Designing an antenna for the M24LRxx-R and M24LRxxE

AN2972
Application note
Designing an antenna for the M24LRxx-R and M24LRxxE-R
dual interface I²C/RFID devices
Introduction
The M24LRxx-R or M24LRxxE-R device is an EEPROM designed for access via two
different interfaces: a wired I2C interface and a standard contactless ISO 15693 RFID
interface.
Figure 1.
Dual interface EEPROM
Both interfaces are widely used industry standards. The M24LRxx-R or M24LRxxE-R can
be integrated into almost any electronic application, provided that the processor offers an
I2C interface. It may also be accessed by any RFID reader that supports the ISO 15693
interface.
The purpose of this application note is also to:
●
explain the basic principle of passive RFID
●
describe the basics of a 13.56 MHz inductive antenna design
●
provide some guidelines for a successful integration, from design to production.
Table 1 lists the products concerned by this application note.
Table 1.
Applicable products
Type
Note:
Applicable products
Dual interface EEPROMs
M24LRxx-R, M24LRxxE-R
Evaluation boards
ANTx-M24LRxxx, M24LR-Discovery, ROBOT-M24LR16E-A
STEVAL-IHP004V1, STEVAL-IPR002V1, STEVAL-IPE020V1
The standard M24LRxx-R and energy-harvesting M24LRxxE-R devices will be referred to
as M24LRxx devices throughout the document.
December 2012
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www.st.com
Contents
AN2972
Contents
Operating mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1
2
3
Basic principles and equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1
Passive RFID technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2
Simplified equivalent inlay circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3
Basic equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.4
Optimum antenna tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
How to design an antenna on a PCB . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1
Inductance of a circular antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2
Inductance of a spiral antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3
Inductance of a square antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.4
ST antenna calculation tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.5
PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.5.1
M24LRxx-antenna distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.5.2
Ground layer considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.5.3
Metal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
How to check the M24LRxx antenna tuning . . . . . . . . . . . . . . . . . . . . . 18
3.1
Antenna tuning measurements with a network analyzer . . . . . . . . . . . . . 18
3.2
Antenna measurements with standard laboratory tools . . . . . . . . . . . . . . 19
4
From design to production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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AN2972
List of tables
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Applicable products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
K1 and K2 values according to layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Antenna features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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List of figures
AN2972
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
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Dual interface EEPROM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
M24LRxx-R or M24LRxxE-R operating modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Power supply in RF mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Power transfer versus reader/M24LRxx orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
From the RFID reader to the M24LRxx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
From the M24LRxx to the RFID reader . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Equivalent circuit of the M24LRxx and its antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Tuning the M24LRxx antenna. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Spiral antenna. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Square antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
User interface screen of the planar rectangular coil inductance calculator. . . . . . . . . . . . . 13
Rectangular planar antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
M24LRxx close to antenna but ground plane distant from antenna . . . . . . . . . . . . . . . . . . 15
Bad implementation No.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Bad implementation No.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Not recommended implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Acceptable implementation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Effect of metal on the antenna frequency tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Measurement equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Example of the resonant frequency response of a prototype antenna . . . . . . . . . . . . . . . . 19
ISO standard loop antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Setting up the standard laboratory equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Example of a frequency response measurement of a prototype antenna . . . . . . . . . . . . . 20
Application examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Detuning effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Impact of housing/packaging material on RF communication . . . . . . . . . . . . . . . . . . . . . . 23
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Operating mode
Operating mode
Integrating the M24LRxx-R or M24LRxxE-R in an application is simple: on the I2C side,
there is no specific design requirement as the device interfaces exactly as any serial I2C
EEPROM device. On the RF side, the M24LRxx-R or M24LRxxE-R needs to be connected
to an external antenna to operate.
Figure 2.
