WO2014202735A1

WO2014202735A1 - An optical disc with a wireless communication device, a wireless communication device and a method for its design and fabrication

Info

Publication number: WO2014202735A1
Application number: PCT/EP2014/062968
Authority: WO
Inventors: Ferran PAREDES MARCO; Jorge Bonache Albacete; Simone ZUFFANELLI
Original assignee: Universitat Autònoma De Barcelona
Priority date: 2013-06-20
Filing date: 2014-06-19
Publication date: 2014-12-24

Abstract

The wireless communication device (D) is mounted or designed to be mounted on the optical disc (OD) and comprises an integrated circuit (M) and an antenna matched thereto having first and second poles respectively comprising first and second electrical conductors, where both conductors form a resonator and are contactless coupled to a metal layer (X) of the optical disc (OD) such that the metal layer (X) becomes a radiant element which constitutes, together with the conductors, the two poles of the antenna. The method for the design and fabrication of the wireless communication device comprises obtaining a combined equivalent electrical model by serially connecting electrical models of the wireless communication device (D) and of the optical disc (OD), performing an impedance matching with the integrated circuit (M) by adjusting the parameters of the electrical model of the wireless communication device (D) and fabricating the latter according to the adjusted parameters.

Description

An optical disc with a wireless communication device, a wireless communication device and a method for its design and fabrication

Field of the Art

The present invention generally relates, in a first aspect, to an optical disc with a wireless communication device, where a metal layer of the disc is used as a radiant element for the wireless communication device, where said metal layer forms part of two respective poles of an antenna of the wireless communication device, and more particularly to an optical disc with a wireless communication device where the antenna forms a resonator.

A second aspect of the invention relates to a wireless communication device configured and arranged to be mounted on an optical disc, and use the metal layer thereof as part of two respective poles of the antenna of the wireless communication device, wherein the antenna forms a resonator.

A third aspect of the invention concerns to a method for the design and fabrication of the wireless communication device of the second aspect, mounted or to be mounted on an optical disc.

Prior State of the Art

Radio frequency identification (RFID) is a widespread technology that allows tagging of objects by using electromagnetic waves. In the last years, the use of such technology has experienced a rapid increase, whereas the cost of the tags has dropped down, and further penetration into the market is expected for the next years. Typical applications of this technology are smart inventory and item tracking, among others. Passive tags operating at the UHF-RFID frequency bands (840-930 MHz [1]) are especially employed for this kind of applications due to the significant achievable read ranges, low cost, small dimensions, and because such tags do not need batteries. A passive UHF-RFID tag consists of an antenna matched to an application specific integrated circuit (ASIC), which contains the information about the tagged item. A passive tag is capable of using the electromagnetic energy from the reader to activate the chip, which generates a modulated backscattered signal to the reader. Typical peak read ranges of UHF-RFID tags are in the order of 5-10 m, depending on the country regulations (i.e. maximum allowed EIRP value), tag characteristics (i.e. antenna design and chip sensitivity) and orientation, object material, and environmental conditions.

Whilst RFID technology allows labelling many kinds of items or objects composed by dielectric materials, it is well known that metallic objects, or objects containing metallic parts, can prevent the correct functionality of the tags [2]-[5], as they can cause mismatching between the tag antenna and ASIC, and degradation of the radiation efficiency of the antenna as well. Due to this effect, CD, DVD and Blu-ray (RTM) discs cannot be labelled with standard passive UHF-RFID tags, since they contain a thin metal layer under the disc surface, which causes a severe degradation of the tag read range.

Some effort have been made in the past by designing bending dipole [6] and meander and annular [7] antenna based tags to be placed in the small central area of the disc, which is metal-free. However, due to the proximity of the disc metallic layer and the reduced tag size, the achieved read ranges are in the order of 0.35-0.4 m, which are inadequate for many typical RFID applications. Moreover, this kind of tags typically presents complex layout geometries, and the design process is not fully explained. For this reason, there exists an important gap when comparing the performance of tags for disc identification to that of general purpose UHF-RFID tags.

There are some proposals in the state of the art which disclose using the metal layer of the disc as a radiating element for the wireless device mounted thereon. One of said proposals is included in patent EP1500042B1 , which discloses capacitively coupling one or two tabs of the wireless communication device to a metalized portion of the disc.

Although the proposal of EP1500042B1 can be considered as a step forward in comparison to the previous state of the art, as the applicants thereof seethe metal layer of the disc not as an interfering element to cope with but as an element which can be used as a radiant element for the wireless communication device, said proposal does not either teach how to take profit of the whole disc metal layer for providing an antenna for the wireless communication device. The read range achieved with the arrangements proposed in EP1500042B1 is a severely limited range, as the predominant radiation is from a relatively small area slot antenna.

International Application WO2004097731 discloses a system which includes the features of the preamble of claim 1 of the first aspect of the present invention. The range of the system proposed in WO2004097731 is said to increase two or three times the range of the prior art arrangements.

Neither WO2004097731 nor EP1500042B1 discloses a resonant arrangement, including the disc metal layer therein, as only capacitive arrangements are proposed in both of said patent documents. REFERENCES:

[I ] GS1 EPCglobal, "Regulatory status for using RFID in the UHF spectrum", 6 November 2012.

[2] D. M. Dobkins and S. Weigand, "Environmental effects on RFID tag antennas," IEEE MTT-S International Microwave Symposium, Long Beach, CA, June 2005, pp. 4-7.

[3] J. D. Griffin, G. D. Durgin, A. Haldi, and B. Kippelen, "RF tag antenna performance on various materials using radio link budgets," Antennas and Wireless Propagation Letters, vol. 5, no. 1 , pp. 247-250, Dec. 2006.

[4] K. M. Ramakrishnan and D. D. Deavours, "Performance benchmarks for passive UHF-RFID tags," 13th GI/ITG Conference on Measurement, Modeling, and Evaluation of Computer and Communication Systems, Nurenberg, Germany, Mar. 2006, pp. 137-154.

[5] S. R. Aroor and D. D. Deavours, "Evaluation of the state of passive UHF-RFID: An experimental approach," IEEE Systems Journal, vol. 1 , no. 2, pp. 168-176, December 2007.

[6] W. T. Luk and K. N. Yung, "Bending dipole design of Passive UHF-RFID tag antenna for CD/DVD discs," IEEE, Macau, pp. 1-4, Asia Pacific Microwave Conference, 16-20 December 2008.

[7] A. S. Andrenko, M. Kai, T. Maniwa and T. Yamagajo, "Compact printed-on-CD UHF- RFID tag antennas," IEEE, Honolulu (USA), pp. 5455-5458, Antennas and Propagation Society International Symposium, 9-15 June 2007.

[8] Fred R. Byers, "Care and Handling of CDs and DVDs - A Guide for Librarians and Archivists", NIST Special Publication 500-252.

[9] R.C. Hansen and W. T. Pawlewicz, "Effective Conductivity and Microwave Reflectivity of Thin Metallic Films," IEEE Transactions on Microwave Theory and Techniques, vol. 30, issue 11 , pp. 2064-2066, November 1982.

[10] Y. Hanaoka, K. Hinode, K. Takeda and D. Kodama, "Increase in electrical resistivity of copper and aluminium fine lines", The Japan Institute of Metals, Materials Transactions, vol. 43, No. 7, pp. 1621-16231, 2002.

