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Texas Instruments RF430FRL15xH NFC ISO 15693 Sensor Transponder Practical Antenna Design (Rev. A) Application notes
Application Report
SLOA217A – April 2015 – Revised July 2015
RF430FRL15xH NFC and ISO/IEC 15693 Sensor
Transponder Practical Antenna Design
Christian Buchberger, Kostas Aslanidis
ABSTRACT
The TI RF430FRL15xH ISO/IEC 15693 NFC Sensor Transponder is an NFC Tag Type 5 device operating
at 13.56 MHz (HF band). Depending on the application communication distance requirements, the
antenna geometry can be adjusted. The device gives the flexibility to be used in combination with various
antenna geometries.
The scope of this document is a short practical guidance on antenna design basics.
Figure 1. RF430FRL152HEVM
MSP430 is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
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Contents
RF430FRL15xH NFC and ISO/IEC 15693 Sensor Transponder ...................................................... 3
RF Interface .................................................................................................................. 5
RF430FRL152HEVM ........................................................................................................ 5
Internal Resonance Capacitor ............................................................................................. 6
Antenna ....................................................................................................................... 7
Reading Performance With Different External Antennas ............................................................. 10
Antenna Quality Factor .................................................................................................... 12
Resonance Frequency Detuning ......................................................................................... 15
Rectangular Antenna Inductance Calculation Examples.............................................................. 16
List of Figures
1
RF430FRL152HEVM ........................................................................................................ 1
2
Typical Application ........................................................................................................... 3
3
RF430FRL15xH Functional Block Diagram .............................................................................. 4
4
Example Application Circuit
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
................................................................................................ 4
RF Interface Module With Antenna ........................................................................................ 5
RF430FRL152H EVM Reference Schematic ............................................................................ 6
RF430FRL152H EVM Internal Resonance Capacitor Values ......................................................... 7
Evaluation Module Antenna Connection .................................................................................. 8
EVM Antenna Connection .................................................................................................. 8
Inductance vs Resonance Capacitance Values ......................................................................... 9
TRF7970A EVM, Reference Reader .................................................................................... 10
ISO Class 6, ISO Class 5, ISO Class 4, ISO Class 3 Antennas ..................................................... 10
3DC15HF-0003K and SDTR1103-HF2-0002K Premo Coils ......................................................... 10
3D15 (88 8035 82) and MS32 ka (88 8036 10) Neosid Coils ........................................................ 11
Resonant Frequency and Q Factor Measurement Setup ............................................................. 13
Test Tool .................................................................................................................... 13
Small Test Tool (optional) ................................................................................................. 14
Antenna Q-Factor Measurement ......................................................................................... 15
Communication Distance vs Resonance Detuning .................................................................... 16
Rectangular Antenna Coil ................................................................................................. 16
Antenna Geometry ......................................................................................................... 17
RF430FRL152H EVM ..................................................................................................... 18
ISO 10373-6 Reference Antenna Class 1 .............................................................................. 19
ISO 10373-6 Reference Antenna Class 6 .............................................................................. 20
Temperature Patch ......................................................................................................... 21
Temperature Patch Board Schematic ................................................................................... 22
Neosid 3D15 (88 8035 82) and MS32 (88 8036 10) .................................................................. 23
Premo 3DC15HF-0003K and SDTR1103-HF2-0002K ................................................................ 23
List of Tables
2
.............................................................................................
1
Internal Resonance Capacitor
2
Onboard Antenna Inductance .............................................................................................. 7
3
Antenna Inductance (fres ≈ 13.7 MHz) ..................................................................................... 7
4
Tuning Capacitor Values .................................................................................................. 11
5
Maximum Communication Distance ..................................................................................... 11
6
Mobile Phones Communication Distance Using RF430FRL15xH With ISO Class 6 Antenna ................... 11
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RF430FRL15xH NFC and ISO/IEC 15693 Sensor Transponder
1.1
Transponder Overview
The RF430FRL15xH device is a 13.56-MHz transponder chip with a programmable 16-bit MSP430™ low
power microcontroller. The device features embedded universal FRAM nonvolatile memory for storage of
program code and user data such as calibration and measurement data. The RF430FRL15xH supports
communication, parameter setting, and configuration through the RF-compliant interfaces: ISO/IEC 15693,
NFC Tag Type 5 (T5T) (draft version) and ISO/IEC 18000-3 and the contacted SPI or I2C. Sensor
measurements can be supported by the internal temperature sensor and the onboard 14-bit sigma-delta
analog-to-digital converter (ADC), an external digital sensor can be connected through SPI or I2C
interface.