M24LRxx-R or M24LRxxE-R operating modes
The design principle of the M24LRxx-R or M24LRxxE-R antenna is very simple: the external
antenna inductance (Lantenna) that needs to be designed on board the PCB should match
the M24LRxx-R or M24LRxxE-R internal tuning capacitance (Ctuning) in order to create a
circuit resonating at 13.56 MHz. The basic equation of the tuning frequency is:
1
f tuning = -----------------------------------------------------------------2Π × L antenna × C tuning
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Basic principles and equations
1
AN2972
Basic principles and equations
Definition
RFID reader: an electronic device used for communication between RFID tags (like the
M24LRxx) and a host computer system. A reader generally consists of an RF transmitter
and receiver and an antenna for communicating with tags. A digital interface enables the
reader to communicate with the host computer system. RFID readers are capable of both
reading and writing the tags.
1.1
Passive RFID technology
The ISO 15693 protocol is based on a passive RFID technology, operating in the highfrequency (HF) band, at 13.56 MHz.
Power transfer
When the M24LRxx operates in the RF mode, it is powered by the RFID reader. No battery
is then required to access it whether in write or read mode. With its external inductive
antenna, the M24LRxx draws all of its operating power from the reader’s electromagnetic
field.
The RFID reader plays the same role as the primary of a voltage transformer that powers
the secondary (in this case, the M24LRxx and its inductive antenna). The energy transfer
ratio from the reader to the M24LRxx is similar to the coupling factor of a voltage
transformer. It is a function of:
6/25
●
how well the M24LRxx and its antenna are tuned to the reader’s carrier frequency
(around 13.56 MHz)
●
the distance between the reader and the M24LRxx board
●
the dimensions of the reader antenna and the M24LRxx board
●
the reader power
●
the M24LRxx antenna orientation with regards to the reader antenna
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Basic principles and equations
Figure 3.
Power supply in RF mode
How the RFID reader provides the required energy to the M24LRxx
M24LRxx external
antenna
M24LRxx
VTag = V1sin(ɐt)
B = B0sin(ɐt)
RFID reader
V = V0sin(ɐt)
Reader antenna
ai17177c
When the M24LRxx is placed in the RFID reader’s electromagnetic field, the amount of
energy powering the device is directly related to the orientation of the M24LRxx antenna
with regards to the RFID reader antenna. Indeed, this energy depends on how the
electromagnetic field lines generated by the reader flow through the M24LRxx antenna. This
directly impacts the M24LRxx/reader read range:
●
The best configuration is obtained when both antennas are parallel and face each
other.
●
The read range can drop to zero when both antennas are perpendicular to each other.
●
Any other orientation is possible and will result in different read ranges.
Figure 5 shows different power transfer configurations.
Figure 4.
Power transfer versus reader/M24LRxx orientation
Best orientation: M24LRxx
antenna facing the RFID
reader antenna
Wrong orientation: M24LRxx
antenna tangent to electromagnetic field
B
Acceptable, but
not optimized
B
~
B
B
Reader antenna
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Data transfer
Placed in the RFID reader’s electromagnetic field, the M24LRxx built-in circuitry
demodulates the information coming from the reader.
Figure 5.
From the RFID reader to the M24LRxx
1
0
1
0
M24LRxx
external
antenna
M24LRxx
Reader antenna
RFID reader
ai17181c
In order to send its response back to the reader, the M24LRxx backscatters the data to the
reader by internally changing its output impedance back and forth, which is detected by the
reader.
Figure 6.
From the M24LRxx to the RFID reader
20 mV
M24LRxx
external
antenna
R
Tag
24 V
0
1
1
0
RFID Reader
Reader antenna
ai17182c
All this is part of the standard protocol and taken care of by the M24LRxx embedded
circuitry and the RFID reader’s electronics.
The main thing designers need to concentrate on is designing the M24LRxx antenna that
meets the application requirements in terms of read range and antenna size.
1.2
Simplified equivalent inlay circuit
The chip and its antenna can be symbolized using their equivalent electrical circuit.
Figure 7 shows the equivalent electrical circuit of the M24LRxx (parallel association of a
resistance which emulates the current consumption of the chip and a capacitance added to
the chip to ease tuning).
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Basic principles and equations
The antenna is a wire, so its equivalent electrical circuit is a wire with a resistance
symbolized by Rant. The antenna also has an inductance denoted by Lant. The capacitance
Cant is the representation of parasitic elements (produced by the bridge).
Figure 7.