[I I ] W. Zhang, S.H. Brongersma, O. Richard, B. Brijs, R. Palmans, L. Froyen, K. Maex "Influence of the electron mean free path on the resistivity of thin metal films", Microelectronic Engineering, Volume 76 Issue 1-4, pp. 146-152, October 2004.

[12] R. A. Matula, "Electrical resistivity of copper, gold, palladium, and silver", J. Phys.

Chem. Ref. Data 8, 1147 (1979), DOI:10.1063/1 .555614. [13] C. A. Grosvenor, R. T. Johnk, J. Baker-Jarvis, M. D. Janezic, and B. Riddle, "Time- domain free-field measurements of the relative permittivity of building materials," IEEE Trans, on Instrum. Meas., Vol. 58, No. 7, pp. 2275-2282, July 2009.

[14] NXP UCODE G2XM datasheet, Available online at www.nxp.com.

[15] Alien HiggsTM-3 datasheet, Available online at www.alientechnology.com.

[16] P. V. Nikitin, K. V. S. Rao, R. Martinez and S. F. Lam, "Sensitivity and Impedance Measurements of UHF-RFID Chips," IEEE Trans. Microw. Theory Tech., vol. 57, no. 5, pp. 1297-1302, May 2009.

[17] E. Bergeret, J. Gaubert, P. Pannier, J. M. Gaultier, "Modeling and Design of CMOS UHF Voltage Multiplier for RFID in an EEPROM Compatible Process," IEEE Trans.

Circuits and Systems, vol. 54, issue 10, pp. 833-837, October 2007.

[18] G. De Vita and G. lannaccone, "Design Criteria for the RF Section of UHF and Microwave Passive RFID Transponders," IEEE Trans. Microwave Theory Tech., vol. 59, no. 9, pp. 2978-2990, September 2005.

[19] K. V. S. Rao, P. V. Nikitin, S. F. Lam, "Antenna Design for UHF RFID Tags: A Review and a Practical Application," IEEE Transactions on Antennas and Propagation, vol. 53, No. 12, December 2005.

Description of the Invention

It is an object of the present invention to provide an alternative to the prior state of the art, which covers the gaps found therein, and which particularly overcomes the drawbacks and lacks of the proposal of WO2004097731 , providing an increase in the read range much higher than the one achieved with the system of said international Application.

To that end, the present invention relates to an optical disc with a wireless communication device, said wireless communication device being mounted on the optical disc and comprising an integrated circuit and an antenna matched thereto, wherein said antenna has at least a first and a second poles respectively comprising a first and a second electrical conductors, said first electrical conductor being contactless coupled to a metal layer buried in the optical disc such that said metal layer becomes a radiant element which constitutes, together with the first electrical conductor, said first pole of the antenna, and said second electrical conductor being also contactless coupled to the metal layer buried in the optical disc such that the metal layer becomes a radiant element which also constitutes, together with the second electrical conductor, said second pole of the antenna, wherein said wireless communication device is attached to an external non-electrically conductive layer of the optical disc, said contactless coupling of the electrical conductors with the metal layer buried in the optical disc being a capacitive coupling where said non-electrically conductive layer acts as a dielectric layer and comprises at least an inner and an outer electrically non-conductive adjacent annular areas, said inner annular area not overlapping the metal layer and having an inner peripheral edge delimiting a central hole of the optical disc and an outer peripheral edge, and said outer annular area extending from said outer peripheral edge of the inner annular area up to the outer peripheral edge of the optical disc, said outer annular area overlapping said metal layer buried therein,

Contrary to the known proposals, in the optical disc of the first aspect of the present invention, in a characteristic manner, each of the first and second electrical conductors includes at least two portions, differentiated at least by having different geometrical parameters, and constituting the next elements:

- a capacitive portion having a length and running over said outer annular area up to said outer peripheral edge of the inner annular area or entering the inner annular area; and

- an inductive portion having a width and running over said inner annular area and having a first end coupled to the integrated circuit, or to an impedance matching element placed parallel thereto, and a second end connected to said capacitive portion, wherein said elements of the first and second electrical conductors form a resonator producing an electrical coupling between the wireless communication device and the metal layer buried therein.

By separately controlling the geometric parameters of the inductive and capacitive sections, it is possible to obtain a high degree of controllability in the system frequency response. As a result, the system resonance frequency can be controlled, and this allows obtaining the required impedance matching level at the working frequency. A good (in the order of -10 dB) impedance matching between the ASIC and the disc metal layer, which acts as a radiating element, is necessary in order to obtain good performance of the tag in terms of read range (in the order of several meters). Since the authors of WO2004097731 do not obtain an important improvement in terms of read range, as compared to the present invention, one can conclude that their design could not provide a good impedance matching. The main reason of this fact is that the authors of WO2004097731 did not design the tag with the idea of a series LC resonator.

For a preferred embodiment, the antenna comprises only said first and second poles, constituting a dipole antenna defined as an antenna having a dipolar charge distribution, although for other less preferred embodiments the antenna has more than two poles, all or part of them being constituted, at least in part, by the metal layer of the optical disc analogously to the first and second poles.

According to a preferred embodiment, said wireless communication device is attached to said external non-electrically conductive layer of the optical disc directly or through an intermediate non-electrically conductive support attached thereto.

Generally, the ratio of length/width between said capacitive portion and said inductive portion is of at least 10, and preferably of at least 50.

For an embodiment of the optical disc of the first aspect of the invention, the dipole antenna is a symmetrical dipole antenna, the wireless communication device being arranged over the external non-electrically conductive layer of the optical disc, directly or through said intermediate non-electrically conductive support, symmetrically with respect to a symmetry plane which passes through the geometric centre of the optical disc orthogonally with respect to the plane containing said external non- electrically conductive layer.

The wireless communication device is preferably configured and arranged, with respect to the metal layer of the optical disc, to differentially and symmetrically excite a natural resonance mode of the optical disc such that an electrical dipole is formed with an effective distance between the poles thereof substantially coincident with the diameter of the optical disc.

The external non-electrically conductive layer of the optical disc onto which the wireless communication device is attached, for an embodiment, comprises at least an inner and an outer electrically non-conductive adjacent annular areas, said inner annular area not overlapping the metal layer and having an inner peripheral edge delimiting a central hole of the optical disc and an outer peripheral edge, and said outer annular area extending from said outer peripheral edge of the inner annular area up to the outer peripheral edge of the optical disc, said outer annular area overlapping said metal layer, and wherein each of said first and second electrical conductors includes at least the next elements:

- a capacitive portion running over said outer annular area up to said outer peripheral edge of the inner annular area or entering the inner annular area; and

- an inductive portion running over said inner annular area and having a first end coupled to the integrated circuit, or to an impedance matching element placed parallel thereto (such as a planar capacitor), and a second end connected to said capacitive portion. For an embodiment:

- said capacitive portion comprises a first annular portion defining an arch whose geometric centre coincides with the geometric centre of the central hole of the optical disc, said first annular portion running over said outer annular area up to said outer peripheral edge of the inner annular area or entering the inner annular area; and

- said inductive portion comprises:

- a second annular portion defining an arch whose geometric centre coincides with the geometric centre of the central hole of the optical disc, said second annular portion running over said inner annular area and having a first end coupled to the integrated circuit, which is placed adjacent the central hole, or to said impedance matching element placed parallel thereto; and

- an intermediate straight or meandered portion with a first end connected to a second end of said second annular portion and a second end connected to a middle point of said first annular portion.

For another embodiment, the geometric centres of the arches of the first and/or second annular portions don't coincide with the geometric centre of the optical disc central hole.