The RF430FRL15xH devices are optimized for operation in fully passive (battery-less) or single-cell
battery powered (semi-active) mode to achieve extended battery life in portable and wireless sensing
applications.
FRAM is a nonvolatile memory that combines the speed, flexibility, and endurance of SRAM with the
stability and reliability of flash, all at lower total power consumption.
For details on the device family, refer to the corresponding product folders:
www.ti.com/product/RF430FRL152H
www.ti.com/product/RF430FRL153H
www.ti.com/product/RF430FRL154H
Figure 2 shows a typical application diagram for these transponders.
2
Sensor or
Periphery
IC
or
SPI
RF430FRL15xH
NFC and
RFID
Reader
Figure 2. Typical Application
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Figure 3 shows the functional block diagram of the transponder.
RST/NMI
P1.0 to P1.7
VDDB VSS VDDH
IO Port
8 I/Os
with
interrupt
capability
Power
Supply
System
LF-OSC
VDDSW
HF-OSC
Clock
System
CLKIN
Reset
Int-Logic
ACLK
8KB
ROM
4KB
RAM
CRC
2KB
FRAM
16 bit
SMCLK
VDD2X
VDDD
CP1
CP2
MCLK
MAB
CPU and
Working
Registers
TMS, TCK,
TDI, TDO
ANT2
4W-JTAG
Debug
support
eUSCI_B0
Timer_A
SPI
I2C
3 CC
Registers
14-Bit
SigmaDelta
ADC
Watchdog
WDTA
32/16 Bit
ISO
15693
Decode
and
Encode
ISO
15693
Analog
Front End
CRES
LRES
ANT1
ADC0/ADC1/ADC2
TEMP1/TEMP2
Figure 3. RF430FRL15xH Functional Block Diagram
Figure 4 shows the typical connections to the transponder for an example application.
JTAG signals
C9
C2
VDDSW
VDDB
CP1
B1
C3
C4
CP2
TDO
TMS
17
2
3
16
4
15
5
14
13
6
7
C5
TCK
18
8
ADC2
ADC1/TEMP1
R2
TST1
SVSS
C7
TST2
ADC0
VDD2X
9 10 11 12
VDD2X
SCL
ANT2
24 23 22 21 20 19
1
SDA
ANT1
RST/NMI
C1
TDI
VDD
L1
VDDH
C8
Analog
Sensor 1
C6
SVSS
Analog
Sensor 2
SVSS
Two analog sensors connected through I2C, supplied by VDD2X (≈3 V)
Figure 4. Example Application Circuit
4
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RF Interface
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RF Interface
The RF communication interface (see Figure 5) is based on ISO15693 and supports the NFC T5T (draft
version) specification. It supports data rates 1-out-of-4 and 1-out-of-256 for reader-to-tag communication
and 26 kbit/s for tag-to-reader communication.
Figure 5. RF Interface Module With Antenna
The interface from the RF module to the outside world is the antenna connection. The two pins ANT1 and
ANT2 can be connected to an external antenna. The antenna dimensions and parameters depend on the
basic application requirements, including:
• communication distance
• available space
• antenna technology
The on-chip resonance capacitor (CINT) has a typical value of 35 pF with a tolerance of ±10%.
A resonance circuit is generated using the external antenna (inductance L), the on-chip resonance
capacitor (CINT), and if necessary an external capacitor (CEXT).The additional external resonance capacitor
(CEXT) can be added to allow antenna inductance variation for lower inductance antennas (L < 3.8 µH)
(see Figure 10). The resonance frequency is calculated using following formula:
1
fres =
2×p× L×C
where
•
•
•
3
L = Antenna inductance
C = Total resonance capacitance (C = CINT + CEXT )
fres = Resonance frequency
(1)
RF430FRL152HEVM
The product family of the RF430RL15xH devices includes the RF430FRL152H, RF430FRL153H, and the
RF430FRL154H variants. This evaluation and development platform uses the RF430FRL152H device.
