NL1016416C2 - Measuring and testing device for radio frequency identification labels, measures magnetic dipole moment of label response signal as function of investigation signal - Google Patents

Abstract

The device measures the size of the magnetic dipole moment of the response signal from an inductive radio frequency identification (RFID) label (12) as a function of an investigation signal with a specific frequency and magnetic field strength. An Independent claim is also included for a measuring and testing device for RFID labels which measures the size of the electric dipole moment of the response signal from an inductive RFID label as a function of an investigation signal with a specific frequency and electric field strength.

Description

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Measuring device for testing RFID labels.

5 RFID (Radio Frequency IDentification) labels are labels that are used for a wide range of applications, such as identifying goods, people, animals, respectively in the service of logistics, access control, livestock management, etc. Shoplifting detection using resonant labels is also covered under this category.

In RFID, a large majority uses magnetic coupling between the interrogation unit (reading station) and one or more labels. These systems are also called inductive identification systems. However, coupling by means of the electric field (capacitive systems), or by means of electromagnetic waves (EM systems), is also possible.

The measuring device according to the invention is mainly focused on testing RFID labels that use magnetic (inductive) coupling.

The principles on which the measuring device according to the invention is based are, however, to a large extent also applicable for capacitive systems and for EM systems. With the aid of some variations in the design of the measuring device, the measuring device according to the invention can also be used for measuring on labels for capacitive systems and for EM systems. To the extent applicable, these applications and associated variations are understood to fall within the scope of the invention.

Figure 1 shows the basic circuit of an inductive RFID tag.

Figure 2 shows the link from an RFID tag to the interrogator.

Figure 3 shows the resonance curves of the resonance circuit in an RFID tag.

Figure 4 shows the vector diagram of the response signal of an RFID tag as a result of stimulation by an interrogation field, and the relationship between the resonance frequency of the tag circuit and the frequency of the interrogation signal.

101 64 1 δ "" - 2 -

Figure 5 shows the black box representation of an RFID label with its interrogation signal and response signal.

Figure 6 shows a test arrangement for RFED labels according to the existing technique.

Figure 7 shows an example of a test arrangement for RFID labels according to the invention. Figure 8 shows the principle of a measuring arrangement for capacitive labels.

Figure 9 shows an example of a realization of a measuring arrangement for capacitive labels.

Figure 10 shows the principle of a measurement setup for EM labels.

Figure 11 shows the principle of an alternative measurement setup for EM labels.

Figure 12 shows the basic electrical circuit of the RFID label measuring device according to the invention.

Figure 13 shows the basic electrical circuit of the RFID label measuring device according to a variant of the invention.

Figure 14 shows the spectrum that occurs with RFID labels, which use a subcarrier, which is ASK or PSK modulated with data stored in the label.

Figure 15 shows the basic electrical circuit of the RFID label measuring device according to a second variant of the invention.

Figure 16 shows the spectrum that occurs with RFED labels that use a subcarrier that is FSK modulated with data stored in the label.

Figure 17 shows the basic electrical circuit of the RFID label measuring device according to the invention for electromagnetic labels.

The basic component of an RFID tag is formed by a resonant circuit as shown schematically in Figure 1.

Resonator 1 is composed of electrical components such as coil 2, capacitor 3. Resistance 4 is usually not present as a separate component, but represents the electrical losses in the coil and in the capacitor.

Resonator 1 can also consist of non-electrical components. An example of this is the magneto-acoustic label. A metal plate resonates in a mechanical vibration, S - “· '* ·,. wherein a magnetic coupling takes place by means of the magneto-strictive effect under the influence of a second plate which provides for a magnetic bias field.

In this case, the components coil 2, capacitor 3 and resistor 4 provide an electrical replacement diagram that represents the physical effects of the magneto-acoustic label. 5

Figure 2 shows the coupling from the label to the interrogation unit. Transmit coil 5 generates a primary magnetic field 6 of the magnitude Hprim as a result of the current in coil 5 of the magnitude Itx. If the label is introduced into the primary magnetic field 6, an electro-motor force will be produced in the coil 2 of the label. of inductance Vinii 10 is generated, represented in Figure 2 by voltage source 7.

For Vind applies:., DHprim

Find ~ ~ μ dt (J) where μ is the magnetic permeability, and Hprim the field strength at the label, where the axis of coil 2 coincides with the direction of the magnetic field.

