction.

[0063] In addition to the above-mentioned elements, the inspection device 20 also includes 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 results. If the spin resonance feature 12 resonates at the coupling frequency, the resonator quality changes and thus the power reflected by the stripline resonator also changes. 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.

[0064] In order to explain the features of the present invention in more detail, FIG. 2 schematically shows a resonator device 40 according to an embodiment of the present invention in a top view in the upper part of the figure, the resonator device 40 having a carrier 42, a resonant frequency f arranged on the carrier A The first stripline resonator 44 and the resonant frequency f arranged on the carrier B The second strip line resonator 46 is provided.

[0065] As shown in the lower part of FIG. 2 , the polarization device 34 generates a uniform polarization field B in the air gap 32. 0 , so that the positions x of the two stripline resonators 44 and 46 are A Or x B Specifically, in the embodiment, the two pole faces 38 of the magnetic core 35 are designed to be parallel to each other and to the plane of the resonator device 40, and the field strength of the polarization field at position x is substantially the same. A Or x B The maximum difference is 2%.

[0066] The two stripline resonators 44, 46 are arranged one behind the other in the transport direction 14 and are therefore swept by the spin resonance feature 12 of the banknote 10 one after the other with a time offset. Both stripline resonators 44, 46 have a square shape but have different resonance frequencies f due to their different edge lengths. A 、f B .

[0067] To further explain the function of the invention, a diagram 50 of FIG. 3 shows a simplified spectrum 52 of a spin resonance line, here for example a spin resonance feature 12 of a banknote 10, as a polarization field B 0 The above-mentioned resonance frequency f of the two resonators 44 or 46 is plotted in the curve 52. A Or f B Two representative spectral components 54A, 54B at.

[0068] If the resonator device 40 of FIG. 2 is at a polarization field strength B 0 When a banknote 10 with a spin resonance feature 12 is scanned by the two stripline resonators 44, 46, each detects the resonance frequency f A Or f B The spectral component 54A or 54B of the spin resonance characteristic 12 is shown in FIG. A The spectrum intensity Int(f A ), and the stripline resonator 46 is at position x BThe spectrum intensity Int(f B ).

[0069] As has already been explained above, in the actual plausibility check, the static magnetic field B of the polarization device 34 0 By means of the ramp coil, the field strength is additionally varied around the resonance and thus the polarization field B is passed at a fixed frequency of the excitation field. 0 of field strength in order to allow the spectrum of the resonance of feature 12 to be recorded.

[0070] The advantages of the design according to the invention are explained in more detail on the example of a spin resonance feature 12 having only one spin resonance line with reference to the diagrams 60, 70 of Fig. 4. The spin resonance line 62 of the feature 12 shown simplified in the figure has a line width of, for example, 10 mT in the polarization field strength space, corresponding to the distance from the minimum to the maximum.

[0071] If you use the 0 =8.41GHz resonant frequency 64 of the conventional single resonator recorded spin resonance spectrum, then the polarization field strength B 0 = 300 mT, in order to fully detect the spectral mark at this line width, a field ramp 66 in the range of about 40 mT is required, as shown in FIG4 (a). The field ramp 66 can be plotted in graphs 60 and 70 because the Larmor frequency of the spin resonance feature is related to the polarization field B. 0 The frequency spectra in Figures 60 and 70 are also equivalent to the frequency spectra in the space of the polarization field strength. Since the Larmor frequency is proportional to the polarization field strength, when the field strength is large, the fixed excitation frequency is less than the Larmor frequency, and when the field strength is small, the fixed excitation frequency is greater than the Larmor frequency.

[0072] In the embodiment of FIG. 4 , for example, at the polarization field strength B 0 = 300 mT, the excitation frequency of 8.41 GHz corresponds exactly to the Larmor frequency of the spin resonance feature 12 to be examined. As shown in FIG4 (a), a field ramp 66 with an amplitude of -20 mT to +20 mT must be passed around the resonance field strength in order to be able to fully measure the spin resonance line 62 and its line width of 10 mT at a fixed excitation frequency. Such a field ramp involves a longer measurement time and a larger current requirement.

[0073] If, on the other hand, a spectrum is recorded according to the invention using a resonator arrangement of a plurality of stripline resonators having different resonance frequencies, significantly shorter measuring times and significantly lower current requirements can be achieved.