The RF430FRL152H EVM is an evaluation and development platform for the RF430FRL152H device.
The development board allows the developers to design and test their systems and become familiar with
the ISO/IEC 15693 and NFC T5T protocol.
For test purposes, an onboard antenna is available on the evaluation and development platform. This
antenna is connected to the RF430FRL154H pins ANT1 and ANT2. The two 0-Ω resistors (R27 and R31)
can be removed to disconnect the onboard antenna if an external antenna is used.
Additional information on the target board can be found at http://www.ti.com/tool/rf430frl152hevm.
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RF430FRL152HEVM
3.1
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RF430FRL152HEVM Schematic
Figure 6 shows the EVM schematic. All RF430FRL152HEVM design files can be found at
http://www.ti.com/tool/RF430FRL152HEVM.
Figure 6. RF430FRL152H EVM Reference Schematic
4
Internal Resonance Capacitor
An on-chip resonance capacitor (CINT) has been implemented. The CINT is connected parallel to the
antenna pins ANT1 and ANT2 to generate a resonance circuit with the antenna (coil) that is connected to
the ANT1 and ANT2 pins.
Table 1. Internal Resonance Capacitor (1)
(1)
6
MIN
TYP
MAX
31.5 pF
35 pF
38.5 pF
CINT = 35 pF ±10% on-chip resonance capacitor (CINT)
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Input Capacitance CINT (pF)
The input impedance of the device is not constant. The value varies with the input voltage, the activation
of the rectifier, and the RF limiter of the device (see Figure 7).
37
35
33
31
29
27
25
23
21
19
17
15
0
0.5
1
1.5
2
2.5
3
3.5
4
Voltage (V)
Figure 7. RF430FRL152H EVM Internal Resonance Capacitor Values
5
Antenna
An external antenna can be connected to JP1 on the RF430FRL152HEVM (pins 1 and 2 on the device).
These pins are parallel to the internal resonance capacitor CINT to generate the resonance circuit.
Depending on the antenna inductance, an additional external resonance capacitor parallel to CINT can be
used.
5.1
Onboard Antenna Inductance
The onboard antenna inductance has a typical value of 1.8 µH. The etched antenna tolerances are
typically in the range of ±2% (see Table 2).
Table 2. Onboard Antenna Inductance
MIN
TYP
MAX
1.8 µH
5.2
External Antenna Inductance Range Without External Capacitor
Theoretically, the antenna inductance can have any value within the logical range of the L/C ratio.
If no external capacitor is used, and assuming a fres ≈ 13.7 MHz, the CINT tolerances must be considered
for the calculation of the antenna inductance (see Table 3).
Table 3. Antenna Inductance (fres ≈ 13.7 MHz)
MIN
TYP
MAX
3.57 µH (at CINT = 38.5 pF)
3.85 µH (at CINT = 35 pF)
4.28 µH (at CINT = 31.5 pF)
Note: In this calculation, antenna tolerances are not considered. Normally these are in the range of ±2%.
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Antenna
5.3
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External Antenna Connection
On the RF430FRL152HEVM, it is possible to disconnect the "onboard" antenna and connect an external
antenna.
The modifications to the board are as follows (see Figure 8):
• Remove R27 and R31 (green)
• Connect an external antenna to the two antenna pins at JP1 (blue)
• Use capacitors parallel to the external coil or antenna to adjust it to the resonance frequency
Figure 8. Evaluation Module Antenna Connection
5.4
Antenna Resonance Circuit
Figure 9 shows the basic input circuit of the RF430FRL15xH. The (onboard) antenna (L) with the internal
parallel capacitor (CINT) generates a resonance circuit at the desired frequency.
Figure 9. EVM Antenna Connection
8
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If an external antenna is used, LEXT is connected at JP1 on the RF430FRL152HEVM as described in
Section 5.3. If there is no intention to use external capacitance CEXT, the inductance of the coil has to be
chosen as described in Section 5.2.