From formula (1) it follows that in case Hprim is a sinusoidal function of time t with (Hprim) / ™ f being the amplitude, Find is also a sinusoidal function of time, but is 90 ° in phase lag with respect to Hprim.

Either as:

Hprim (t) (Hprim) max cos (lftfct) (2) then 20 is:

Find (t) = qM (Hpriu max COS (2jtfct - 71 f2.) (3)

Find (0 = (Hprif ^ max SlTl (2.Jtf ct) (4)

The induction voltage Vind runs a current Itabel in the series circuit consisting of coil 2 with inductance L \, capacitor 3 with capacitance C1} and serial loss resistor 4 the size of. The magnitude and phase of Iiabei depends on the frequency fc of the interrogation signal and on the resonance frequency / 0 of the label circuit. We can write: Λ ii bi '' '· "a - 4 - 1label _ _ \ _ _ J_ 1 V'w * <1 +! Q & -u>) X *' 1 + Λ2ν (5) with _ fc fo V ~ Fo ~ 7c (6) 5 and e = to

The following applies to the absolute value of the current Iim: I label ^ v ~ i ~ VoTëV) <8)

The following applies to the phase angle between the current fable and the voltage Find:, 0 “* fê) = -arctg (öv) rn

Relation (8) gives the known resonance curve, as shown in the upper part of Figure 3. In the lower part, the phase relationship between Ilabel and Vind is drawn. If the frequency of the interrogation signal fc is equal to the resonance frequency f 0, thus v = 0 according to relation (6), then the phase difference between Ilabet and Vind is zero. For fc <f0, so v <0, the phase difference is between 0 and 90 degrees, so the current // aèe / through the label coil 2 is ahead of the induction voltage Vim /. For fc> f0, so v> 0, it is phase difference between 0 and -90 degrees. I! Abet is therefore lagging behind Vind.

However, the induction voltage Vind is already 90 degrees behind the primary magnetic field Hprim, so that the phase difference between Itabei and Hprim is between 0 ° and -180 °. The scale for the phase difference between ΙιΜ and Hprim is drawn on the right-hand side of the lower part of Figure 3.

a - 5 -

In figure 4, the vector diagram is drawn with the vectors Hprim, Vm, and Itabel - in which the direction of Vind is fixed (with respect to vector but that of I 1 ^ 1 is dependent on the frequency. The direction of 11 ^ 1 coincides with that of Find if the frequency of the interrogation signal is equal to the resonance frequency of the label.

In figure 4 circle 8 is also drawn. This circle is the geometric location of all possible vectors Ilabel as a function of the standardized frequency v.

Arrow 9 indicates the direction in which the circle 8 is traversed if the frequency is varied from low to high. The points v = and v = ^ indicate the frequencies at which the phase angle is -45 ° and -135 °. The amplitude there is 1 / V2 times the peak value, or 10 -3 dB. The frequency difference between the two points is called the -3 dB bandwidth and has the size

The current Ilabel through coil 2 of the label causes a secondary magnetic field 10, the size Hsec, in the same phase with Ilabel. This secondary field is the response of the label to the stimulus of the label by the primary field Hprim, and induces a voltage in receiving coil 11 in Figure 2.

The strength of the secondary field depends on the location in the space relative to the label, and is therefore not an unequivocal parameter representing the response of the label. However, because the label generally has small dimensions relative to the distance between the transmitter coil 5 and the label, the shape of the magnetic field of the label can be sufficiently well represented by the field of a magnetic dipole. The magnitude of the field strength at each point in the space can then be calculated from the magnetic dipole moment, m, of the label. The magnitude is given by: 25 m = NΟΙΐΜ (10) where N is the number of turns of label reel 2, and O is the area covered.

The relationship between label response and stimulus is now given by: m

Hprim 101641Gm d - 6 -

The magnitude of this ratio is given by the effective (magnetic) volume: ,, m v-eff = 77— (11) H-prim

The phase relationship is given by: “* Ê) = -m ** m - § 5

The label described above is a passive label as used in theft detection systems. If the tag is used for strict identification, where a number or other information is stored digitally in the tag, the response signal is modulated with the relevant information. That is, one or more parameters of the response signal is or become varied over time. This type of labels is referred to here as the active labels.

With regard to the response signal, two parameters for modulation are available: • the amplitude, • the phase.

From equations (8), (10) and (11) it follows that the amplitude is determined by Q, and since Rs is a loss resistance, the effect thereof is also expressed in Q according to formula (7), so that there is a direct and There is a linear relationship between v_eff and Q.