[0074] 4 (b), the resonator device of the sensor element according to the present invention comprises, for example, five stripline resonators spaced apart and arranged successively in the transport direction, which have a resonance frequency f A=7.96GHz, f B =8.18GHz, f C =8.41GHz, f D =8.63GHz and f E =8.85GHz. The resonant frequency of the centered stripline resonator is f C This corresponds to the resonant frequency f of a single resonator in FIG4(a). 0 , and is exactly equivalent to the polarization field strength B 0 =300mT is the center frequency of the spin resonance line 62.

[0075] Therefore, the resonator device 40 can be used to generate a polarization field strength B 0 At the same time, at five different frequencies f A to f E The intensity is measured at the frequency f A to f E This is depicted by a dotted line in Figure 4(b).

[0076] In order to be able to fully measure the spin resonance line 62 and its line width of 10 mT, an additional field ramp 78 is also required here. However, it can also be seen from FIG. 4( b ) that in order to fully detect the spectral signature of the spin resonance line 62 here, a significantly smaller field ramp 78 with an amplitude of only approximately −3.5 mT to 3.5 mT is sufficient, which corresponds to approximately one fifth of the amplitude of the conventionally required field ramp 66 of FIG. 4( a ).

[0077] Since the polarization field in the air gap is uniform, and therefore for all five stripline resonators, the polarization field strength B 0 are the same, so the upper axis of the graph 70 is at the resonant frequency f A to f E The polarization field strength B is given at 0 = 300 mT. The arrow of the field ramp 78 indicates the scanning of the shape of the spectral line 62 by the field ramp 78 with a total amplitude of about 7 mT. By measuring simultaneously at five frequencies, the spectral line 62 can be measured with the same spectral resolution, while the measurement time and thus the current requirement are significantly reduced.

[0078] In the embodiment shown in FIG. 2 , the resonator arrangement comprises only two stripline resonators for illustration purposes, but it should be understood that a greater number of stripline resonators may also be used, in particular to achieve a better spectral resolution, as described in conjunction with FIG. 4 .

[0079] In the embodiments described so far, the stripline resonator is used to detect the spectral component of a single spin resonance line. If the banknote 10 or in general the data carrier to be checked is provided with a spin resonance signature representing multiple spectral lines, the resonator arrangement can easily be provided with additional stripline resonators which detect the spectral components of additional lines.

[0080] The individual stripline resonators of the resonator arrangement according to the invention can be operated with the aid of independent signal sources. However, this also requires that the resonators be connected to independent signal branches, for which, especially in the case of a plurality of resonators, a large amount of installation space is required for implementing the circuit.

[0081] FIG5 shows an alternative circuit 80 for connecting the resonator device 40 of the sensor element 30 according to the invention to only a single signal branch. All stripline resonators 44, 46 of the resonator device 40 are connected to this signal branch, so that the required installation space is very small. For simplicity, only two resonators 44, 46 are shown in the circuit 80 of FIG5. However, the number of resonators can be easily increased as required by connecting other excitation sources and receiving mixers in parallel.

[0082] The circuit 80 of FIG. 5 is divided into an excitation circuit 82 and a receiving circuit 84. In addition, the circuit is divided into a digital part and an analog part. The separation is performed in the digital-to-analog (D / A) or analog-to-digital (A / D) converter shown. The digital part of the circuit can advantageously be implemented with the aid of an FPGA. Therefore, additional signal sources and receiving mixers can be added in a simple manner.

[0083] A high frequency source 86 is located in the analog part of the circuit and provides a signal at a carrier frequency which corresponds approximately to the resonant frequency of the plurality of resonators. For example, it may correspond to the average of the resonant frequencies, or it may be slightly below the lowest resonant frequency. In particular, the carrier frequency is in the GHz range.

[0084] In the FPGA there are a plurality of signal sources 22, which operate in the baseband of the circuit 80, i.e. at low frequencies, for example in the range of a few kHz to 1 GHz. The frequencies of the signal sources 22 preferably have a fixed frequency distance relative to the resonant frequency of the associated resonator. This frequency distance is preferably the same for all signal source and resonator pairs and is defined by the carrier frequency.

[0085] The signal sources 22 are summed, D / A converted, and then mixed with the high frequency carrier 86. For better representation, the filter bank is not shown in the figure. After signal amplification, the bandpass signal obtained is sent to the circulator 88. If the frequency of a single signal source 22 is equal to the carrier frequency and the corresponding resonant frequency f A 、f BThe individual spectral components of the bandpass signal are then distributed to the corresponding resonators 44 , 46 , since each resonator can only be effectively excited at its resonant frequency.

