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The circuit exploits a distributed microstrip structure that is fabricated using a copper ... II. CIRCUIT DESIGN. The frequency doubler schematic is shown in Fig. 2.
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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 24, NO. 12, DECEMBER 2014

Low-Power Frequency Doubler in Cellulose-Based Materials for Harmonic RFID Applications Valentina Palazzi, Federico Alimenti, Senior Member, IEEE, Paolo Mezzanotte, Member, IEEE, Marco Virili, Chiara Mariotti, Student Member, IEEE, Giulia Orecchini, and Luca Roselli, Senior Member, IEEE

Abstract—This letter presents the design of a Schottky diode frequency doubler suitable for harmonic RFID tags. A microwave frequency doubler is implemented in a cellulose-based (paper) substrate, i.e., an ultra-low cost, recyclable and biodegradable material. The circuit exploits a distributed microstrip structure that is fabricated using a copper adhesive laminate to have low conductor losses. The measurements show a conversion loss of 13.4 dB at the output frequency of 2.08 GHz. This is achieved with an available input power of 10 dBm only. Finally a harmonic RFID experiment proves a reading range of 50 cm, obtained by transmitting 0 dBm and receiving a second harmonic of 60 dBm, i.e., well above the sensitivity of a typical microwave receiver. Index Terms—Cellulose materials, flexible substrates, frequency multipliers, green electronics, harmonic RFID, internet of things (IoT), paper-based substrates, Schottky diodes.

I. INTRODUCTION

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VER the years the intriguing idea of Ubiquitous Intelligence (UI) has been pushing forward technological advances and investments, particularly in Radio Frequency IDentification (RFID) systems, and has given rise to new collateral concepts, like Wireless Sensor Networks (WSN), Internet of Things (IoT), smart items and Machine-to-Machine (M2M) communication. To accomplish the above evolution, however, there are two limiting factors that must be overcome. Firstly, the diffusion of distributed (and often disposable) sensors imposes a tremendous cost reduction. Secondly, designers have to consider the pollution risks involved in the pervasive and uncontrolled presence of electronic devices everywhere in the environment. The latter problem can be faced using recyclable and biodegradable materials such as cellulose-based composites. Cellulose, the most common natural polymer, is biodegradable, flexible and very cheap, thus representing an interesting opportunity for the fabrication of disposable wireless sensors [1], [2]. In this framework, several RFID systems were proposed, based on the harmonic generation concept [3], [4]. The main advantage of these is the immunity to clutter returns, i.e., the capability to clearly detect the tag even at medium reading Manuscript received June 12, 2014; revised August 18, 2014; accepted September 25, 2014. Date of publication October 20, 2014; date of current version December 01, 2014. This work was supported in part by the GRETA project (PRIN, call 2011) and by Agilent Technologies and Computer Simulation Technologies (CST). The authors are with the Department of Engineering, University of Perugia, Perugia 06125, Italy (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LMWC.2014.2361431

Fig. 1. 1 b harmonic RFID system. The main system parameters are: transreceived power; distance; specific antenna gain ( mitted power; fundamental, 2nd harmonic, tag, reader).

ranges. The basic architecture of a 1 b harmonic RFID system is shown in Fig. 1 and consists of a reader illuminating the and having the receiver scene at a fundamental frequency . As a consequence, only tags able to generate tuned to a second harmonic can be detected. The passive frequency block in Fig. 1) is a critical building block since doubler ( its performance contributes to the determination of the reading range. In particular, it should feature a low conversion loss even at low input power with a minimum component count and board area [5]. In this work a microwave Schottky diode frequency doubler in microstrip technology is designed and implemented by using a flexible cellulose-based (i.e., photo paper) material. The prior art is related to 3–30 MHz circuits [6] or quasi-optical devices [7] only. The doubler adopts a distributed circuit for the matching and harmonic termination networks and requires two additional components (a diode and a coupling capacitor). The layout is fabricated with a copper (Cu) adhesive laminate ( 5.8 10 S/m) shaped by a photo-lithographic process and then transferred to the hosting substrate via a sacrificial layer [8]. II. CIRCUIT DESIGN The frequency doubler schematic is shown in Fig. 2. A HSMS-2850 Schottky diode from Agilent [9] is adopted to perform the frequency multiplication, exploiting the non-linearity of its I–V characteristic. The circuit works as follows: the diode distorts the input waveform at the fundamental frequency . Such a distortion generates harmonics and the two 4 stubs (at ), placed on both sides of the diode, select the proper frequency component. The short-circuit stub at the input side tone to reach the diode, whereas ( in Fig. 2) allows the toward the output. On the it reflects the second harmonic other hand, the open-circuit stub at the output side ( in Fig. 2), short-circuits the component without affecting the signal , the (i.e., is an open circuit for it). This happens because, at above stubs are half wave long. Stubs isolates input and output

