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

Abstract

The invention relates to a sensor element (30) for testing a flat data carrier (10), in particular a banknote, having a spin resonance feature (12). The sensor element comprises a magnetic core (35) having an air gap, into which the flat data carrier (10) can be inserted in order to be tested, a polarization device (34) for generating a static magnetic flux in the air gap, and a resonator device (40) for exciting the spin resonance feature of the data carrier to be tested in the air gap. According to the invention, the resonator device (40) contains at least two stripline resonators (44, 46) which are designed to operate at different excitation frequencies.

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] When testing a data storage medium, for example, as part of an authenticity check, a spin resonance spectrum of the spin resonance feature is often determined and compared with an expected spectrum based on characteristic features. Typically, spin resonance spectra are recorded using a time-consuming Bo ramp method (also called the Bo sweep method). In this process, the static polarization field Bo is slowly varied around the resonance field strength at a fixed frequency of the excitation field Bi, thus sweeping through the field strength of the polarization field Bo. Since the Larmor frequency of a spin resonance feature to be tested is proportional to the polarization field strength Bo, this effectively shifts the excitation frequency towards the Larmor frequency, allowing the recording of a frequency spectrum of the spin resonance feature.Since the temporal change of the field strength of the polarization field Bo in the Bo-Ramp method is much slower than the temporal change of the modulation field Bmod and the excitation field Bi, Bo is referred to as a static magnetic field or static magnetic flux in the context of this application, even when a ramp field is present.

[0007] However, especially in high-speed banknote processing machines, sensor operation requires short measurement times, which are insufficient to measure the full spectrum of a spin resonance feature with a single ramp (often also called a sweep). Frequency spectra can then only be recorded with a few measurement points, i.e., with low resolution or over a narrow frequency band. For many applications, however, a high-resolution, broadband measurement is desirable, for example, to distinguish between feature substances with different Larmor frequencies. High spectral resolution also allows spin resonance features with a spectral code, for example, for different currencies or different denominations.

[0008] Based on this, the invention is based on the object of specifying an improved device for testing data carriers with spin resonance features, and in particular of providing a sensor element that allows a spectrally high-resolution and / or broadband measurement of the spin resonance of a data carrier to be tested in a short time.

[0009] This object is achieved by the features of the independent claims. Further developments of the invention are the subject of the dependent claims.

[0010] 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.

[0011] The resonator device contains at least two stripline resonators designed and configured for operation 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 operating at different excitation frequencies enables the simultaneous measurement of spin resonance at several different frequencies, thus allowing for higher spectral resolution and / or shorter measurement times. The requirements for a field ramp for measuring a spectral line are also significantly reduced.

[0013] 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.

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

[0015] In another, likewise preferred embodiment, the stripline resonators of the resonator device form a multi-track arrangement with a plurality of parallel tracks, in which each track is formed by a one-dimensional array of stripline resonators.

[0016] 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 arrangement with two or three tracks, each with five stripline resonators, can also be advantageous. Increasing the number of stripline resonators has the advantage of better spectral resolution or a shorter required measurement time.

[0017] In an advantageous embodiment, the stripline resonators of the resonator device all have different resonant frequencies. The resonant frequencies can in particular be distributed equidistantly, so that the resonant frequency for a number N of resonators for the i-th resonator is given by the relationship fi = fo + (il)*Af, with i = 1, ..., N, with a minimum frequency fo and a constant frequency spacing Af. The resonator with the highest frequency has a resonant frequency of fN = fo + (Nl)*Af. For example, the resonant frequencies of any two stripline resonators differ by at least 1%, preferably by at least 2%. To increase the signal-to-noise ratio, several stripline resonators can also be provided at each of the frequencies ß.

[0018] The stripline resonators are advantageously geometrically similar, i.e., they have the same shape but different sizes. In particular, the stripline resonators can have different edge lengths; for example, square stripline resonators with different edge lengths 1 can be provided, or rectangular stripline resonators with different edge lengths 1 X / ly, but same aspect ratio l x / ly, or rectangular stripline resonators with different gaps but the same width. The air gap is advantageously defined by two plane-parallel pole faces of the magnetic core. The magnetic core preferably consists of a ferromagnetic material with a magnetic permeability g at the pole faces. r »1, so in particular |ii greater than 1 X IO 2, but the pole faces can also be made of a paramagnetic material with gr « 1, i.e. in particular g r at most 1+10 2 , are formed. The polarization device advantageously generates a static magnetic flux in the air gap that is substantially equally strong at the location of each of the stripline resonators. In particular, it is provided that the static magnetic flux at the location of the stripline resonators has a maximum deviation of 2%.