Depending on the antenna inductance, an additional external resonance capacitor CEXT may be added
parallel to LEXT. When calculating the corresponding CEXT value, the internal capacitance CINT has to be
taken into account. The sum of the parallel capacitors is the value for the total resonance capacitance.
Cres = CINT+ CEXT
During the development phase, it is recommended to use an external adjustable (trimming) capacitor for
fine-tuning. This will help to eliminate component tolerance and board parasitics. For production, the value
of this variable capacitor can be measured and replaced by an external fixed capacitor with the same
value (or combination of several capacitors).
The recommended operating resonance frequency (fres) is about fres ≈ 13.7 MHz for optimal performance
according to Section 8. For stable operation, make sure that the resonance frequency, including all of the
tolerances, stays between 13.56 MHz and 13.7 MHz. Using resonance circuits tuned outside of that range
results in performance degradation.
Figure 10 shows the inductance and capacitance values to generate resonance at 13.7 MHz.
Resonance Capacitance (pF)
140
120
100
80
60
40
Resonance at 13.7 MHz
20
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Inductance (µH)
Figure 10. Inductance vs Resonance Capacitance Values
The passive quality factor (Q) of the resonance circuit should be Q < 50. In case of a higher Q, an
external resistor parallel to the inductor Lext can be added to reduce Q. A suitable value could be in the
range of 10 kΩ to 20 kΩ.
Note: In this document, parasitic capacitances from the layout and the connections are not considered.
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Reading Performance With Different External Antennas
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Reading Performance With Different External Antennas
Depending on the application, the antenna size may be restricted. To give an overview of how different
antenna sizes can change the performance of the system, some example measurements of the reading
distance are given.
The TRF7970A EVM, a fully integrated NFC/RFID transceiver with an onboard PCB antenna, is used as
reference reader (http://www.ti.com/tool/trf7970aevm) (see Figure 11).
Figure 11. TRF7970A EVM, Reference Reader
Figure 12 through Figure 14 show the different antennas that were used on the RF430FRL15xH side.
Figure 12. ISO Class 6, ISO Class 5, ISO Class 4, ISO Class 3 Antennas
Figure 13. 3DC15HF-0003K and SDTR1103-HF2-0002K Premo Coils
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Figure 14. 3D15 (88 8035 82) and MS32 ka (88 8036 10) Neosid Coils
The antennas were tuned to a resonance frequency of fres = 13.7 MHz in all cases using external tuning
capacitors parallel to the inductors. Table 4 lists the inductances and the corresponding capacitance
values to achieve 13.7-MHz resonance.
Table 4. Tuning Capacitor Values
Antenna
Inductance
Total Resonance Capacitor
ISO Class 3
2.4 µH
56 pF
ISO Class 4
2.4 µH
56 pF
ISO Class 5
2.4 µH
56 pF
ISO Class 6
2.4 µH
56 pF
SDTR1103-HF2-0002K
2 µH
68 pF
3DC15HF-0003K
3 µH
45 pF
3D15 (88 8035 82)
3.25 µH
41 pF
MS32 (88 8036 10)
2.1 µH
64 pF
Table 5 lists the performance results using TRF7970AEVM as reference reader. The communication
distance described in Table 5 and Table 6 gives the range at which the reader and the tag can
communicate and exchange data. The charging distance is larger. Place the reader and the tag antenna
parallel to each other for the best performance.
Table 5. Maximum Communication Distance
RF430FRL152H External Antenna
Maximum Communication Distance (cm)
RF430FRL152H EVM (onboard antenna)
9.5
ISO Class 3
6.5
ISO Class 4
7.5
ISO Class 5
8
ISO Class 6
8.5
3DC15HF-0003K
4.5
SDTR1103-HF2-0002K
3.5
3D15 (88 8035 82)
5
MS32 (88 8036 10)
3.5
NFC enabled mobile phones can also be used to read the RF430FRL15xH. Table 6 lists the reading
distances using different mobile phones and the RF430FRL15xH with an external ISO Class 6 antenna.
