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Texas Instruments Low-Frequency RFID in a Nutshell Application notes
Application Report
SWRA284 – September 2011
Low-Frequency RFID in a Nutshell
Stefan Recknagel .............................................................................................................................
ABSTRACT
This application report describes the principles of Texas Instruments low-frequency RFID products, how to
choose the right components, and shows best practices for a good PCB layout.
This application report can be used as a guideline for designing a system with the TMS37157 PaLFI chip.
1
2
3
4
5
6
Contents
Introduction .................................................................................................................. 2
Low-Frequency RF Communication ...................................................................................... 2
2.1
Downlink - From the RFID Base Station to the RFID Transponder ......................................... 2
2.2
Uplink - From the RFID Transponder to the RFID Base Station ............................................. 6
How to Choose the Right Passive Components ........................................................................ 7
3.1
Charge Capacitor CL and Resonance Capacitor CR ......................................................... 7
3.2
Antenna ............................................................................................................. 8
Resonance Frequency and System Quality ............................................................................. 9
4.1
How to Calculate the Resonance Frequency of a Resonance Circuit ...................................... 9
4.2
How to Measure the Quality Factor and Resonance Frequency of an RF Circuit ....................... 10
4.3
Influence of Resonance Frequency and System Quality on an RFID System ........................... 11
4.4
Recommended PCB Layout .................................................................................... 12
Summary ................................................................................................................... 12
References ................................................................................................................. 12
List of Figures
1
Correct Placement of a RFID Stick Antenna Near a Base Station ................................................... 2
2
Read Page Command With Response (4 ms/div) ...................................................................... 3
3
Program Page Command With Correct BCC and Response (10 ms/div) ........................................... 4
4
Program Page With Incorrect BCC and No Response (10 ms/div) .................................................. 4
5
PPM Timing for a Read Command ....................................................................................... 5
6
PPM Timing for a Program Command ................................................................................... 6
7
Response Format for a Transponder LF Command
6
8
Transponder Only Application Circuit TMS37157
7
9
10
11
12
13
...................................................................
......................................................................
Quality Factor and Capacity of a NPO Capacitor ......................................................................
Quality Factor and Inductivity of an Antenna ............................................................................
Internal Capacitances of TMS37157 ....................................................................................
Resonance Curve of an RFID System..................................................................................
Recommended PCB Layout for an Antenna ...........................................................................
8
9
10
11
12
List of Tables
.............................................................................
1
Signal Overview of Scope Print Outs for to
2
Example Timings for the TMS37157 PaLFI Device .................................................................... 5
3
Description of the Different Fields of a Transponder Response ...................................................... 6
4
Recommended Components for RFID Transponders Based on the TMS37157 ................................... 7
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1
Introduction
1
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Introduction
RF design is often perceived as being very difficult and hard to understand. This application report guides
you from the principles of the low-frequency (LF) RF communication to the right passive components and
finally to a good layout.
The transmit frequency of the TI low-frequency RF products is 134.2 kHz. The communication range for
passive devices (powered by the magnetic field of a RFID reader) is typically up to 10 cm.
2
Low-Frequency RF Communication
Texas Instruments offers different types of low-frequency RFID devices. All passive TI RFID devices
require a charge phase in which a capacitor is charged (TI half duplex (HDX) principle). Read-only devices
then send their information (typically 96 bits). Read/write devices need to receive a command to perform a
desired action and answer to the downlink command. Active 3D devices (for example, for passive entry or
passive start devices) use the downlink channel to receive a command via LF and wake a microcontroller
if the right wake pattern is received. The uplink is usually transmitted via UHF.
This application report describes the complete communication (downlink and uplink) that is valid for all
types of transponders.
Low-frequency RF communication can be used for short distances even in challenging environments. The
direction of the antenna matters. Always use the antenna in its preferred direction. This should be kept in
mind while designing a RFID system. Figure 1 shows how to position a stick antenna near an RFID base
station antenna.
Base Station Antenna
Transponder Antenna
Figure 1. Correct Placement of a RFID Stick Antenna Near a Base Station
2.1
Downlink - From the RFID Base Station to the RFID Transponder
An RFID communication is always initiated by the base station. The RFID transponder usually has no
local voltage supply and needs the LF field, which is sent by the base station, to function.
