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

Abstract

The invention relates to a sensor element (30) for testing a planar data carrier (10), in particular a banknote, that has a spin resonance feature (12). The sensor element comprises: a magnetic core having an air gap into which the planar data carrier (10) can be inserted for testing purposes; a polarisation device (34) for generating a static magnetic flux in the air gap; 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) comprises a stripline resonator (46) and a supply structure (44) for the stripline resonator (46), and is designed to generate a high-frequency field with circular polarisation owing to the geometry of the stripline resonator (46) and / or the geometry of the supply structure (44).

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, 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 flat data storage devices are tested.

[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. Conventional stripline resonators generate a linearly polarized high-frequency field, but only a fraction of this energy can be used to excite spin resonance.

[0006] Proceeding from this, the object of the invention is to provide an improved device for testing data carriers with spin resonance features, and in particular to provide a sensor element that allows effective excitation of the spin resonance of the data carriers to be tested.

[0007] This object is achieved by the features of the independent claims. Further developments of the invention are the subject of the dependent claims.

[0008] The invention provides a sensor element for testing, in particular authenticity testing, a flat data carrier with a spin resonance feature. The flat data carrier can be a banknote, for example. The sensor element contains a magnetic core with an air gap into which the flat data carrier can be inserted for testing, a polarization device for generating a static magnetic flux in the air gap, and a resonator device for exciting the spin resonance feature of the data carrier to be tested in the air gap. The spin resonance feature is preferably an ESR feature.

[0009] The resonator device comprises a stripline resonator and a feed structure for the stripline resonator, and is designed by the geometry of the stripline resonator and / or the geometry of the feed structure to generate a high-frequency field with circular polarization, in particular in the near field.

[0010] 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.

[0011] Furthermore, the present invention is based on the observation that conventional stripline resonators in banknote processing machines generate a linearly polarized field with an amplitude Bi. Such a linearly polarized field can always be split into two counter-rotating circularly polarized fields, although only one of the two components is active in the excitation and detection of the spin resonance feature. Therefore, only half of the radio-frequency power is usable for spin excitation; the unused portion leads to increased waste heat and is therefore disruptive.

[0012] In a conventional sensor, therefore, only the appropriately circularly polarized portion of the bi-field experiences absorption in the spin resonance feature under test, while the oppositely polarized portion does not. This results in a low signal-to-noise ratio when using a linearly polarized resonator, especially the one used for excitation, to detect absorption in the feature, since the portion that is not altered by the spin resonance feature, due to its "incorrectly" circularly polarized nature, is always measured.

[0013] The sensor element according to the invention, with a resonator device that generates a circularly polarized field, allows for more efficient excitation and detection of spin resonance. Specifically, the sensitivity can be increased by up to a factor of 2, or the required excitation power can be reduced by a factor of 2.

[0014] In an advantageous embodiment, the resonator device is a passive, circularly polarized resonator device, in particular a resonator device without active electronic phase shifters. Active electronic phase shifters require a lot of energy and space and are therefore not suitable for use in a testing device for testing flat data carriers, especially in banknote processing machines.

[0015] Preferably, the supply line structure of the resonator device contains at most two supply lines for the stripline resonator, since a larger number of supply lines leads to increased signal distortion.

[0016] In an advantageous embodiment, the resonator device comprises a stripline resonator with a conductive structure having a characteristic length 1. The supply line structure contains exactly two supply lines to the stripline resonator and a delay line for generating a phase shift between signals in the two supply lines.

[0017] In another equally advantageous embodiment, the resonator device comprises a stripline resonator with a conductive structure having a characteristic length 1, and the supply line structure contains precisely one supply line to the stripline resonator, which is connected to the stripline resonator at a contact point. The contact point is in particular in the middle of the boundary line between the supply line and the stripline resonator. The stripline resonator is preferably designed symmetrically to at least two axes of symmetry. In particular, a contact axis is defined by the connection between the contact point and the intersection point of the axes of symmetry. Furthermore, the stripline resonator has edges that are preferably at an angle between 40° and 50°, in particular substantially at a 45° angle, to the contact axis.

