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Bandwidth Limitations of Ultra High Frequency–Radio Frequency

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Bandwidth Limitations of Ultra High Frequency–Radio Frequency www.ietdl.org Published in IET Microwaves, Antennas & Propagation Received on 20th November 2012 Revised on 3rd May 2013 Accepted on 19th May 2013 doi: 10.1049/iet-map.2013.0158 ISSN 1751-8725 Bandwidth limitations of ultra high frequency–radio frequency identification tags Gerard Zamora1, Ferran Paredes1, Francisco Javier Herraiz-Martínez2, Ferran Martín1, Jordi Bonache1 1GEMMA/CIMITEC, Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona), Spain 2Department of Signal Theory and Communications, Carlos III University in Madrid, Leganés (Madrid), 28911, Spain E-mail: [email protected] Abstract: Fundamental bandwidth limitations on the design of ultra high frequency (UHF) radio frequency identification (RFID) tags are investigated. This study was conducted by considering the optimum equivalent-circuit network necessary for bandwidth broadening in single resonant UHF–RFID tags with conjugate matching. This equivalent-circuit network is simply a parallel combination of an inductor and a resistor cascaded to the UHF–RFID chip. Bandwidth optimisation of the resulting network is validated by means of the Bode criterion. According to this analysis, a broadband UHF–RFID tag prototype able to operate worldwide is designed and fabricated. In order to reduce tag size and cost, the external matching network is omitted in the reported implementation. The measured read ranges are above 5 m within the whole UHF–RFID frequency band (840– 960 MHz), with a peak value of 9 m at 870 MHz. 1 Introduction reader can read information from the tag. Since the impedance of RFID chips is complex and depends on the Radio frequency identification (RFID) is a rapidly developing manufacturer, specific RFID tag antennas must be designed technology that provides wireless identification and tracking for the considered chips available on the market. RFID tags capability [1]. Although the first paper on the basic are typically designed to achieve conjugate matching principles of passive RFID technology was published in between the chip and the antenna (this maximises the read 1948 [2], it took a long time before the technology range). Several techniques for achieving complex advanced to the current level [3]. Nowadays, many impedance matching by means of matching networks can applications, such as electronic toll collection, asset be found in the literature [6–8]. In [8], matching at two of identification, retail item management, access control, the UHF–RFID regulated bands (Europe and USA) was animal tracking and vehicle security, among others, require considered, and it was demonstrated that optimum matching RFID systems [4]. can be achieved by using dual-band tags, rather than A passive back-scattered RFID system consists of a reader broadband ones. Nevertheless these matching networks and a tag, which includes an antenna and an application introduce losses, which may degrade the tag performance. specific integrated circuit (ASIC) chip. The reader transmits However, apart from the losses introduced by the matching a modulated signal with periods of un-modulated carrier, network, cost and size issues sometimes force to omit which is received by the tag antenna. The chip converts the external matching networks and direct matching between un-modulated signal to DC (for external feeding) and sends the antenna and the chip is a due. back its information by varying its complex RF input The qualitative behaviour of antenna impedance, chip impedance. This impedance typically toggles between two impedance and read range as a function of frequency for a different states, corresponding to conjugate matching and to typical UHF–RFID tag is illustrated in Fig. 1 [5]. It can be some other impedance, effectively modulating the seen that chip impedance is always capacitive and has a back-scattered signal [5]. The UHF–RFID regulated bands small resistance [9, 10]. Obviously, this complex impedance vary in the different world regions, and include frequencies is frequency dependent, and adds difficulty in obtaining between 840 and 960 MHz. Thus, UHF–RFID is operated broadband matching with simple matching networks or at 840–845 MHz in China, at 866–869 MHz in Europe, at without matching networks. For this reason, the design of 902–928 MHz in USA and at 950–956 MHz in Japan. worldwide UHF–RFID tags without external matching Appropriate impedance matching between the antenna and networks represents an important challenge in RFID the chip is of substantial importance in RFID [5]. It directly technology. influences RFID system performance, such as the read In this paper, we investigate the fundamental bandwidth range of the tag, that is, the maximum distance at which a limitations on the design of single resonant UHF–RFID 788 IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 