For different variants of said embodiments, the conductor defining said annular portions run following a curve path or a meandered path, depending on the capacity and/or inductive values required and on the electric equivalent circuit required to be implemented and the performance thereof.

For other embodiments, the capacitive and/or inductive portions don't include annular portions but conductor portions having non-ring shapes, selected also depending on the equivalent circuit required to be implemented thereby, the performance thereof, and the values of theirs components.

According to an embodiment, said resonator constitutes, through semi-lumped elements, a series stepped-impedance resonator, where each of said capacitive portions constitutes, together with the inner metal layer and the non-electrically conductive layer of the optical disc, a semi-lumped capacitor and each of said inductive portions constitutes a semi-lumped series inductance, whose geometrical parameters are chosen to provide an appropriate impedance matching between the integrated circuit and the metal layer of the optical disc for a desired radio frequencies band.

The optical disc of the first aspect of the invention comprises, for an embodiment, a counter mass arranged on the optical disc at a point crossed by said symmetry plane, whose weight and location is calculated to compensate a mass unbalance produced by the wireless communication device, in order to make coincident the mass centre with the geometric centre of the optical disc.

Due to the symmetry, for a preferred embodiment, of the arrangement of the wireless device on the optical disc, said calculus of the weight and location of the counter mass is considerably simplified in comparison with a non-symmetric arrangement of the wireless device on the optical disc.

The provision of such counter mass is of special importance for aligning the barycentre (or mass centre) of the whole structure with the disc rotation centre, thus preventing vibrations during rotation, in order to allow a good reading/writing of the data stored or to be stored therein and avoid a possible damage of the disc than a tilted rotation could cause.

Preferably, the wireless communication device is a RFID tag, and more preferably a passive tag operating at UHF band, although alternative wireless communication devices of another kind, active or passive and working at the same or at other frequency bands, are also covered by other less preferred embodiments of the present invention.

A second aspect of the present invention concerns to a wireless communication device mounted or to be mounted on an optical disc, comprising an integrated circuit and an antenna having an impedance matched to the impedance of the integrated circuit, said wireless communication device being configured and arranged to be mounted on an optical disc, wherein said antenna has at least a first and a second poles respectively comprising a first and a second electrical conductors, said first electrical conductor being configured and arranged for being contactless coupled to a metal layer buried in the optical disc such that said metal layer becomes a radiant element which constitutes, together with the first electrical conductor, said first pole of the antenna

the second electrical conductor being also configured and arranged for being contactless coupled to the metal layer buried in the optical disc such that the metal layer becomes a radiant element which also constitutes, together with the second electrical conductor, said second pole of the antenna, wherein said wireless communication device is configured and arranged to be attached to an external non-electrically conductive layer of the optical disc, said contactless coupling of the electrical conductors with the metal layer buried in the optical disc being a capacitive coupling where said non-electrically conductive layer acts as a dielectric layer and comprises at least an inner and an outer electrically non-conductive adjacent annular areas, said inner annular area not overlapping the metal layer and having an inner peripheral edge delimiting a central hole of the optical disc and an outer peripheral edge, and said outer annular area extending from said outer peripheral edge of the inner annular area up to the outer peripheral edge of the optical disc, said outer annular area overlapping said metal layer buried therein.

Contrary to the known proposals, in the wireless communication device of the second aspect of the invention each of the first and second electrical conductors includes at least two portions, differentiated at least by having different geometrical parameters, and constituting the next elements:

- a capacitive portion having a length and configured and arranged for running over said outer annular area up to said outer peripheral edge of the inner annular area or entering the inner annular area; and

- an inductive portion having a width and configured and arranged for running over said inner annular area and having a first end coupled to the integrated circuit, or to an impedance matching element placed parallel thereto, and a second end connected to said capacitive portion;

wherein said elements of the first and second electrical conductors form a resonator producing an electrical coupling between the wireless communication device and the metal layer buried therein.

For an embodiment, the wireless communication device of the second aspect of the invention constitutes the wireless communication device of the optical disc of the first aspect.

For an embodiment the wireless communication device of the second aspect of the invention includes the above mentioned intermediate non-electrically conductive support, the integrated circuit and first and second electrical conductors being attached to a front face of the intermediate non-electrically conductive support, the latter being prepared for its attachment to the optical disc, for example by including an adhesive layer on its back face or other kind of attachment means appropriate for that purpose.

A third aspect of the invention concerns to a method for the design and fabrication of a wireless communication device to be mounted on an optical disc, where the wireless communication device is the wireless communication device of the second aspect of the invention, the method comprising, for a desired radio frequencies band:

- constructing an electrical model of the optical disc, which describes approximately its input impedance;

- constructing an electrical model of the wireless communication device plus part of the optical disc which is electrically coupled thereto when the wireless communication device is mounted thereon; - obtaining a combined equivalent electrical model for the whole structure by serially connecting said electrical model of the wireless communication device and said electrical model of the optical disc, the latter with the values of its parameters varied by taking into account the presence of the wireless communication device;

- performing an impedance matching between the impedance of the integrated circuit of the wireless communication device and the impedance of the combined equivalent electrical model by adjusting the parameters of the electrical model of the wireless communication device plus part of the optical disc electrically coupled thereto; and

- fabricating the wireless communication device according to said adjusted parameters.

For an embodiment of the method of the third aspect of the invention, said electrical model of the wireless communication device plus part of the optical disc is a semi-lumped element approximation of a series stepped-impedance resonator producing an electrical coupling between the wireless communication device and the optical disc metal layer, and which comprises, for each of the first and second electrical conductors of the antenna of the wireless communication device, the next elements:

- a capacitive portion designed to run, directly or through an independent intermediate non-electrically conductive support, over at least an outer annular area of an external non-electrically conductive layer of the optical disc, said outer annular area overlapping the metal layer, up to an outer peripheral edge of an adjacent inner annular area of the external non-electrically conductive layer, which doesn't overlap the metal layer, or entering said inner annular area; and

- an inductive portion designed to run over said inner annular area, directly or through said intermediate non-electrically conductive support, and having a first end coupled to the integrated circuit, or to an impedance matching element placed parallel thereto, and a second end connected to said capacitive portion;

where each of said capacitive portions constitutes, together with the inner metal layer and the non-electrically conductive layer of the optical disc, a semi-lumped capacitor and each of said inductive portions constitutes a semi-lumped series inductance, the method comprising:

- performing said adjusting of the parameters of the electrical model of the wireless communication device plus part of the optical disc by selecting the geometry (ring-shaped or non-ring shaped, straight or meandered lines, etc.) and adjusting the geometrical parameters of said capacitive and inductive portions; and - fabricating said capacitive and inductive portions with said adjusted geometrical parameters, including at least different geometrical parameters for each other, and attaching the so fabricated portions and the integrated circuit of the wireless communication device directly on said external non-electrically conductive layer of the optical disc or on said independent intermediate non- electrically conductive support to be attached thereto.