[0018] Advantageously, the stripline resonator is designed to be flat with a main extension plane which is perpendicular to the direction of the static magnetic flux generated by the polarization device.

[0019] Since in this description the direction of the static magnetic flux is also referred to as the z-direction, the main extension plane or main plane of the stripline resonator extends accordingly in the xy-plane perpendicular to the z-direction.

[0020] The resonator device is advantageously arranged in the air gap such that a flat data carrier introduced for testing is located in the near field of the excitation field generated by the stripline resonator.

[0021] 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.

[0022] 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.

[0023] The stripline resonator 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 backward to the resonator.

[0024] 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.

[0025] The resonator device advantageously comprises a flat carrier on which the stripline resonator and the supply line structure are applied. 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.

[0026] The invention also includes a testing device for testing a flat data carrier, in particular a banknote, with a spin resonance feature having a sensor element of the type described above and precisely one signal source from which the resonator device is fed with a predetermined excitation frequency.

[0027] In an advantageous embodiment of the test device, the resonator device comprises a stripline resonator with a conductive structure having a characteristic length of 1. The supply line structure contains exactly two supply lines to the stripline resonator and a delay line for generating a phase shift between signals in the two supply lines. The two supply lines are fed from the same signal source, and the delay line's delay path corresponds to a phase shift of 90° at the predetermined excitation frequency.

[0028] Advantageously, the testing device further comprises a transport device which brings the flat data carriers to be tested along a transport path into a test position in the air gap or through a test position in the air gap of the magnetic core, wherein the resonator device is arranged in the air gap such that the test position is located in the near field of the excitation field generated by the stripline resonator.

[0029] The transport device is designed and configured in particular for high-speed transport, for example between 1 m / s and 12 m / s, of the flat data carriers to be tested along the transport path.

[0030] The invention also includes a method for testing a flat data carrier, in particular a banknote, with a spin resonance feature by means of a sensor element of the type described or a testing device of the type described. In the method, a flat data carrier to be tested is introduced into the air gap of the magnetic core of said sensor element, a static magnetic flux is generated in the air gap using the polarization device and, preferably, a time-varying magnetic modulation field is generated using a modulation device, and the spin resonance feature of the data carrier to be tested is excited using the resonator device. Advantageously, a response signal of the spin resonance feature generated by the excitation is also recorded by the resonator device and output to a detector.The excitation of the spin resonance feature and / or the recording of the response signal of the spin resonance feature can be carried out using a continuous wave (CW) method, a pulsed method or a rapid scan method.

[0031] Further embodiments and advantages of the invention are explained below with reference to the figures, in which a true-to-scale and true-to-proportion reproduction has been omitted in order to increase clarity.

[0032] They show:

[0033] Fig. 1 shows schematically a testing device of a banknote processing system for measuring spin resonances of a banknote test specimen,

[0034] Fig. 2 schematically shows in (a) a cross-section and in (b) a plan view of a resonator device of the test device of Fig. 1, and

[0035] Fig. 3 in (a) to (d) further embodiments of passive circularly polarized resonator devices in which the feed line structure has only one feed line in each case.

[0036] The invention will now be explained using the example of banknote authentication. Figure 1 schematically shows a testing device 20 of a banknote processing system for measuring spin resonances of a banknote test item 10.

[0037] The banknote specimen 10 has a spin resonance feature 12 to be tested, the characteristic properties of which serve to verify the authenticity of the banknote. For the authenticity test, the banknote specimen 10 is guided along a transport path 14 through a sensor element 30 according to the invention of the testing device 20. To detect spin resonance signatures of the spin resonance feature 12, the sensor element 30 generates three different magnetic fields in the measuring area.

[0038] First, a polarization device 34 generates a static magnetic flux parallel to the z-axis in the measuring area. To generate a strong polarization field, the height of the air gap in the z-direction is advantageously less than 10 mm, in particular even less than 5 mm.

[0039] Second, a modulation device 36 generates a time-varying magnetic modulation field in the air gap, which also runs parallel to the z-axis and has a modulation frequency fMod in the range between 1 kHz and 1 MHz. Finally, a resonator device 40 generates an excitation field in the air gap that induces energy transitions between the spin energy levels in the spin resonance feature 12. The excitation field typically has frequencies above 1 GHz and is polarized perpendicular to the z-direction.