Table 6. Mobile Phones Communication Distance Using RF430FRL15xH With
ISO Class 6 Antenna
NFC Enabled Mobile Phone
Communication Distance (cm)
Nokia Lumia 830
3
HTC One M8
2.5
Samsung Galaxy S3
3.5
Samsung Galaxy S5
4
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Antenna Quality Factor
7
Antenna Quality Factor
7.1
Basics
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The quality factor of a resonance circuit antenna gives a quantification of how well the antenna behaves in
the frequency spectrum given its resonance frequency. A higher Q factor results in a narrow band
behavior. If the operational band of the antenna is too narrow (high Q), information can no longer be
received or transmitted due to the limited bandwidth (BW) and communication MAY fail. On the other
hand, if the operational band is too wide (low Q), the system performance decreases due to lower RF field
leading to lower power supply and communication fails caused by noise due to large BW.
The antenna Q factor can be calculated using the following formulas:
BW3dB = f2 – f1
f
Q = res
BW
where
•
•
•
•
7.2
BW3dB is the 3-dB bandwidth of the antenna
f2 is the upper frequency
f1 is the lower frequency
fres is the desired resonance frequency (in this case, fres = 13.7 MHz).
(2)
Setup and Measurement
To calculate the Q Factor, the resonant frequency and bandwidth must be measured. That can be done
using a spectrum analyzer with tracking generator (such as the R&S FSP) in combination with a special
text fixture. The test fixture should consist of a pickup coil connected to the input of the spectrum analyzer
and a larger coil connected to the output of the spectrum analyzers tracking generator as shown in
Figure 15.
Spectrum analyzer setup:
• All measurements must be done with the antenna connected to the rest of the system (including the
evaluation board and IC). Otherwise, internal and parasitic capacitance are ignored.
• Connect test fixture to analyzer. In Figure 15, connect the large coil (red connector) to output of
tracking generator, and connect the small pickup coil (blue connector) to the input channel.
• Place the device under test (DUT) on top of the fixture and center and align it horizontally to the pickup
coil.
• Enable tracking generator output, center to the expected inlay frequency (13.6 MHz) with a span in the
range of 1 to 5 MHz. If there is no resonance curve to be seen, adjust the span and reference level. A
vertical scale of 1 to 3 dB/div is recommended.
• Use markers to determine and measure the 3-dB bandwidth of the resonance curve. (–3 dB from the
resonance peak in both negative and positive f-direction).
• The quality factor can now be calculated using the formulas in Section 7.1. See Section 7.3 for an
example.
NOTE: Temperature, humidity, and proximity of metals or organic materials affect the resonant
frequency and Q of antennas.
12
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Figure 15. Resonant Frequency and Q Factor Measurement Setup
7.2.1
Test Tool
Figure 16. Test Tool
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Antenna Quality Factor
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Figure 17. Small Test Tool (optional)
7.3
Example
The values that Figure 18 shows are:
f1 = 13.594 MHz
f2 = 13.796 MHz
BW3dB = f2 – f1= 202 kHz
Alternatively, set the marker settings to read the delta between them directly.
With our resonance frequency given as fres = 13.7 MHz, the quality factor of the antenna (or DUT) is:
Q=
14
fres
13.7 MHz
=
= 67.8
BW3dB
202 kHz
(3)
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Resonance Frequency Detuning
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Figure 18. Antenna Q-Factor Measurement
8
Resonance Frequency Detuning
The resonance frequency (in combination with a stable quality factor) should be tuned for the best
communication distance between the reader and the transponder. Variations of the resonance frequency
will cause performance degradation.
The variations normally come from the tag's internal capacitor tolerances, antenna parameters,
connections and external influences such as metallic or organic objects in close proximity.
Table 1 gives the internal resonance capacitor tolerances of the RF430FRL15xH IC. For practical reasons,
it is recommended to compensate all the tolerances using an external capacitor connected between ANT1
and ANT2. During the development phase, an adjustable capacitor can be used to fine tune to achieve a
maximum communication distance. This capacitor can be replaced with a fixed value for the final product.
It is difficult to compensate the resonance frequency variations caused by unpredictable external
influences in advance as these are not known. In these cases a special antenna design may be used to
reduce the influences.
