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Simple Low Cost UHF RFID Reader
Pavel V. Nikitin, Shashi Ramamurthy, Rene Martinez
Intermec Technologies Corporation6001 36thAve W, Everett, WA,
98203, USA
{pavel.nikitin , shashi.ramamurthy, rene.martinez} @
intermec.com
Abstract We describe a UHF RFID reader which uses low cost
off-the-shelf components such as MSP430 microcontroller and
the TH7203x transmitter IC. The receiver circuit is based on
envelope detector and analog data slicer. The design is very
simple and can be used as an educational tool for academic
research.
I. INTRODUCTION
There exists a number of UHF RFID reader designs:IC-based,
discrete designs, or software defined radio (SDR)platforms. For
example, two well-known reader IC familiesare AS399x [1] and
R1000/2000 [2] and both require a
number of external components including an
externalmicrocontroller running a significant amount of
software.There exists many discrete based reader designs [3-5] and
anumber of SDR platforms, such as Ettus USRP radio [6-8].Many of
those reader designs are complex and expensive andeven low power
evaluation kits cost hundreds of dollars. Atthe same time, there
exist readily available low cost designkits and platforms for HF
RFID [9] and even for 2.45 GHzactive RFID [10]. Also, on the tag
end of the spectrum, thereis a variety of open tag platforms such
as WISP [11].
We describe a simple low cost reader which can beused as an
educational tool. Some features of our reader are:
Minimalistic Gen2 (ISO 18000-6C) protocol support(Query ->
RN16 -> ACK-> EPCID) with low costmicrocontroller (such as
MSP430);
Low power transmitter based on off-the shelf IC;
Simple receiver circuit structure (diode based envelope
detector and analog comparator based data slicer).
II. THE READER
The block diagram of the reader is shown in Figure 1.
Figure 1. Block diagram of the reader.
A.Microcontroller
We decided to use an Olimex microcontroller kit based
onMSP430F5510 processor [12]. It includes standard JTAGinterface
besides the standard I/O interfaces, an LCD (usefulfor displaying
the tag information), SD card slot (to potentiallystore data from
read tags), charger for the externalrechargeable 850 mAh battery
and a prototyping area.
B.
Transmitter
We found and used readily available off-the-shelf lowpower
transmitter from the TH7203x IC family by Melexis.These ICs operate
on low voltage battery and can generatecarrier in 850-930 MHz band.
It is a simple inexpensivesolution which can be readily integrated
into the reader design.We used TH72035 evaluation board [13] tuned
to 915 MHz.To increase the output power, we fed it directly from
thebattery rather than from the Olimex onboard 3.3 V
voltageregulator. Transmitter IC is modulated using direct ASK
datainput pin on TH72035. For the front end isolator, we used alow
cost surface mounted 10 dB coupler with integrated lowpass filter
(Johanson 0910CF15B0100), also on evaluationboard but other coupler
or circulator could be used too. Theoutput power of the reader at
the antenna port was 9 dBm.
C.
Receiver
We used a very simple receiver circuit based on anenvelope
detector and a comparator, similar to the one foundin many UHF RFID
ICs. The schematic is shown in Figure 2,we largely followed the
application note by Maxim onbuilding a data slicer circuit for UHF
ASK receivers [14]. Forcircuit simulation, we used free simulator
CircuitLab. Wepicked the values of the circuit
components(R1=R2=R3=931K, C1=75 pF) so that the time constant
R1C1is roughly 3 bit intervals for 44 KHz backlink rate. For
diodes,we used SOT-23 Schottky pairs. For an op amp, we usedMAX931
but any other comparator can be used too. Thecircuit was assembled
on the prototyping area of the Olimexmicrocontroller board.
Figure 2. Receiver circuit schematic.
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III. EXPERIMENTAL RESULTS
A. Tag Read Range
Figure 3 shows the photo of our prototype. The antennawas a
homemade dipole tuned to 915 MHz. Our firmware hasnot been fully
completed yet, so we used an oscilloscope toverify that the tag is
responding and that the receiver circuitcan detect it and pass a
clean demodulated and data sliced
signal to the microcontroller. From the distance of up to
6inches (15 cm) away we could clearly read various ImpinjMonza 3
tags (provide properly demodulated and data slicedsignal to the
microcontroller). Figure 3 also shows the signals(screenshots from
oscilloscope) at circuit nodes 1, 5, 6, and 7.