[0019] The stripline resonators are advantageously designed with a flat main plane of extension that is plane-parallel to at least one of the pole faces of the magnetic core that define the air gap. The main plane of extension is also 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 plane of extension of the stripline resonators then extends in the xy-plane perpendicular to the z-direction.

[0020] In an advantageous embodiment, the sensor element further comprises a modulation device for generating a time-varying magnetic modulation field in the air gap, wherein the modulation frequency is preferably the same for all stripline resonators of the resonator device. For example, the modulation frequency at the location of any two stripline resonators differs by a maximum of 2%. The modulation device is advantageously formed by a single modulation coil, in particular a single planar coil, arranged in the air gap.

[0021] 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.

[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, which are set up and designed for operation at different resonant frequencies, 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 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 includes either a plurality of signal sources with different excitation frequencies, from which the stripline resonators of the resonator device are fed, or a single signal source with an excitation signal with several different frequency components, from which the stripline resonators are fed. In the latter case, the resonator device expediently includes stripline resonators with different resonance frequencies, each corresponding to one of the frequency components of the excitation signal, so that each of the stripline resonators is excited by the excitation signal with the appropriate frequency component at its resonance frequency. This is explained in connection with Fig.5 is explained in more detail using an example.

[0029] Advantageously, the testing device further includes a transport device that guides the flat data carriers to be tested along a transport path through the air gap of the magnetic core. The transport device is particularly designed and configured 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.The invention also includes a method for testing a flat data carrier, in particular a banknote, with a spin resonance feature by means of a sensor element of the type described or a testing device of the type described, wherein in the method a flat data carrier to be tested is guided along a transport path through the air gap of the magnetic core of said sensor element, wherein a plurality of stripline resonators of the resonator device are arranged one behind the other parallel to the transport path, a static magnetic flux is generated in the air gap by the polarization device and preferably a time-varying magnetic modulation field is generated by a modulation device, and the spin resonance feature of the data carrier to be tested is excited by the resonator device.

[0030] In an advantageous method, the data carrier to be tested is guided past the stripline resonators arranged one behind the other, and a temporal measurement series of the response signal of the spin resonance feature generated upon excitation is recorded by each of the stripline resonators. Measurement data belonging to the same measurement spot is identified from the temporal measurement series of the stripline resonators. Spectral information about the spin resonance feature is derived from the identified measurement data. The data carrier is evaluated based on the derived spectral information, in particular for authenticity and / or belonging to a data carrier class. The measurement data are advantageously spatially resolved or spatially averaged.

[0031] According to an advantageous development of the method, it is provided that a spatially homogeneous ramp field is superimposed on the static magnetic flux, so that the total static magnetic flux in the air gap varies over time between a minimum value and a maximum value, the spectral information is derived from the identified measurement data taking into account the field strength of the static magnetic flux at the respective measurement time, and the authenticity of the tested data carrier and / or the affiliation of the tested data carrier to one of several data carrier classes with different spectral signatures is determined on the basis of the derived spectral information.

[0032] As described, in a preferred embodiment, the stripline resonators are arranged one behind the other along a transport direction of the data carrier. This has the advantage that all resonators measure the same track on the data carrier, i.e., the same measurement points with a certain time offset. This facilitates the evaluation and testing of the data carrier.

[0033] A multi-track setup is also advantageous for generating spatial resolution perpendicular to the transport direction. For this purpose, several tracks are constructed, each with a one-dimensional array of resonators for spectral resolution.

[0034] Further embodiments and advantages of the invention are explained below with reference to the figures, which are not drawn to scale or proportions for the sake of clarity. They show:

[0035] Fig. 1 shows schematically a testing device of a banknote processing system for measuring spin resonances of a banknote test specimen,

[0036] Fig. 2 in the upper part schematically in plan view the resonator device of the testing device of Fig. 1 and the supplied banknote test piece and in the lower part schematically the course of the homogeneous polarization field,

[0037] Fig. 3 the simplified spectrum of a spin resonance unit as a function of the excitation frequency at a fixed value of the polarization field,

[0038] Fig. 4 diagrams illustrating the recording of the spin resonance spectrum of the spin resonance line of Fig. 3, in (a) with a conventional sensor element and in (b) with a sensor element according to the invention,

[0039] Fig. 5 shows a circuit for connecting the resonator device of a sensor element according to the invention with only a single signal branch,

[0040] Fig. 6 the spectrum of the spin resonance feature of a paper sample, and

[0041] Fig. 7 Signal curves when measuring the spin resonance feature of Fig. 6 with a sensor element according to the invention.

