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OverviewOperating PrinciplesInductive CouplingMicrowave CouplingMatching of RFID Tags Summary

twi / v1.0 / 09. Nov 2006

CST STUDIO SUITE™ 2006BApplication Note

RFID Simulation

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Overview

• Fundamental tool for Automatic Identification:authentication, ticketing, access control, supplymanagement, 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, veryrobust, 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 keepantenna small

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Inductive CouplingRFID tags are mostly planar coils with small dimensions compared to λ

Hexahedral or tetrahedral F-Solver are typically most suited.

Example for13.56 MHz

with courtesy and permission of Legic Identsystems AG

At 13.56 MHz Measurement: (7.15 + 398i) ΩSimulation: (7.0 + 334i) Ω

MeasurementCST MWS simulation

Imaginary partof impedance

Curves are overlying each other, excellent agreement!

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Microwave Coupling900 MHz RFID Tag Antenna

www.alientechnology.com/products/rfid_tags.php

Simulation of the planar structurewith a standard 50 Ω port

Mesh view

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50 Ω

Non-resonant simulation, 3 min sim. time

Wrong resonance frequency, low couplingAntenna is poorly matched

900 MHz RFID Tag Antenna

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Matching Procedure

Matching impedance: 1.55 Ω

Lumped Capacitance for shifting theresonance frequency:

Z_im = 89.5i Ω L = 15.8 nH

(or: L from „Template Based Postprocessing“)

(or: Macros -> Calculate -> Calculate L-C-fres)

pF 96.112 ==

ωLC

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Matching in CST DESIGN STUDIO™

1.5 Ω

Resonance frequency correct (900 MHz), excellent matching (-30 dB)

Simulation run in CST DESIGN STUDIOTM: 5 sec.

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Alternative ApproachInclusion of lumped element in CST MWS simulation

Resonant simulation, 20 min sim. time

Excellent agreement between matching in Design Studio and matching in CST MWSCST DS approach is 7 times faster

1.5 Ω 1.96 pF

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Field Monitors

Current Density

Electric Field

FarfieldDipole field

With lumped element included in CST MWS all 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

Unsymmetricfields

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Summary

• RFID is a general concept using different technical 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