RFID Readers for the HDX Protocol - Design,
Simulation and Testing
D. T. Vuza, R. Frosch 1), and H. Koeberl 1)
Institute of Mathematics of the Romanian Academy, Bucharest, Romania,
1)
Frosch Electronics OEG, Graz, Austria
danvuza@hotmail.com
Topic: E
Presentation: O
Summary: Radio Frequency Identification (RFID) is a rapidly growing technology of the
present time, with applications in many areas of industrial and social activities such as animal
identification, access control, and supply chain [1]. There is a growing demand for RFID readers
with high standards of performance, reliability and easy adaptability to the needs of the concrete
application. Previous work [2] - [4] presented the contribution of the first author in collaboration
with Frosch Electronics (FE) to the development of readers for communication with tags
according the FDX protocol. Such readers were divided into two classes [4], voltage driven and
current driven. Recently, FE decided to add a new feature to the existing readers, providing them
with the possibility of communicating with tags that use the HDX protocol. We present here
circuit topologies achieving the required extension for each of the above-mentioned classes of
readers. We discuss the important issue of transients, which has to be taken into account when
striving for data integrity, and hence reliability. We present tools that aid in the process of reader
design, tuning and testing: a CAD tool consisting of a Spice simulation of transients and a tag
simulator.
Keywords: RFID, reader, transponder, simulator, FSK.
Motivation
Readers produced so far by FE are classified as voltage driven and current driven,
according to which circuit variable is driven by the reader and which is driven by the tag (and
sensed by the reader). A voltage driven reader powers the antenna with an AC voltage of
constant amplitude and decodes the answer from the tag by sensing the voltage at the tap point
(the junction between the antenna and the resonant capacitor). A current driven reader powers
the antenna with an AC current of constant amplitude and senses the voltage across the whole
antenna circuit (antenna plus capacitor), in which case the tap point connection is no longer
needed. Both classes of readers were intended to cover the FDX (Full Duplex) protocol, in which
the tag transmits data while simultaneously receiving power from the reader. In order to stay
competitive, FE decided to add to its readers the HDX (Half Duplex) protocol, of interest in
application such as animal identification, so that the same reader could cover a larger variety of
applications. In the latter protocol, the tag is first charged by an RF pulse of limited duration
from the reader, then it transmits the data using the energy stored during the first step. For each
of the above-mentioned classes, a distinct circuit topology that brings the new HDX feature is
required, according to whether the tap point connection is accessible or not in the initial design.
Unlike FDX, where the tag transmits data by load modulation, FSK is employed for HDX, the
tag signaling data bits by switching between its resonant frequency and a lower frequency.
Transient effects have been discussed in [4] for load modulation and have to be discussed again
in the new setting, since transients manifest themselves when the tag changes the frequency and
may have deleterious effects on data integrity if their duration is too long.
Results
We present circuit topologies that achieve the HDX extension for each of the two classes
of readers. For current driven readers, we consider the options of decoding FSK by either a
dedicated IC or by building our decoder from general purpose ICs supplemented by a software
component, which could in principle offer more control over the decoding process. For both
topologies we discuss transient effects, both theoretically and with the aid of a Spice simulation.
The latter can also serve as a CAD tool for reliability analysis, being of use in visualizing the
transients and measuring their duration (figure 1). For practical measurement of transient effects,
a tag simulator was devised which expands upon the concepts presented in [5]. Observing the
frequency transition is not easy on a scope, as the frequency difference is rather small and the
transition is gradual precisely because of the transient effects. By providing, in addition to the
frequency switch in the simulated tag signal, an output that triggers the scope at the moment of
the transition, the simulator acts as a convenient signal source for analyzing the system response
of the reader together with its antenna (figure 2).
Fig. 1: Spice simulation of transients induced by
transition from 134.4 KHz to 123 KHz in tag
response. Upper: voltage at tap point. Lower:
transient induced by transition at t = 500 us.
Fig. 2: Frequency transition generated with tag
simulator. Upper: signal from reader antenna.
Middle: FSK signal generated by simulator. Lower:
trigger at transition provided by simulator.
References
[1]
[2]
[3]
[4]
[5]
K. Finkenzeller, “RFID Handbook”, John Wiley & Sons Ltd, West Sussex, England, 2003.
E. Gelinotte, R. Frosch, L. Pascu and D. T. Vuza, “An RFID Reader Based on the Atmel
AT91SAM7S64 Micro-Controller”, Proceedings of the 1st Electronics Systemintegration
Technology Conference, Dresden, pp. 1158-1165, 2006.
D. T. Vuza, R. Frosch and H. Koeberl, “A Long Range RFID Reader Based on the Atmel
AT91SAM7S64 Micro-Controller”, 30th ISSE Conference Proceedings, Cluj, pp. 445-450, 2007.
D. T. Vuza, R. Frosch, H. Koeberl and D. Boissat, “A Low Cost Anticollision Reader”,
Development and Implementation of RFID Technology, I-Tech Education and Publishing, Vienna,
pp. 201-216, 2009.
D. T. Vuza, S. Chi u and P. Svasta, “An RFID Tag Simulator Based on the Atmel AT91SAM7S64
Micro-Controller”, 33th ISSE Conference Proceedings, Warsaw, pp. 229-234, 2010.