Equivalent circuit of the M24LRxx and its antenna
Rant
A
Rchip
Ctun
Cant
Lant
B
M24LRxx
External antenna
ai17178b
In first-order equations, Rchip, Cant and Rant are negligible. This is why the basic equations
that follow will only take Lant and Ctun into consideration.
1.3
Basic equations
Resonant frequency
The resonant frequency of the LC circuit is defined by the equation:
LCω² = 1
where:
1.4
●
L is the inductance in Henry
●
C is the capacitance in Farad
●
ω is the angular frequency in radians per second (ω = 2 × π × f, with f = frequency in Hz)
Optimum antenna tuning
The total impedance of an LC loop is given by the sum of the inductive and capacitive
impedances:
Z = ZL + ZC
By writing the inductive impedance as ZL = jωL and the capacitive impedance as ZC = 1/jωC,
and then substituting in the previous equation, you have:
Z = jωL + 1/jωC
Now, extracting a common denominator yields:
Z = (1 – LCω²) /jωC
Note:
The total impedance Z is zero at the resonant frequency of the LC circuit (the numerator is
zero when LCω² = 1). The resonant frequency corresponds to the maximum current
received by the [L,C] loop; in this case, the M24LRxx (capacitor C) and the antenna
(inductor L).
Consequently, the dual interface device’s antenna must be tuned so that its resonating
frequency matches the RFID reader antenna’s tuning frequency as much as possible. At this
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Basic principles and equations
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point, the coupling factor between the RFID reader and the dual interface EEPROM antenna
is the best, meaning the best possible read range from the application standpoint.
Figure 8.
Tuning the M24LRxx antenna
Energy
Reader antenna tuning
Tag #1
antenna
tuning
Tag #2 antenna tuning
Tag #3
antenna
tuning
Frequency
ai17183
In Figure 8, Tag #2 is best tuned for this application configuration.
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2
How to design an antenna on a PCB
How to design an antenna on a PCB
Designing an inductive antenna is about impedance matching. The antenna impedance
must match the conjugated impedance of the M24LRxx in order to obtain the needed tuning
frequency.
A 13.56 MHz antenna can be designed with different shapes, depending on the application
requirements. As explained previously, the major parameter is the inductance L of the
antenna. The following paragraphs offer a way of computing the antenna dimensions for a
determined value of inductance L.
2.1
Inductance of a circular antenna
L ant = μ 0 × N
2.2
1.9
r
× r × ln  ---- , where:
r0
●
r is the radius in millimeters
●
r0 is the wire diameter in millimeters
●
N is the number of turns
●
µ0 = 4π · 10–7 H/m
●
L is measured in Henry
Inductance of a spiral antenna
d
2
L ant = 31.33 × μ 0 × N × ----------------------- , where:
8d + 11c
●
d is the mean antenna diameter in millimeters
●
c is the thickness of the winding in microns
●
N is the number of turns
●
µ0 = 4π · 10–7 H/m
●
L is measured in Henry
Figure 9.
Spiral antenna
ai15812
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2.3
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Inductance of a square antenna
d
2
L ant = K1 × μ 0 × N × ------------------------- , where:
1 + K2 ⋅ p
●
d = (dout + din)/2 in millimeters, where:
●
p = (dout – din)/(dout + din) in millimeters
●
K1 and K2 depend on the layout (refer to Table 2 for values)
dout = outer diameter
din = inner diameter
Figure 10. Square antennas
Table 2.
2.4
K1 and K2 values according to layout
Layout
K1
K2
Square
2.34
2.75
Hexagonal
2.33
3.82
Octagonal
2.25
3.55
ST antenna calculation tool
ST provides a simplified software tool (antenne.exe) to compute inductances of rectangular
planar antennas. The purpose of this tool is to give good approximations: the obtained
results should be verified.
This tool uses the Grover method (see Equation 1). Figure 11 shows the user interface.
