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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
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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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