Figure 3. Reader prototype and signals at nodes 1, 5, 6, and 7
(labeledon Fig. 1) when Monza 3 tag is 4 inches away from the
antenna.
B.Reader Sensitivity
We measured our reader sensitivity using conducted RFIDchip
assembly (Monza 2 IC matched to 50 Ohm) with thethreshold
sensitivity of -11.5 dBm and threshold modulationefficiency of -10
dB. The transmit and receive paths wereisolated using circulators
so that their attenuations could becontrolled independently as
shown in Figure 4. The measuredreader sensitivity was approximately
-35 dBm.
Figure 4. Conducted sensitivity measurements.
C.Power Consumption
The power consumption of our reader is 60 mW (theaverage current
is 15 mA at 4 V battery voltage). It canoperate continuously for
more than 50 hrs on a single charge.
IV. DISCUSSION AND CONCLUSIONS
This project is still a work in progress. The currentread range
of our reader is small (~15 cm with Monza 3 tags).However, it is
often sufficient in many practical scenarios forUHF RFID, such as
sparse item level inventory where only ashort range is required and
only a few tags need to be read.
Even though we used fairly expensive developmentboard, the total
cost of our proof-of concept reader is still a
fraction of any commercially available low power UHF RFIDreader
board, such as this [15]. Our current cost, not includingthe
antenna, is under 40$ (23$ for the Olimex board, 9$ for
therechargeable battery, and 7$ for the coupler, transmitter,
andreceiver components). The cost can potentially be
significantlyreduced by using lower cost battery and other
componentssuch as this MSP430 kit [16]. The reader is also simple
whichmeans that even students with little experience in RF
designcan build and experiment with it. Some of the future work
is:
Firmwareo Finish the code so that our reader can read tag
EPC ID, display it on LCD, and flash LEDs;o Add support for tag
singulation (multiple tag
reads) and several backlink rates; Hardware
o Improve the receiver sensitivity by usingmultistage charge
pump and increase thetransmitter power by adding RF amplifier;
o Integrate all RF components on one PCB which
fits into the Olimex board prototyping area.In summary, we
believe that this simple reader platform is
a good educational tool which can allow many students
andresearchers to study various aspects of UHF RFID.
REFERENCES
[1] AMS RFID products: http://www.ams.com/eng/Products/RF-
Products/RFID[2] Impinj Indy UHF RFID reader chips:
http://www.impinj.com/Indy_RFID_Reader_Chips.aspx [3]
Experimental front end for UHF RFID reader:
http://www.elektrorevue.cz/en/articles/analogue-technics/0/experimental-front-end-for-uhf-rfid-reader/
[4]
Designing an FPGA based RFID reader, Xcell Journal,
2006:www.linear.com/docs/40013
[5] Chen Ying and Zhang Fu-Hong, A system design for UHF
RFIDreader, IEEE ICCT conference, pp. 301-304, 2008
[6] Ettus Research: www.ettus.com/ [7] L. Catarinucci, D. De
Donno, et.al., "A cost-effective SDR platform for
performance characterization of RFID tags," IEEE Transactions
onInstrumentation and Measurement, vol. 1, no. 4, pp.903-911, April
2012
[8] M. Buettner and D. Wetherall, "A software radio-based UHF
RFIDreader for PHY/MAC experimentation," IEEE RFID Conference,
2011
[9] OpenPCD Passive RFID Project: http://www.openpcd.org/
[10]
Open Beacon active RFID project: http://www.openbeacon.org/ [11]
WISP RFID: http://sensor.cs.washington.edu/WISP.html[12]
Olimex MSP430-5510STK development and prototype board
kit::https://www.olimex.com/Products/MSP430/Starter/MSP430-5510STK
[13]
Melexis TH72035 transmitter IC - 868/915MHz,
FSK/ASK:http://www.melexis.com/General/General/TH72035-131.aspx
[14] Data slicing techniques for UHF ASK receivers, Maxim
applicationnote:
http://www.maximintegrated.com/app-notes/index.mvp/id/3671
[15] SolidDigi UHF RFID
reader:http://www.soliddigi.com/product/showproduct.php?lang=en&id=32
[16] TI MSP430 LaunchPad Kit:
http://www.ti.com/tool/msp-exp430g2
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