Amplitude modulation is thus generated with the help of modulation of Q. One way to realize that technically is to connect an additional damping resistor in parallel with tuning capacitor C1.

According to formulas (12) and (6), phase modulation is generated by modulating the resonance frequency of the label circuit, f0. This can be achieved by making the tuning capacitor C1 variable, for example by connecting an additional capacitor in parallel.

d - 7 -

The data can be modulated directly on the response signal (NRZ), via a channel coding such as Manchester coding, or via a subcarrier, which can be modulated with data in a large number of ways, as is known to those skilled in the telecommunications art.

5

The practical application places functional requirements on the label. These functional requirements therefore determine the specification, measurement and testing of labels; the technical solutions with which the label is realized is not important for measuring, and therefore not for the invention.

The task for testing and measuring on RFID labels is to measure the response of a label to the applied stimulus, whereby the label is considered as a black box. Figure 5 illustrates this approach. Box 12 indicates the label to be tested, whereby the label is driven with stimulus 13, and the label responds with response 14. To stimulus 13 a number of parameters of the stimulus or interrogation signal can be coupled. Examples are: the carrier sequence fc, the field strength Hprim, and the modulation depth md.

Parameters can also be linked to the response signal 14: magnetic dipole moment m, phase of the response signal relative to the interrogation signal 0, subcarrier frequency fsub, magnetic dipole moment of a subcarrier component 20 tftsubi etc.

An RFID tag can spontaneously start sending the response signal as soon as it enters the interrogation field of the reading station. This type of label is called: Tag Talk First. However, some types of labels only start sending a response signal when they are invited to do so by the reading station by means of a wake-up signal: Reader Talk First. In particular, these labels are used in systems in which it is possible that several labels may be present simultaneously in the interrogation field. Such a wake-up signal is then part of an anti-collision protocol, which prevents multiple labels from simultaneously transmitting their information and thereby disturbing each other.

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The consequence for a label measuring device is that it must be possible to modulate the interrogation signal with the wake-up signal. More generally, it means that the measuring device must be able to execute the communication protocol that applies to the label to be tested.

In all cases this concerns amplitude modulation (Amplitude Shift Keying or On-Off 5 Keying) of the interrogation signal, since only this modulation form can be demodulated properly in the circuit of the label.

An RFID tag, which includes an electronic circuit for modulating the response signal, an active tag, will generally have a lower limit for the level of the interrogation signal. This is due to two aspects: firstly because the interrogation signal often provides the energy supply for the functioning of the electronic circuit, and secondly because the interrogation signal is used as a clock signal for the internal digital circuit.

On the other hand, there is an upper limit of the level of the interrogation signal, whereby the electronic circuit no longer functions properly, or is even damaged.

In summary, the following requirements are set with regard to the generation of the interrogation signal: • The field of the interrogation signal must be homogeneous at the location of the label to be tested.

• The carrier frequency must be adjustable.

• The amplitude must be adjustable.

The amplitude must be capable of being calibrated as a magnetic field strength for the inductive labels, as an electric field strength for the capacitive labels, and as a field strength in a test cell in EM systems.

• The amplitude must be modular with adjustable modulation depths, and the modulation must be programmable according to the communication protocols used in the labels.

The following parameters of the response signal can be measured: - 9 - • The amplitude (the magnetic dipole moment for the inductive labels, the electric dipole moment for the capacitive labels, and the radiated power in EM systems). For inductive and capacitive systems, this means that the field of the measuring antenna must be homogeneous at the location of the label to be tested; 5 for EM labels means that the labels must be located in the far field area.

• The phase in relation to the interrogation signal.

• The modulation depth in direct modulating labels, or • the amplitude of subcarrier sidebands (the magnetic dipole moment for the inductive labels, electric dipole moment for the capacitive labels, and the radiated power in EM systems).

• The effective magnetic volume and Q factor with passive labels.

A number of specific requirements are set for the measurement setup of a label measuring device. Thus, it must be possible to measure the response signal without this measurement being influenced by the presence of interrogation signal, although both signals are of the same carrier frequency.

The measuring arrangement must be such that the labeling circuit is not affected by the measuring arrangement, in particular the coil that generates the magnetic interrogation field, and the coil that picks up the response signal, must be coupled so weakly to coil 2 in the label that the label resonance circuit this does not cause additional damping.

