2013 IEEE International Conference on RFID
Simple Low Cost UHF RFID Reader
Pavel V. Nikitin, Shashi Ramamurthy, Rene Martinez
Intermec Technologies Corporation
6001 36th Ave 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.
A. Microcontroller
We decided to use an Olimex microcontroller kit based on
MSP430F5510 processor [12]. It includes standard JTAG
interface besides the standard I/O interfaces, an LCD (useful
for displaying the tag information), SD card slot (to potentially
store data from read tags), charger for the external
rechargeable 850 mAh battery and a prototyping area.
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 families
are AS399x [1] and R1000/2000 [2] and both require a
number of external components including an external
microcontroller running a significant amount of software.
There exists many discrete based reader designs [3-5] and a
number of SDR platforms, such as Ettus USRP radio [6-8].
Many of those reader designs are complex and expensive and
even low power evaluation kits cost hundreds of dollars. At
the same time, there exist readily available low cost design
kits and platforms for HF RFID [9] and even for 2.45 GHz
active RFID [10]. Also, on the tag end of the spectrum, there
is a variety of open tag platforms such as WISP [11].
We describe a simple low cost reader which can be
used as an educational tool. Some features of our reader are:
• Minimalistic Gen2 (ISO 18000-6C) protocol support
(Query -> RN16 -> ACK-> EPCID) with low cost
microcontroller (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.
B. Transmitter
We found and used readily available off-the-shelf low
power transmitter from the TH7203x IC family by Melexis.
These ICs operate on low voltage battery and can generate
carrier in 850-930 MHz band. It is a simple inexpensive
solution 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 the
battery rather than from the Olimex onboard 3.3 V voltage
regulator. Transmitter IC is modulated using direct ASK data
input pin on TH72035. For the front end isolator, we used a
low cost surface mounted 10 dB coupler with integrated low
pass filter (Johanson 0910CF15B0100), also on evaluation
board but other coupler or circulator could be used too. The
output power of the reader at the antenna port was 9 dBm.
C. Receiver
We used a very simple receiver circuit based on an
envelope detector and a comparator, similar to the one found
in many UHF RFID ICs. The schematic is shown in Figure 2,
we largely followed the application note by Maxim on
building a data slicer circuit for UHF ASK receivers [14]. For
circuit simulation, we used free simulator CircuitLab. We
picked
the
values
of
the
circuit
components
(R1=R2=R3=931K, C1=75 pF) so that the time constant R1C1
is roughly 3 bit intervals for 44 KHz backlink rate. For diodes,
we used SOT-23 Schottky pairs. For an op amp, we used
MAX931 but any other comparator can be used too. The
circuit was assembled on the prototyping area of the Olimex
microcontroller board.
THE READER
The block diagram of the reader is shown in Figure 1.
Figure 1. Block diagram of the reader.
Figure 2. Receiver circuit schematic.
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2013 IEEE International Conference on RFID
III.
EXPERIMENTAL RESULTS
IV.
DISCUSSION AND CONCLUSIONS
This project is still a work in progress. The current
read range of our reader is small (~15 cm with Monza 3 tags).
However, it is often sufficient in many practical scenarios for
UHF RFID, such as sparse item level inventory where only a
short range is required and only a few tags need to be read.
Even though we used fairly expensive development
board, the total cost of our proof-of concept reader is still a
fraction of any commercially available low power UHF RFID
reader board, such as this [15]. Our current cost, not including
the antenna, is under 40$ (23$ for the Olimex board, 9$ for the
rechargeable battery, and 7$ for the coupler, transmitter, and
receiver components). The cost can potentially be significantly
reduced by using lower cost battery and other components
such as this MSP430 kit [16]. The reader is also simple which
means that even students with little experience in RF design
can build and experiment with it. Some of the future work is:
• Firmware
o 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 using
multistage charge pump and increase the
transmitter 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 and
researchers to study various aspects of UHF RFID.
A. Tag Read Range
Figure 3 shows the photo of our prototype. The antenna
was a homemade dipole tuned to 915 MHz. Our firmware has
not been fully completed yet, so we used an oscilloscope to
verify that the tag is responding and that the receiver circuit
can detect it and pass a clean demodulated and data sliced
signal to the microcontroller. From the distance of up to 6
inches (15 cm) away we could clearly “read” various Impinj
Monza 3 tags (provide properly demodulated and data sliced
signal 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 (labeled
on Fig. 1) when Monza 3 tag is 4 inches away from the antenna.
REFERENCES
B. Reader Sensitivity
We measured our reader sensitivity using conducted RFID
chip assembly (Monza 2 IC matched to 50 Ohm) with the
threshold sensitivity of -11.5 dBm and threshold modulation
efficiency of -10 dB. The transmit and receive paths were
isolated using circulators so that their attenuations could be
controlled independently as shown in Figure 4. The measured
reader sensitivity was approximately -35 dBm.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
Figure 4. Conducted sensitivity measurements.
[14]
C. Power Consumption
The power consumption of our reader is 60 mW (the
average current is 15 mA at 4 V battery voltage). It can
operate continuously for more than 50 hrs on a single charge.
[15]
[16]
127
AMS
RFID
products:
http://www.ams.com/eng/Products/RFProducts/RFID
Impinj
Indy
UHF
RFID
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reader”, IEEE ICCT conference, pp. 301-304, 2008
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performance characterization of RFID tags," IEEE Transactions on
Instrumentation and Measurement, vol. 1, no. 4, pp.903-911, April 2012
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reader for PHY/MAC experimentation," IEEE RFID Conference, 2011
OpenPCD Passive RFID Project: http://www.openpcd.org/
Open Beacon active RFID project: http://www.openbeacon.org/
WISP RFID: http://sensor.cs.washington.edu/WISP.html
Olimex MSP430-5510STK development and prototype board kit::
https://www.olimex.com/Products/MSP430/Starter/MSP430-5510STK
Melexis TH72035 transmitter IC - 868/915MHz, FSK/ASK:
http://www.melexis.com/General/General/TH72035-131.aspx
“Data slicing techniques for UHF ASK receivers”, Maxim application
note: http://www.maximintegrated.com/app-notes/index.mvp/id/3671
SolidDigi
UHF
RFID
reader:
http://www.soliddigi.com/product/showproduct.php?lang=en&id=32
TI MSP430 LaunchPad Kit: http://www.ti.com/tool/msp-exp430g2