For a preferred variant of said embodiment of the method of the third aspect of the invention:

- said capacitive portion comprises a first annular portion defining an arch whose geometric centre coincides with the geometric centre of the central hole of the optical disc, said first annular portion being designed to run over said outer annular area up to said outer peripheral edge of the inner annular area or entering the inner annular area; and

- said inductive portion comprises:

- a second annular portion defining an arch whose geometric centre coincides with the geometric centre of the central hole of the optical disc, said second annular portion being designed to run over said inner annular area and having a first end coupled to the integrated circuit, which is placed adjacent the central hole, or to said impedance matching element placed parallel thereto; and

Brief Description of the Drawings

The previous and other advantages and features will be more fully understood from the following detailed description of embodiments, with reference to the attached drawings, which must be considered in an illustrative and non-limiting manner, in which:

Figs. 1 a and 1 b schematically show the a simplified structure of a DVD+R disc, by, respectively, a top and a lateral view, where the disc has a metal layer embedded between two polycarbonate layers; layer thicknesses are not drawn to scale;

Fig. 2 (a) is graph showing a comparison between a simulated input impedance of the DVD+R disc of Fig. 1 and the input impedance inferred from a circuit model (Z_RLc) thereof, while Fig. 2 (b) schematically shows the current profile on the metal layer of the disc at 885 MHz;

Fig. 3 shows the radiation patterns at 885 MHz for a DVD+R without any wireless communication device mounted thereon and for the optical disk of the first aspect of the present invention for an embodiment where the disc is a DVD+R and the wireless communication device mounted thereon is a RFID tag: (a) E-plane; (b) H- plane.

Fig. 4 is an electrical model of the optical disc plus RFID tag, for the embodiments of Figs. 5a and 5b (a), and a simplified electrical model using electrical wall (E-wall) properties (b);

Figs. 5a to 5c show three different embodiments of the wireless communication device and of the optical disc plus RFID tag of the invention, where the non-electrically conductive outer annular area of the optical disk has not been depicted, for clarity sake in order to allow an enlarged view in detail of the elements of the RFID tag, the three embodiments differing in the tag layout, particularly in the geometry and number of electrically conductive portions their RFID tags include, and in that in Fig. 5a the optical disc includes a counter mass;

Fig. 6a depicts a graph which represents the simulated power wave reflection coefficient of the embodiment of Fig. 5(a), while the graph of Fig. 6b shows a comparison between the impedance of the circuit model and the one inferred from electromagnetic (EM) simulation.

Fig. 7a is a schematic top view of the optical disc with a wireless communication device of the invention, for an embodiment where the wireless device is the RFID tag of Fig. 5a and is directly attached to the non-electrically conductive outer annular area of the optical disc;

Fig. 7b is a schematic top view of the optical disc with a wireless communication device of the invention, for an embodiment similar to the one of Fig. 7a but differing therefrom in that the RFID tag is attached to an intermediate support, the latter being attached to the non-electrically conductive inner and outer annular areas of the optical disc;

Fig. 8 is a graph showing the measured read range of the final tag mounted on a

DVD+R disc according to the layout of Figs. 5a and 7a; and

Fig. 9 show the symmetric current distribution generated over the optical disc of Fig. 5a for a differential and symmetric excitation thereof, said distribution corresponding to a natural resonance mode of the disc. Detailed Description of Several Embodiments

For the embodiments described in the present section, the wireless communication device of the present invention is a UHF-RFID passive tag, and the optical disc is a DVD disc.

In order to design the UHF-RFID tag of the invention, first an electrical model of the DVD disc must be built. Next the description of the DVD structure and of the building of its corresponding electrical model is made.

- DVD disc structure and electrical model:

DVD Disc structure:

The substrate of DVD discs is composed of two identical circular-shaped polycarbonate layers (0.6 mm thickness each) bonded together, which provide structural rigidity and protect the internal layers from mechanical damage (scratches) and oxidation caused by atmospheric agents (humidity, oxygen, dust, etc.) [8]. A very thin metal layer (50-100 nm) is grown between the two polycarbonate layers by means of sputtering deposition. In Fig. 1 b said inner metal layer is indicated as X while the top and bottom polycarbonate layers are Z1 and Z2, respectively. The constitutive material of this metal layer can be aluminium, gold, silver, or silver alloys, depending on the disc type [8]. Such layer, which is found in all kind of optical storage devices (CDs, DVDs, Blu-Ray (RTM) discs), is necessary to reflect the laser beam during read and write operations of the data content.

Data layer material also varies depending on the disc type. Moulded polycarbonate, organic dye or phase-changing metal alloy film is commonly used for DVD-ROM, DVD+R, DVD+RW, respectively [8]. Data is stored in this layer in the form of nanometre scale pits and lands, which modulate the intensity of the reflected laser beam detected by the reader photodiode. Some kinds of DVDs can also include a semi- reflective metal layer (e.g., double data layer DVDs) and even a labelling layer such as a thermal-printable, inkjet-printable, or silkscreen-printable layer [8]. However, in order to perform an electromagnetic simulation of the structure, it is possible to simplify the problem by modelling the disc considering only the substrate layers and the metal layer. This is because these are the most important layers in terms of electrical properties of the disc, as the other layers are very thin and do not present critical electrical properties (i.e., high permittivity or high conductance). This approximation can only be assumed in the case of single data layer discs only. Other DVD types should be modelled including additional metal layers. Electrical model of the DVD disc:

The main concept provided in this invention is to consider the metal layer of the disc as an antenna. By properly exciting electrical currents on the disc surface, an oscillating dipolar moment can be generated, acting as the main radiating element of the system. Therefore, the tag can be designed to operate by coupling electromagnetic energy to the metal layer of the disc, resulting in a larger antenna size, as compared to the sizes of conventional CD/DVD tag antennas. Moreover, the radiation efficiency degradation due to the image currents, which occurs in the conventional disc tags, is avoided with this approach. The reason is that the disc metal layer is used as the main radiator, therefore image currents are not present.

Because electromagnetic (EM) coupling to the metal layer is required, the impedance matching between the antenna and the ASIC will directly depend on the input impedance of the disc metal layer. Therefore, in order to design the proposed tag, it is very useful to understand the electrical behaviour of the disc in terms of its impedance. To analyse the frequency response of the disc, the CST Microwave Studio commercial software (the frequency domain solver) was used. The simulated structure, presented in Figs. 1 a and 1 b, is a simplified model of a DVD+ disc, according to the approximations described previously. The metal layer X is modelled as an ohmic sheet with a square resistance given by

f

Jt = ^ (1)

t

where p ^fA_g is the effective resistivity of the silver film and t is the film thickness. It has been demonstrated that the effective electrical resistivity of thin metal films can vary depending on the fabrication process and the thickness of the deposited layer [9]-[10]; typically, the effective resistivity increases with respect to the material bulk resistivity as the film thickness decreases. This effect becomes very important when the thickness is in the order of the electron mean-free-path p of the conductive material [9]-[10]. The value of p for silver is roughly 50 nm [11 ] at room temperature. Hence, for a reflecting layer thickness of 50 nm, the thickness to mean-free-path ratio t/p approaches unity. It is possible to predict the effective resistivity ' of a thin metal layer, given the values of the bulk resistivity p and the ratio t/p [9]. Based on [9], the value of p ^fA_g for t/p = 1 can be found as 1 .5 pA_g (where pA_g = 16 ηΩ-m is the bulk resistivity of silver [12]), leading op^fA_g = 24 ηΩ-m. Therefore, according to (1 ) the square resistance value is Rn = 0.5Ω. The relative permittivity and dielectric loss tangent values used for polycarbonate plastic are s_r = 2.88 and tanS = 0.012, respectively [13]. It is important to remark that the electrical properties of the polycarbonate plastic and silver layers may vary depending on many factors related to the fabrication process. In this regard, and considering the wide range of different DVD manufacturers, the choice of one set of electrical parameters for the structure is arbitrary and slight variations in the tag performance can result for different brands.