[0086] For example, two signal sources 22 located in the FPGA can be operated at 100 MHz or 700 MHz. Mixing with a high-frequency carrier wave 86 with a frequency of 9.0 GHz then allows coupling of the 9.1 GHz and 9.7 GHz resonators.

[0087] The signals reflected by the resonators 44, 46 are fed to the receiving amplifier via a circulator 88, down-mixed (phase shifters are not shown for simplicity) and A / D converted. Further signal processing can then be performed in the FPGA, such as a field-modulated lock-in detection. The figure shows, for example, two evaluation units 90 for evaluating the response signals obtained at the excitation frequencies fA, fB of the spin resonance feature being examined.

[0088] In order to demonstrate the mode of operation of the present invention, the behavior of a sensor element having a resonator arrangement with two square λ / 2 stripline resonators according to FIG. 2 was simulated.

[0089] The stripline resonators 44, 46 are mounted here on a printed circuit board 42 with a thickness of 1.5 mm and a dielectric constant of 3.66. The resonators 44, 46 are spaced 15 mm apart in the banknote transport direction 14. The edge length of the first resonator 44 is 8.1 mm, corresponding to a resonance frequency f A =8.8 GHz, the edge length of the second resonator 44 is 7.1 mm, corresponding to the resonance frequency f B =9.8GHz.

[0090] The two resonators 44, 46 are operated via circulators with independent 50Ω signal sources, which are operated at the corresponding resonant frequencies in continuous wave (CW) mode with the same power. For coupling to the signal source, the impedance of the resonator is converted to 50Ω by means of a λ / 4 converter.

[0091] The resonator device 40 constructed in this way is installed in the air gap of the magnetic circuit, wherein a uniform polarization field with a strength of 300 mT is generated.

[0092] Then a paper sample of length 100 mm is loaded, and a spin resonance feature is uniformly applied to its surface, and its spectrum 112 is shown in the graph 110 of FIG6. The resonance frequencies f of the two resonators 44, 46 are also shown. A and f B and the related relative signal strength Int(f A ) and Int(f B ).

[0093] The paper sample with this spin resonance feature is conveyed through the resonator arrangement 40 and the signal intensity of the spin resonance feature is recorded by the two resonators 44, 46. The obtained signal curves 122A (resonator 44) and 122B (resonator 46) are shown in the diagram 120 of FIG7, which shows the measured signal intensity depending on the position x. The signal curves are normalized to the average signal intensity of the signal curve 122A.

[0094] By averaging the signal intensities in the plateau region of each signal curve 122A, 122B, the ratio of the signal intensity of the first resonator 44 to the signal intensity of the second resonator 46 is obtained, namely 1.0 / -0.85, which is consistent with the ratio Int(f A ) / Int(f B ) are very consistent.

[0095] In the arrangements described so far, the stripline resonators of the resonator arrangement are designed such that their resonance frequency lies essentially within the line width of the spin resonance line to be measured. However, it is also possible to provide stripline resonators in the resonator arrangement whose resonance frequency does not correspond to the expected Larmor frequency. With such resonators, a negative proof can be performed, i.e., no spin resonance signal is expected for this resonator for a genuine banknote.

[0096] Reference numerals list

[0097] 10 banknote inspection samples

[0098] 12 Spin resonance characteristics

[0099] 14Conveying Path

[0100] 20 Inspection device

[0101] 22 Signal Source

[0102] 24 Duplexer

[0103] 26 detector diodes

[0104] 28 evaluation units

[0105] 30 sensor elements

[0106] 32 Air Gap

[0107] 34 Polarization device

[0108] 35 core

[0109] 36 Modulation device

[0110] 38 polar surface

[0111] 40 Resonator Device

[0112] 42 carrier

[0113] 44, 46 Stripline Resonator

[0114] 50 Charts

[0115] 52 Spin resonance line spectrum

[0116] 54A, 54B spectral components

[0117] 60 Charts

[0118] 62 spin resonance lines

[0119] 64 Resonance Frequency

[0120] 66 slopes

[0121] 70 Charts

[0122] 78 slopes

[0123] 80 Circuit

[0124] 82 Excitation circuit

[0125] 84 receiving circuit

[0126] 86 high frequency carrier

[0127] 88 Circulator

[0128] 110 Charts

[0129] 112 Spin Resonance Characteristic Spectrum

[0130] 120 Charts

[0131] 122A, 122B signal curve

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) into which a planar data carrier (10) can be introduced for testing, - the sensor element has polarization means (34) for generating a static magnetic flux in the air gap (32), - a sensor element with a resonator device (40) 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 arrangement (40) comprises at least two stripline resonators (44, 46) which are designed and arranged to operate at different excitation frequencies.

