Application Report
SCBA027 – July 2010
TMS3705 Passive Antenna Solution
Langner Siegfried, Juergen Mayer
.............................................................................. MCU-RF Systems
ABSTRACT
The Texas Instruments low-frequency transponder technology provides the possibility to use a simple
passive antenna in combination with various antenna cable lengths. This solution significantly reduces
system costs because the active part of the transceiver can be added to the already existing host system,
e.g., the body control module (BCM) of a vehicle.
In comparison to a standard system, no additional components are necessary, i.e., voltage regular and
blocking capacitors. Due to the possibility of varying the cable length, it is possible to develop a universal
control module that can be used in many different architectures.
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Contents
Introduction .................................................................................................................. 2
Main Features ............................................................................................................... 2
System overview ............................................................................................................ 3
System Options ............................................................................................................. 4
Recommended Antenna Parameter for Long Cable Systems ........................................................ 6
Schematic for a Long Cable Reader System .......................................................................... 10
References ................................................................................................................. 13
List of Figures
.......................................................................................
1
HDX RFID System Block Diagram
2
Communication Overview .................................................................................................. 3
3
HDX RFID Reader Configuration With Short Antenna Cable ......................................................... 4
4
HDX RFID Reader Configuration With Long Antenna Cable
5
5
Antenna Cable Types
8
6
7
8
9
10
11
.........................................................
......................................................................................................
Schematic Used for Long Cable Simulation .............................................................................
Simulation Result - Frequency Shift Due to Long Antenna Cable ..................................................
HDX RFID Reader Schematic ...........................................................................................
HDX RFID Reader – Schematic Low-Pass Filter .....................................................................
HDX RFID Reader Schematic – DC Path Highlighted ...............................................................
HDX RFID Reader Schematic – ESD Protection Diodes ............................................................
SCBA027 – July 2010
TMS3705 Passive Antenna Solution
Copyright © 2010, Texas Instruments Incorporated
2
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1
Introduction
1
www.ti.com
Introduction
This application report provides supplementary information about the Texas Instruments 134.2 kHz RFID
Base Station IC TMS3705x. In particular, the document shows the possibility of using a remote passive
antenna using a simple two-wire antenna cable with a length up to 4 meters.
In principle, the TMS3705x serves as an interface between the transponder and a Microcontroller using a
bi-directional serial interface. The transponder is supplied with energy and data by on-off keying of the
carrier. According to the TRP specification, the bit encoding must be handled by the Microcontroller. At the
end of the telegram sent to the transponder, the transmitter is completely switched off and the transponder
sends the data back by frequency modulation (FM) using two frequencies.
The TMS3705x is a highly integrated device for communication with all of the Texas Instruments
low-frequency transponders; the amount of necessary external components is minimized.
2
Main Features
•
•
•
•
•
•
•
Only a few external components
Reference designs available
Increased passenger safety as no module on steering column is required
Increased flexibility in housing and system design
High degree of flexibility for the antenna placement
Easy design to support: Left-/Right- steering vehicles
Reduced shock/reliability stress as only the antenna is at the steering column
134.2kHz RFID reader ASIC
Antenna
Loop
Host Controller
Transponder
TMS3705x
S3705x
Controller
μμController
Antenna Cable up to 4 m
Body Control Module (BCM)
Figure 1. HDX RFID System Block Diagram
All trademarks are the property of their respective owners.
2
TMS3705 Passive Antenna Solution
SCBA027 – July 2010
Copyright © 2010, Texas Instruments Incorporated
System overview
www.ti.com
3
System overview
A typical passive transponder system consists of a transponder and a reader (exciter). The air interface
between the transponder and the reader behaves like a magnetic transformer with a large air gap. The
resulting coupling factor between the transponder and reader is significantly lower compared to traditional
magnetic transformers.
There are two LF RFID communication technologies on the market: full duplex (FDX) and half duplex
(HDX). The HDX technology is Texas Instruments proprietary RFID technology. The fundamental
difference to FDX, which uses load modulation for the communication between the transponder and the
reader, is that the HDX transponder generates an active, frequency shift keying (FSK) modulated signal
after the reader charges up the charge capacitor inside the transponder.
Therefore, the TI system is specified by the field strength level required to activate the transponder,
Charge & Downlink, as well as the field strength that is generated by the transponder during the Uplink
response phase.
