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D. Meyers's D. Meyers's Document Transcript

  • Technical Review Meyers – 4007 Passive Radio Frequency Identification Systems Introduction Radio-frequency identification (RFID) is rapidly becoming a common place technology. Found in both commercial and personal markets including mobile devices, car navigation systems, logistics, inventory management, health care, pet tracking, and distributed sensor networks. The basic RFID system includes a transponder (tag), a reader, and a host. Developments in antenna design, chip size, power management and frequency use have driven RFID into the competitive marketplace. The two types of RFID are active and passive, referring to the type of transponder used. Active tags contain a power source and more memory but are expensive compared to its counterpart. Passive tags are small, relatively inexpensive with extremely long life spans, however, read distances are much shorter. This paper will focus on passive RFID systems focusing on transponders, readers and antenna. Application of Passive RFID Several frequency bands have been standardized for RFID. Low-Frequency (LF, 125-134 kHz), high-frequency (HF, 13.56 MHz), ultra-high-frequency (UHF, 860-928 MHz), and microwave (2.4 GHz and 5.8 GHz) [1]. Communication between transponders and readers occurs via either inductive coupling or capacitive coupling, depending on the frequency and congruently the read distance. Short reads are performed in the near field and involve inductive coupling which is the interaction of an alternating magnetic field. HF 13.56 MHz and LF 134.2 kHz use near field coupling; the near field for 13.56 MHz ends at approximately 3.5 meters [2]. Longer reads are performed in the UHF ~900 MHz or microwave 2.4 GHz range because power degradation is less, dropping as r^-2 as opposed to r^-6 [2]. The following sections will discuss the individual parts of an RFID system and implementation of that system. Underlying RFID Technology Tags Design of an RFID tag is extremely important. It must be small, inexpensive, and include a microchip and an antenna. Designs determine the required orientation of the tag and play a very important role in the read distance [3]. Tag development began with 1
  • Technical Review Meyers – 4007 Generation 1 Class 0 that were programmed by the manufacture and have progressed to Generation 2 Class 1 that are now programmed by the end user and standardized for global acceptance. The Gen. 2 Class 1 tags are WORM (write once read many) with up to 96 Bits of readable memory a 32 Bit password and a 32 Bit kill password [4]. Passive tags do not contain their own power supply, therefore the energy delivered by the continuous wave (CW) transmitted by the reader must be used to charge capacitors to a minimum operating voltage to transmit its identity back to the reader. Voltage multipliers can be used to store up charge on capacitors in order to deliver enough power; a concatenation of voltage multipliers may be necessary to obtain the minimum operating voltage [4]. Antennas Both the tag and the reader contain an antenna that is vital for the RFID system to function as the two act as a transformer where induction occurs on each one by the other. For the tag it is very important that the antenna is designed to match the impedance of the microchip on the tag as well as keeping the entire design inexpensive. The basic technique used to return information is by reflecting most of the energy from the reader or absorbing and temporarily storing it, creating a modulated signal (i.e. backscattering). The dipole is the simplest antenna that can be used for the passive RFID tag, however, not the best. Variations of the dipole such as meandering, inductive coupling, capacitive coupling, shunt inductors and size changes, can be made so the wavelength, gain and impedance of the load and antenna can produce the correct scattering cross section [5]. Readers The reader consists of a transmitter and receiver as well as amplifiers, mixers, oscillators, filters, and analog to digital converters. The reader must communicate with the tag using its antenna and also be able to communicate with the host to process the received information. Standard communications between the host and reader include Ethernet, RS-232, RS-485 and USB. Implementation of a Gen 2 RFID System A Generation 2 Class 1 RFID communication system can communicate at variable speeds depending upon the needs of the implementation. With the possibility of 2
  • Technical Review Meyers – 4007 communicating with anywhere between 100 and 1000 tags per second; A specific implementation depends upon the reliability required as well as the noise in the environment [6]. Placement of a reader is extremely important in communicating with a tag, whether reading or writing. Techniques such as polarized antenna, improved data sorting, tag signal isolation, and read/write verification are used to improve the reliability of the system. The result of the Gen. 2 Class 1 system is a highly robust and adaptable system that will allow RFID technology to flourish in the future. 3
  • Technical Review Meyers – 4007 [1] G. Marrocco, "The Art of UHF RFID Antenna Design: Impedance-Matching and Size Reduction Techniques," IEEE Antennas and Propagation Magazine, vol. 50, no. 1, Feb., 66-79, 2008. [2] K. Mahaffey. Black Hat digital self defense. Lecture. Topic: "Passive RFID Security," Black Hat Security Conference, July 2005. [3] Product Manager Joint-Automatic Identification Technology, “Passive RFID,” Product Manager Joint-Automatic Idenification Technology, 2007. [Online]. Available: http://www.eis.army.mil/AIT/technology/rfid_passive.asp. [Accessed August 29, 2008]. [4] J. T. Protho, “Improved Performance of a Radio Frequency Idenification Tag Antenna on a Metal Ground Plane,” M.S. thesis, The Georgia Institute of Technology, Atlanta, GA, 2007. [5] D. M. Dobkin, The RF in RFID. Oxford, UK: Elsevier Inc., 2008. [6] Alien, EPCglobal Class 1 Gen 2 RFID Specification, Alien, 2005. 4