In order to carry out the simulation, a differential input port (P in Fig. 1 a) is placed directly on the metal layer, and aligned to one of the infinite symmetry planes of the structure, as depicted in Fig. 1 a. The reason for choosing a symmetric position of the port P is simple: the final structure (tag layout and DVD) should present (at least in a first order approximation) symmetry in order to prevent a displacement of the barycenter. Actually, unbalancing the rotating mass can cause read/write problems and physical damage of the disc and the reader.

The simulated input impedance Z_disc of the structure (depicted in Fig. 2(a)) shows a parallel-type resonance occurring at f₀ = 2.61 GHz, suggesting that a shunt RLC circuit model can be used to describe the disc impedance to a good approximation. Based on the resistive part of Z_disc and the reactance slope values at f₀, it is possible to perform a parameter extraction. The calculated values for the circuit parameters are R_p = 6350 Ω, L_p = 57 nH, C_p =65 fF. Good agreement between the circuit model (impedance Z_RLc) and EM simulation (impedance Z_di_SC) is observed in Fig. 2(a).

Since the first resonance occurs at high frequencies, a short dipole charge profile and an inductive impedance behaviour at the UHF-RFID band is expected. Given that the structure provides two symmetrical paths for the electric current, a two-elements array is expected to form the radiation pattern. The effective distance between the two dipoles is a function depending on the current density profile along the section of the metal layer, and is comprised between the physical dimensions of the disc: 38 mm ( ./10 at 885 MHz) and 120 mm {λ/2.8 at 885 MHz). Such dipole array is thus expected to present a maximum directivity value higher than the single dipole, and the radiation pattern is expected to present a maximum at Θ = 0° (direction normal to the disc plane). Moreover, a directional pattern is expected for the H-plane (yz), with a minimum directive gain at Θ = 90° due to the destructive interference between the two radiating elements. However, since the two elements of the array are in-phase excited and their distance is smaller than I2, the directive gain minimum value on the H-plane is not expected to reach zero. Far-field polarization is expected to be linear, with E-field oriented along the x-axis (see Fig. 2(b)). The simulated current pattern on the metal layer at the center frequency (885 MHz) is depicted in Fig. 2(b). As expected, each side of the disc exhibits a dipolar type current. The simulated radiation pattern (see Fig. 3) confirms the behavior of the disc as an array of two short dipoles, with a maximum directive gain value D₀ = 3.3 dBi at Θ = 0°. Minimum directive gain on the yz-plane is -1.4 dBi. Due to the width of the conductive path on the metal layer, the radiation efficiency is reasonably high, reaching the value of Brad = 25% at 885 MHz, resulting in a maximum antenna gain of Go = -2.7 dB. This means that, providing a good (i.e. an appropriate) impedance matching, it is possible to obtain a read range in the order of several meters. As an example, considering EIRP = 4 W (which is the maximum allowed value in many countries [1 ]), the read range is 7 m for ASICS with _iA = -15 dBm [14] and 10 m with _iA = -18 dBm [15], according to the well- known equation [19]:

where EIRP is the equivalent isotropic radiated power, λ is the free-space wavelength at the working frequency, G_r is the gain of the tag antenna and τ = (7- | s \ ²) is the power transmission coefficient. This theoretical value corresponds to the read range in free- space, with both antennas (reader and tag) correctly oriented at the maximum gain direction. - Tag design, synthesis and simulation: Design principles:

In the previous paragraphs, the capability of a DVD to work as an efficient antenna at the UHF- FID frequencies was demonstrated. In order to design an RFID tag capable of taking advantage of these potentialities, the coupling method problem has to be solved. In fact, coupling is necessary because the metal layer is not directly accessible in the DVD post-production stage, and a RFID tag can only be placed on top of the polycarbonate structural layer. The solution adopted by this invention, for a preferred embodiment, is to produce an electrical coupling between the tag and the disc by means of a series stepped-impedance resonator (SIR) synthetized through semi- lumped elements. The capacitance between the tag metal (top layer) and the disc metal (inner layer), C_tag, provides electrical coupling. The series inductance L_tag, realized through a thin line (e.g., 0.4 mm), forces a controllable series resonance, which provides a virtual short circuit between the ASIC and the metal layer of the disc.

An equivalent circuit model for the whole structure can be obtained by means of a series connection of the disc (described previously) and the tag circuit models, as shown in Fig. 4(a). Notice that due to the series connection between the disc and the tag, the impedance seen from the ASIC is simply Z_A = RA+JXA, where RA = Rtag+R'disc and XA = tag+X'disc- Differential excitation and symmetry considerations, allows to apply the electric wall concept (Fig. 4(b)). Although the disc can still be modeled by an RLC parallel resonator, the parameter values vary due to the presence of the tag. In particular, a shorter effective current path on the metal disc results as a function of the capacitor length l_c. Thus, the new values of inductance L'_p and resistance R_p' are smaller compared to L_p and R_p. The resulting disc impedance is designated as Z'd_hc = R 'disc+jX'disc- The tag capacitance C_tag and inductance L_tag can be controlled by means of simple geometrical parameters (e.g., the area of the semi-lumped capacitor and the width of the semi-lumped inductor). By working in the vicinity of the LC series resonance, it is possible to control the tag series reactance X_tag, which can be inductive or capacitive. Thus, the total reactance XA = X'disc+Xtag can be adjusted in order to match the capacitive reactance Xc of the RFID chip (ASIC). Since ohmic losses of the LC resonator are omitted in the model, the resistive part of the antenna system R_A is directly equal to R 'disc- Its value cannot be easily controlled for impedance matching, since it presents a very small variation as a function of the tag geometry. However, according to the simulation results, the value of R 'disc at UHF is in the range of 8-12 Ohms, thus providing a good matching level with many ASICs available on the market, without the need for an additional matching network.

It is important to note that the circuit proposed above is a simplified circuit model which allows an easy design of the tag in the UHF-RFID band. In order to describe the impedance behavior of the system in a wider band, it is necessary to take into account an additional capacitance introduced by the presence of the tag conductors, which is in the order of 100 fF and is placed in shunt with the impedance Z_A.

This equivalent circuit models of Figs. 4(a) and 4(b) are valid for the two layouts shown in Figs. 5a and 5b (which will be described later in more detail), while for the layout of Fig. 5c an additional capacitance in parallel between the integrated circuit M and impedance Z_A must be included in the equivalent circuit, due to the inclusion of the impedance matching element placed parallel to the chip M, which implements a capacitor C3. Layout synthesis and simulation results:

The tags presented in this section, generally indicated as D in the appended Figures, with reference to Figs. 5a to 5c, are some examples of layout synthesis based on the concepts illustrated in the previous section. The layout simulated and finally synthetized is the one of Fig. 5a, although the ones of Figs. 5b and 5c, and many other possible modifications thereof (regarding the shape of the different portions shown, their number and their geometric parameters), are also covered by the present invention, for other embodiments.

Said three embodiments of Figs. 5a, 5b and 5c have in common that each of the first and second electrical conductors of their RFID tag D include at least the next elements:

- a capacitive portion comprising a first annular portion C1 ; C2 defining an arch whose geometric centre coincides with the geometric centre of the central hole of the optical disc OD, the first annular portion C1 ; C2 running over the electrically non- conductive outer annular area F of one of the polycarbonate layers Z1 , Z2 (preferably the top layer Z1 ) up to the outer peripheral edge Ne of the inner annular area N (for other embodiments, not shown, it can enter said inner annular area N); and

- an inductive portion comprising:

- a second annular portion Li b; L2b defining an arch whose geometric centre coincides with the geometric centre of the central hole of the optical disc OD, the second annular portion Li b; L2b running over the inner annular area N and having a first end coupled to the integrated circuit M, which is placed adjacent the central hole, for the embodiments of Figs. 5a and 5b, or to an impedance matching element C3 placed parallel thereto, for the embodiment of

Fig. 5c; and

- an intermediate straight or meandered portion L1 a; L2a (if meandered inductance increases) with a first end connected to a second end of said second annular portion Li b; L2b and a second end connected to a middle point of said first annular portion C1 ; C2.