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PALAZZI et al.: LOW-POWER FREQUENCY DOUBLER IN CELLULOSE-BASED MATERIALS FOR HARMONIC RFID APPLICATIONS

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Fig. 2. Schematic of the developed frequency doubler. The main circuits pa0.4 mm, 51 mm, for the short-circuit stub; 1 mm, rameters are: 49.4 mm, for the open-circuit stub. The Input Matching Network 0 mm and (IMN) is made with a tapped impedance transformer having 50.2 mm (tap at 14.4 mm from ground). The Output Matching Network 6.8 nH and 0.9 pF. (OMN) is a LC circuit with

Fig. 3. Conversion loss of the frequency doubler versus the load impedance , i.e., the impedance seen by the diode looking toward the is reported as the x-axis, whereas the is used output matching network. 10 dBm at as a parameter. The input signal has an available power 1.04 GHz, with a multiplying factor 2. A minimum conversion loss 160 80 . of 13.4 dB is obtained for an optimum impedance

Fig. 4. Layout (left panel) and prototype fabricated in paper substrate (right panel). The layout of the distributed microstrip structure is folded to reduce the area occupation. The Schottky diode is soldered on the Cu laminate as in standard PCBs. Active area: 18 mm in length and 19 mm in width.

from each other facilitating the design of matching circuits on both sides. 1.04 GHz and for an availThe doubler is targeted for able input power 10 dBm. Such a frequency is chosen to demonstrate operation in the low-GHz frequency range, thus proving the potential of the proposed technology. The Input Matching Network (IMN) consists of a tapped impedance transsource former, the purpose of which is to increase the 50 impedance to the conjugate, large-signal input impedance of the

Fig. 5. Comparison between measurements and simulations: conversion loss versus input power (top panel) and reflection coefficient versus frequency dBm level the (bottom panel). For an input signal at 1.04 GHz with a dB. measured conversion loss is 13.4 dB, whereas

diode. This is about 266 in magnitude (i.e., 255 ) for the considered input power. The tapped transformer can be completely printed and is superior to L-type lumped networks, especially for high impedance ratios. The Output Matching Network (OMN), instead, is used to (output) port impedance in the optimum transform the 50 that gives the minimum conversion loss for load impedance the diode. To this purpose an analysis was performed by the Harmonic-Balance (HB) simulator. According to the load-pull methodology, the diode was terminated (output side) on a com. Then both and were plex impedance 350 and varied within a certain range (i.e., 50 150 ) and, for each impedance value, the conver0 sion loss was determined. Fig. 3 clearly shows that the minimum conversion loss is obtained for an optimum impedance 160 80 . Finally a simple LC network ( and in Fig. 2) is designed to transform the port impedance into the opalso acts as a dc return for the diode. timum load. Note that ) are All the components of Fig. 2 (except the diode and implemented as distributed elements. The equivalent microstrip parameters are quoted in [8] and account for both the cellulose

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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 24, NO. 12, DECEMBER 2014

Fig. 6. Harmonic RFID experiment: received signal power versus tag-to-reader distance . The system parameters, according to Fig. 1, are: 4.9 dBi, 4.3 dBi, 0 dBm, 1.04 GHz. The experiment is in good agreement with the link-budget based on the Friis’ formula. Similarly . to radar, the received power decays as TABLE I COMPARISON WITH THE STATE-OF-THE-ART

substrate and the adhesive Cu laminate. The circuit parameters are reported in the caption of Fig. 2. III. RESULTS To validate the proposed circuit, the layout is fabricated and transferred from the Cu laminate to the paper substrate via a sacrificial layer, see Fig. 4. A sheet of adhesive Cu is attached to the bottom side of the paper substrate to realize the microstrip ground plane. The via-hole contacts are implemented by wire soldering. The measurements of the implemented frequency doubler are shown in Fig. 5. In particular, the top panel represents the conversion loss as a function of the available power of the input source, whereas the bottom panel depicts the input reflection coversus frequency. The CAD study was carried-out efficient in two steps exploiting a co-simulation approach. Firstly, the distributed part of the microstrip circuit was analyzed with electromagnetic solvers (both CST Microwave Studio and ADSMomentum). Then the computed results were imported within ADS, interfaced with the diode model and simulated with the Harmonic Balance. For an input power of 10 dBm, the measurements show a minimum conversion loss of 13.4 dB, the ADS co-simulation indicates 13.5 dB whereas the CST co-simulation gives 12.8 dB. Once experimentally validated, the frequency doubler was assembled with suitable antennas, according to the harmonic RFID configuration of Fig. 1. In the tag, a helix antenna (gain 4.9 dBi) was used at the fundamental frequency, while a patch antenna (gain 4.3 dBi) was adopted for the second harmonic.