[0042] 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 or, as in the illustrated embodiment, can extend over the entire surface of the banknote test specimen.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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 arranged in the air gap 32 generates an excitation field Bi that induces energy transitions between the spin energy levels in the spin resonance feature 12. The resonator device 40 contains at least two stripline resonators operated at different excitation frequencies.

[0047] For this purpose, the test device 20 contains one or more signal sources 22, whose excitation signals are fed, for example, via a duplexer 24, to the resonator device 40, where they generate alternating magnetic fields with two or more different frequencies for the simultaneous measurement of the spin resonance feature 12 at different frequencies. For this purpose, the test device 20 can contain multiple signal sources with different excitation frequencies or just a single signal source 22 with an excitation signal with several different frequency components, which generates alternating magnetic fields with different frequencies due to the special design of the stripline resonators. The excitation field typically has frequencies above 1 GHz and is polarized perpendicular to the z-direction.

[0048] 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.

[0049] To explain the special features of the present invention in more detail, Fig. 2 shows in the upper part of the figure a schematic plan view of a resonator device 40 according to an embodiment of the invention with a carrier 42, with a first stripline resonator 44 arranged on the carrier with a resonance frequency fA and a second stripline resonator 46 arranged on the carrier with a resonance frequency fß.

[0050] As indicated in the lower part of Fig. 2, the polarization device 34 generates a homogeneous polarization field Bo in the air gap 32, so that the field strength of the polarization field is essentially the same at the positions XA and XB of the two stripline resonators 44, 46. Specifically, in the exemplary embodiment, the two pole faces 38 of the magnetic core 35 are plane-parallel to each other and to the plane of the resonator device 40, and the field strength of the polarization field at the locations XA and XB differs by a maximum of 2%.

[0051] The two stripline resonators 44, 46 are arranged one behind the other in the transport direction 14 and are therefore swept over one after the other by the spin resonance feature 12 of the banknote 10 with a time offset. The two stripline resonators 44, 46 are both square in shape, but have different resonance frequencies fA, h due to their different edge lengths.

[0052] To further explain the functioning of the present invention, diagram 50 of Fig. 3 shows the simplified spectrum 52 of a spin resonance line, in this case, for example, the spin resonance line of the spin resonance feature 12 of the banknote 10 as a function of the excitation frequency f at a fixed value of the polarization field B0. Two characteristic spectral components 54A, 54B at the above-specified resonance frequencies fA and fß of the two resonators 44 and 46 are plotted in the curve 52.

[0053] If the resonator device 40 of Fig. 2 is swept over by the banknote 10 with the spin resonance feature 12 at the polarization field strength Bo, each of the two stripline resonators 44, 46 detects the spectral component 54A or 54B of the spin resonance feature 12 belonging to its resonance frequency fA or fß. Specifically, the stripline resonator 44 at position XA measures the spectral intensity Int(fA) of the spectral component 54A and the stripline resonator 46 at position XB measures the spectral intensity Jnt(fß) of the spectral component 54B.

[0054] As already explained in principle above, in a real authenticity test, the static magnetic field Bo of the polarization device 34 is additionally varied around the resonance field strength with the aid of a ramp coil and thus the field strength of the polarization field Bo is passed through at a fixed frequency of the excitation field in order to allow the recording of a frequency spectrum of the resonance of the feature 12.

[0055] The advantage of inventive designs is explained in more detail with reference to diagrams 60, 70 of Fig. 4 using the example of a spin resonance feature 12 with only one spin resonance line. The spin resonance line 62 of feature 12, shown in simplified form in the figures, has, for example, a line width of 10 ml in the space of the polarization field strength, corresponding to the distance from minimum to maximum.