[0040] The frequency of the excitation field is tuned to the Larmor frequency of the spin resonance feature 12 to be detected in order to measure its spin resonance signature and use it for authenticity testing. For this purpose, the test device 20 contains a signal source 22 whose excitation frequency fiaw corresponds to the expected Larmor frequency of the spin resonance feature 12. The excitation signal from the signal source 22 is fed via a duplexer 24 to a resonator device 40, where it generates an alternating magnetic field of frequency fiaw.

[0041] 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 fww, the resonator quality factor changes and thus 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 resulting measurement signal is amplitude-modulated at the modulation frequency.

[0042] In the present invention, the resonator device 40 contains a stripline resonator 46 and a feed structure 44 for the stripline resonator 46, wherein the resonator device 40 is formed by the geometry of the stripline resonator 46 and / or the geometry of the feed structure 44 for generating a high-frequency field with circular polarization.

[0043] For further explanation, Figure 2 schematically shows in (a) a cross-section and in (b) a plan view of such a resonator device 40. The resonator device 40 comprises a flat carrier, for example in the form of a printed circuit board 42, with an upper side facing the test object 10 during the measurement and an underside facing away from the test object.

[0044] A conductive structure with a characteristic length 1 is arranged on the top side of the circuit board 42 as a stripline resonator 46, and a ground plane 48 is arranged on the bottom side. The supply line structure and the coupling of the conductive structure 46 are not shown in the schematic cross-section of Fig. 2(a) for the sake of clarity.

[0045] As already explained above, conventional stripline resonators generate a linearly polarized magnetic field, of which only a portion, namely one of the two counter-circularly polarized components, can be used for spin excitation of a spin resonance feature. This unused component leads to increased heat generation and a lower signal-to-noise ratio during detection, since—when using the same resonator for excitation and detection—the inactive circularly polarized component is always measured. Significantly improved detection can be achieved by using a resonator device according to the invention for generating a circularly polarized magnetic field.

[0046] With reference to the top view of Fig. 2(b), the resonator device 40 of the exemplary embodiment comprises a stripline resonator 46 as a conductive structure of characteristic length 1 and a feed line structure 44 with two feed lines and a passive phase shift. The two signal branches are phase-shifted by 90° via a delay line of length A = X / 4, so that a circularly polarized excitation field Bi is generated in the stripline resonator 46. X is the wavelength of the excitation signal generated by the signal source 22.

[0047] Figure 3 shows further advantageous embodiments of passive circularly polarized resonator devices, in which the supply line structure has only a single supply line and in which the circular polarization of the generated excitation field arises because the stripline resonator has edges that are inclined to the supply line. In particular, the supply line is connected to the stripline resonator at a contact point 100, wherein the stripline resonator is formed symmetrically to at least two axes of symmetry 56, wherein a contact axis 101 is defined by the connection between the contact point 100 and the intersection of the axes of symmetry 56, and wherein the stripline resonator has edges that are substantially at a 45° angle to the contact axis 101, for example, at an angle between 40° and 50°.For illustration purposes, the stripline resonator in these embodiments is formed on the basis of a square microstrip resonator 50 with a side length of 1.

[0048] Figure 3(a) initially shows a resonator device 70 in which a stripline resonator 46 with a single supply line 52 is arranged on a circuit board 42. The stripline resonator 46 is formed by a square microstrip resonator 50 with a diagonal, central recess 54, which is at a 45° angle to the supply line 52. The axes of symmetry 56 intersect in the center of the recess 54. In this example, the contact axis 101 corresponds to the axis of the supply line 52 and is at a 45° angle to the edges of the recess 54.

[0049] In the resonator device 72 according to the embodiment of Fig. 3(b), the stripline resonator 46 is formed by a microstrip resonator 50 in which two diagonally opposite corners 58 of a complete square are truncated. The symmetry axes 56 of the stripline resonator 46 intersect at the center of the structure. In this example, the contact axis 101 thus corresponds to the axis of the supply line 52 and is at a 45° angle to the beveled edges 58.

