NXP Antenna Simulation

AN 1538 Fashion Reference Antenna Design for the UCODE G2XM / G2XL IC

Rev. 1.0 — 14 March 2008 Application note 1538

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Keywords UCODE EPC G2, G2XM, G2XL, Reference Design, Antenna Design, Fashion, Aluminum

Abstract This application note describes a label antenna design for the UCODE G2XM / G2XL, optimized for fashion applications. This design represents a reference design to demonstrate performance of the NXP UCODE G2XM / G2XL.

NXP Semiconductors AN 1538 Reference Label Antenna Design for Fashion Applications

© NXP B.V. 2008 All rights reserved.

Application note 1538 Rev. 1.0 — 14 March 2008 2 of 21

Revision history

Rev Date Description

1.0 20080314 First initial release; Authors: Barbara Ribic, Alexey Nazarov, Rainer Lutz




1. Introduction

The fashion industry is quickly adopting RFID on item level. RFID is being used for a

variety of applications including fast inventory taking, POS check out, customer service,

etc.

The requirements for fashion labels are significantly different to standard retail labels

used on boxes.

1) Size

Retail labels with a size of e.g. 100*10mm do fit only into few fashion hang tags. The size

of 50 * 30 mm of the fashion reference design is compatible with most hang tags design

and gives sufficient range for the application.

2) Omnidirectional readability

Traditional UHF tags can be read very well from some directions, but only poorly from

others. The difference in read range from best direction to worst direction (short side) can

be 20:1. In a fashion shop inventory it is highly preferable that the tag gets read equally

well from all directions. Reading a hangtag on a shirt in a shelf is exactly worst direction.

The fashion reference design is providing almost equal read range from all directions.

Traditional logistics tags (highly direction sensitive) Omnidirectional reference design

good read range from this angle

Only 5-10% read range from this angle

Omni- directional




3) Readability in stacked mode

In fashion, tags are often densely stacked e. g. T-shirts on a shelf. Traditional logistics

UHF tags dramatically loose range in these conditions. The NXP fashion reference

design shows consistently strong read range even densely stacked.

2. Technical requirements on a Fashion Design • Region: world wide 860 – 960 MHz; • Read Range 3,5 – 4,5 m • ASIC: G2X (ZIC = 24 – j 195 Ohm @ 915 MHz @ PIC = PIC min ); • Packaging type: direct chip attach; • Performance in presence of additional dielectrics: paper and plastic laminate, apparel

material (from thin shirts to thick jeans) and still operate world wide. • Operates well in typical fashion applications (mobile inventory, smart shelf, POS) • Less orientation sensitivity compared to conventional logistics UHF Tags • Operates in close proximity of other labels • Symmetrical radiation properties according to X and Y axes. • Be competitive with actual benchmark label(s) used for fashion applications

Simulation of densely stacked fashion tags The NXP fashion reference design show significantly better range than conventional tags in stacked mode.

Range needed for inventory




3. Fashion Reference Antenna Design

3.1 Geometry • Dimensions of the design: 50 mm x 30 mm; • Antenna material: copper; thickness 18um; • Substrate material: PET; thickness 50um; • Antenna should be matched to following assembled IC impedance:

(Z ass. IC = 22 – j 147 Ohm @ 915 MHz @ PIC = PIC min );

Cass = 0,3 pF;

Fig 1. Fashion Reference Design




3.2 Design description

To meet all of the above listed requirements a series of special design concepts have been worked out.

The requirement of long read range calls for a utilization of resonance dipole.

To reach the necessary dipole length on a given electrically small label area (50 x 30 mm) on one side and necessary/sufficient radiation in desired directions (the directions, poor for a conventional dipole) on the other side a technique of multiple bending the dipole is used.

To meet the requirements of symmetrical radiation with respect to Y axis the central part of the dipole is placed / shifted close to Y axis. This way however decreases the current in the transverse parts of the dipole (perpendicular to the Y axis) and as a consequence radiation in Y-axis direction. For this reason the central part of the dipole is not exactly symmetric to the Y-axis (see Fig 1).

To make the conjugate-complex matching between the antenna and capacitive ASIC possible, a small inductive loop is utilized.

To ensure the required increased bandwidth of the label the classical conjugate-complex matching technique is set aside. The antenna is tuned in such a manner that a good compromise between a required label performance (Read Range) and a required bandwidth (more than 100 MHz) is made.

The tuning elements are the lengths of both dipole and the loop as well as the way of connection them.

In the present design the loop-dipole connection is created using a mixed magnetic and galvanic coupling.