For other embodiments, not illustrated, the shape and geometric parameters of the capacitive and inductive portions can differ from the ones illustrated and described above (for example the capacitive portions can be non-ring portions), all of them can follow different paths, straight, curved, meandered, etc., with the purpose of optimizing parameters (bandwidth, radiation efficiency, etc.) and/or provide a good matching for a specific chip model (that can be the case of the inclusion of the matching capacitor C3 of Fig. 5c).

The next description refers to the layout of Fig. 5a, although with the addition of a intermediate substrate S (as in Fig. 7b) onto which the chip M and conductive portions C1 , C2, L1 a, Li b, L2a and L2b are attached.

Although it is not possible to achieve a perfect symmetry of the tag that guarantees that the barycenter is located at the center of the disc, the layout design has been focused on optimizing it. The chip, which introduces mass unbalance, has been chosen to be placed as near as possible to the rotation center of the disc, i.e., adjacently to the inner hole of the polycarbonate structure, in order to minimize the barycenter shift of the system. The electrical connection between the coupling capacitance C_tag (at the edge of the tag) and the ASIC (at the center of the tag) was designed in order to work as the tag inductance L_tag, resulting in a very simple final layout (Fig. 5a). The UHF-RFID chip used in the design is the NXP UCODE G2XM [14]; according to the manufacturer the input impedance is Z_c = 16-/148 Ω and the threshold power level at 915 MHz is P_th = -15 dBm. However, the value of P_th for this ASIC has been experimentally measured over the UHF band [16], yielding results in the order of -13 dBm at the center frequency. A parallel RC type circuit model can be used to describe the ASIC input impedance in the whole UHF-RFID band [17]-[18]. Based on the input impedance at 915 MHz, the values were found to be R_c = 1385 Ω and C_c = 1.16 pF.

The substrate S used for the prototype is the Rogers RO3010 (with thickness t = 0.254 mm, s_r = 10.2, ΐαηδ = 0.0023), which has a structural function only. In fact, the high permittivity of the material does not have a role, neither in size reduction, nor in increasing the tag performance. Instead, it was chosen as a mechanical support for fabrication. Commercial versions of the tag can be engineered without any substrate, i.e. directly attached to the disc surface, as shown in Figs. 5a-5c and 7a, such as by screen- printing or by another technique known in the art, in order to minimize the cost.

Once the layout strategy has been defined, the problem presents 3 degrees of freedom (l_c, Wc, W_∑) in the geometry of the tag D (see Fig. 5a). Obviously, many different solutions exist in order to obtain the required reactance. As a starting point, the value of Wc was fixed at 2 mm, an arbitrary value that ensures a minimal overlapping of the tag within the DVD graphical label layer. A proper combination of lc and W_∑ was then chosen in order to obtain the matching peak at the desired frequency (885 MHz). At this stage it is useful to consider that, as a first order approximation, the capacitor length l_c controls the capacitance value C_tag, and the line width W_∑ controls the inductance value Ltag. Final values for the geometrical parameters are Wc = 2 mm, l_c = 16.5 mm, W_∑ = 0.4 mm, 4 = 15 mm (corresponding to the diameter of the DVD inner hole) and B = 42 mm.

Simulation results, in terms of return loss, are depicted in Fig. 6(a). As expected, good matching is achieved at 885 MHz ( | s \ 2 = -9.5 dB, corresponding to τ = 1- | s \ 2 = -0.6 dB). Based on the simulated input impedance of the structure, a parameter extraction for the electrical model was performed, in order to demonstrate its validity. The extracted circuital values are 2^ = 19 nH, i?^ = 1500 Ω, C'_p = 58 fF, C_tag = 2 pF, Ltag = 20 nH. Very good agreement between electromagnetic and circuital impedance in the UHF-RFID band can be observed in Fig. 6(b).

The radiation pattern for the whole system is very similar to that of the DVD alone, as depicted in Fig. 3, demonstrating that the tag is actually using the metal layer of the DVD as the main radiating element. Radiation efficiency for the system remains reasonably high, with a value of 23% at 885 MHz, resulting in a maximum gain value of -2.4 dB. According to these results, it is possible to estimate a maximum theoretical read range of 5.4 m (considering τ = -0.6 dB, P_th = -13 dBm and EIRP = 4W), on the basis of equation (2).

- Fabrication and measurements: Experimental setup and read range equations:

The measurement setup consists of an Agilent N5182A vector signal generator, capable of generating RFID interrogation frames, connected to a TEM cell by means of a 50 Ω coaxial cable. A circulator is used to send the backscattered signal from the TEM cell to an Agilent N9020A signal analyzer, in order to decode the digital RFID frames generated by the tag, which is placed inside the TEM cell and oriented along the maximum directivity axis.

To measure the tag read range, an RFID interrogation frame is sent to the TEM cell at different power levels, in order to determine the minimum power level P_min required to activate the tag, that is, to receive a backscattered response frame. An electric field probe is then placed at the tag position in order to measure the root mean square of the electric field E_rms generated by the interrogation frame at the power P_min. Therefore, E_rms is the minimum electric field required for the tag operation, and it is possible to calculate the read range directly from its value. In fact, the average power density S associated to a plane wave is determined from the value of the electric field E_rms according to S = ^≡^ (3)

η where η is the intrinsic impedance of free space. For a radiating antenna it is possible to calculate the far field Poynting vector module in a given direction as

_ P_t ' G_t _ EIRP

7 2

4π Anr

where P_t is the total transmission power, G_t is the antenna gain and r is the distance from the antenna. Thus, the read range r can be obtained by equating (2) and (3), resulting in slWEIRP

r =— (5)

^rms

The method described above is then repeated for each frequency of interest (e.g., at 5 MHz steps) in order to obtain the read range in the whole UHF band.

Experimental results:

To validate the simulated results, the layout of Fig. 7b (i.e. the one of Fig. 5a but with the addition of the support S) was fabricated by means of a PCB drilling machine (LPKF-H100). The measurements (not shown) present a frequency shift of the tag response, with a read range peak at 955 MHz. This effect has been attributed to the intrinsic indetermination of the electrical properties of the DVD materials at the UHF band, and to the simplifications made in the modelling of the disc. In fact, unlike most substrates used for tag implementation, DVD disc materials are not intended for microwave design. Therefore, manufactures neither report nor control their electrical properties. Moreover, as discussed previously, some geometrical parameters are not specified as well (e.g., the thickness of the reflecting layer).

Based on the measured results, a second layout of the tag was synthetized and fabricated in order to correct the frequency shift. The matching position was easily adjusted by simply tailoring the length /c of the tag capacitance. The corrected value of the geometrical parameter is lc = 26 mm. The final tag, mounted on a DVD-R disc, is shown, schematically, in Fig. 7b. The values for the W∑ Wc and lc parameters are the same used for the simulations described above. The measured read range of the new layout, depicted in Fig. 8, shows the curve correctly centred around 885 MHz, with a peak value of 5.2 m, which is very close to the 5.4 m predicted from the simulation. Furthermore, the results reveal that the read range is above 3 m in the whole RFID-UHF frequency band, confirming that the tag is capable of worldwide operation with very good performance.