Figure 2. Communication Overview
By splitting the overall communication into Charge & Downlink (Reader to TRP) and Uplink (TRP to
reader), both communication phases can be optimized for a very high reading distance. The signal
separation uses an operation with a high-resonance Q-factor and a receiver with high sensitivity (below 1
mVPP) to receive very small transponder signals. There is no distortion by any reader signal since the
transmitter is switched off during Uplink. In addition, the frequency modulation FSK is much more immune
to interference and distortion since the information is not in the amplitude. Due to the greater read range,
the Texas Instruments RFID system gives you much more freedom for the mechanical design.
SCBA027 – July 2010
TMS3705 Passive Antenna Solution
Copyright © 2010, Texas Instruments Incorporated
3
System Options
www.ti.com
4
System Options
4.1
Conventional System – Short Antenna Cable
The circuitry in Figure 3 shows the standard TMS3705x schematic if no antenna cable is used. For more
information, see the TMS3705 Transponder Base Station IC Datasheet (SCBS881).
To Band Pass
and Limiter
2
SFB
Diagnosis
R2
150 k
1
SENSE
Vref
R1
47 k
TMS3705
Cres.
7
C2
Ant2
R4
4R7
Full
Bridge
6
L1
GND
Antenna
Driver
C3
5
Ant1
R3
4R7
Figure 3. HDX RFID Reader Configuration With Short Antenna Cable
4
TMS3705 Passive Antenna Solution
SCBA027 – July 2010
Copyright © 2010, Texas Instruments Incorporated
System Options
www.ti.com
4.2
Remote Antenna System – Long Antenna Cable
The generic circuitry for the long cable application shows the additional recommended components that
are needed (see Figure 4).
To Band Pass
and Limiter
2
SFB
Diagnosis
R2
150 k
Vref
1 SENSE
R1
47 k
TMS3705
Cres.
7
C2
Full
Bridge
6
L1
Long Antenna cable
C3
Ant2
R4
4R7
C4
100n
5
GND
Antenna
Driver
Ant1
R3
4R7
R5
1K
(10K)
Figure 4. HDX RFID Reader Configuration With Long Antenna Cable
4.3
Reader Antenna
A typical immobilizer system is working with an air-coil antenna mounted around the mechanical lock
cylinder. The final installation environment must always be considered in the antenna design to avoid any
change of the resonance frequency.
The lock cylinder normally lowers the inductance as well as the Q-factor of the antenna coil.
For certain applications, a pre-detuning using a metal plate can avoid future massive de-tuning of the
system in a moving environment.
SCBA027 – July 2010
TMS3705 Passive Antenna Solution
Copyright © 2010, Texas Instruments Incorporated
5
Recommended Antenna Parameter for Long Cable Systems
4.4
www.ti.com
Resonance Frequency and Component Calculation
The recommended inductance is in a range of 300 µH to 700 µH. The typical inductance used for an
immobilizer system is 440 µH, which results in combination with a 3.3 nF capacitor in a resonance
frequency of 132 kHz.
fres =
1
2P L * Cres
Frequency
L[mH]
0.44
Cres.[pf]
3300
Ccable[pf]
0
Cfix[pf]
3300
f[kHz]
132.08
For a remote antenna system with a variable cable length, an inductance in the range of 300 µH is
recommended to minimize the resonance shift caused by varying the cable length.
4.5
Resonance Frequency and Q-Factor Consideration
The antenna resonance frequency is mainly determined by the antenna inductance and the fixed
resonance capacitor. The antenna Q-factor is mainly determined by the frequency and all resistive loses
of the antenna, typically, the DC resistance of the coil.
The inductive reactance is calculated with:
X L = 2P * f * L
Example using a typical inductance of 440 µH:
X L = 2P * 134.2 kHz * 440 m H
X L = 371 W
The antenna quality factor is given with:
XL
QL =
R DC
The resistor RDC should be measured at the resonance frequency in case magnetic or electric conductive
material is located in the vicinity of the coil. RDC is the sum of resistive, eddy current, and hysteresis
losses; the resistive losses are the sum of the copper resistance of the antenna. For high-Q antennas,
litz-wire should be used to reduce the impact of the skin effect.
5
Recommended Antenna Parameter for Long Cable Systems
5.1
Antenna Inductance
To reduce the impact of the parasitic antenna cable capacitance of a long antenna cable system, it is
recommended to lower the antenna inductance to about 300 µH.
Using a lower antenna inductance, a larger fixed capacitance value is required to keep the resonance
frequency of the system almost constant. As a result of using a larger fixed capacitor value, the influence
of the parasitic antenna cable capacitance is reduced.