Figure 19 shows an example of influence of a detuned ISO Class 6 antenna on the RF430FRL152H tag
over the communication distance using the NFC Reader TRF7970AEVM.
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Rectangular Antenna Inductance Calculation Examples
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10
Communication Distance (cm)
9.5
13.7 MHz
9
8.5
8
7.5
7
6.5
6
13
13.25
13.5
13.75
14
14.25
Resonance Frequency (MHz)
Figure 19. Communication Distance vs Resonance Detuning
9
Rectangular Antenna Inductance Calculation Examples
Calculation of the antenna inductance is very complex. For engineering purposes, there are some
simplified formulas that use certain assumptions to get a good estimation of the inductance. There are
various simplified formulas with different assumptions. These assumptions must be considered when
choosing the formula.
Figure 20. Rectangular Antenna Coil
Equation 4
(1)
estimates the inductance value of a rectangular coil such as Figure 20 shows.
é
æ
u0
h + h2 + w 2
L := NP ×
× ê –2(w + h) + 2 h2 + w 2 – h × ln ç
ê
ç
p
w
è
ëê
ö
æ
2
2
÷ – w × ln ç w + h + w
÷
ç
h
ø
è
ù
ö
÷ + h × ln æ 2 × h ö + w × ln æ 2 × w ö ú
ç
÷
ç
÷
÷
è a ø
è a øú
ø
ûú
(4)
Where,
N = Number of turns
w = Average width of the rectangle
h = Average height of the rectangle
hout = Outer height
wout = Outer with
hin = Inner height
win = Inner width
a = Trace width
µ0 = Permeability of the medium
P = Correction factor
(1)
16
From http://emclab.mst.edu/inductance/rectgl/
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Best experience using the average values for the parameters w and h (see Figure 21). It is calculated as:
h :=
hout – hin
2
w :=
wout – win
2
(5)
The correction factor P is given in the literature as:
Wired: 1.8 to 1.9
Etched: 1.75 to 1.85
Printed: 1.7 to 1.8
Further antenna calculation methods can be found on various websites including:
http://emclab.mst.edu/inductance/rectgl/
http://emclab.mst.edu/inductance/
Figure 21. Antenna Geometry
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Rectangular Antenna Inductance Calculation Examples
9.1
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RF430FRL152H EVM Onboard Antenna
The following example shows the calculation for the RF430FRL152HEVM onboard antenna (see
Figure 22).
Figure 22. RF430FRL152H EVM
N: 6
w: 37 mm
h: 19 mm
hout: 25 mm
wout: 43 mm
hin: 13 mm
win: 31 mm
a: 0.5 mm
P: 1.8
Measured L = 1.8 µH
Calculated value with P = 1.8
L := NP ×
é
æ
u0
h + h2 + w 2
× ê –2(w + h) + 2 h2 + w 2 – h × ln ç
ê
ç
w
p
è
ëê
ö
æ
2
2
÷ – w × ln ç w + h + w
÷
ç
h
ø
è
L = 1.776 × 10 –6
18
ù
ö
÷ + h × ln æ 2 × h ö + w × ln æ 2 × w ö ú
ç
÷
ç
÷ú
÷
è a ø
è a ø
úû
ø
(6)
RF430FRL15xH NFC and ISO/IEC 15693 Sensor Transponder Practical
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SLOA217A – April 2015 – Revised July 2015
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9.2
ISO10373-6 Reference Antenna Class 1
The following example shows the calculation for an ISO Class 1 antenna (see Figure 23).
Figure 23. ISO 10373-6 Reference Antenna Class 1
N: 4
w: 69 mm
h: 39 mm
hout: 42 mm
wout: 72 mm
hin: 36 mm
win: 66 mm
a: 0.5 mm
P: 1.9
Measured L = 2.3 µH
Calculated value with P = 1.9
L := NP ×
é
æ
u0
h + h2 + w 2
× ê –2(w + h) + 2 h2 + w 2 – h × ln ç
ê
ç
w
p
è
ëê
ö
æ
2
2
÷ – w × ln ç w + h + w
÷
ç
h
ø
è
ù
ö
÷ + h × ln æ 2 × h ö + w × ln æ 2 × w ö ú
ç
÷
ç
÷
÷
è a ø
è a øú
ø
ûú
L = 2.307 × 10 –6
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(7)
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Rectangular Antenna Inductance Calculation Examples
9.3
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ISO 10373-6 Reference PICC 6 Antenna
The following example shows the calculation for an ISO Class 6 antenna (see Figure 24).