An example of the existing technology is the test method as described in the international standard Identification cards - Test Methods - Part 7: Vicinity cards, ISO / IEC 25 FCD 10373-7.

Figure 6 shows the measurement setup. Placed on three non-conductive printed circuit boards 15, 16, and 17 are respectively a measuring antenna coil 18, an antenna coil 19 generating the interrogation field, and a compensation antenna coil 20 identical to the measuring antenna coil 18 but placed on a printed circuit board 17 such that it is a mirror image forms of measurement antenna coil 18 30 relative to interrogator antenna coil 19.

101841 Sa - 10 -

The label 12 to be tested is placed within measuring antenna coil 18. Thus, it is in the magnetic field of antenna coil 19 that carries the current of the interrogation signal. Induction voltage Vimi is induced in coil 2 of the label in accordance with Figure 2.

The current induced in coil 2, whether or not modulated with a data signal, generates the secondary magnetic field Hsec, which in turn induces a voltage in measuring antenna coil 18.

The essence of this arrangement is that a voltage induced in the measurement antenna coil 18 as a result of the interrogation field generated by antenna coil 19 is compensated and supplemented by an opposite voltage generated in compensating antenna coil 20. Since the label 12 to be tested mainly couples to measuring antenna coil 18 and not with compensation coil 20, a voltage generated in measuring antenna coil 18 as a result of the response signal from label 12 will not be compensated by compensating antenna coil 20, 15 and thus this voltage will be measurable in the sum signal of the measuring antenna coil and compensation coil.

The disadvantages of this arrangement are threefold. Firstly, the positioning of the printed circuit boards 15 and 17 relative to the printed circuit board 16 is too critical to obtain a reliable compensation for the induction of the primary magnetic field in the measuring antenna 18. All the more so because the label may contain metal components that incorporate this compensation in to a large extent.

Secondly, measuring antenna coil 18 is placed closely around the label to be tested. The resulting strong coupling on the one hand ensures a strong induction voltage in measuring antenna coil 18, but also makes this coupling highly dependent on the current dimensions of coil 2 in the label. As a result, the relationship between the magnetic dipole moment of the response signal of the label and the voltage induced in measuring antenna coil 18 is uncertain and already very different for labels with only small deviating dimensions of the label coil 2.

if O '<:' · 'ti W 1 v / Λ *

However, a strong coupling is desirable to solve the aforementioned tolerance problems in compensating for the induction voltage due to the primary magnetic field.

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A third disadvantage of this arrangement is also caused by the strong coupling between label coil and measuring antenna coil. This coupling, which is mainly magnetic in nature, but which also has a capacitive element at high frequencies and a short distance, causes an influence on the label properties due to the load due to the measuring antenna coil.

This measuring arrangement is therefore intrinsically unsuitable for measuring the magnetic dipole moment of the response signal from labels, and therefore does not meet the most basic design requirement for a label measuring device.

Figure 7 shows a measuring arrangement according to the invention. A circular or square antenna coil 21 is arranged in combination with a circular or square measuring antenna coil 22, such that the centers of both coils coincide, and that the coils are perpendicular to each other. In addition, coil 22 has a slightly smaller size than coil 21, so that coil 22 falls entirely within coil 21.

Platform 23 carries the label 12 to be tested, wherein the label makes an angle of 45 degrees with both antenna coil 21 and measuring antenna coil 22.

Antenna coil 21 generates the primary field Hprim (6), the field lines of which, as shown in Figure 7, run symmetrically on either side of the plane of measuring antenna coil 22, but do not intersect that plane. Thereby no voltage is induced in coil 22. The field lines do pass through the label 12 to be tested, and the induction voltage Find is induced in coil 2 of the label in accordance with Figure 25. The current induced in coil 2, whether or not modulated with a data signal, generates the secondary magnetic field Hsec (10), which intersects the plane of measuring antenna coil 22 and induces a voltage in it.

Since the dimensions of the coil 2 in the label to be tested are small with respect to the interrogator antenna coil 21, its field at the location of the label is practically homogeneous. The dimensions of the coil 2 in the label to be tested are also small with respect to the measuring antenna coil, so that the coupling factor with the measuring antenna coil is practically proportional to the area covered by label coil 2. Since the magnetic dipole moment of the response signal is also proportional to this area, a good measurement of the magnetic dipole moment is possible with this measuring arrangement, even if the dimensions of a label coil are not clearly defined.

Also, due to the difference in size, the coupling of the label coil with both interrogator antenna coil 21 and measurement antenna coil 22 is so small that influence of the label properties by these antenna coils is practically negligible.