The reader antennas are equal to the tag ones. These antenna gains are typical in many RFID systems. The reader transmitter was implemented with a HP8657A source and the receiver was an Agilent N9320B Spectrum Analyzer. The transmitted power was 0 dBm and the tag-to-reader distance was varied. Fig. 6 shows the measured power as a function of distance with a calculated curve. The calculation is based on the Friis’ formula and on the assumption of a constant conversion loss of 13.4 dB. Both the measured and calculated curve show that the received power is above the noise floor of a typical receiver below tag-to-reader distances of 0.5 m. Finally, a comparison with the state-of-the-art for passive frequency multipliers is reported in Table I [10]. Note that, the cellulose-based frequency doubler compares well with the standard designs with respect to all the main performances. On the other hand it uses a low-cost green material, opening the door to a variety of applications involving RFID. IV. CONCLUSION In this letter a microwave frequency doubler in microstrip technology is implemented exploiting a cellulose-based (i.e., paper) substrate. The doubler uses a circuitry in adhesive copper laminate for low conductor loss and requires only two external devices (Schottky diode and capacitor). The experiments show a conversion loss of 13.4 dB at the output frequency of 2.08 GHz. This result is obtained for an input power of 10 dBm and enables the realization of green harmonic tags for real environments and applications. REFERENCES [1] F. Alimenti, P. Mezzanotte, S. Giacomucci, M. Dionigi, C. Mariotti, M. Virili, and L. Roselli, “24-GHz single-balanced diode mixer exploiting cellulose-based materials,” IEEE Microw. Wireless Compon. Lett., vol. 23, no. 11, pp. 596–598, Nov. 2013. [2] F. Alimenti, C. Mariotti, P. Mezzanotte, M. Dionigi, M. Virili, and L. Roselli, “A 1.2 V, 0.9 mW UHF VCO based on hairpin resonator in paper substrate and Cu adhesive tape,” IEEE Microw. Wireless Compon. Lett., vol. 23, no. 4, pp. 214–216, Apr. 2013. [3] J. Song, V. Viikari, N. Pesonen, I. Marttila, and H. Seppa, “Optimization of wireless sensors based on intermodulation communication,” IEEE Trans. Microw. Theory Tech., vol. 61, no. 9, pp. 3446–3452, Sep. 2013. [4] F. Alimenti and L. Roselli, “Theory of zero-power RFID sensors based on harmonic generation and orthogonally polarized antennas,” Progress Electromag. Res., vol. 134, pp. 337–357, 2013. [5] S. Presas, “Microwave Frequency Doubler Integrated With Miniaturized Planar Antennas,” M.S. thesis, Tampa, FL, USA, May 2008. [6] M. Virili, G. Casula, C. Mariotti, G. Orecchini, F. Alimenti, P. Cosseddu, P. Mezzanotte, A. Bonfiglio, and L. Roselli, “7.5–15 MHz organic frequency doubler made with pentacene-based diode and paper substrate,” in Proc. IEEE Int. Microw. Symp., Tampa Bay, FL, USA, June 2014, pp. 1–4. [7] G. Orecchini, V. Palazzari, A. Rida, F. Alimenti, M. M. Tentzeris, and L. Roselli, “Design and fabrication of ultra-low cost radio frequency identification antennas and tags exploiting paper substrates and inkjet printing technology,” IET Microw. Antennas Propag., vol. 5, no. 8, pp. 993–1001, Jul. 2011. [8] F. Alimenti, P. Mezzanotte, M. Dionigi, M. Virili, and L. Roselli, “Microwave circuits in paper substrates exploiting conductive adhesive tapes,” IEEE Microw. Wireless Compon. Lett., vol. 22, no. 12, pp. 660–662, Dec. 2012. [9] HSMS-2850 Series—Surface Mount Zero Bias Schottky Detector Diodes Agilent Technologies, 1999, [Online]. Available: [Online]. Available: http://www.semiconductor.agilent.com [10] P. Rajanaronk, A. Namahoot, and P. Akkaraekthalin, “A single-diode frequency doubler using a feed-forward technique,” in Proc. Asia-Pacific Microw. Conf., Yokohama, Japan, Dec. 2006, pp. 1413–1416.