Feasibility Study of a Fully Organic Frequency Doubler ... - IEEE Xplore

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fully organic, low cost, frequency doubler for harmonic ... Shottky diode able to double the frequency of the received ... electronic circuit for RFID applications.
Feasibility study of a fully organic frequency doubler for harmonic RFID applications L. Roselli 1, F. Alimenti 1, M. Virili 1, F. Lolli 1, B. Popescu 2, D. Popescu 2, S. Locci 2, P. Lugli 2 1

Department of Electronic and Information Engineering of the University of Perugia, Italy 2 Institute for Nanoelectronics, Technische Universitat Munchen, Munich, Germany

Abstract — This paper describes a feasibility study of a fully organic, low cost, frequency doubler for harmonic RFID applications. The proposed structure is formed by two antennas (RX and TX) printed on paper and an organic Shottky diode able to double the frequency of the received signal. To the author knowledge this contribution proofs for the first time the feasibility of a fully organic non linear electronic circuit for RFID applications.

Therefore low cost RFID transponders are one of the major goals for OE. In this paper an organic frequency doubler for a harmonic RFID tag is presented; a brief description of the complete system (transponder plus reader) is first given, then we will go into the tag description (transponder antennas and organic Schottky diode); eventually the expected results will be shown.

Index Terms — RFID tags, microwave antennas, diodes, inductors, resonator filters, organic electronics, pentacene.

II. HARMONIC RFID

Similarly to a conventional RFID system, the harmonic RFID concept [1] consists of a reader, able to interrogate a tag. The difference between the conventional RFID and the harmonic one is that the latter doubles the frequency of the received signal and sends back the information just utilizing a self-generated harmonic of the signal received by the reader.

I. INTRODUCTION In the last years several idioms came out in the area of ICT. Among them: Internet of Things (IoT), Ubiquitous Electronics (UE), Anywhere, Anything, Anytime (3As)... All these concepts have the common characteristic of an enormous demand of electronics diffused everywhere, not easily traceable neither recoverable. It is evident that a barrier to the development of these disruptive approaches is the non complete environmental compatibility of present electronic systems based on traditional materials. In this scenario Organic Electronics (OE) appears a natural way to overcome the abovementioned limitations; thus enabling a vast variety of new solutions for old and new societal issues in many field of daily life. OE, in fact, is based on the combination of new materials (paper, PET, etc. for substrates and packages and organic semiconductors for electronic devices) and potentially cost-effective industrial processes suitable for massively distributed solutions (above all: inkjet printing and spraying). OE hence represents a technological platform to face the most of ICT related challenges of the future. A question comes up spontaneously at this point: “which is the ICT related architectural approach suitable for combination with OE?” The answer of course is not unique, but certainly RadioFrequency IDentification (RFID) systems are becoming increasingly common and are well recognized as one of the most suitable approaches to cope with new ICT challenges in a non conventional way.

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CONCEPT

Fig. 1.

Harmonic RFID concept.

Such a system, with apparently no information added on the back scattered harmonic (just the signal itself) is often called “harmonic radar” [2]; in fact, this system, like a radar, provides information about tag presence. It is worth mentioning that this feature would not be possible with a conventional tag without information modulated on a carrier, because, in this case, the reflected signal from the

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The antenna structure conceived can be seen in the part of Fig. 1 representing the tag (the part on the right) and consists of two inductors, each of them in parallel with two parallel plate capacitors, implementing the two tag aerial resonators. First a campaign of full-wave simulations using commercial FDTD based software has been carried on in order to refine the dimensions of the inductors. To design the inductors a value of 47 pF have been assumed for the capacitors (the capacitors can be precisely dimensioned only after the device modeling, to take into account the parasitic effects of the organic diode). Table II summarizes the relevant characteristics of the inductors.

tag would be confused with the back scattering from the environment. Fig. 1 summarizes the harmonic RFID concept. III. SYSTEM DESCRIPTION The whole system, consists of a reader and a tag. The reader sends a signal at f0 frequency (7.5 MHz) and is able to receive the signal at 2f0 frequency. The tag has a receiving antenna tuned at f0 detecting the signal coming from the reader. This signal drives a non linearity represented by a Schottky organic diode which generates the second harmonic feeding the transmitting antenna tuned at 2f0. Fig. 2 shows a circuital representation of the whole system: The reader is modeled on the transmitting side by means of a source at f0 along with the primary circuit of a transformer; on the receiving side, instead, it is accounted for by means of a load resistor RL and the secondary circuit of a transformer. The tag is easily described by the diode and by the two resonant circuits at frequency f0 and 2f0 on the left and on the right of the diode respectively.

TABLE II RELEVANT CHARACTERISTICS OF THE INDUCTORS Parameters internal diameter number of turns track width track spacing

Inductor @ f0 140 mm 5 4 mm 2 mm

Inductor @ 2f0 60 mm 4 4 mm 2 mm

As an example the evaluation of the inductance of the 2f0 antenna as a function of the inner square spiral width is shown in Fig. 3.

Fig. 2. Circuit representation of the Harmonic RFID system.

IV. TAG MODELING

A. Antenna modeling For the sake of compactness a planar square spiral solution has been adopted. On the basis of preliminary computation [3] a first guess structure has been dimensioned accounting for paper substrates and metallizations.

Fig. 3. 2f0 antenna inductance (unit H) as a function of the inner width (“din” in the picture – unit mm) of the square spiral. The electromagnetic simulations have than been used to extract the equivalent circuits of the two resonators. Fig. 4a shows the resonator equivalent circuit and table III summarizes the relevant parameters: the inductances L and the resistances R have been obtained from the EM simulations, the capacitances C have been computed to have a resonance at f0 and 2f0 respectively. The input (coupling between reader and tag at f0) and output

TABLE I THICKNESS, r AND  OF MATERIALS Paper substrate Thickness 260 m Relative dielectric constant r 3.3 Silver-ink Thickness 2 m 7 conductivity  2.5·10 S/m

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(coupling between tag and reader at 2f0) coupled resonators have been modeled starting from the isolated resonators and using the equivalent model of the transformer proposed in [3] and shown in Fig. 4b where the coupling coefficient k depends on the distance between reader and tag and on the mutual orientation of them.