Equation 1: Grover method
L ant = L 0 +  M , where:
●
M is the mutual inductance between each of the antenna segments
●
L0 is as defined in Equation 2
s
Equation 2: L 0 =
 Lj , where:
j=1
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●
s is the number of segments
●
Lj is the self inductance of each segment
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How to design an antenna on a PCB
Figure 11. User interface screen of the planar rectangular coil inductance calculator
Examples:
The following antenna parameters have to be fed to the software to compute the antenna
coil inductance:
●
the number of turns
●
the number of segments
●
w: the conductor width in millimeters
●
s: the conductor spacing in millimeters
●
the conductor thickness in micrometers
●
Length in millimeters
●
Width in millimeters
The number of turns is incremented each time a segment is added to a complete turn.
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Figure 12. Rectangular planar antennas
1
1
10
8
3 turns, 10 segments
2 turns, 8 segments
w
s
Width
thickness
(cross-section)
Length
ai15815
Once the antenna coil inductance has been calculated, a prototype coil is realized. The
value of the so-obtained prototype must then be validated by measurement. This can be
done using either a contactless or a non-contactless method.
2.5
PCB layout
2.5.1
M24LRxx-antenna distance
The M24LRxx must be laid out as close as possible to the antenna (a few millimeters). Any
additional wire/trace would change the antenna characteristics and tuning.
2.5.2
Ground layer considerations
Designing an inductive antenna on a PCB means that special attention must be paid to
ground plane design:
●
no ground plane above or below the antenna
●
no ground plane surrounding the antenna
Figure 13 shows a correct layout.
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How to design an antenna on a PCB
Figure 13. M24LRxx close to antenna but ground plane distant from antenna
Ground layer
M24LRxx
Front PCB side
Back PCB side
ai17194b
The signal and energy transfers between the reader and the M24LRxx board are good as
long as the antenna and the ground layer do not overlap.
Examples of bad implementations
Figure 14 and Figure 15 show two examples of bad implementation. In both cases, the
electromagnetic flux cannot flow through the antenna, there is no energy transfer between
the reader and the M24LRxx antenna.
Figure 14. Bad implementation No.1
ai17195
Figure 15. Bad implementation No.2
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Figure 16 shows an example of a not recommended implementation. The electromagnetic
flux is greatly attenuated by the short-circuited loop surrounding the M24LRxx antenna.
Figure 16. Not recommended implementation
ai17197
Figure 17 shows an acceptable implementation, if the antenna and the ground plane do not
overlap.
Figure 17. Acceptable implementation
ai17198
Figure 13 remains the best solution.
STMicroelectronics recommends designers to allocate a dedicated area of the PCB layout
to the antenna only, with no surrounding ground layer.
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2.5.3
Metal considerations
What does it happen when there is metal near the antenna?
When the antenna of the M24LRxx is close to metal, it changes its own resonance
frequency, as shown in Figure 18.
Figure 18. Effect of metal on the antenna frequency tuning
13.56
13.56 + x%
F [MHz)]
S11 log
magnitude [dB]
Legend
Frequency tuning of an antenna in the open air
:
Frequency tuning of an antenna close to metal
MS30739V1
Due to the modification of the tuning frequency, when an antenna is close to metal, the
frequency tuning of a tag should be modified: tuned at 13.56 MHz in the metal environment.
In other words, when a tag is placed in a metal environment, the tuning frequency of the tag
in the open air must be set to compensate for future shift. The metal environment induces
eddy current, quality factor downgrading, frequency detuning and a modification of the fieldstrength distribution.
As a conclusion, when an antenna, which has been tuned in order to operate in the open air,
is to operate near metal, it is necessary to redesign a new antenna which will be precisely
tuned with the global application.
Table 3.
Antenna features
Features
Antenna 1: ANT1-M24LR16E
Antenna 2: ANT1-M24LR16E with 74 pF
in parallel of the antenna
Antenna size
45 mm x 75 mm
45 mm x 75 mm
Frequency tuning in the air
13.7 MHz
7.5 MHz
Frequency tuning stuck on the
metal (1)
25 MHz
14 MHz
Read range in the open air (1)
7.5 cm
0.5 cm
No detection
2.5 cm
Read range close to metal
Status
(1)(2)
This antenna is tuned to operate This antenna is tuned to operate close to
in the open air
metal
1. The measurement has been done with DEMO-CR95HF-A.
2. The measurement has been done as per an antenna stuck on the full metal table.
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How to check the M24LRxx antenna tuning
3
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How to check the M24LRxx antenna tuning
The methods of antenna design described in the previous section may lead to an
inductance slightly different from the value that would offer optimum performance in the end
application. This is because the overall inductance of the antenna might slightly drift in the
application (with magnetic and ferromagnetic materials in the proximity of the antenna). It is
therefore necessary to run actual measurements of the resonant frequency of the antenna.