[0056] If a conventional single resonator with a resonance frequency 64 of fo = 8.41 GHz is used to record the spin resonance spectrum, then with a polarization field strength Bo = 300 ml, a field ramp 66 over a range of approximately 40 ml is required for complete detection of the spectral signature at this linewidth, as illustrated in Fig. 4(a). The field ramp 66 can be plotted in the diagrams 60, 70 because, due to the proportionality of the Larmor frequency of the spin resonance feature to the strength of the polarization field Bo, the frequency spectrum of the diagrams 60, 70 simultaneously corresponds to a spectrum in the space of the polarization field strength. Since the Larmor frequency is proportional to the polarization field strength, at a high field strength the fixed excitation frequency is lower than the Larmor frequency, and at a low field strength the fixed excitation frequency is higher than the Larmor frequency. In the embodiment of Fig.4, for example, at a polarization field strength of Bo = 300 mT, the excitation frequency of 8.41 GHz corresponds precisely to the Larmor frequency of the spin resonance feature 12 to be tested. As can be seen from Fig. 4(a), a field ramp 66 with an amplitude of -20 mT to +20 mT must be traversed around the resonance field strength in order to be able to fully measure the spin resonance line 62 with its line width of 10 mT at a fixed excitation frequency. Such a field ramp is associated with long measurement times and high power consumption. If, on the other hand, a resonator device with several stripline resonators of different resonance frequencies according to the present invention is used to record the spectrum, a significantly shorter measurement time and a significantly lower power consumption can be achieved.

[0057] With reference to Fig. 4(b), the resonator device of a sensor element according to the invention contains, for example, five stripline resonators arranged one behind the other at a distance in the transport direction, with the resonant frequencies fA = 7.96 GHz, fr = 8.18 GHz, fr = 8.41 GHz, fr> = 8.63 GHz, and fr = 8.85 GHz. The resonant frequency fr of the central stripline resonator corresponds to the resonant frequency fr of the individual resonator in Fig. 4(a) and precisely to the central frequency of the spin resonance unit 62 at a polarization field strength B0 = 300 mT.

[0058] With the resonator device 40, the intensity at five different frequencies fA to fr can thus be measured simultaneously for a fixed value of the polarization field strength Bo. The frequencies fA to fr are indicated by dashed lines in Fig. 4(b).

[0059] In order to fully measure the spin resonance line 62 with its line width of 10 mT, an additional field ramp 78 is also required here. However, as further shown in Fig. 4(b), a much smaller field ramp 78 with an amplitude of only approximately -3.5 mT to 3.5 mT, corresponding to approximately one-fifth of the amplitude of the conventionally required field ramp 66 in Fig. 4(a), is sufficient to fully capture the spectral signature of the spin resonance line 62.

[0060] Since the polarization field in the air gap is homogeneous and the polarization field strength Bo is therefore the same for all five stripline resonators, the polarization field strength Bo = 300 mT is indicated on the upper axis of diagram 70 at the resonance frequencies fA to fr. The arrows of the field ramp 78 indicate the scanning of the shape of the spectral line 62 by the field ramp 78 with a total amplitude of approximately 7 mT. By simultaneously measuring at five frequencies, the spectral line 62 can be measured with the same spectral resolution, with significantly reduced measurement time and thus significantly lower power consumption.

[0061] In the embodiment shown in Fig. 2, the resonator device contains only two stripline resonators for illustration purposes, but it is understood that a larger number of stripline resonators can also be used, in particular to achieve better spectral resolution, as explained, for example, in connection with Fig. 4.

[0062] In the embodiments described so far, the stripline resonators serve to detect the spectral components of a single spin resonance line. If the banknote 10 or, in general, a data carrier to be verified is equipped with a spin resonance feature that exhibits multiple spectral lines, the resonator device can easily be equipped with additional stripline resonators that detect spectral components of the additional lines.

[0063] The individual stripline resonators of a resonator device according to the invention can be operated using independent signal sources. However, this also requires that the resonators be connected to independent signal branches, which requires a lot of space for circuit implementation, especially with a large number of resonators.

[0064] Figure 5 shows an alternative circuit 80 for connecting a resonator device 40 of a sensor element 30 according to the invention with 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 minimal. For the sake of simplicity, only two resonators 44, 46 are shown in the circuit 80 of Fig. 5. However, by paralleling additional excitation sources and receive mixers, the number of resonators can easily be increased as desired.