A typical communication consists of a charge phase of 25 ms to 50 ms,which is followed by the command
phase. A read and a program command are shown in Figure 2 and Figure 3, respectively. In both cases
the transponder sends its response after it detects a loss of the LF field.
Figure 2 to Figure 4 show a complete communication exchange between a base station and an RFID
transponder. All signals are measured at the transponder and have the following meaning:
Table 1. Signal Overview of Scope Print Outs for Figure 2 to Figure 4
2
NO.
NAME
COLOR
UNIT
1
RF
Yellow
5 V/div
RF voltage at the transponder antenna
MEANING
2
VCL
Blue
2 V/div
Voltage at the VCL charge capacitor
3
EOB
Magenta
1 V/div
End of burst detector output of transponder
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Low-Frequency RF Communication
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The RF voltage is the voltage that is generated by the magnetic field of the RFID base station at the
transponder antenna. The RF voltage is limited by the internal RF limiter of the transponder. The RF
voltage is used to charge the VCL capacitor (typically 220 nF); VCL is the operating voltage for the
transponder and is limited to approximately 6 V. For proper operation, make sure that the VCL limit is
reached before the end of charging. The required charging time is a function of distance, antenna size,
inductance, Q factor, and base station field strength.
After the charge phase, data is transmitted using On/Off Keying (see Table 2 for timings). The transponder
decodes the received data. The EOB output of the transponder shows the detection of the bits. If the time
between two rising edges of EOB is longer than tbitH (see Table 2) a high bit is detected; otherwise, a low
bit is detected.
For a program page command, a 2-byte BCC is transmitted together with the data. Additionally, a second
power burst (Power Burst 2, see Figure 3) is needed for programming the EEPROM. If the received BCC
(needed for program and lock commands) is wrong or if the VCL level does not reach the VCL checker
limit, the transponder performs a discharge without responding (see Figure 4 for a program command with
a wrong BCC).
Read Page
Command
Response
Charge VCL Capacitor
VCL Limit = 6 V
Reached
Figure 2. Read Page Command With Response (4 ms/div)
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Low-Frequency RF Communication
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Charge VCL Capacitor
Program Page Command +
Power Burst 2
Response
5 byte Data + Right BCC
Figure 3. Program Page Command With Correct BCC and Response (10 ms/div)
Charge VCL Capacitor
Program Page Command +
Power Burst 2
No
Response
5 Byte Data + Wrong BCC
Figure 4. Program Page With Incorrect BCC and No Response (10 ms/div)
The downlink uses On/Off Keying for transmitting the data from the base station to the transponder. The
base station switches the 134.2 kHz LF field on and off to modulate the data.
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Low-Frequency RF Communication
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The bits are modulated with Pulse Position Modulation (PPM). Figure 5 shows the PPM timing for a read
page command; Figure 6 shows the timing for a program page command. The program page command
needs one additional transmitter off phase and a power phase. See the device-specific data sheet for the
correct timings. For TMS37157 the recommend timings are shown in Table 2.
Table 2. Example Timings for the TMS37157 PaLFI Device
SIGN
TIME
UNIT
NODE
ttx
25
ms
Charge time to charge VCL capacitor
toffi
170
µs
Initial off time
toff
170
µs
Standard off time
tonH
350
µs
Transmitter on time for a high bit
tbitH
520
µs
Period for a high bit
tonL
230
µs
Transmiter on time for a low bit
tbitL
400
µs
Period for a low bit
tprg
15
ms
Program time necessary for programming the EEPROM
tHdet
510
µs
High bit period must be 210 µs (minimum) to 1730 µs (maximum)
Figure 5. PPM Timing for a Read Command
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Figure 6. PPM Timing for a Program Command
2.2
Uplink - From the RFID Transponder to the RFID Base Station
After reception of a valid command, the transponder processes the data and sends a response. The
response is always 96 bits long and has the structure shown in Figure 7; a detailed description of the
fields is shown in Table 3.
High Bit
PREBITS
START
16 Bits
8 Bits
DATA
Low Bit
48 Bits
LSB
R/W
ADDR.
FRAME BCC
8 Bits
16 Bits
96 Bits
DISCHARGE
MSB
Figure 7. Response Format for a Transponder LF Command
One high bit is represented by 16 sinus oscillations with a frequency of 123.7 kHz (length: 129.3 µs). One
low bit is represented by 16 sinus oscillations with a frequency of 134.7 kHz (length: 128.3 µs).