In order to compensate a mass unbalance produced by tag D, for making coincident the mass centre with the geometric centre of the optical disc OD, a counter mass G, whose weight and location is duly calculated, is arranged, for the embodiments of Fig. 5a and 7a, on the optical disc OD at a point Y2 (see Fig. 9) crossed by symmetry plane E.

Fig. 9 shows the symmetric current distribution generated over the optical disc of Figs. 5a and 7a for a differential and symmetric excitation thereof, where said distribution corresponding to a natural resonance modes of the disc, and also show the mass centre location, at point Y1 , and the point Y2 for the location of the counter mass G, in the symmetry plane E, to correct the shifting of the mass centre with respect to the disc rotation centre.

Although not shown in Fig. 9, said differential and symmetric excitation of the optical disc also causes a symmetric charges distribution which, together with the current distribution shown in Fig. 9 corresponds, as indicated above, to one of the natural resonance modes of the disc, due to said symmetrical and differential excitation, which, according to the present inventors, is the best manner of taking profit of the whole dimensions of the disc, due to the fact that the positive and negative charges are accumulated at opposed ends of the disc, forming a dipole having the dimensions of the whole disc, i.e. of its diameter, which provides read ranges much higher than the ones which could be achieved by the device of WO2004097731 and with any other known system.

The present invention provides a novel strategy for the design of UHF-RFID tags mounted on optical discs has been presented. The main advantages are the substantial improvement in the read range and the simplicity of layout synthesis and geometry. Moreover, the radiating element is the metallic layer of the optical disc, for both poles of its antenna. As an example of application, a tag for DVD+R discs has been designed and fabricated. The experimentally measured read range has a peak value of 5.2 m, very close to the 5.4 m predicted by electromagnetic simulation. In addition, the read range is above 3 m in the whole RFID-UHF band, that is, one order of magnitude higher than the read ranges of conventional DVD tags reported in the literature. These results confirm the validity of the proposed circuit model of the optical disc, the coupling strategy between the ASIC and the radiating element, and the equivalent circuits of either element, which are at the basis of the design approach.

The potential of the presented methodology is clear on account of the RFID chip used in the design (the one available in the inventors' laboratory), that has a relatively low performance in terms of activation power (-13 dBm), as compared to the latest commercially available ASICs (e.g. [15]). Therefore it is possible to significantly improve the read range by considering the latest RFID chips available on the market.

A person skilled in the art could introduce changes and modifications in the embodiments described without departing from the scope of the invention as it is defined in the attached claims.

Claims

'\ - An optical disc with a wireless communication device, said wireless communication device (D) being mounted on the optical disc (OD) and comprising an integrated circuit (M) and an antenna having an impedance matched to the impedance of the integrated circuit (M), wherein said antenna has at least a first and a second poles respectively comprising a first and a second electrical conductors, said first electrical conductor being contactless coupled to a metal layer (X) buried in the optical disc, such that said metal layer becomes a radiant element which constitutes, together with the first electrical conductor, said first pole of the antenna, and said second electrical conductor being also contactless coupled to said metal layer (X) buried in the optical disc (OD) such that said metal layer (X) becomes a radiant element which also constitutes, together with the second electrical conductor, said second pole of the antenna, wherein said wireless communication device (D) is attached to an external non-electrically conductive layer (Z1 ) of the optical disc (OD), said contactless coupling of the electrical conductors with the metal layer (X) buried in the optical disc (OD) being a capacitive coupling where said non-electrically conductive layer (Z1 ) acts as a dielectric layer and comprises at least an inner (N) and an outer (F) electrically non-conductive adjacent annular areas, said inner annular area (N) not overlapping the metal layer (X) and having an inner peripheral edge (Ni) delimiting a central hole of the optical disc (OD) and an outer peripheral edge (Ne), and said outer annular area (F) extending from said outer peripheral edge (Ne) of the inner annular area (N) up to the outer peripheral edge of the optical disc (OD), said outer annular area (F) overlapping said metal layer (X) buried therein,

wherein the optical disc is characterised in that each of said first and second electrical conductors includes at least two portions, differentiated at least by having different geometrical parameters, and constituting the next elements:

- a capacitive portion (C1 ; C2) having a length (l_c) and running over said outer annular area (F) up to said outer peripheral edge (Ne) of the inner annular area (N) or entering the inner annular area (N); and

- an inductive portion (L1 a-L1 b; L2a-L2b) having a width (W_L) and running over said inner annular area (N) and having a first end coupled to the integrated circuit (M), or to an impedance matching element (C3) placed parallel thereto, and a second end connected to said capacitive portion (C1 ; C2), wherein said elements of the first and second electrical conductors form a resonator producing an electrical coupling between the wireless communication device (D) and the metal layer (X) buried therein.

2. - The optical disc of claim 1 , wherein said antenna comprises only said first and second poles, constituting a dipole antenna defined as an antenna having a dipolar charge distribution.

3. - The optical disc of claim 1 or 2, wherein said wireless communication device (D) is attached to said external non-electrically conductive layer (Z1 ) of the optical disc (OD) directly or through an intermediate non-electrically conductive support (S) attached thereto.

4. - The optical disc of claim 3, wherein said dipole antenna is a symmetrical dipole antenna, said wireless communication device (D) being arranged over said external non-electrically conductive layer (Z1 ) of the optical disc (OD), directly or through said intermediate non-electrically conductive support (S), symmetrically with respect to a symmetry plane (E) which passes through the geometric centre of the optical disc (OD) orthogonally with respect to the plane containing said external non-electrically conductive layer (Z1 ).

5. - The optical disc of claim 3 or 4, wherein said wireless communication device (D) is configured and arranged, with respect to said metal layer (X) of the optical disc (OD), to differentially and symmetrically excite a natural resonance mode of the optical disc (OD) such that an electrical dipole is formed with an effective distance between the poles thereof substantially coincident with the diameter of the optical disc (OD).

6. - The optical disc of any of the previous claims, wherein the ratio of length/width (IJWL ) between said capacitive portion (C1 ; C2) and said inductive portion (L1 a-L1 b; L2a-L2b) is of at least 10.

7. - The optical disc of any of the previous claims, wherein:

- said capacitive portion (C1 ; C2) comprises a first annular portion (C1 ; C2) defining an arch whose geometric centre coincides with the geometric centre of the central hole of the optical disc (OD), said first annular portion (C1 ; C2) running over said outer annular area (F) up to said outer peripheral edge (Ne) of the inner annular area (N) or entering the inner annular area (N); and

- said inductive portion (L1 a-L1 b; L2a-L2b) comprises:

- a second annular portion (Li b; L2b) defining an arch whose geometric centre coincides with the geometric centre of the central hole of the optical disc (OD), said second annular portion (Li b; L2b) running over said inner annular area (N) and having a first end coupled to the integrated circuit (M), which is placed adjacent the central hole, or to said impedance matching element (C3) placed parallel thereto; and

- an intermediate straight or meandered portion (L1 a; L2a) with a first end connected to a second end of said second annular portion (Li b; L2b) and a second end connected to a middle point of said first annular portion (C1 ; C2).