The resonance frequency is calculated with:
fres =
6
1
2P L * Cres
TMS3705 Passive Antenna Solution
SCBA027 – July 2010
Copyright © 2010, Texas Instruments Incorporated
Recommended Antenna Parameter for Long Cable Systems
www.ti.com
The total resonance capacitance is calculated with:
Cres = Cfix + Cable ;
Frequency;
L[mH]
Cres .[pF]
Ccable[pF]
Cfix[pF]
f[kHz]
Frequency;
0.44
4.40E-04
3300
0
3300
132.08
3.3E-09
L[mH]
Cres .[pF]
Ccable[pF]
Cfix[pF]
f[kHz]
0.44
4.40E-04
3540
240
3300
127.52
3.54E-09
Lant = 440 µH
Frequency;
L[mH]
Cres .[pF]
Ccable[pF]
Cfix[pF]
f[kHz]
Frequency;
0.3
4850
0
4850
131.94
3.00E-04
L[mH]
0.3
3.00E-04
4.85E-09
Cres .[pF]
5090
240
4850
128.80
5.09E-09
Ccable[pF]
Cfix[pF]
f[kHz]
Lant = 300 µH
SCBA027 – July 2010
TMS3705 Passive Antenna Solution
Copyright © 2010, Texas Instruments Incorporated
7
Recommended Antenna Parameter for Long Cable Systems
5.2
www.ti.com
Antenna Cable Options
Figure 5 shows the following cable types that have been tested.
The capacitance of the cable is determined by the mechanical construction of the cable and its dielectric.
Depending of the cable type and length, the antenna inductances may need to be recalculated; it is
recommended to use a twisted pair in order to minimize unwanted radiations. see Section 5.1.
Figure 5. Antenna Cable Types
A
C = S r S0 ×
d
8
TMS3705 Passive Antenna Solution
SCBA027 – July 2010
Copyright © 2010, Texas Instruments Incorporated
Recommended Antenna Parameter for Long Cable Systems
www.ti.com
5.3
Simulation
A simulation with a cable capacitance of 60 pF/meter is shown in Figure 6.
R8
PARAMETERS:
2
4
SWEEP
length
470k
V
R7
OPAMP
–
47k
+
U2
0
R1
500k
L2
+
V2
Cparasitic
20p
Cres
4.7n
Rdamp
15k
100
C_Cable
300uH
{60p length}
–
RL
13
0
Figure 6. Schematic Used for Long Cable Simulation
The reader antenna forms a resonance circuit with the tuning capacitors. It is important to notice that the
reader operates in series resonance during transmit (Charge&DownLink). In receive mode (Uplink), the
reader forms a parallel resonance circuitry with the tuning capacitor and the antenna. In receive mode
(parallel resonance) the Q-factor of the antenna resonance circuit is slightly higher.
Therefore, the simulation was done using a parallel resonance circuitry model.
SCBA027 – July 2010
TMS3705 Passive Antenna Solution
Copyright © 2010, Texas Instruments Incorporated
9
Schematic for a Long Cable Reader System
www.ti.com
Figure 7 shows that there is no significant shift of the resonance frequency.