3.1
Antenna tuning measurements with a network analyzer
The tuning frequency of the M24LRxx antenna can be measured using a network analyzer
with a loop probe.
The RF electromagnetic field is generated by connecting a loop probe (like the 7405-901
Eaton/Alitech 6 cm loop) to the output of the network analyzer set in reflection mode (S11
measurement).
Figure 19. Measurement equipment
Network analyser
Loop probe
Antenna prototype with the M24LRxx
(represented as a capacitor)
ai16083b
This equipment setup will directly display the system’s resonant frequency.
Experiments
As the objective is to find an [Lantenna + M24LRxx Ctuning] tuned at 13.56 MHz, the
frequency sweep range has to be set around this value, that is:
●
Start frequency: 5 MHz
●
End frequency: 20 MHz
●
Output power: –10 dBm
●
Measurement: reflection or S11
●
Format: log magnitude
Place the antenna within the field generated by the network analyzer + loop probe. The
resonant frequency corresponds to the minimum observed on the S11 measurement curve.
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How to check the M24LRxx antenna tuning
Figure 20. Example of the resonant frequency response of a prototype antenna
12
12.5
13
13.5
14
14.5
15
13.56 MHz
S11 Log
magnitude
(dB)
Frequency (MHz)
Resonant frequency (13.56 MHz)
ai17199
3.2
Antenna measurements with standard laboratory tools
The antenna resonant frequency can also be measured with standard laboratory equipment
like:
●
a signal generator
●
an oscilloscope
●
two standard loop antennas
Experiment setup
Connect the first ISO 10373-7 standard loop antenna (see Figure 21) to the signal generator
to provide the RF electromagnetic field.
Connect the second ISO 10373-7 standard loop antenna to the oscilloscope (see Figure 22)
by using either a standard oscilloscope probe (1M or 10M input impedance) or a 50 Ω BNC
cable (oscilloscope input set to 50 Ω in this case).
Place the [antenna+M24LRxx] inside the RF electromagnetic field.
Figure 21. ISO standard loop antenna
ISO/IEC 7810 ID-1 outline
72 mm × 42 mm coil
1 turn
connections
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Figure 22. Setting up the standard laboratory equipment
Oscilloscope
Synchronization frequency
Prototype antenna to be measured
Loop antennas
Signal generator
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Experiments
Set the signal generator to output a sine wave with a peak-to-peak amplitude in the range of
200 mV. Starting from 5 MHz, increase the signal generator frequency until you reach the
maximum amplitude of the signal measured with the oscilloscope. The signal generator
frequency then corresponds to the resonant frequency of the [antenna+M24LRxx] pair.
Figure 23 provides the frequency response curve of the prototype antenna, based on
measurements of the received signal amplitude at different frequencies.
Figure 23. Example of a frequency response measurement of a prototype antenna
Resonant frequency = 13.56 MHz
Voltage on
the second
ISO 10373-7
antenna
12
12.5
13
13.5
Frequency (MHz)
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14.5
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From design to production
From design to production
Designers should expect some difference between the theoretical and the real performance
of the antenna on the PCB in the end application.
Here are a few considerations:
System level validation
It is paramount to take great care when validating the antenna tuning for the various
application use cases, whether it be programming traceability information on the
manufacturing line, performing inventory of several end-products in the warehouse or
reading data (end user).
Different reader profiles would result in distinct performance levels on a given M24LRxx
board.
Figure 24. Application examples
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Considerations on the actual system tuning frequency
Even though all readers transmit at 13.56 MHz, the optimal tuning frequency of the
M24LRxx antenna is not necessarily exactly 13.56 MHz.
Some mutual mechanisms such as detuning/coupling between the reader antenna and the
tag antenna may lead to an M24LRxx antenna with an optimum tuning frequency different
from 13.56 MHz.