[0050] In the resonator device 74 according to the embodiment of Fig. 3(c), the square stripline resonator 50 contains a via 60 on the diagonal of the square for signal coupling through the back of the circuit board 42. This via does not form a symmetry-breaking element, but rather defines the contact point 100. The stripline resonator 50 has four axes of symmetry 56 that intersect at the center of the square. The contact axis 101 corresponds to a diagonal of the square and is therefore at a 45° angle to its edges.

[0051] In the resonator device 76 of the embodiment shown in Fig. 3(d), the single lead 52 is coupled to a corner 62 of a square stripline resonator 50. The stripline resonator 50 has four axes of symmetry 56 that intersect at the center of the square. The contact axis 101 corresponds to a diagonal of the square and is therefore at a 45° angle to its edges.

[0052] The designs shown in Fig. 3, which have only a single supply line, have the advantage over a design with two supply lines according to Fig. 2 in that fewer signal conductors are located on the top side of the structure, thus suppressing potential distortions in the phase behavior of the signal feed. To demonstrate the advantages of sensor elements according to the invention with a circularly polarized resonator device, the behavior of a sensor element with a square λ / 2 stripline resonator according to Fig. 2 was simulated.

[0053] The stripline resonator is mounted on a 1.5 mm thick circuit board with a dielectric constant of 3.66. The edge length of the resonator is 7.1 mm, corresponding to a resonant frequency of 9.8 GHz. The resonator is coupled via the feed line structure shown in Fig. 2. For this purpose, the resonator impedance was transformed to 100 Ω at two perpendicular edges using λ / 4 impedance transformers. The signal in one of the two lines is delayed by a phase difference of 90° using an additional conductor. The subsequent parallel connection of the two feed lines results in an input impedance of 50 Ω. The resulting stripline resonator generates an excitation field with circular polarization via the coupling from two sides and the 90° delay line.

[0054] As a comparison example, the behavior of a comparison stripline resonator was simulated, which has the same geometry as the resonator according to the invention, but is coupled to the signal line at only one edge without a delay line and thus generates a linearly polarized excitation field in a conventional manner.

[0055] The resonator device according to the invention shown in Fig. 2 has a Q factor that is approximately 15% lower than the reference resonator. According to current understanding, this lower Q factor is due to losses and imperfections in the coupling network.

[0056] In order to quantify the detection properties of the resonator device according to the invention and the reference resonator, a spin resonance measurement was subsequently simulated with both resonator devices. For this purpose, the resonator devices were each placed in the air gap of a permanent magnetic circuit. Also located within this air gap were a paper sample loaded with a spin resonance feature and a planar modulation coil for generating the modulation field. The spin resonance feature was designed such that the spin transitions were excited with an excitation field Bi that was right-handed circularly polarized with respect to the Bo flux direction.

[0057] The resonator device shown in Fig. 2 was introduced into the air gap in two different orientations during the simulation: one in a manner that generates a left-handed polarized field relative to the Bo flux direction, and the other in a manner that generates a right-handed polarized field. Due to the aforementioned design of the spin resonance feature, significant excitation of spin transitions is expected only in the latter orientation.

[0058] The spin resonance signal of the paper sample was recorded using each of the three sensor elements constructed in this way. As a result, the sensor element equipped with the inventive resonator device configured for right-handed polarization obtained a signal 1.27 times higher than the sensor element with the linearly polarized reference resonator. The fall short of the maximum possible intensity increase by a factor of 2 « 1.41 is, according to current understanding, due to the somewhat lower Q of the circularly polarized resonator.

[0059] With the resonator set up for left-handed polarization, which is not adapted to the spin resonance of the paper sample, a nearly vanishing signal amplitude was measured, as expected.

