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Design and development of a RFID reader with carrier
frequency of 125 KHz
Lucian - Nicolae Cojocariu, Octavian - Modest Manu
Ștefan cel Mare University
Suceava, Romania
[email protected], [email protected]
Abstract: This paper presents the development of a low frequency Radio Frequency Identification
transceiver system aimed at providing non-contact solutions for identifying, monitoring, and tracking people,
animals, and objects. The communication between Radio Frequency Identification reader and transponder is
done wireless. When the transponder is placed in the electromagnetic field emitted by the RFID reader antenna,
it is activated and the information stored is transmitted in digital form by modulating the field emitted by the
reader antenna. Various antenna configurations were analyzed and the optimal configuration was chosen for the
implementation.
Keywords: RFID Reader, Antenna, microcontroller system, transceiver, graphic interface.
I. Introduction
Radio Identification (RFID) systems received
an increasing amount of interest during the last
years due to the great potential for application
applications in the areas of control access [1],
indoor localization [2-5] healthcare monitoring
systems [6].
Numerous international research centers and
companies aim at developing smart electronic
chip for RFID systems capable of operating in a
variety of environmental conditions with a high
level of data integrity. The main component of a
RFID reader is the transceiver which must
provide the carrier wave frequency and the
modulation type suitable to the envisioned
application. The developed RFID reader
presented in this article is based on a MLX90109
integrated transceiver produced by Melexis
Company which has an operating frequency of
125 KHz and uses Amplitude-Shift Keying (ASK)
modulation. These characteristics are required
for low frequency RFID applications [7].
The main parts of the developed RFID reader
are an antenna, a MLX90109 integrated
transceiver, an Atmega8 microcontroller and a
FT232RL convertor. The communication at the
functional block level consists of transponder tag
interrogation by the RFID reader using radio
electromagnetic wave, which reads the data
contained in the transponder transmits them to
a computer via USB.
The function block diagram of the developed
RFID reader is presented in Fig. 1
Computer
USB
FT232RL
Convertor
UART
Atmega8L
Microcontroller
Transceiver
MLX90109
Antenna
FIELD
Transponder
Figure 1 . RFID system block diagram
R
F
I
D
R
e
a
d
e
r
The MLX90109 Integrated Circuits is
controlled by a microcontroller and is connected
to a coil antenna and a capacitor which forms a
parallel LC circuit tuned to a 125 KHz frequency.
The transceiver transmits to the microcontroller
the data read from the transponder memory
which are already demodulated and decoded.
The microcontroller Atmega8 switches from plus
or minus some of the transceiver control pins in
order to allow the identification of a large class
of transponders. The data provided by the
transceiver to microcontroller are transmitted via
a Universal Asynchronous Receiver – Transmitter
(USART) interface to the FT232RL convertor.
The FR232RL convert data received from UART
interface to data transmitted via Universal Serial
Bus interface (USB). Finally, the graphical
interface allows the users to interact with the
RFID reader and tag in an easy manner.
II. Hardware Design
The hardware architecture of the RFID reader
is based on the MLX90109 transceiver, which is
schematically presented in Fig. 2. In order to
serve as a transceiver, the MLX90109 integrated
circuit requires additional electronic components
such as resistive voltage divider connected to pin
MODU, low pass filter, antenna, fast decay
circuit.
The resistive voltage divider provides a
voltage drop equals to 0.8 V on pin MODE of
integrated transceiver when the terminal of R4
resistor is switched to the negative pole of the
power supply by the PC0 pin of Atmega8
microcontroller. Thus the antenna field is
activated. If the microcontroller switches the
terminal of R4 resistor to the positive pole of the
power supply the voltage drop across the MODU
pin of the transceivers is higher than 0.8V and
the antenna field disappears. By knowing the
voltage supply and the resistance values, the
voltage drop value is obtained from the formula
for the resistive voltage divider as follows:
VM ODE 
V D D R4
R 4  R5
(1)
For the selected values of 30 KΩ for R5 and
150 KΩ for R4, the voltage drop on pin MODE is
VMODE=0.83V when the resistive voltage divider
is supplied at VDD=5V.
A low pass filter is designed to stop stray
signals with frequencies below 60 Hz, from
electric power supply network, and it is compose
of the capacitor C8 and resistor network R4 and
R 5.
Figure 2 . MLX90109 transceiver with external
components.
The value of C8 capacitor can be determined
by using Equation 2.