Table 3. Description of the Different Fields of a Transponder Response
6
FIELD
LENGTH
PREBITS
16 bits
Always 16 low bits - 0x00 00
NODE
START
8 bits
Depends on type of transponder - TMS37157 : 0x7E
DATA
48 bits
Depends on the performed command
R/W ADDRESS
8 bits
Depends on the performed command
FRAME BCC
16 bits
Calculated from START, DATA, and R/W ADDRESS
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How to Choose the Right Passive Components
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The response is always 96 bits long, resulting in a maximum duration for the response of approximately
12.2 ms. After transmitting the data, the transponder discharges the VCL capacitor. If the transponder
receives an invalid command, it shows only the 16 pre-bits followed by discharging the VCL capacitor (see
Figure 4).
3
How to Choose the Right Passive Components
PUSH
BUSY
CLKA/M
SPI_CLK
SPI_SOMI
NPOR
SPI_SIMO
A Texas Instruments RFID low-frequency transponder system needs only a few passive components. This
application report describes the components for 1D systems using the TMS37157. The application circuit
for the transponder function of the TMS37157 is shown in Figure 8.
VBATI
EOB
RF1
TMS37157 PaLFI
LR
VBAT
CR
VCL
TEST I/F
CL
GND
TDAT
TCLK
TEN
VCL
RL
Figure 8. Transponder Only Application Circuit TMS37157
The components and recommended values are listed in Table 4.
Table 4. Recommended Components for RFID Transponders Based on the TMS37157
3.1
SYMBOL
FUNCTION
TYPE
VALUE
LR
Antenna
Ferrite stick antenna
2.66 mH
CR
Resonance capacitor
NPO
470 pF
CL
VCL charge capacitor
X7R
220 nF
RL
Termination resistor
Arbitrary
1 MΩ
Charge Capacitor CL and Resonance Capacitor CR
A half duplex (HDX) RFID system requires a charge capacitor to store energy for phases during which no
magnetic field from the RFID base station is present. Texas Instruments recommends using a 220-nF
surface mount capacitor, X7R type, with a maximum operating voltage of at least 16 V.
The resonance circuit of a RFID circuit is determined by the antenna and the resonance capacitor.
Therefore, a resonance capacitor with a low variation in capacity and a high quality factor is required.
Texas Instruments strongly recommends using a 470-pF ±2% surface-mount capacitor, NPO (CGO) type,
with a maximum operating voltage of at least 20 V.
Only NPO (CGO) type capacitors have a high enough quality factor to ensure a properly working RFID
system. Using another type of capacitor will likely result in a non-working system.
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How to Choose the Right Passive Components
3.1.1
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How to Measure the Quality Factor and Capacitance of a Capacitor
To measure the quality factor and the capacitance of a capacitor, an impedance analyzer is needed.
Figure 9 shows the measurement performed with a Wayne Kerr impedance analyzer. Attach the capacitor
between the two measurement pins of the impedance analyzer. The measurement frequency is 134.2 kHz
(the nominal frequency of LF RFID). Be aware that the quality measurement is not precise for such high
values. The quality should be above 1000.
Figure 9. Quality Factor and Capacity of a NPO Capacitor
3.2
Antenna
The quality factor of a NPO type resonance capacitor is normally above 1000. Thus, one of the most
importing factors for the overall quality factor of the resonance circuit is the antenna. In combination with
the TMS37157, Texas Instruments recommends using a ferrite stick antenna with an inductivity of 2.66
mH ±2.8%. To ensure an overall system quality of at least 30 (required for proper operation of the
TMS37157) the quality factor of the antenna should be between 40 and 75.
3.2.1
How to Measure the Quality Factor and Inductivity of an Antenna
To measure the quality factor and the inductivity of an antenna, an impedance analyzer is needed.
Figure 10 shows the measurement performed with a Wayne Kerr impedance analyzer. Attach the antenna
between the two measurement pins of the impedance analyzer. The measurement frequency is 134.2 kHz
(the nominal frequency of LF RFID).
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Resonance Frequency and System Quality
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Figure 10. Quality Factor and Inductivity of an Antenna
4
Resonance Frequency and System Quality
4.1
How to Calculate the Resonance Frequency of a Resonance Circuit
The resonance frequency of a resonance circuit can be calculated with Equation 1.