[0060] 10 banknote test specimen

[0061] 12 Spin resonance feature

[0062] 14 Transport path

[0063] 20 Test device

[0064] 22 Signal source

[0065] 24 duplexers

[0066] 26 detector diode

[0067] 28 Evaluation unit

[0068] 30 sensor element

[0069] 34 Polarization device

[0070] 36 Modulation device

[0071] 40 Resonator device

[0072] 42 circuit board

[0073] 44 Supply line structure

[0074] 46 stripline resonator

[0075] 48 Ground plane

[0076] 50 square microstrip resonator

[0077] 52 supply line

[0078] 54 diagonal central recess

[0079] 56 axes of symmetry

[0080] 58 diagonally opposite corners

[0081] 60 vias

[0082] 62 Corner

[0083] 70, 72, 74, 76 Resonator devices

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 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, and a resonator device (40) for exciting the spin resonance feature of the data carrier to be tested in the air gap, characterized in that the resonator device (40) comprises a stripline resonator (46) and a feeder structure (44) for the stripline resonator, and is designed by the geometry of the stripline resonator (46) and / or the geometry of the feeder structure (44) to generate a high-frequency field with circular polarization.

2. Sensor element (30) according to claim 1, characterized in that the resonator device (40) is a passive, circularly polarized resonator device, in particular a resonator device without active electronic phase shifters.

3. Sensor element (30) according to claim 1 or 2, characterized in that the lead structure of the resonator device (40) contains at most two leads for the stripline resonator, 4. Sensor element (30) according to at least one of claims 1 to 3, characterized in that the resonator device (40) as a stripline resonator (46) comprises a conductive structure with a characteristic length 1, and that the feeder structure (44) has exactly two feeders to the stripline resonator and a It contains a delay line for generating a phase shift between signals in the two feed lines.

5. Sensor element (30) according to at least one of claims 1 to 3, characterized in that the resonator device (40) comprises a conductive structure with a characteristic length 1 as a stripline resonator, and that the lead structure contains exactly one lead to the stripline resonator, which is connected to the stripline resonator (46) at a contact point (100), wherein the stripline resonator (46) is symmetrical about at least two axes of symmetry (56), wherein a contact axis (101) is defined by the connection between the contact point (100) and the intersection of the axes of symmetry (56), and wherein the stripline resonator (46) has edges which are formed at an angle between 40° and 50° to the contact axis (101).

6. Sensor element (30) according to at least one of claims 1 to 5, characterized in that the stripline resonator (46) is designed planar with a principal extent plane which is perpendicular to the direction of the static magnetic flux generated by the polarization device.

7. Sensor element (30) according to at least one of claims 1 to 6, characterized in that the resonator device (40) is arranged in the air gap such that a planar data carrier introduced for testing is located in the near field of the excitation field generated by the stripline resonator.

8. Sensor element (30) according to at least one of claims 1 to 7, characterized in that the resonator device (40) is designed to excite spin resonance signals with a frequency above 1 GHz, in particular between 1 GHz and 10 GHz.

9. Sensor element (30) according to at least one of claims 1 to 8, characterized in that the air gap has a height of less than 10 mm, preferably less than 5 mm.

10. Test device (20) for testing a planar data carrier, in particular a banknote, with a spin resonance feature having a sensor element (30) according to one of claims 1 to 9, and exactly one signal source (22) from which the resonator device (40) is fed with a predetermined excitation frequency.

11. Test device (20) according to claim 10, characterized in that the resonator device (40) as a stripline resonator (46) comprises a conductive structure with a characteristic length 1, and that the feeder structure (44) contains exactly two feeders to the stripline resonator and a delay line for generating a phase shift between signals in the two feeders, and that the two feeders are supplied from the same signal source (22), and the delay path of the delay line corresponds to a phase shift of 90° at the predetermined excitation frequency.

12. Test device (20) according to claim 10 or 11, with a transport device which places the planar data carriers (10) to be tested along a transport path (14) into a test position in the air gap or passes through a test position in the air gap of the magnetic core, wherein the resonator device is arranged in the air gap such that the test position is located in the near field of the excitation field generated by the stripline resonator.

13. Test device (20) according to claim 12, characterized in that the transport device is designed and set up for a high-speed transport of the planar data carriers to be tested along the transport path (14).

14. 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 9 or a test device (20) according to one of claims 10 to 13, wherein in the method a planar data carrier (10) to be tested is introduced into the air gap of the magnetic core of said sensor element (30), a static magnetic flux is generated in the air gap with the polarization device (34) and preferably a time-varying magnetic modulation field with a modulation device (36), and the spin resonance feature (12) of the data carrier (10) to be tested is excited with the resonator device (40).

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