2. The sensor element (30) according to claim 1, characterized in that The stripline resonators (44, 46) of the resonator device (40) are arranged in the form of a one-dimensional array.

3. The sensor element (30) according to claim 1 or 2, characterized in that The stripline resonators (44, 46) of the resonator arrangement (40) all have different resonant frequencies.

4. The sensor element (30) according to at least one of claims 1 to 3, characterized in that The stripline resonators (44, 46) are designed geometrically similarly, ie have the same shape but different dimensions, in particular different edge lengths.

5. The sensor element (30) according to at least one of claims 1 to 4, characterized in that The air gap (32) is defined by two plane-parallel pole faces (38) of the magnetic core (35).

6. The sensor element (30) according to at least one of claims 1 to 5, characterized in that The polarizing device (34) generates a static magnetic flux in the air gap, which has substantially the same strength at the location of each stripline resonator, and in particular the static magnetic flux at the location of the stripline resonators has a maximum deviation of 2%.

7. The sensor element (30) according to at least one of claims 1 to 6, characterized in that The stripline resonator (44, 46) is designed in a planar manner with a main extension plane which is parallel to at least one pole face plane of the magnetic core which defines an air gap.

8. The sensor element (30) according to at least one of claims 1 to 7, 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 at the location of each stripline resonator of the resonator arrangement.

9. The sensor element (30) according to claim 8, characterized in that The modulation device (36) is formed by a single modulation coil, in particular a single planar coil, arranged in the air gap.

10. The sensor element (30) according to at least one of claims 1 to 9, characterized in that The stripline resonators (44, 46) are designed in a planar manner with a main extension plane which is perpendicular to the direction of the static magnetic flux generated by the polarization device.

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

12. The sensor element (30) according to at least one of claims 1 to 11, characterized in that The sensor element has a ramp coil for generating a ramp function of the static magnetic flux.

13. The sensor element (30) according to at least one of claims 1 to 12, characterized in that The stripline resonators of the resonator arrangement form a multi-line layout with a plurality of parallel lines, wherein each line is formed by a one-dimensional array of stripline resonators.

14. 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 13, and - a test device or a plurality of signal sources with different excitation frequencies, by which the stripline resonators of the resonator device are supplied, - or the testing device has a single signal source of an excitation signal with a plurality of different frequency components, from which the stripline resonator is supplied.

15. The testing device (20) as claimed in claim 14, comprising a transport device which guides the flat data carrier (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 arranged to rapidly transport the flat data carrier to be checked along the transport path (14).

16. 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 13 or a checking device (20) according to claim 14 or 15, 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), wherein a plurality of stripline resonators of the resonator arrangement are arranged one after the other parallel to the transport path, - 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 - Exciting a spin resonance feature (12) of the data carrier (10) to be checked by means of a resonator device (40).

17. The method according to claim 16, characterized in that - guiding the data carrier (10) to be checked past the stripline resonators (44, 46) arranged in succession and recording a time measurement sequence of a response signal of the spin resonance feature (12) generated after excitation by each of the stripline resonators, - respectively identifying measurement data belonging to the same measurement position from the temporal measurement sequence of the stripline resonator, - deriving spectral information about the spin resonance feature (12) from the identified measurement data, and - evaluating the data carrier (10) based on the derived spectral information, in particular with regard to authenticity and / or attribution to a data carrier class.

18. The method according to claim 16 or 17, characterized in that The measured data are either spatially resolved or spatially averaged.

19. Method according to at least one of claims 16 to 18, characterized in that - superimposing a spatially uniform ramp field on the static magnetic flux so that the total static magnetic flux in the air gap (32) varies in time between a minimum value and a maximum value, - deriving spectral information from the identified measurement data taking into account the field strength of the static magnetic flux at the respective measurement time point, and - determining the authenticity of the checked data carrier (10) and / or the affiliation of the checked data carrier (10) to one of a plurality of data carrier categories having different spectral signatures based on the derived spectral information.

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