10, pp. 788–794 & The Institution of Engineering and Technology 2013 doi: 10.1049/iet-map.2013.0158 www.ietdl.org Fig. 1 Antenna impedance, chip impedance and read range as functions of frequency for a typical RFID tag Fig. extracted from [5] tags with complex conjugate matching at roughly the where s denotes the reflection coefficient, ω the angular intermediate frequency of the UHF–RFID worldwide band frequency and RC and CC are the equivalent resistance and (915 MHz). According to this analysis, we also propose a capacitance, respectively, of the chip. Since ln(1/|s|) can be broadband tag exhibiting conjugate matching at this expressed in terms of the return loss (in dB) at the input of frequency, without using an external matching network. the matching network, the expression (1) yields The presented tag operates worldwide, with a significant read range ( >5 m) within the whole UHF–RFID band 1 (840–960 MHz), and a peak value of 9 m. − || ≤ 10 log e s dBdf (2) RCCC 2 Physical bandwidth limitations and 0 optimisation Expression (2) forces the area given by the return loss curve to 2.1 Bode criterion be less than, or equal, to a constant. Bandwidth optimisation implies assuming |s| to be a constant over the band of interest, – The input impedance of a UHF RFID chip is mainly and |˙s|=1 (RL = 0 dB) elsewhere. This is the response with determined by the voltage multiplier stage of the integrated maximum bandwidth, but it cannot be realised in practice circuit transponder, which can be modelled by a parallel because it would require an infinite number of elements in combination of a resistance RC (that accounts for the losses the matching network [13]. The necessity of multiple stages of the multiplier) and a capacitance CC (that includes all the to approximate the optimum response makes this objective capacitive effects) [9, 10], as it is shown in Fig. 2. prohibitive because of the increment of cost and tag According to Bode’s limit [11, 12], the reflection dimensions. In fact, UHF–RFID tags are typically designed coefficient achievable by any passive and lossless network with a single resonant frequency in their frequency placed between a purely resistive generator and an RC response. Therefore in this work, the main objective is parallel load is limited by bandwidth optimisation by considering a single resonant UHF–RFID tag with complex conjugate matching at the 1 intermediate frequency. p 1 v ≤ ln || d (1) s RCCC 0 2.2 Choice of passive circuit network for conjugate matching and bandwidth optimisation The purpose of this subsection is to obtain the passive circuit network which provides complex conjugate matching at the intermediate frequency and the broader possible bandwidth when it is cascaded to a given UHF–RFID chip. Let us consider the schematic depicted in Fig 2, where the load to be matched consists of a parallel combination of a capacitance CC and a resistance RC. An eventual impedance matching network is inserted between the antenna and the Fig. 2 Schematic of a typical UHF–RFID tag in the most general load, to consider the most general case. Maximum power case transfer between the antenna and the chip is achieved for = ∗ = ∗ An impedance matching network is located in between the chip (modelled as conjugate matching, that is, Zchip ZN (or Ychip YN ), a RC parallel load) and the tag antenna since this forces the power reflection coefficient, given by IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 10, pp. 788–794 789 doi: 10.1049/iet-map.2013.0158 & The Institution of Engineering and Technology 2013 www.ietdl.org the authors of [14, 15] the antenna or the antenna plus the matching network (see Fig. 2) for maximum bandwidth. 2 It is important to point out that a series combination of an ∗ 2 ∗ 2 − + 2 Z −Z Y −Y GN Gchip B inductor and a resistor cascaded to the chip, providing ||s 2= chip N = N chip =(3) + + 2 conjugate matching at f , can be considered to achieve a Zchip ZN YN Ychip + + 2 0 GN Gchip B bandwidth very close to the optimum. This is a direct consequence of the fact that a low-loss network is required to be zero. In (3), YN = GN +jBN, Ychip = Gchip +jBchip and B = for bandwidth optimisation. In order to demonstrate it, let BN + Bchip. Since the chip impedance depends on the input us consider a low-loss network formed by a series power applied to the chip [5], for the evaluation of the combination of a resistor with resistance Rs and an inductor χ ≫ x ≫ maximum read range, Zchip must be considered at the with reactance s (Q 1, therefore s Rs). The threshold power. conductance GN and susceptance BN of such a network can Obviously, conjugate matching cannot be fulfilled in a be approximated by broad band. Therefore we choose the intermediate – R R frequency of the UHF RFID frequency span, f0, to force G = s ≃ s (6) N 2 + x2 x2 conjugate matching. This means that the conductances and Rs s s susceptances must satisfy GN = Gchip and BN = −Bchip, fl −x −1 respectively, at f0.
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