[0065] The circuit 80 of Fig. 5 is divided into an excitation circuit 82 and a reception circuit 84. The circuit is further divided into a digital part and an analog part. The separation occurs here with the digital-to-analog (D / A) and analog-to-digital (A / D) converters shown. The digital part of the circuit can advantageously be implemented using an FPGA. This allows additional signal sources and reception mixers to be added in a simple manner. In the analog part of the circuit there is a radio-frequency source 86 which supplies a signal at a carrier frequency which approximately corresponds to the resonance frequencies of the multiple resonators. For example, it can correspond to the mean value of the resonance frequencies, or it can be slightly lower than the lowest resonance frequency. In particular, the carrier frequency is in the GHz range.The FPGA contains several signal sources 22 that operate in the baseband of circuit 80, i.e., at low frequencies, for example, in the range from a few kHz to 1 GHz. Preferably, the frequency of a signal source 22 has a fixed frequency spacing from the resonant frequency of the associated resonator. This frequency spacing is preferably the same for all pairs of signal sources and resonators and is defined by the carrier frequency.

[0066] The signal sources 22 are added, subjected to a D / A conversion, and then up-converted with the high-frequency carrier 86. For clarity, no filter banks are shown in the figure. After signal amplification, the resulting bandpass signal is fed to a circulator 88. If the frequencies of the individual signal sources 22 are tuned to the carrier frequency and the respective resonant frequencies fA, fß, the individual spectral components of the bandpass signal are distributed among the respective resonators 44, 46, since each resonator can only be effectively excited at its resonant frequency.

[0067] For example, the two signal sources 22 located in the FPGA can be operated at 100 MHz and 700 MHz, respectively. Subsequent mixing with a 9.0 GHz high-frequency carrier 86 allows the coupling of a 9.1 GHz and a 9.7 GHz resonator.

[0068] The signal reflected by the resonators 44, 46 is fed to a receiving amplifier via the circulator 88, downconverted (phase shifters are not shown for simplicity), and subjected to an A / D conversion. Further signal processing, such as lock-in detection of the field modulation, can then be performed in the FPGA. The figure shows two evaluation units 90 for evaluating the response signals of the tested spin resonance feature obtained at the excitation frequencies fA, fß.

[0069] In order to demonstrate the operation of the invention, the behavior of a sensor element with a resonator device with two square X / 2 stripline resonators according to Fig. 2 was simulated.

[0070] The stripline resonators 44, 46 are constructed on a 1.5 mm thick printed circuit board 42 with a dielectric constant of 3.66. The resonators 44, 46 are spaced 15 mm apart along the banknote transport direction 14. The edge length of the first resonator 44 is 8.1 mm, corresponding to a resonant frequency of fA = 8.8 GHz; the edge length of the second resonator is 7.1 mm, corresponding to a resonant frequency of fß = 9.8 GHz.

[0071] The two resonators 44, 46 are driven by circulators with independent 50 Ω signal sources, which operate at the respective resonant frequencies with equal power in continuous wave (CW) mode. For coupling to the signal source, the impedance of the resonators is transformed to 50 Ω using a λ / 4 transformer.

[0072] The resonator device 40 constructed in this way is installed in the air gap of a magnetic circuit in which a homogeneous polarization field with a strength of 300 mT is generated.

[0073] Subsequently, a paper sample of length 100 mm was homogeneously loaded over its surface with a spin resonance feature, the spectrum 112 of which is shown in diagram 110 of Fig. 6. The resonance frequencies fA and fß of the two resonators 44, 46 and the corresponding relative signal intensities Int(fA) and Int(fß) are also shown.

[0074] The paper sample with this spin resonance feature is transported through the resonator device 40, and the signal intensity of the spin resonance feature is recorded using the two resonators 44, 46. The resulting signal curves 122A (resonator 44) and 122B (resonator 46) are shown in diagram 120 of Fig. 7, which shows the measured signal intensities as a function of location x. The signal curves were normalized to the average signal intensity of signal curve 122A.

[0075] By averaging the signal intensity in the plateau region of each signal curve 122A, 122B, a ratio of the signal intensity of the first resonator 44 to the signal intensity of the second resonator 46 of 1.0 / -0.85 is obtained, which agrees very well with the ratio Int(fA) / Int(fß) expected from the resonance spectrum of Fig. 6.

[0076] In the arrangements described so far, the stripline resonators of the resonator device are designed such that their resonance frequency lies essentially within the linewidth of the spin resonance line to be measured. However, it is also possible to provide a stripline resonator in the resonator device whose resonance frequency does not coincide with any of the expected Larmor frequencies. Such a resonator can then be used for negative detection, i.e., for a genuine banknote, no detection is required for this resonator.

