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8/12/2019 Cst Application Note Rfid
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www.cst.com1
Overview
Operating Principles
Inductive CouplingMicrowave Coupling
Matching of RFID Tags
Summary
twi / v1.0 / 09. Nov 2006
CST STUDIO SUITE 2006B
Application Note
RFID Simulation
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Overview
Fundamental tool forAutomatic Identification:authentication, ticketing, access control, supply
management, parking, payment, vending, surveillance
Advantages:
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)
Radio Frequency IDentification
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General Principle
Typical characteristics of RFID:
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
Reader RFID tag
Data
Energy
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Frequencies
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 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 CouplingRFID tags are mostly planar coils with small dimensions compared to
Hexahedral or tetrahedral F-Solverare typically most suited.
Example for
13.56 MHz
with courtesy and permission of Legic Identsystems AG
At 13.56 MHz Measurement: (7.15 + 398i)
Simulation: (7.0 + 334i)
Measurement
CST MWS simulation
Imaginary part
of impedance
Curves are overlying each other,
excellent agreement!
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Microwave Coupling
900 MHz RFID Tag Antenna
www.alientechnology.com/products/rfid_tags.php
Simulation of the planar structure
with a standard 50 port
Mesh view
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50
Non-resonant simulation,
3 min sim. time
Wrong resonance frequency, low coupling
Antenna is poorly matched
900 MHz RFID Tag Antenna
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Matching Procedure
Matching impedance: 1.55
Lumped Capacitance for shifting theresonance frequency:
Z_im = 89.5i L = 15.8 nH
(or: L from Template Based Postprocessing)
(or: Macros -> Calculate -> Calculate L-C-fres)
pF96.11
2==
LC
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Matching in CST DESIGN STUDIO
1.5
Resonance frequency correct (900 MHz),
excellent matching (-30 dB)
Simulation run in
CST DESIGN STUDIOTM: 5 sec.
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Alternative Approach
Inclusion of lumped element in CST MWS simulation
Resonant simulation,
20 min sim. time
Excellent agreement between matching in
Design Studio and matching in CST MWS
CST DS approach is 7 times faster
1.5 1.96 pF
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Field Monitors
Current Density
Electric Field
Farfield
Dipole field
With lumped element included in CST MWSall field monitors are available
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Test under Realistic Surroundings
RFID tag in front
of metal can
RFID tag in front
of water bottles
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Field Distortion by PEC-Cans
poorer matching
Second resonance
Unsymmetric
fields
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Summary
RFID is a general concept using differenttechnical principals
CST complete technology approach offersbest 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