In this structure, ohmic contact for hole transport is obtained at Au-PEN heterojunction (Au= -4.9 eV) and rectifying contact is obtained at Al-PEN heterojunction (Al= -4.06 eV); being TIPS PEN-HOMO and -LUMO 5.1 eV and -3.35 eV respectively, with reference to the vacuum level. In order to extract the SPICE diode model [4] parameters, I-V and C-V device-level simulations have been performed by means of a modified drift-diffusion model implemented in the industry tool CENTAURUS (former ISETCAD). The simulator input parameters are: the device geometry, the electronic levels in pentacene, and the properties of the material interfaces. The simulations allow the I-V and the C-V characteristics to be obtained. SPICE model parameters are summarized in table IV.

TABLE III VALUES OF L, R, C L R C

Resonator @ f0 10.62 nH 4.69  43 pF

Resonator @ 2f0 2.23 nH 1.17  50 pF

TABLE IV DIODE MODEL PARAMETERS Saturation current IS Ohmic resistance RS Emission coefficient N Transit time Tt Linear capacitance Cd Zero-bias junction capacitance Cj0 Junction potential Vj Grading coefficient M Parallel capacitance Cp Fig. 4. a) Equivalent circuit of the resonator, b) equivalent circuit of the coupling transformer (b).

-12

2.28·10 122  1.08 5 ns 2.62 pF 2.18 pF 0.48 V 0.85 8 pF

A

C. System modeling The schematic of the complete model is shown in Fig. 6. The sections on the right and on the left of the two ideal transformers represent the tag and the reader respectively. The maximum transmission coefficients of the coupled resonators have been obtained for k1 and k2 equal to 0.04. Beyond this value we run into overcoupling and the transmission coefficients decrease. The reader resonators, R1 L1 C11 and R2 L2 C22, have a resonant frequency equal to f0 and 2f0 respectively; the same resonance frequencies have been obtained for the tag resonators (R1 L1 C12 and R2 L2 C21) just tuning the capacitances C12 and C21 in order to compensate the diode parasitics. Input and output impedances have been purposely tuned in order to account for optimum power transfer on the reader side. All the parameters of the schematic in Fig. 6 are summarized in Table V.

B. Diode modeling The required non-linear I-V behavior can be obtained organically by realizing a sandwich structure of pentacene between gold and aluminium contact slices (Fig. 5). In this way a Schottky barrier is formed at the Al – PEN heterojunction. The device realized is an ultra – thin diode (150 nm PEN layer) with an area of 4500 μm2.

Fig. 5. Organic Schottky diode structure.

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The performance of a harmonic doubler is described by the conversion loss defined as the difference between the input and output power expressed in decibel. The conversion loss as a function of the input power is shown in Fig. 7. For a radiated power between -25 and -10 dBm at an optimum tag reader distance of 20 cm it is equal to 10 dB thus confirming that a fully organic realization of a tag is consistent with practical RFID applications. VII. CONCLUSION Fig. 6.

This contribution aims at proposing a fully organic realization of an RFID tag in view of future massively distributed electronic applications. An harmonic RFID tag, as a clear example of subsystem requiring aerial realization along with non linear electronic devices has been adopted. Rigorous modeling of aerials as well as of non linear Schottky organic diode have been applied to extract relevant lumped element equivalent circuit parameters. Eventually simulation of the whole structure, including tag-reader coupling has been performed. The obtained results testify the feasibility of a fully organic RFID tag by using present technologies and opens new horizons in the scenario of future ICT applications and solutions.

Schematic of the complete model.

TABLE V CIRCUIT PARAMETERS Input impedance RS Output impedance RL Reader resonator f0 capacitance C11 Reader resonator 2f0 capacitance C22 Tag resonator f0 capacitance C12 Tag resonator 2f0 capacitance C21 Reader/tag resonator f0 inductance L1 Reader/tag resonator 2f0 inductance L2 Reader/tag resonator f0 resistance R1 Reader/tag resonator 2f0 resistance R2 Transformer f0 coupling coefficient k1 Transformer 2f0 coupling coefficient k2

10 k 3.5 k 43 pF 50 pF 28 pF 43 pF 10.62 nH 2.23 nH 4.69  1.17  0.04 0.04

ACKNOWLEDGEMENT The authors wish to acknowledge Prof. Luca Pierantoni for the stimulating discussions we had on the material dealth with in this contribution.

V. RESULTS

REFERENCES [1] L. Pierantoni, D. Mencarelli, T. Rozzi, F. Alimenti, L. Roselli, P. Lugli, “Multiphysics analysis of harmonic RFID tag on paper with embedded nanoscale material,” in Proc. of the 5th European Conf. on Antennas and Propagation, April 2011. [2] B.G. Colpitts, G. Boiteau, “Harmonic radar transceiver design: miniature tags for insect tracking,” IEEE Trans. on Antennas and Propagation, vol. 52, no. 11, pp. 2825-2832, November 2004. [3] I. Bahl, “Lumped Elements for RF and Microwave Circuits”, Artech House, 2003. [3] P. Antognetti, G. Massobrio, “Semiconductor Device Modeling with SPICE”, Second Edition, McGraw-Hill, 1993

Fig. 7. Simulated behavior of the RFID system with a fully organic TAG and a reader optimized for the maximum power transfer.

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