radio frequency identification (rfid) antenna and … and syst… · rfid antenna and system design...

Radio Frequency Identification (RFID) Antenna and System

DesignMarkus Laudien ( ANSOFT Corp)

Application Engineer

RFID antenna and system design

AgendaRequirements of UHF-RFID systems and limitations of analytical treatmentReader antennaTransponder unit ( chip, package, antenna ) Field-Simulation of a whole transmission setup.Integration of nonlinear components into the transmission setup. Conclusions

What is RFID (Radio FrequencyIDentification)

Wireless identifications of piece goods

Frequencies of operation : status :

LF: 125 kHz ; 13.56 MHz ( near field coupling ) established, massproduction, shortrange (typ <1 m )

UHF: 433 MHz, 868 MHz, 915 MHz , 2.45 GHz market introduction, ( far field operation ) larger range ( <8m)

Reader:Communication withTags, tracking of piecegoods

Block diagram of an RFID transmission system

Transponder IC

RECEIVERDEMODULATOR

TRANSMITTERMODULATOR

LOGIC

MEMORY(READ/WRITE)

antenna

READERSYSTEM

Variable near field conditions. •Change of material properties •Neighboring transponders.

The transponder circuit is powered by the Incident field at the antenna

•constant near field conditions at the reader (stationary)•Sometimes variable near field conditions (mobile reader)

ASK

PSK orASK

The operating distance and system reliability in most cases is given by the forward link which determines the power supply of the transponder. The backward link is mostly notcritical due to the high receiver sensitivity

Transmitter33dBm output

Antenna (Omni)1.4dB gain

0.5 dB mismatch

Free SpaceTransmission-40 dB loss

Antenna (Omni)1 dB Gain

1 dB matching loss

Receiver-14 dBm sensitivity

33dBm

+5dB-3dB

35dBm

-40dB-5dBm

0dBi-5dBm

-5dBm

Antenna (circ)5dBi gain

3dB Polasation-loss

Link Budget

Example:

Contributions from the transmitter and the reader antenna are relatively easy to evaluate, however, in the operational environment the transmission setup and antenna are subject to strong variations due to the strongly variable environment.

Modern systems are targeted for communication up to a few hundred tags thus requiring good reliability

i.e. safety marginof approx 9 dB

Strongly varying region: Dependence on the distance, objects in the vicinity of the transponder antenna, material variation

The use of RF simulation tools in all parts of the system helps to predict the range of reliable operation

Due to the indirect power supply it is essential to make a careful calculation of the power budget.Safety margins have to be included

Design of transponders for RFID Systems

System Specification

Selection of Hardware ( Reader, Chips )

Decision for manufacturing technique(printing, etching … ) and assemblyprocess; possible size of label, …

Antenna design

Prototype assembly of transponder( chip & antenna )

Test under „laboratory conditions“

Customer Samples..Test under „real“ conditions ( customer)

Simulation potential ( system )

Parametric antenna studies & opti-misation, increase of bandwidth

sensitivity analysis

Simulation sensitivity towards processPackaging parasitics, statistical

Simulation of simplified transmissionbetween reader and tag

(only if reader ant model available )

Modeling of a „real“ situation withreader , many tags and other

objects in betweenRELIABILITY ???

Theoretical estimation of the transmission distance ( Friis equation)

TRANSMIT ANTENNAGAIN* = GT

Polarization = ρTREFL. COEFF = ΓT

RECEIVE ANTENNAGAIN* = GR

Polarization = ρRREFL. COEFF = ΓR

DISTANCE = RWAVELENGTH OF SIGNAL = λ

( )( )2222

11ˆˆ4 RTRTRT

T

R GGRP

PΓ−Γ−•⎟

⎠⎞

⎜⎝⎛= ρρπλ

Analytical approaches like the Friis equation assume

•Non-disturbed near field conditions (no proximity of dielectric and metal objects )•Known antenna characteristics•No diffraction and reflection effects

… this is not the case in most real RFID situations which meansthat this is a very rough estimation. A full system simulation

including reader , tags and the environment is needed.

Reader-Antenna

A reader antenna in most cases uses circular polarization in order to avoid potential strong polarization losses due to a linear polarization of the transponder antenna

With HFSS & Optimetrics different quantities (e.g. S11, axial ratio, field quantities ) can be improved in one single optimization setup:

Goal quantity calculation range Goal Value Weighting

Reader-antenna

Optimization Process ( at 915 MHz )

Model similar to: L. Boccia, G. Amendola, G. Di Massa: Design a high-precision Antenna for GPS ; Microwaves&RF onlineJanuary 2003

Reader-antenna: Results

Return LossGain

Axial Ratio vs.ThetaRealized gain ( Phi vs Theta polarisation )

Reader-antenna: E-Field and mesh

Mag E-Field ( log scale, vertical excitation )

Transponder: Antenna Matching of the tag

1) Chip Impedance: given by input circuit, technology, power level of incident signal

2) Package: Depending on the carrier technology different package techniques may apply like wire bonds, Flip Chip and SOC housing. Due to parasitic behaviour this part ís not neglectable. Here the user has a certain amount of influence

3) Antenna: The antenna impedance is given by the shape of the antenna, the used materials and changes in the environment. Here the user has the largest influence to adapt it to a specific application

Mainly three elements are influencing the matching characteristic of an UHF RFID-Tag:

Influence of thetag designeron performance

-

0

++

Transponder: Antenna Matching of the tag

LossChipAntenna

ChipChiprelativeChip PPP

PP

++== η

The power efficiency of the transponder isstrongly influenced by the antenna lossesand the package losses

Antenna matching for conjugatecomplex impedance of chip ANDpackage

Transponder: input impedance of the chip

UHF transponder, which uses the energy from the incident field, exhibits a cascaded rectifier switch on behalf to the antenna. There are many different topologies that have been extensively examined *,**:

These circuit parts (if necessary an additional overvoltage protection) affect considerably the chip impedance, which is seen by the antenna. This impedance depends on the input power.