Capacitive RFID labels make use of the capacitive coupling between interrogation station and label, i.e. use is made of electric fields instead of magnetic fields as coupling medium. To this end, the label is provided on the outside with two capacitor plates 24 as indicated in figure 8. Two large capacitor plates 25, fed by interrogation signal source 26, generate a vertically directed homogeneous electric field Eprim, 27.

A second set of capacitor plates 28 is arranged symmetrically and perpendicularly to capacitor plates 25. As a result, the interrogation field 27 will not induce voltage between plates 28. Both sets of capacitor plates are located on cube 29, as shown in Figure 9. The label 12 with capacitor plates 24 to be tested arranged in the center of the cube, such that the electric dipole formed by the capacitor plates 24 is located in the plane perpendicular to both sets of capacitor plates 25 and 28, and directed at an angle of 45 degrees with the directions of capacitor plates 25 and 28, as indicated in Figures 8 and 9.

In this measurement arrangement according to the invention, the RFID tag to be tested is located in a homogeneous electric field that transmits the interrogation signal, and it also couples homogeneously to the measurement capacitor plates 28. In this arrangement, it is thus possible by means of e.g. the receiver, tuned to the relevant (subcarrier) frequency component and represented by voltmeter 30, to measure the response signal of the label to be tested as an electrical dipole moment.

4 - 13 -

Electro Magnetic (EM) labels use propagating radio waves (EM waves) for the connection between the interrogation station and label. Typical frequency bands for such RFID systems are 900 MHz (UHF) and 2450 MHz (Microwave).

The essential parameters for describing the functional behavior of an EM tag 5 are for the interrogation signal frequency, field strength at the tag, and modulation depth; for the response signal, that is the effectively radiated power in the desired direction of a subcarrier component per sideband.

Also measuring on EM labels is a system of mutually disconnected interrogation signal transmitting antenna and a measuring receiving antenna. A realization thereof is given in figure 10. Interrogator antenna dipole 31, arranged vertically, emits a vertically polarized EM wave 32. Measuring / receiving antenna 33, arranged horizontally, is sensitive to horizontally polarized waves, and will therefore not receive the interrogation signal.

The label 12 to be tested, provided with a dipole antenna 34, is arranged at an angle of 45 degrees with the vertical axis, and in a plane perpendicular to the propagation direction 15 of the EM wave. The interrogation signal is thus received, although it is weakened by 3 dB due to the 45 degree skew. This calibration can be corrected in the calibration of the measuring device.

An electrical vibration in the dipole antenna of the label to be tested generates a secondary EM wave with a polarization direction, which also has an angle of 45 degrees with respect to the vertical direction, and parallel to the dipole antenna of the label to be tested. Measuring antenna 33 does receive this signal, although with a further 3 dB attenuation. Assuming that the distance between antennas 31 and 33 on the one hand, and the label to be tested on the other, is known, it becomes possible to use m.b.v. the receiver, tuned to the relevant (subcarrier) frequency component and represented by voltmeter 30, to measure the power actually emitted by the label to be tested in the direction of measuring antenna 33.

The antenna in the label does not necessarily have to be a dipole. All other forms of EM antennas can be used, for example also patch antennas, even if they are sensitive to circularly polarized EM waves, and also if they emit circularly polarized EM waves.

An alternative to the two crossed dipole antennas 31 and 33 is a patch antenna with two orthogonal feeding points. Each feeding point related to one linear 1016416a - 14 - polarization direction, with both polarization directions perpendicular to each other. Such a type of patch antenna with two orthogonal feeding points is normally used to transmit circularly polarized EM waves, but can also be used as a duplexer circuit. In Fig. 11, the interrogation signal 5 to be transmitted is applied to a first supply point 35 of patch antenna 36, the interrogation signal 32 being transmitted, for example, vertically polarized. The feedback signal received is at supply point 37, while the interrogation signal is suppressed at this supply point.

Figure 12 provides an interpretation of the circuit of the label measuring device according to the invention.

Part 38 therein is the measuring arrangement for the inductive labels. Interrogation antenna coil 21 is driven by an interrogation signal generated by the first DDS (Direct Digital Synthesizer) 39. The label response signal received by the measurement antenna coil 22 is correlated in the product detectors 40 and 41 with the interrogation signal itself in first product detector 40 (I reference signal), which yields the first measurement signal I (t), and with the Q reference signal, which is 90 degrees delayed compared to the interrogation signal (second product detector 41), which produces the second measurement signal Q (t).

