8/12/2019 Cst Application Note Rfid

1/15

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

8/12/2019 Cst Application Note Rfid

2/15

www.cst.com2

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

8/12/2019 Cst Application Note Rfid

3/15

www.cst.com3

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

8/12/2019 Cst Application Note Rfid

4/15www.cst.com4

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

8/12/2019 Cst Application Note Rfid

5/15

www.cst.com5

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

8/12/2019 Cst Application Note Rfid

6/15

www.cst.com6

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!

8/12/2019 Cst Application Note Rfid

7/15

www.cst.com7

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

8/12/2019 Cst Application Note Rfid

8/15

www.cst.com8

50

Non-resonant simulation,

3 min sim. time

Wrong resonance frequency, low coupling

Antenna is poorly matched

900 MHz RFID Tag Antenna

8/12/2019 Cst Application Note Rfid

9/15

www.cst.com9

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

8/12/2019 Cst Application Note Rfid

10/15

www.cst.com10

Matching in CST DESIGN STUDIO

1.5

Resonance frequency correct (900 MHz),

excellent matching (-30 dB)

Simulation run in

CST DESIGN STUDIOTM: 5 sec.

8/12/2019 Cst Application Note Rfid

11/15

www.cst.com11

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

8/12/2019 Cst Application Note Rfid

12/15

www.cst.com12

Field Monitors

Current Density

Electric Field

Farfield

Dipole field

With lumped element included in CST MWSall field monitors are available

8/12/2019 Cst Application Note Rfid

13/15

www.cst.com13

Test under Realistic Surroundings

RFID tag in front

of metal can

RFID tag in front

of water bottles

8/12/2019 Cst Application Note Rfid

14/15

www.cst.com14

Field Distortion by PEC-Cans

poorer matching

Second resonance

Unsymmetric

fields

8/12/2019 Cst Application Note Rfid

15/15

www.cst.com15

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