CST STUDIO SUITE™ 2006B
Application Note
RFID Simulation
Overview
Operating Principles
Inductive Coupling
Microwave Coupling
Matching of RFID Tags
Summary
1 twi / v1.0 / 09. Nov 2006
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Overview
Radio Frequency IDentification
• Fundamental tool for Automatic Identification:
authentication, ticketing, access control, supply
management, parking, payment, vending, surveillance
• Advantages:
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–
–
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Contains more information than e.g. Barcodes
Can be read/write
Contactless ID (in contrast to phone or bank cards)
May become cheap mass product (e.g. in supermarkets)
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General Principle
Data
Reader
RFID tag
Energy
Typical characteristics of RFID:
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•
•
•
•
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Tag is a passive device, energy is transmitted from reader
Distance mm to 10m (typically ~20 cm)
Contains silicon chip, can be read only or read/write
Responds with modulated signal
Mostly printed (planar) structures
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Frequencies
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125/134 kHz
Animal identification, industrial applications, very
robust, low data transmisstion (64 bit)
7.4 - 8.8 MHz
Electronic Article Surveillance (EAS)
13.56 MHz
"Smart Labels" widely used for product/article ID
868 - 928 MHz
Several applications
2.4 GHz
Vehicle identification, electronic toll collection
5.8 GHz
electronic toll collection in Europe
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Operating Principles
• Inductive Coupling (125 kHz – 15 MHz)
– Very small dimensions compared to λ
– Coupling only through magnetic field
– Tag typically a planar coil
• Microwave Coupling (868 MHz – 5.8 GHz)
– typically a regular antenna
(e.g. planar folded dipole)
– Matching network important to keep
antenna small
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Inductive Coupling
RFID tags are mostly planar coils with small dimensions compared to λ
Hexahedral or tetrahedral F-Solver are typically most suited.
Example for
13.56 MHz
At 13.56 MHz Measurement: (7.15 + 398i) Ω
Simulation:
(7.0 + 334i) Ω
Measurement
CST MWS simulation
Imaginary part
of impedance
Curves are overlying each other,
excellent agreement!
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with courtesy and permission of Legic Identsystems AG
Microwave Coupling
900 MHz RFID Tag Antenna
Simulation of the planar structure
with a standard 50 Ω port
www.alientechnology.com/products/rfid_tags.php
Mesh view
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900 MHz RFID Tag Antenna
50 Ω
Wrong resonance frequency, low coupling
Antenna is poorly matched
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Non-resonant simulation,
3 min sim. time
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Matching Procedure
Matching impedance: 1.55 Ω
Lumped Capacitance for shifting the
resonance frequency:
Z_im = 89.5i Ω
L = 15.8 nH
(or: L from „Template Based Postprocessing“)
C=
1
Lω
2
= 1.96 pF
(or: Macros -> Calculate -> Calculate L-C-fres)
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Matching in CST DESIGN STUDIO™
1.5 Ω
Simulation run in
CST DESIGN STUDIOTM: 5 sec.
Resonance frequency correct (900 MHz),
excellent matching (-30 dB)
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Alternative Approach
Inclusion of lumped element in CST MWS simulation
1.5 Ω 1.96 pF
Excellent agreement between matching in
Design Studio and matching in CST MWS
CST DS approach is 7 times faster
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Resonant simulation,
20 min sim. time
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Field Monitors
With lumped element included in CST MWS
all field monitors are available
Current Density
Electric Field
Farfield
Dipole field
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Test under Realistic Surroundings
RFID tag in front
of metal can
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RFID tag in front
of water bottles
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Field Distortion by PEC-Cans
Unsymmetric
fields
Second resonance
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poorer matching
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Summary
• RFID is a general concept using different
technical principals
• CST complete technology approach offers
best solution for each case
– Frequency Domain / CST EMS for coil type
– Transient Simulator for microwave type
• Tags often contain lumped elements
– Possible in both CST MWS or CST DS
– CST DS typically more efficient
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