20

If the response signal consists of an unmodulated carrier, such as with a passive EAS label, a phase-selective measurement of the response signal takes place in this way. If the resonance frequency of the tag is equal to the frequency of the interrogation signal, the phase of the response signal will be 90 degrees in phase behind the interrogation signal. As a result, the output of first product detector 40 will remain at zero volts, but that of second product detector 41 will give an output signal proportional to the amplitude of the response signal.

More generally: 30 If the response signal is represented by: -ij r5 ·· = v c / l 'V.' i; R (t) = A (t) cos (2 nfct + <p (t)) (13) with A (t) the amplitude of the response signal and 0 (t) the phase with respect to the interrogation signal, then with path filtering of the double frequency components: I (t) = A (t) cos {2nfct) cos (2jrfct + φ (/)) => A (t) cos (φ (ή) (14) 5 And: Q (t) = A (t) sin (2 nfct) cos (2 yrfct + φ (ή) => A (t) sin (φ (t)) (15)

This means that with an arbitrary phase angle φ (t) the amplitude of the response signal can be determined from: 10 / W2 + Q (O2 = (A (t) cos (φ (O)) 2 + (A (t) sin ( φ (O)) 2 = A2 (t) (16)

Thus: A (i) = + <22 (Ö (17))

And the phase angle from: A (r) rin (^ (f)) QV) φ (,) = arc, gm ^ m) = (18) 15

This method of measuring the amplitude and phase of a signal is known as quadrature detection.

The quadrature components I (t) and Q (t) are digitized in the A / D converters 42 and 43, and then processed in computer 44, with which amplitude and phase are recorded in accordance with formulas (17) and (18).

First DDS 36 is directly controlled by computer 44. In modern integrated DDS circuits, such as AD9854 from Analog Devices Ine., It is possible to modulate the output signals in phase, in frequency and in amplitude. The ASK modulation required for the communication protocols to labels to be tested is carried out in this way.

However, a fast data connection between computer 44 and first DDS 39 is necessary for this. A realization by means of a DDS circuit on an expansion card in a desktop computer, in which the DDS is controlled directly from the internal data bus of the computer, therefore forms part of the invention.

A modern DDS like the AD9854 gives the possibility to generate two signals, which show a phase difference of 90 degrees with high accuracy. It is part of the invention to utilize this possibility for generating the reference signals for quadrature detection, i.e. for the product detectors 40 and 41.

In an alternative solution, shown in Figure 13, the reference signals for the product detectors 40 and 41 are not directly supplied by first DDS 39, but are generated in a Phase Lock Loop consisting of VCO 45, phase splitter 46, and phase comparator 47. VCO 45 oscillates at the frequency of the interrogation signal, or a multiple thereof. The phases of the reference signals are thus also coupled to that of the interrogation signal, but by means of a phase correction voltage applied to phase comparator 47, a phase offset can be made which can compensate for other phase offsets. These phase offsets can arise from the technical realization of the interrogation field antenna and the measurement antenna, and can also be frequency dependent. By means of a DAC, the computer can generate this phase correction voltage frequency-dependent; the DDS of the AD9854 type has the option of using the DAC of the Q output for this purpose.

20

With a number of types of RFID tags, the data on the response signal is modulated by first generating a subcarrier, and in turn modulating it with the data signal. For example an RFID tag, which is interrogated with an interrogation signal with a frequency of 13.56 MHz, generates a subcarrier with a frequency of 424 kHz by dividing the interrogation frequency 25. This subcarrier is then ASK modulated with the Manchester coded data signal. The speed thereof is, for example, 26 kbaud.

Figure 14 shows this spectral classification graphically.

In the circuits according to Figs. 12 and 13, the modulated subcarrier becomes available at the outputs of the product detectors 40 and 41, which is then digitized in the A / D converters 42 and 43. In computer 44, this modulated 1016418¾ subcarrier must be demodulated before the data signal is available, including the signal level of the subcarrier.

This requires so much computing power from the computer that the use of a digital signal processor becomes necessary, which would unnecessarily complicate the design of the measuring device.

A more attractive solution is therefore to demodulate the subcarrier signal directly, without first recovering this signal. Figure 15 gives an example of such a circuit. Second DDS 48 generates a reference signal for the product detectors 40 and 41 with a frequency that is equal to one of the absolute frequencies of the subcarrier components, in the example 13136 kHz (below sideband) or 13984 kHz (upper sideband). Second DDS 48 is also set directly by computer 44. Subcarrier frequency and sideband selection thus belong to the software settings.