C8 
R1  R 2
2  R1 R 2 f
(2)
For a signal with a frequency of 60Hz and for
the selected values of R4=150 KΩ and R5=30
KΩ, the value of condenser C8 = 100 nF is
obtained.
The antenna connected to the integrated
transceiver MLX90109 must have impedance
between 1 KΩ and 5 KΩ. In order to obtain the
125 KHz carrier frequency a parallel LC oscillator
circuit is used, including the antenna coil
inductance and the C7 capacitor as shown in
Fig. 2.
At resonance the capacitive reactance of C7
and the inductive reactance of the antenna coil
become equal leading to the following
expression for C7:
C7 
1
2
( 2 f 0 ) L
(3)
where f0 is the carrier frequency. For a
resonant frequency f0=125 KHz and an
inductance L=44 µH, a value of C7=37 nF was
determined. The diameter of a single conductor
insulated with email used for the antenna coil is
determined from the following current density
formula:
d 2
I ant
 J Cu
The electrical current density
jCu=11 A/mm2 and the maximum
flows into the antenna coil at
Iant=218 mA. Thus the obtained
(4)
of copper is
current which
resonance is
value for the
wire diameter d is 0.28 mm. The standardized
value for wire diameters is chosen as d=0.3 mm
for a wire without insulation, and d=0.337mm
for wire with email insulations. Next, the number
of turns for the antenna coil can be calculated as
follows:
N 
2L
  8D

 0 D  ln 
 2 
d

 
interface, decodes and sends them via UART
interfaces to microcontroller and vice versa.
(5)
were N represents the number of windings, L
represents the coil inductance measured in H, D
is the inner coil diameter measured in mm, d is
the wire diameter measured in mm and µ0
represents the magnetic permeability of air with
a value of 4  π  107[Wb/A m].
Various
antenna
configurations
were
designed and made based on the Eqs. 4 and 5.
Two of them are presented in Fig. 3. In order to
decrease the time decay of the antenna
oscillations below the maximum recommended
value, a fast decay circuit was developed with a
P-MOS transistor noted with T and a Schottky
diode noted with D in Fig. 2. The transistor
works in switching mode and the diode D has
the role of protecting the transistor T from the
occurrence of reverse voltage on antenna coil.
Figure 4 . Atmega8l
microcontroller
external
components and connections to the RFID transceiver
Figure 5 . FT232RL
convertors
Figure 6 . Practical
implementations
components
with
externals
Figure 3 . RFID antennas with diameter of 20mm
(left) and 135mm (right).
The Atmega8L microcontroller handles the
RFID transceiver via the followings pins: MODU,
MODE, and SPEED. The data provided by the
MLX90109 integrated transponder are read by
the microcontroller via CLOCK and DATA pins.
The Atmega8L microcontroller was chosen for its
low energy consumption, the reduced number of
pins and its high processing speed. Due to the
powerful instructions, most of them executed in
one single cycle, this microcontroller achieves
throughputs close to 8MIPS at the working
frequency of 8MHz. The design of RFID reader is
oriented to low power consumptions such that
the entire application can be powered from a
USB port. The FT232RL convertor, presented in
Fig. 5, receives data from a computer via USB
reader designed.
of
RFID
The decay time is given by the following
relation:
t d eca y  Q a n t
1
(6)
f0
where Qant represents the quality factor of
antenna coil.
The hardware part of RFID reader prototype
developed according to the proposed design is
shown in Fig. 6.
III. Software Components
The software of the developed RFID reader
consists of the program implemented in the
microcontroller that allows it to control and
communicate with MLX90109 transponder and
the user interface applications running on the
computer which displays information received
from transponder and allows user to configure
the MLX90109 integrated transponder.
The first step into programming the Atmega8
microcontroller was to choose the fuse bits
because it uses an external quartz oscillator in
order to ensure frequency stability of the clock
signal. The microcontroller program was written
in C and contains roughly 200 lines of code. An
external interrupt service routine serves the data
readings from the transponder memory. An
internal timer interrupt is called 30000 times per
seconds and sends data to user interfaces
application via the USB.
The graphical user interface running on the
computer was developed using Microsoft Visual
C++ integrated development environment. The
source code for the interface project totals 1000
lines of code. It runs an event that reads from
the USB port and the two threads that are called
every 10 ms. One thread is dealing with the
selection of the port to which the hardware is
connected, and the other thread is dealing with
the processing of data received from
microcontroller via FT232L converter.