A good example is ST’s reference antenna (gerber files available from www.st.com) whose
tuning frequency is 13.74 MHz ((a)) to provide the best performance with the Feig MR101
reader.
a. Using the method described in Section 2.5.3: Metal considerations.
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From design to production
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The read range varies depending on whether the M24LRxx board is read alone or stacked
with others (detuning effect). Figure 25 illustrates the detuning effect.
Figure 25. Detuning effect
The vicinity of another M24LRxx board may change the inductance dynamics. The boards
may couple with each other, leading to a resultant antenna resonant frequency different
from the individual one.
These are just examples of what may induce a difference between theory and real use
cases. They are meant to emphasize the need for real life validation of antenna designs.
PCB manufacturing process validation
The PCB fabrication parameters (such as the copper or epoxy layer thickness) have an
impact on the antenna inductance. Variations happen if the parameters of the PCB
fabrication process change or in case of a change of PCB supplier.
Departments such as quality, operations and manufacturing should therefore be made
aware of this.
Product packaging/housing considerations
The read range of the dual interface M24LRxx board can be greatly affected by the housing
of the final product.
The most obvious case is when a metallic housing is used. The product packaging then
behaves as a Faraday cage, preventing the reader energy and signal from attaining the dual
interface EEPROM device.
The housing might also influence the PCB antenna’s tuning frequency, which is why it is
always recommended to measure the RF performance of the application in the final product
configuration.
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From design to production
Figure 26. Impact of housing/packaging material on RF communication
Dual interface EEPROM
Nonconductive housing:
RF communication OK
Conductive housing:
no RF communication
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Process flow
●
Design:
–
Start from the dual interface EEPROM’s internal tuning frequency (Ctuning).
–
Calculate the theoretical Lantenna value based on Ctuning and ftuning.
Hint: check the device datasheet.
Hint: use the simplified models in this application note or other more sophisticated
models developed in the RF literature.
●
–
Define the antenna dimensions.
–
Compute the theoretical antenna design and layout.
Prototyping
–
Define an antenna matrix with different values centered around the targeted
Lantenna value.
Hint: select 6 to 10 antennas with inductances that vary around Lantenna by steps
of 5 %.
–
Fabrication of the antennas and M24LRxx mounting.
For each prototype:
●
●
–
Measure the antenna’s tuning frequency.
–
Measure the read range with all types of selected RFID readers.
–
Measure the read range in configurations close to the actual product usage.
Industrialization
–
Characterize the tuning frequency dispersion on a significant number of samples.
–
Measure the read range of the lowest and highest tuning frequency boards with
various readers and in the various configurations.
–
Validate that the selected target Lantenna value is appropriate versus the process
variation.
Production
–
Process monitoring
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Revision history
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AN2972
Revision history
Table 4.
Document revision history
Date
Revision
26-May-2009
1
Initial release.
06-Aug-2009
2
Modified:
– Introduction
– Section 1.1: Passive RFID technology
– Section 1.2: Simplified equivalent inlay circuit
– Section 1.4: Optimum antenna tuning
– Section 2.3: Inductance of a square antenna
Added: Section 4: From design to production
18-Aug-2009
3
Corrected equation allowing to compute the tuning frequency on
cover page.
04-Sep-2009
4
Figure 3: Power supply in RF mode, Figure 5: From the RFID reader
to the M24LRxx and Figure 6: From the M24LRxx to the RFID reader
modified.
Section 2.5: PCB layout added.
Section 3.1: Antenna tuning measurements with a network analyzer
and Section 3.2: Antenna measurements with standard laboratory
tools modified.
Considerations on the actual system tuning frequency added. PCB
manufacturing process validation modified.
Product packaging/housing considerations and Process flow added.
Small text changes.
11-Feb-2010
5
Document classification level changed to public.
Power transfer updated in Section 1.1: Passive RFID technology.
Section 1.4 title modified.
6
M24LR64-R replaced by M24LRxx-R and M24LRxxE-R on the cover
page, then by M24LRxx (see Note:).
Moved former 3rd and 4th paragraphs on the cover page to an
Operating mode section.
Added Table 1: Applicable products.
Added Section 2.5.3: Metal considerations.
21-Dec-2012
24/25
Changes
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