[0077] Spin resonance signal expected.

[0078] List of reference symbols

[0079] Banknote test specimen

[0080] Spin resonance feature

[0081] Transport path

[0082] Test device

[0083] Signal source

[0084] Duplexer

[0085] detector diode

[0086] Evaluation unit

[0087] Sensor element

[0088] air gap

[0089] Polarization device

[0090] magnetic core

[0091] Modulation device

[0092] Pole faces

[0093] Resonator device

[0094] carrier, 46 stripline resonators

[0095] diagram

[0096] Spectrum of a spin resonance lineA, 54B spectral components

[0097] diagram

[0098] Spin resonance line

[0099] Resonance frequency

[0100] Field ramp

[0101] diagram

[0102] Field ramp

[0103] circuit

[0104] Excitation circuit 84 Receiving circuit

[0105] 86 high-frequency carriers

[0106] 88 Circulator

[0107] HO Diagram 112 Spectrum of the spin resonance feature

[0108] 120 Diagram

[0109] 122A, 122B Signalkurven

Claims

Patent claims 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) contains at least two stripline resonators (44, 46) which are designed and configured for operation at different excitation frequencies.

2. 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. Sensor element (30) according to claim 1 or 2, characterized in that the stripline resonators (44, 46) of the resonator device (40) all have different resonance frequencies.

4. Sensor element (30) according to at least one of claims 1 to 3, characterized in that the stripline resonators (44, 46) are geometrically similar, i.e. they have the same shape but different sizes, in particular different edge lengths.

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

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

7. Sensor element (30) according to at least one of claims 1 to 6, characterized in that the stripline resonators (44, 46) 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.

8. Sensor element (30) according to at least one of claims 1 to 7, 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 at the location of each of the stripline resonators of the resonator device.

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

10. Sensor element (30) according to at least one of claims 1 to 9, characterized in that the stripline resonators (44, 46) are formed planarly with a principal extent plane which is perpendicular to the direction of the static magnetic flux generated by the polarization device.

11. 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. 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. Sensor element (30) according to at least one of claims 1 to 12, characterized in that the stripline resonators of the resonator device form a multi-track arrangement with several parallel tracks, in which each track is formed by a one-dimensional array of 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 different excitation frequencies from which the stripline resonators of the resonator device are fed, or a single signal source (22) with an excitation signal with several different frequency components from which the stripline resonators are fed.

15. Test device (20) according to claim 14, comprising a transport device which guides the planar data carriers (10) to be tested along a transport path (14) through the air gap of the magnetic core, wherein the transport device is advantageously designed and configured for high-speed transport of the planar data carriers to be tested along the transport path (14).

16. 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 or 15, wherein in the method a planar data carrier (10) to be tested is guided along a transport path through the air gap of the magnetic core of said sensor element (30), wherein a plurality of stripline resonators of the resonator device are arranged parallel to the transport path, a static magnetic flux is generated in the air gap by the polarization device (34) and preferably a time-varying magnetic modulation field is generated by a modulation device (36), and the spin resonance feature (12) of the data carrier (10) to be tested is excited by the resonator device (40).

17. Method according to claim 16, characterized in that the data carrier (10) to be tested is passed by the stripline resonators (44, 46) arranged one behind the other, and a time series of measurements of the response signal of the spin resonance feature (12) generated after excitation is recorded by each of the stripline resonators, measurement data belonging to the same measurement spot are identified from the time series of measurements of the stripline resonators, spectral information about the spin resonance feature (12) is derived from the identified measurement data, and the data carrier (10) is evaluated on the basis of the derived spectral information, in particular for authenticity and / or belonging to a data carrier class.

18. Method according to claim 16 or 17, characterized in that the measurement data are spatially resolved or spatially averaged.

19. Method according to at least one of claims 16 to 18, characterized in that a spatially homogeneous ramp field is superimposed on the static magnetic flux, so that the total static magnetic flux in the air gap (32) varies temporally between a minimum value and a maximum value, the spectral information is derived from the identified measurement data taking into account the field strength of the static magnetic flux at the respective measurement time, and the authenticity of the tested data carrier (10) and / or the belonging of the tested data carrier (10) to one of several data carrier classes with different spectral signatures is determined on the basis of the derived spectral information.

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