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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 www.cst.com

Overview
Radio Frequency IDentification
? Fundamental tool for Automatic Identification:
authentication, ticketing, access control, supply management, parking, payment, vending, surveillance

? Advantages:
– – – –
2

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
Energy

RFID tag

Typical characteristics of RFID: ? ? ? ? ?
3

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
125/134 kHz Animal identification, industrial applications, very robust, low data transmisstion (64 bit) Electronic Article Surveillance (EAS) "Smart Labels" widely used for product/article ID Several applications Vehicle identification, electronic toll collection electronic toll collection in Europe
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7.4 - 8.8 MHz 13.56 MHz 868 - 928 MHz 2.4 GHz 5.8 GHz
4

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!

6
with courtesy and permission of Legic Identsystems AG

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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
8

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)

10

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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
11

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
12 www.cst.com

Test under Realistic Surroundings

RFID tag in front of metal can
13

RFID tag in front of water bottles
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Field Distortion by PEC-Cans

Unsymmetric fields

Second resonance

14

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
15 www.cst.com


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