The performance in a coupling mode is synthesized based on theoretical and experimental investigations as well as based on simulations. The shaping of the loop and dipole as well as changing their geometrical parameters (length, width) were used to tune the antenna for close coupling operation.

Design is optimized for paper laminate!




4. HFSS Simulation Results

The following simulations are solved using HFSS, a commercial 3-D finite element method solver for electromagnetic structures used for antenna design and the design of complex RF electronic circuit elements.

4.1 Antenna Impedance

One of the key characteristics of the label antenna is its complex input impedance as a function of frequency. The curves of real R_antrenna and imaginary X_antrenna parts of the optimized design is shown in Fig.2 (solid lines). For the purpose of orientation the reader (reading person) the IC impedance is shown as well (dashed lines).

Fig 2. Fashion Generic design: Impedance




4.2 Return Loss The antenna impedance on one side and the IC impedance on the other let calculate the

return loss, Γ, which show a degree of matching between them (Equation 1).

ICA

ICA

ZZZZ

+

−=Γ

*

(1)

The corresponded curve is shown in Fig 3. The curve is based on the assumption that

the IC impedance remains constant for all frequencies and corresponds to those,

measured at 915MHz by PIC = Pmin IC

Fig 3. Fashion Generic design: Return Loss




The matched frequency area covers the whole UHF RFID frequency band (860-960

MHz) as well additional frequency band up to 1,12 GHz, thus building a margin which

accounts for frequency shift which takes place when additional dielectrics (laminate,

garments) come in contact with label in an application.

4.3 Antenna Gain The label radiation properties are shown in Fig. 4 - Fig. 6. The maximal Gain is 0,7 dBi.

The minimum Gain is -8 dBm. The antenna has null-filling thanks to its special bent

geometry and radiates in the direction of its axis (Axis Y). The difference is 8 dB.

Fig 4. Fashion Generic design: 3D Antenna Gain @ 915MHz




Fig 5. Fashion Generic design: 2D Antenna Radiation Pattern @ 915MHz. E-Plane




Fig 6. Fashion Generic design: 2D Antenna Radiation Pattern @ 915MHz. H-Plane




4.4 Minimum Power Threshold Using the above derived parameters it is now possible to determine a label threshold

level, so called Pmin-value (minimum Power at the label enough to power up the label)

(Fig. 7), (Equation 2).

Fig 7. Threshold power (Pmin) derivation

MismatchAssTagsensIC GPP Γ+Γ+−=min (2)

Pmin [dBm] …. ………………Minimum label threshold power

PsensIC [dBm] = -15 dBm …..Minimum G2X IC threshold power

GTag [dB] = 0,7 dBi…. ……...Label antenna gain

ΓAss [dB] = -2,5 dB…. ……….Assembly loss

ΓMismatch [dB]…. ……………..Antenna – IC mismatch loss, value based on return loss

The resultant Pmin values over frequencies are shown in Fig. 8.




Fig 8. Fashion Generic design: Threshold Power ( Pmin )

4.5 Read Range Using the Friis formula the maximum operation distance of the label (Read Range), R,

can be derived (Equation 3).

(3)

Where EIRP – Emitted Isotropic Radiated Power and Pmin – label threshold power in Watt

min4 PEIRPR ×⎟

⎠⎞

⎜⎝⎛

⋅=

πλ




Fig 9. Fashion Generic design: Read Range

Table 1 shows the simulated read ranges in typical RFID frequency bands, based on the

simulation results shown in Fig 9.

Table 1. Simulated read range in typical RFID regions

EU (868 MHz) USA (915 MHz) Japan (952 MHz)

3,5 m 4,5 m 4,2 m

4.6 Close coupling effects

In application typical for fashion (clothes on a hanger, clothes on a shelf, etc.) clothes as

well as associated tags are located very close to each other. Due to presence of other

tags the characteristics of each tag change compared to those in isolated mode. The

following effects are known from the general antenna theory as a result of close located

tags:




1. Input impedance changes

2. Beamforming

3. Gain and efficiency reduction

These effects should be taken into account during designing of a fashion tag in order to

maintain the performance in close coupled mode. Fig. 10 shows the typical label relative

locations for such applications as mobile inventory and smart shelves.

Fig 10. Fashion Generic Design in stacked mode: 3 labels




Fig. 11 shows the label performance in stacked mode as a function of number of tags for

a conventional label and for the fashion label. For this scenario the performance of the

central label in Fig.10 is evaluated. The picture makes it clear that a performance of the

conventional logistics label degrades rapidly for the number of tags more than 2 (blue

line). On the other side the fashion label still provides the performance sufficient for the

inventory (red line).