8. - The optical disc of any of the previous claims, wherein said resonator is a series stepped-impedance resonator, where each of said capacitive portions (C1 ; C2) constitutes, together with the inner metal layer (X) and the non-electrically conductive layer (Z1 ) of the optical disc (OD), a semi-lumped capacitor (C_tag) and each of said inductive portions (L1 a-L1 b; L2a-L2b) constitutes a semi-lumped series inductance (Ltag), whose geometrical parameters are chosen to provide an appropriate impedance matching between the integrated circuit (M) and the metal layer (X) of the optical disc (OD) for a desired radio frequencies band.

9. - The optical disc of any of the previous claims, comprising a counter mass (G) arranged on the optical disc (OD) at a point (Y2) crossed by said symmetry plane (E), whose weight and location is calculated to compensate a mass unbalance produced by the wireless communication device (D), in order to make coincident the mass centre with the geometric centre of the optical disc (OD).

10. - The optical disc of any of the previous claims, wherein said wireless communication device (D) is a RFID tag.

11. - A wireless communication device to be mounted on an optical disc, comprising an integrated circuit (M) and an antenna having an impedance matched to the impedance of the integrated circuit (M), said wireless communication device (D) being configured and arranged to be mounted on an optical disc (OD), wherein said antenna has at least a first and a second poles respectively comprising a first and a second electrical conductors, said first electrical conductor being configured and arranged for being contactless coupled to a metal layer (X) buried in the optical disc (OD) such that said metal layer (X) becomes a radiant element which constitutes, together with the first electrical conductor, said first pole of the antenna, said second electrical conductor being also configured and arranged for being contactless coupled to said metal layer (X) buried in the optical disc (OD) such that said metal layer (X) becomes a radiant element which also constitutes, together with the second electrical conductor, said second pole of the antenna, wherein said wireless communication device (D) is configured and arranged to be attached to an external non-electrically conductive layer (Z1 ) of the optical disc (OD), said contactless coupling of the electrical conductors with the metal layer (X) buried in the optical disc (OD) being a capacitive coupling where said non-electrically conductive layer (Z1 ) acts as a dielectric layer and comprises at least an inner (N) and an outer (F) electrically non-conductive adjacent annular areas, said inner annular area (N) not overlapping the metal layer (X) and having an inner peripheral edge (Ni) delimiting a central hole of the optical disc (OD) and an outer peripheral edge (Ne), and said outer annular area (F) extending from said outer peripheral edge (Ne) of the inner annular area (N) up to the outer peripheral edge of the optical disc (OD), said outer annular area (F) overlapping said metal layer (X) buried therein,

wherein the wireless communication device (D) is characterised in that each of said first and second electrical conductors includes at least two portions, differentiated at least by having different geometrical parameters, and constituting the next elements:

- a capacitive portion (C1 ; C2) having a length (l_c) and configured and arranged for running over said outer annular area (F) up to said outer peripheral edge (Ne) of the inner annular area (N) or entering the inner annular area (N); and

- an inductive portion (L1 a-L1 b; L2a-L2b) having a width (W_L) and configured and arranged for running over said inner annular area (N) and having a first end coupled to the integrated circuit (M), or to an impedance matching element (C3) placed parallel thereto, and a second end connected to said capacitive portion (C1 ; C2),

wherein said elements of the first and second electrical conductors form a resonator producing an electrical coupling between the wireless communication device (D) and the metal layer (X) buried therein.

12. - The wireless communication device of claim 11 , wherein it constitutes the wireless communication device (D) of the optical disc (OD) of any of claims 1 to 10.

13. - A method for the design and fabrication of a wireless communication device mounted or to be mounted on an optical disc, characterised in that the wireless communication device (D) is said wireless communication device of claim 11 or 12, and in that the method comprises, for a desired radio frequencies band:

- constructing an electrical model of said optical disc (OD), which describes approximately its input impedance;

- constructing an electrical model of the wireless communication device (D) plus part of the optical disc (OD) which is electrically coupled thereto when the wireless communication device (D) is mounted thereon;

- obtaining a combined equivalent electrical model for the whole structure by serially connecting said electrical model of the wireless communication device (D) and said electrical model of the optical disc (OD), the latter with the values of its parameters varied by taking into account the presence of the wireless communication device (D); - performing an impedance matching between the impedance of the integrated circuit of the wireless communication device (D) and the impedance of the combined equivalent electrical model by adjusting the parameters of the electrical model of the wireless communication device (D) plus part of the optical disc (OD) electrically coupled thereto; and

- fabricating the wireless communication device (D) according to said adjusted parameters.

14.- The method of claim 13, wherein said electrical model of the wireless communication device (D) plus part of the optical disc (OD) is a semi-lumped element approximation of a series stepped-impedance resonator producing an electrical coupling between the wireless communication device (D) and the optical disc metal layer (X), and which comprises, for each of the first and second electrical conductors of the antenna of the wireless communication device (D), the next elements:

- a capacitive portion (C1 ; C2) designed to run, directly or through an independent intermediate non-electrically conductive support (S), over at least an outer annular area (F) of an external non-electrically conductive layer (Z1 ) of the optical disc (OD), said outer annular area (F) overlapping the metal layer (X), up to an outer peripheral edge (Ne) of an adjacent inner annular area (N) of the external non- electrically conductive layer (Z1 ), which doesn't overlap the metal layer (X), or entering said inner annular area (N); and

- an inductive portion (L1 a-L1 b; L2a-L2b) designed to run over said inner annular area (N), directly or through said intermediate non-electrically conductive support (S), and having a first end coupled to the integrated circuit (M), or to an impedance matching element (C3) placed parallel thereto, and a second end connected to said capacitive portion (C1 ;C2);

where each of said capacitive portions (C1 ; C2) constitutes, together with the inner metal layer (X) and the non-electrically conductive layer (Z1 ) of the optical disc (OD), a semi-lumped capacitor (C_tag) and each of said inductive portions (L1 a-L1 b; L2a-L2b) constitutes a semi-lumped series inductance (L_tag), the method comprising:

- performing said adjusting of the parameters of the electrical model of the wireless communication device (D) plus part of the optical disc (OD) by selecting the geometry and adjusting the geometrical parameters of said capacitive (C1 ; C2) and inductive (L1 a-L1 b; L2a-L2b) portions; and

- fabricating said capacitive (C1 ; C2) and inductive (L1 a-L1 b; L2a-L2b) portions with said adjusted geometrical parameters, and attaching the so fabricated portions and the integrated circuit (M) of the wireless communication device (D) directly on said external non-electrically conductive layer (Z1 ) of the optical disc (OD) or on said independent intermediate non-electrically conductive support (S) to be attached thereto.

15.- The method of claim 14, wherein:

- said capacitive portion (C1 ; C2) comprises a first annular portion (C1 ; C2) defining an arch whose geometric centre coincides with the geometric centre of the central hole of the optical disc (OD), said first annular portion (C1 ; C2) being designed to run over said outer annular area (F) up to said outer peripheral edge (Ne) of the inner annular area (N) or entering the inner annular area (N); and

- said inductive portion (L1 a-L1 b; L2a-L2b) comprises:

- a second annular portion (Li b; L2b) defining an arch whose geometric centre coincides with the geometric centre of the central hole of the optical disc (OD), said second annular portion (Li b; L2b) being designed to run over said inner annular area (N) and having a first end coupled to the integrated circuit (M), which is placed adjacent the central hole, or to said impedance matching element (C3) placed parallel thereto; and

- an intermediate straight or meandered portion (L1 a; L2a) with a first end connected to a second end of said second annular portion (Li b; L2b) and a second end connected to a middle point of said first annular portion (C1 , C2).