By means of this quadrature detection of the subcarrier component, its amplitude level can be measured, and modulation forms such as amplitude modulation, phase modulation, or combinations thereof can be demodulated directly. The characteristics of these modulations can also be measured.

In the case of frequency modulation of the subcarrier, FSK, the circuit of Fig. 15 is also applicable. In that case, however, the frequency of both reference signals is taken equal to a value located midway between two possible frequencies. For example, when the two FSK subcarrier frequencies are 424 and 484 kHz, which in the case of an interrogator carrier frequency of 13560 kHz, and low sideband, results in a frequency for the reference signals of 13106 kHz (the center of 13076 and 13136 kHz), see also 25 figure 16.

The outputs of product detectors 40 and 41 now show signals with the difference frequency (33 kHz according to the example). Since both output signals have a mutual phase difference of 90 degrees as a result of that phase difference between the two reference signals, it is now possible to detect whether the measured frequency was higher than the reference frequency or lower. The techniques required for this are known to those skilled in the art, and are described inter alia in European patent 0608961 of applicant.

Hf? 1. "" · L 'W [...

ê> - 18 -

The measuring circuit for testing RFID labels of the Electro-Magnetic type requires an extension to the circuit of Figure 15 since it is not possible to directly generate a DDS circuit with signals of the frequencies 900 or 2450 MHz. Figure 17 shows a solution for this. First VCO 49 generates the interrogation signal transmitted by antenna 31 in antenna arrangement 50. A Phase Lock Loop circuit consisting of first frequency divider 51, first phase comparator 52, in addition to the aforementioned first VCO 49, locks the frequency of the interrogation signal to a reference signal first DDS 49 whose frequency is the Nth part of the 10 interrogation frequency.

The interrogation signal is modulated in modulator 53 with the help of a modulation signal generated by the Q / Control output of first DDS 39.

The response signal received by measuring antenna 33 is multiplied in product detectors 40 and 41 by the I and Q component of a reference signal multiplied for measuring the amplitude and demodulating the data modulation.

Due to the large transit times for the interrogation signal and the response signal between the antennas 31 and 33 on the one hand and the label 12 on the other, which, due to the use of EM 20 waves for the propagation, comprise the duration of several periods, there is no longer a fixed relationship between the phase of the transmitted interrogation signal and the phase of the received response signal. In fact, because in practical systems the distance between interrogation unit and label can vary rapidly, the above-mentioned phase relationship can therefore vary considerably with time. In other words: the response signal can exhibit a Dopper shift 25 and therefore the interference of the received response signal with the interrogation signal directly coupled in the receiver will be so strong that the response signal can no longer be demodulated properly. For the system design of such EM RFID systems, this means that, in contrast to the inductive and capacitive RFID systems, EM RFID labels always return a response signal in which the data is modulated on a subcarrier whose frequency is at least so high that no nuisance is more affected by Doppler shifts.

-n * - 19 -

The I and Q reference signal for the product detectors 40 and 41 is generated by synchronizing a second PLL consisting of second YCO 54, second frequency divider 55, and second phase comparator 56 on the reference signal from second DDS 48.

Phase splitter 46 then forms the I-signal for first product detector 40 and the 90-degree phase-shifted Q signal for second product detector 41. The reference signal with frequency (fc ± fSUb) / N, required to control this PLL, is supplied by second DDS 48.

In the solutions where two DDS circuits are used, for example for demodulating response signals with a subcarrier, it is necessary to be able to accurately adjust the mutual frequency difference of reference signals from both DDS circuits. This requirement is met if the two DDS circuits are driven from a common clock circuit.

f1016416 ”

Claims (16)