1
fres = −
−
2π √Cres • Lantenna
(1)
Where:
• fres – Resonance frequency of the resonance circuit
• Cres – Resonance capacitor
• Lantenna – Inductivity of the antenna
The resonance frequency of the components from Figure 9 and Figure 10 is 140.44 kHz. The TMS37157
has an additional internal parasitic capacitance (Cint, see Figure 11) of approximately 30 pF, which must
be added to Cres. In addition, the TMS37157 has an integrated trimming function that offers the possibility
to switch on additional resonance capacitors to trim fres to 134.2 kHz.
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Resonance Frequency and System Quality
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TMS37157 LF Interface
RF1
Cint
L antenna
Cres
Trim Capacitor Ar ray
VCL
CL
RL
Figure 11. Internal Capacitances of TMS37157
4.2
How to Measure the Quality Factor and Resonance Frequency of an RF Circuit
For measurement of the quality and resonance frequency of an RF circuit, a spectrum analyzer is needed.
Connect a send coil to the output of the spectrum analyzer and set the output power to minimum. For
Texas Instruments low-frequency systems, choose a center frequency of 134.2 kHz. Connect another
cable to one antenna pin and ground, the other end to the input of the spectrum analyzer. Be aware that
high voltages can appear at the antenna, which can damage the input of the spectrum analyzer. It is
recommended to place the RF circuit away from the send coil initially (20 cm or more), and then close the
distance until a superelevation at the spectrum analyzer can be seen (see Figure 11). The peak
determines the resonance frequency. The quality factor can be calculated from the resonance frequency
and the 3-dB bandwidth. The 3-dB bandwidth is the difference between the two frequencies where the
power is half (3 dB lower) as in resonance. The quality factor can be calculated with Equation 2.
f
Q = res
B3dB
(2)
Where:
• Q – Quality factor of the resonance circuit
• fres – Resonance frequency of the resonance circuit
• B3dB – 3-dB bandwidth
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Figure 12. Resonance Curve of an RFID System
The quality factor of the RFID system measured in Figure 12 is calculated with Equation 3.
f
134.06kHz
Q = res =
= 39
B3dB
3.440kHz
4.3
(3)
Influence of Resonance Frequency and System Quality on an RFID System
The TMS37157 requires a quality factor of at least 30 to work properly. The quality factor of the resonance
circuit is determined by different factors:
• Antenna should have a high Q (52 to 60)
• Resonance capacitor must be NPO (CGO) type
• In the PCB design, no metal layer should be underneath or around the antenna
• No signal routing under the antenna
• Keep PCB traces to the antenna short, straight, and thin
• Keep antenna pads as small as possible
• Quality factor and resonance frequency change with temperature
An RFID system is described by the parts of the resonance circuit (antenna, capacitor) but also by the
RFID chip and the other active and passive parts mounted on the PCB. The last step during production of
an RFID system is to trim the circuit. Because of component tolerances, every RFID system must be
trimmed—this can be easily achieved with the integrated trim function of the TMS37157.
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Summary
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A low quality factor and untrimmed resonance circuit decrease the performance of the whole RFID
system. Inadequate energy is induced into the RF circuit of the transponder, resulting in a low voltage at
the antenna (RF pin) and a poorly charged VCL capacitor. Metal under the resonance circuit detunes the
resonance circuit and lowers the quality factor immensely; this should be considered while designing a
mount-on-metal system.
4.4
Recommended PCB Layout
For a good PCB layout, keep in mind the factors which determine the system quality, mentioned in
Section 4.3 . An example for a recommended PCB layout is shown in Figure 13, where the L1 antenna
and the C11 resonance capacitor are shown. The PCB traces to the antenna are straight, thin, and short
and are not routed under the antenna. The antenna pads are as small as possible, and no metal layer is
under or around the antenna.
Figure 13. Recommended PCB Layout for an Antenna
5
Summary
This application report gives a short introduction about low-frequency RFID from Texas Instruments. The
reader is introduced to LF RFID principles, is advised how to choose the right passive components, and
should be aware of the influence of resonance frequency and system quality in an RFID transponder
system.
6
References
1. TMS37157 Passive Low-Frequency Interface (PaLFI) Device With EEPROM and 134.2-kHz
Transponder data sheet (SWRS083)
2. eZ430-TMS37157 Development Tool User's Guide (SLAU281)
3. Communicating With the RFID Base Station (SWRA283)
12
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