(A) Long Cable_and opamp. dat (active)
0
–5
–10
–15
110KHz
130KHz
115KHz
120KHz
125KHz
DB (V (L2:1)) – MAX (DB (V (L2:1) ) )
135KHz
140KHz
145KHz
150KHz
Frequency
Figure 7. Simulation Result - Frequency Shift Due to Long Antenna Cable
Schematic for a Long Cable Reader System
+5V
6
C3
D2
C1 NPO (High Q), Value depending on Inductance
GND GND GND
C2, C3 must be optimized during EMC tests
Footprint for 10nF/NPO shall be planned
C5 is for DC DC Decoupling of Antenna pin (short to Vbat)
R9 for providing DC path to GND (Receiver)
C9
+
C10
100nF/X7R
ww
GND
1k
R5
TXCT_
1k
R8
DATA
220R
13
optional/NPO
Q2
11
2
12
GND
14
TP1
3
D_TST SCIO
4 A_TST
VSSB
R3
5
ANT1 VSS
4R7/100mA 6
VSSA OSC1
R4
7
ANT2 OSC2
4R7/100mA 8
VDDA VDD
TMS3705
15
1 JP1
2
3
ww
SFB F_SEL_
16
test connector for
connecting external CTL
C4
CSTCRCR4M00G55B MURATA
10
9
GND
C6
optional/NPO
C11
100nF/X7R
C12
GND
D1
3.3nF/NPO
/2%/100V
FSEL High
for 4MHz
R6
1
C1
TP6
ww
2
SENSE TXCT_
220µF/50V Low ESR
2
GND C2
C5
Value TBD
1
R2
150k 2%
22pF/NPO/COG)
220pF/NPO/COG)
JP1_S
JP12
R10
1
L1
Antenna 2pin connector
10k
100nF/X7R
IC1
GND
R1
47k 2%
R9
TP4
ww
HIGH Voltage during TX
TP2
ww
TP3
R7 10k
C4, C6 is only needed if internal
load capacitance is not correct
1nF
GND
D1, D2 Low capacitance bidirectional ESD protection diodes in SOD323
NXP:PESD12VL1BA (12Volt) up to 25kV
GND
R10 damping for reduction of Q Value, TBD
R3, R4 must NOT be a MINIMELF
L Antenna Inductance, pay attention to cable capacitance
Figure 8. HDX RFID Reader Schematic
10
TMS3705 Passive Antenna Solution
SCBA027 – July 2010
Copyright © 2010, Texas Instruments Incorporated
Schematic for a Long Cable Reader System
www.ti.com
6.1
Low-Pass Filter
The two 4.7Ω resistors (R3, R4) in series to antenna output (Ant1, Ant2) in combination with the two
capacitors (C2, C3) to GND are forming a low-pass filter to suppress unwanted harmonics at higher
frequencies.
To Band Pass
and Limiter
2
SFB
Diagnosis
R2
150 k
Vref
1 SENSE
R1
47 k
TMS3705
Cres.
7
C2
6
L1
Antenna Cable
C3
Ant2
R4
4R7
C4
100n
5
Full
Bridge
GND
Antenna
Driver
Ant1
R3
4R7
R5
1K
(10K)
Figure 9. HDX RFID Reader – Schematic Low-Pass Filter
Depending on the cable type, the value of C2 and C3 has to be adapted to minimize unwanted emissions.
It is important that C2 and C3 are NPO(COG) capacitor types.
The low pass cut-off frequency (R4/C2 and R3/C3) can be calculated as follows:
fc =
1
2P * R * C
SCBA027 – July 2010
TMS3705 Passive Antenna Solution
Copyright © 2010, Texas Instruments Incorporated
11
Schematic for a Long Cable Reader System
6.2
www.ti.com
Fault Protection – Shorts Against Vbat
The TMS3705 is internally protected against shorts to GND but not to Vbat. In a standard system where
the antenna is combined with the immobilizer module, a short to Vbat is very unlikely.
If a long antenna cable is used, the probability for a short to Vbat is higher, especially if the antenna cable
is fed in a cable harness together with other supply lines.
To Band Pass
and Limiter
2
SFB
Diagnosis
R2
150 k
Vref
1 SENSE
R1
47 k
TMS3705
Cres.
7
C2
6
L1
Antenna Cable
C3
Ant2
R4
4R7
C4
100n
5
Full
Bridge
GND
Antenna
Driver
Ant1
R3
4R7
R5
1K
(10K)
Figure 10. HDX RFID Reader Schematic – DC Path Highlighted
The IC internal FET of ANT1 provides the DC path to GND. To discharge any unwanted load on the
antenna cable to GND (see red lines in Figure 10, a DC path to GND needs to be set up. This is done by
adding R5 in parallel to C4.
12
TMS3705 Passive Antenna Solution
SCBA027 – July 2010
Copyright © 2010, Texas Instruments Incorporated
References
www.ti.com
6.3
Electrostatic Discharge (ESD) Protection
The ESD protection of the TMS3705x IC is ±2 kV according to the MIL STD 883. The complete reader
circuitry (module) normally needs a higher ESD immunity.
In a passive cable system where the TMS3705x is integrated in a centralized control module, the two
antenna connections for the long cable are most critical.
To prevent a potential ESD damage, two bi-directional protection diodes are added. They improve the
ESD immunity up to 25 kV.
Figure 11. HDX RFID Reader Schematic – ESD Protection Diodes
7
References
•
TMS3705 Transponder Base Station IC Datasheet (SCBS881)
SCBA027 – July 2010
TMS3705 Passive Antenna Solution
Copyright © 2010, Texas Instruments Incorporated
13
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