By the number of stacked tags around 5 to 7 a kind of saturation in the performance is

reached. Further increasing the number of labels introduces no considerable

performance reduction for the central label.

Fig 11. Performance in stacked mode: Fashion Label vs. Conventional Label

Range needed for inventory




5. Assembly process

5.1 Equipment

• Thermode Test Station TTS 300 from Mühlbauer • Low force thermode

5.2 Recommended assembly parameters

• Antenna: Copper 18um • Substrate: PET 50um • Glue: Kyocera TAP0604F • Temperature

− Upper thermode: 185°C

− Lower thermode: 180°C

• Bonding time: 10 sec. • Bonding pressure: 1,9 N




6. Measurement Method

The measurements were conducted with label prototypes. Therefore possible performance tolerances have to be taken into account.

6.1 Pmin measurement

The minimum power measurements are carried out in an anechoic chamber, according to the measurement setup described in the EPC global document “Tag Performance Parameters and Test Methods Version 1.1.1”.

The information gained from this measurement method is the minimal required power level at the label for powering the IC. This minimal power (Pmin) is measured for a defined frequency range from 840 MHz to 990 MHz. Fig 12 shows the Pmin measurement results of all assembled antennas.

7. Measurement Results

7.1 Fashion design - Pmin measurements results

Fig 12. Measurement Results: Pmin-curve




8. Conclusions

Fashion applications, such as inventory or self check out, have specific requirements for UHF label antennas in order to reach reliable read rates.

Intense physical studies have translated the required features like broad bandwidth, orientation insensitivity and good performance in stacked mode into design concepts. These have been optimized with the 3D electromagnetic simulation tool Ansoft HFSS.

Measurements of prototypes have confirmed the simulated results.

This application note described a reference label antenna design optimized for fashion applications.




9. Legal information

9.1 Definitions Draft — The document is a draft version only. The content is still under internal review and subject to formal approval, which may result in modifications or additions. NXP Semiconductors does not give any representations or warranties as to the accuracy or completeness of information included herein and shall have no liability for the consequences of use of such information.

9.2 Disclaimers General — Information in this document is believed to be accurate and reliable. However, NXP Semiconductors does not give any representations or warranties, expressed or implied, as to the accuracy or completeness of such information and shall have no liability for the consequences of use of such information.

Right to make changes — NXP Semiconductors reserves the right to make changes to information published in this document, including without limitation specifications and product descriptions, at any time and without notice. This document supersedes and replaces all information supplied prior to the publication hereof.

Suitability for use — NXP Semiconductors products are not designed, authorized or warranted to be suitable for use in medical, military, aircraft, space or life support equipment, nor in applications where failure or malfunction of a NXP Semiconductors product can reasonably be expected to result in personal injury, death or severe property or environmental damage. NXP Semiconductors accepts no liability for inclusion and/or use of NXP Semiconductors products in such equipment or applications and therefore such inclusion and/or use is for the customer’s own risk.

Applications — Applications that are described herein for any of these products are for illustrative purposes only. NXP Semiconductors makes no representation or warranty that such applications will be suitable for the specified use without further testing or modification.

9.3 Trademarks Notice: All referenced brands, product names, service names and trademarks are property of their respective owners.

UCODE G2XM/G2XL — is a trademark of NXP B.V.

NXP Semiconductors AN 1538 Label Antenna Reference Design for Fashion Applications

Please be aware that important notices concerning this document and the product(s) described herein have been included in the section 'Legal information'.

© NXP B.V. 2008. All rights reserved.

For more information, please visit: http://www.nxp.comFor sales office addresses, email to: [email protected]

Date of release: 14 March 2008Document identifier: AN_1538

10. Contents

1. Introduction .........................................................3 2. Technical requirements on a Fashion Design ..4 3. Fashion Reference Antenna Design ..................5 3.1 Geometry ...........................................................5 3.2 Design description..............................................6 4. HFSS Simulation Results....................................7 4.1 Antenna Impedance ...........................................7 4.2 Return Loss........................................................8 4.3 Antenna Gain .....................................................9 4.4 Minimum Power Threshold...............................12 4.5 Read Range .....................................................13 4.6 Close coupling effects ......................................14 5. Assembly process.............................................17 5.1 Equipment ........................................................17 5.2 Recommended assembly parameters..............17 6. Measurement Method .......................................18 6.1 Pmin measurement ............................................18 7. Measurement Results .......................................18 7.1 Fashion design - Pmin measurements results ...18 8. Conclusions.......................................................19 9. Legal information ..............................................20 9.1 Definitions ........................................................20 9.2 Disclaimers.......................................................20 9.3 Trademarks ......................................................20 10. Contents.............................................................21

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