Through the developed software one can
manipulate the integrated MLX90109 RFID
transceiver. By checking the appropriate radio
button fields on the interface, it is possible to
read a variety of RFID transponders operating
on frequency of 125 KHz. Accessing the
application entitled "RFID Reader" and
connecting the developed RFID reader to the
computer via USB cable determine the
application to automatically choose the
corresponding USB port. In order to read data
stored in a RFID transponder, one must know
the header of the tag, data transmission speed
(bandwidth), data coding mode and the memory
size of the tag.
IV.
Prototype testing
Numerous tests of the developed RFID reader
prototype were performed by using various tags,
antenna coils, geometries and environments. By
connecting the antenna presented on the left
side of Fig. 3, a range of 4 cm was determined
while by using the antenna presented on the
right side the range was extended to 15 cm. The
received antenna signals were analyzed by using
a LeCroy WaveSurfer digital storage oscilloscope,
and sample results are presented in Fig. 8. The
measured carrier frequency was 124.98 kHz
which is very close to the target frequency of
125 kHz.
a)
b)
Figure 8 . a) The carrier frequency, b) the
modulated signal.
Figure 7 . Graphic user Interface.
understanding of the operation of RFID systems.
Due to the fact that the integrated transceiver
that can handle antennas with impedance
ranging between 1 KΩ to 5 KΩ a wide range of
antennas can be developed and the benefits and
disadvantages of them can be analyzed. The
presented RFID reader has been designed to
work with power supply from the USB port of
the computer that is connected to. Also, the
small size of the RFID reader provides
portability. Given the above considerations the
developed RFID reader can be used as a study
platform for low frequency RFID systems.
VI.
Figure 9 . Free decay of antennas oscillations.
Acknowledgements
L.C. thanks Lucian Andrieș for his support
during
the
development
of
software
components.
O.M. thanks CNCSIS-UEFISCSU PROJECT
NUMBER RU-107/2010 for the financial support.
VII. References
[1]
[2]
[3]
Figure 10 . The decay of antennas oscillations via
the developed decay circuit.
The efficiency of the decay circuit is apparent
from Figs. 9 and 10. In the case of antenna coil
with 130 mm diameter the decay time of the
antenna oscillations is 280 µs when no decay
circuit is used. This value is greater than the
maximum imposed by the manufactures which is
200 µs. By using the decay circuit with the
same antenna, the decay time of antenna
oscillations drops to 71 µs.
[4]
RFID Systems - Localization and Speed
Measurement”, chapter 8 în “Radio Frequency
Identification Fundamentals and Applications,
Bringing Research to Practice”, Intech
Publisher, Wien, (pp. 113-130), 2010.
WILLIS S., HELAL S., LFID information grid for
blind
navigation
and
wayfinding,
In
Proceedings
ninth
IEEE
international
symposium on wearable computers (ISWC’05)
[5]
(pp. 34–37) Osaka, Japan, 2005
HIHNEL D., BURGARD W., FOX D., et al.
Mapping and localization with LFID technology.
In Proceedings IEEE international conference
on robotics & Automation (pp. 1015–1020)
[6]
V. Conclusions
The MLX90109 RFID integrated transceiver
can be the basis of the development for many
RFID applications that use a carrier of low
frequency. Developing applications that use this
integrated transceiver can lead to a better
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P.,
BHARGAVA
N.,
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[7]
[8]
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FINKENZELLER K., RFID Handbook (second
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MLX90109 Datasheets from MELEXIS available
at: www.melexis.com
[9]
Atmega8L Datasheets from Atmel available at:
www.atmel.com/dyn/resources/prod_documen
ts/2486s.pdf
[10]
FT232RL Datasheets from FTDI available at:
www.ftdichip.com/Documents/DataSheets/DS_
FT232R.pdf
Lucian – Nicolae COJOCARIU
Msc student, Ştefan cel Mare University from
Suceava, Faculty of Computer Science and
Electrical Engineering, Msc thesis: Designing and
developing a radio-frequency identification sensor
network for healthcare application, supervisor:
Prof. Eng. Mihai DIMIAN, PhD.
Octavian – Modest MANU
PhD student, Ştefan cel Mare University from
Suceava, Faculty of Computer Science and
Electrical Engineering, PhD thesis: Contributions to
the development of smart antennas and
applications, PhD supervisor: Prof. Eng. Adrian
GRAUR, PhD.