A measuring device for measuring and testing RFID labels, characterized in that the measuring device is adapted to measure the magnitude of the magnetic dipole moment of the response signal of an inductive RFID label as a function of an interrogation signal with a specified frequency and magnetic field strength. A measuring device for measuring and testing RFID labels according to claim 10, characterized in that said magnetic dipole moment is measured by means of a measuring antenna coil, which magnetically couples to the coil in the label to be tested, and which diameter is considerably larger, about 2 to 4 times, than the diameter of the coil in the label. A measuring device for measuring and testing RFID labels according to claims 1 or 2, characterized in that the interrogator antenna coil and the measuring antenna coil are arranged such that the direct magnetic coupling between the interrogator antenna coil and the measuring antenna coil is zero, while both coils link to the label to be tested. A measuring device for measuring and testing RFID labels according to claim 3, characterized in that the interrogator antenna coil, the measuring antenna coil, and coil 2 of the label to be tested are arranged such that their centers coincide and that the planes in which the interrogator antenna coil and the measuring antenna coil are located at an angle of 90 degrees from each other, while coil 2 of the label to be tested is located in the bisector line between the surfaces of the interrogating antenna coil and the measuring antenna coil. 5. A measuring device for measuring and testing RFID labels, characterized in that the measuring device is adapted to measure the size of the electric <P, · '. P :: '·' j.-iö «T I I ·. a ·· S a - 21 - dipole moment of the response signal of a capacitive RFID tag as a function of an interrogation signal with a specified frequency and electric field strength. A measuring device for measuring and testing RFID labels according to claim 5, characterized in that the interrogating antenna and the measuring antenna are arranged such that the direct capacitive coupling between the interrogating antenna and the measuring antenna is zero, while both antennas couple with the label to be tested. A measuring device for measuring and testing RFID labels according to claim 10, characterized in that the interrogation antenna, in the form of two opposite capacitor plates 20, and the measuring antenna, in the form of two opposite capacitor plates 23, are located on a body with at least one axis of symmetry, such as, for example, a cube or a cylinder, such that the connecting line between the centers of gravity of capacitor plates 20, and the connecting line between the centers of gravity of capacitor plates 23 intersect at a point perpendicular to the point in question axis of symmetry, at which point also the center of the testing capacitive label is placed in such a position that the electrical dipole of the label is oriented perpendicular to that axis of symmetry and has angles of 45 degrees with said connecting lines. 20 A measuring device for measuring and testing RFID labels according to one or more of the preceding claims, characterized in that the measuring device is adapted to measure the magnitude of the effectively radiated power of the response signal of an Electromagnetic RFID label as a function of an generated electromagnetic interrogation signal with specified frequency and field strength. 9. A measuring device for measuring and testing RFID labels according to claim 8, characterized in that the interrogating antenna and the measuring antenna are arranged such that the direct electromagnetic coupling between the interrogating antenna and the measuring antenna is zero, while both link antennas to the EM RFID label to be tested. A measuring device for measuring and testing RFID labels according to claim 5, characterized in that the interrogated dipole antenna and the measuring dipole antenna are in the same plane, the centers of both antennas coincide, while the dipoles form a perpendicular angle to each other, so that the polarization directions of both antennas are perpendicular to each other and therefore no direct coupling takes place between the two antennas, but that a response signal is received from an EM RFID tag with a linear or circularly polarized antenna. A measuring device for measuring and testing RFID labels according to claim 9, characterized in that the interrogation antenna and the measuring antenna are combined in one patch antenna, wherein two different connection points on the patch are used for connection of generator 26 for generating of the interrogation signal and for connection of receiver and measuring circuit 30, wherein the patch antenna can resonate in two modes with the polarization directions of both vibration modes perpendicular to each other so that no direct coupling takes place between the two antenna connections, but that an interrogation signal 20 is transmitted to, and that a response signal is received from an EM RFID tag with a linear or circularly polarized antenna. 12. A measuring device for measuring and testing RFID labels according to one or more preceding claims, characterized in that for generating an interrogation or interrogation signal, and for generating reference signals for the phase synchronous measurement of the magnetic dipole moment, use is made of a first integrated Direct Digital Synthesizer (DDS) circuit, which circuit forms part of an expansion card of a computer and is directly controlled from the data bus of that computer. : i ... | · £ - 23 - A measuring device for measuring and testing RFID labels according to claim 12, characterized in that the DDS circuit generates two sinusoidal signals that have the same frequency but differ 90 degrees in phase. A measuring device for measuring and testing RFID tags according to claim 12 or 13, characterized in that reference signals generated in a second DDS circuit are used for measuring subcarrier signal components of the tag response signal. A measuring device for measuring and testing RFID labels according to claims 12, 13 or 14, characterized in that the second DDS circuit also forms part of an expansion card of a computer and is directly controlled from the data bus of that computer . A measuring device for measuring and testing RFID labels according to claims 14 or 15, characterized in that the first DDS circuit and the second DDS circuit use a common clock signal generator circuit. 1016416 ^