Figure 24. ISO 10373-6 Reference Antenna Class 6
N: 8
w: 69 mm
h: 39 mm
hout: 19.5 mm
wout: 24.5 mm
hin: 13.5 mm
win: 18.5 mm
a: 0.3 mm
P: 1.85
Measured L = 2.3 µH
Calculated value with P = 1.85
L := NP ×
é
æ
u0
h + h2 + w 2
× ê –2(w + h) + 2 h2 + w 2 – h × ln ç
ê
ç
w
p
è
ëê
ö
æ
2
2
÷ – w × ln ç w + h + w
÷
ç
h
ø
è
L = 2.282 × 10 –6
20
ù
ö
÷ + h × ln æ 2 × h ö + w × ln æ 2 × w ö ú
ç
÷
ç
÷
÷
è a ø
è a øú
ø
ûú
(8)
RF430FRL15xH NFC and ISO/IEC 15693 Sensor Transponder Practical
Antenna Design
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9.4
Temperature Patch Application Example
In this application example, the RF430FRL152H is used as a temperature sensor. It is mounted on a small
(3-cm diameter) round PCB that includes all external capacitors necessary for operation as well as a
printed inductor loop used as antenna (see Figure 25). To measure the temperature, the RF430FRL152H
gathers information from an external temperature sensor. For more information, refer to the application
report Battery-Less NFC/RFID Temperature Sensing Patch (SLOA212).
Figure 25. Temperature Patch
Using the TRF7970A EVM as reader device, the maximum reading distance is 8 cm.
Equation 9 (from http://emclab.mst.edu/inductance/circular/) estimates the inductance of a small circular
coil.
:84;
. = 0 2 Û 4 Û äK Û äN [ln F
G F 2]
S+P
(9)
Where,
L = Inductance
N = Number of turns = 5
R = Radius of coil = 15 mm
μo = Electromagnetic permeability = 1.2566370614…×10−9 H/mm
μr = Relative permeability = 0.9996
w = Trace width = 0.540 mm
t = Trace thickness = 0.035 mm
t
w
The calculated and measured inductance values of the temperature patch antenna (Figure 25) are:
:8Û15;
. = 52 Û 15 Û 1.2566 Û 0.9996 Bln @
A F 2C * [H]
0.540+0.035
>9
L =1565.432 * 10 [H]
(10)
(11)
Calculated value: L = 1.56 µH
Measured value: L = 1.49 µH
SLOA217A – April 2015 – Revised July 2015
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Rectangular Antenna Inductance Calculation Examples
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Figure 26. Temperature Patch Board Schematic
22
RF430FRL15xH NFC and ISO/IEC 15693 Sensor Transponder Practical
Antenna Design
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9.5
Antenna References
Neosid Pemetzrieder GmbH & Co. KG (http://www.neosid.de)
Figure 27. Neosid 3D15 (88 8035 82) and MS32 (88 8036 10)
Premo (http://www.grupopremo.com)
https://www.grupopremo.com/en/548-nfc-antennas
Figure 28. Premo 3DC15HF-0003K and SDTR1103-HF2-0002K
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Revision History
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Revision History
Changes from April 30, 2015 to July 20, 2015 ................................................................................................................. Page
•
•
•
•
In Table 4, Tuning Capacitor Values: changed column heading from "Add External Capacitor" to "Total Resonance
Capacitor"; switched "3DC15HF-0003K" and "SDTR1103-HF2-0002K" in first column ........................................ 11
Replaced Figure 25, Temperature Patch, with a new image ...................................................................... 21
Deleted former Figure 26, Temperature Patch Board Layout; renumbered all following figures .............................. 21
Added calculation for antenna inductance (from the paragraph that starts "Equation 9 shows how to calculate..." through
"Measured value: L = 1.49 µH") ....................................................................................................... 21
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
24
Revision History
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