*Z. Zhu, B. Jamali, P.Cole: Brief Comparison of Different Rectifier structures for HF and UHF RFID ; (Phase II, Draft version 0.0 ) University of Adalaide 24.04.2004

**Qiang Li, Yfeng Han, Hao Min, feng Zhou : Fabrication and Modeling of SChottky Diode Integrated in CMOS Process; State Key Lab of ASIC& System; Fudan University, Shang Hai 200433

Transponder: input impedance of the chip

The most critical range is, depending upon the technology, at the minimum power levels which still allow an operation of the transponder circuit (approx. -23 dBm... -10 dBm). Input circuit, antenna, and receiver should be optimized for the range of the lowest permitted power levels.

Example of a cascaded input circuit of a transponder IC . Today's common CMOS processes allow the integration of Schottky diodes. The input power at 915 MHz is swept within the relevant input power range of -40 dBm... +10 dBm.

Depending on the circuit topology and input power level Re (Z_chip) and Im ( Z_chip ) may change 20 % and more within the range of operation condition.

Transponder: resulting chip impedance

Impedance match: CMOS technology of the UHF transponders provides low real part of impedance to the antenna (approx.. 5... 30 ohms) and a high reactive part around -200... -500 ohms.

The Smith plot above shows that typical output impedance of the transponder chips lie in the marked range. Due to the low Re(Z) the matching to the antenna requires some inductive series element(s) in combination with shunt capacitance(s). Matched circuits are sensitive towards

•power dependent changes of the chip impedance•Parasitic contributions of the package

Transponder: contribution of the package

Different package technology have a strong influence on impedance matching and have to be taken into account by simulation or measurement:

Bondwires Flip Chip TSSOP

Simplified equivalent circuit of different packages


Example*: mounting on a label antenna in flip chip technique

*: pictures with permission of Philips AG / Gratkorn ( Austria )

Metallized bumps on the chip surface are pressed into the metallisation on the tag carrier.It can happen that overlaps between bumps and chip metallisation are forming a significantcapacititance due to an overlap.


Example: mounting in flip chip technique, Simulation of parasitic effects with HFSS

Extraction of parasitic elementswith 3D Simulation in HFSS,

Extraction shows a sensitivitytowardsChange of position, angle of mounting, distance between chipmetallisation and laben substrate.

Transponder: Antenna Matching

As the UHF labels use mostly printed or etched metallisation almost no design uses discrete components. This means the matching will be done within the antenna.Several antenna topologies are common which match to the complex chip impedance.

Usual antenna topologies are e.g.:

These antenna topologies mostly contain an inductive sections close to the chip and one or more capacitive sections

Position of the chip


This subject is very suitable for the use of 2.5D or 3D field simulation tools, which make the adjustment by parametric variation of the antenna geometry. It is important to merge the complex chip impedance directly into the model.

Example of a parametric variation in HFSS:

Xsize_carr

Ysize_carr

mperiodwidth

msize

distbar

The material of the smart card (thickness: thick_card) can also be a variable selected based on the material of the carrier which is under it (xsize_carr, ysize_carr, thick_carr)


Tuning the antenna on an initial configuration (e.g. a certain carrier material with well-known permittivity and loss tangent): A variation of one of the sizes is connected with a simultaneous change of the resonant frequency and antenna impedance

Variation of m_period:

Effect on the resonant frequency Effect on re ( Z_ant )


Variation of load_bar distance:

The variation “load_bar" of the distance can be used for the variation of the resonant frequency and by Re(Z_ant). By variation of two (not completely independent ) parametersit is quite simple to achieve the desired resonance frequency and a good matching:

msize, mperiod f_res down f_res updist_bar f_res up f_res downw f_res down f_res up

Parameter Enlargement Reduction

Transponder antenna: sensitivity towards changes in the proximity of the antenna

For a particular configuration (e.g. a 5mm thick mother board from PA) the transponder antenna shows a very strong sensitivity of the resonance behavior in relation to variations (e.g. the thickness or the material properties):

It is interesting to ask, from a system point of view, how such a change affects the receiver behavior, especially considering the nonlinear elements within the transmission circuit.

Sensitivity towards changes of the antenna and in the proximity of the antenna

Many more scenarios in the near field of a tag antenna can be simulated

Metal objects inthe proximity

Effect of humidityabsorption of the

carrier

Material changesof carrier (tolerances )

Near Field interactionwith other resonant

tags

Variation of conductorthickness and width

Simulation of a realistic UHF RFID transmission setup in HFSS (64 bit)

Assumed is a close-to-reality model of a wall-mounted 915 MHz reader antenna and at a distance of approx. 2.20 m a pallet with 12 transponder antennas. This setup is simulated with HFSS 10.

Pallet with 12 boxes ( PS-foam) and RFID transponders

Distance from reader to the center of the pallet approx.. 2.20 m

•Total of 14 Ports•Frequency range from 800 MHz..1GHz

Detail image of two neighboring transponder antennas

Reader antenna withtwo polarisation setups

Simulation of a realistic UHF RFID transmission setup

Computation on 2 GHz dual Opteron PC: used memory: 4.5 GB RAMComputing time approx. 3.5h ( including 8 adaptive passes to a convergence of <2 % )

Computing time may be further reduced by:

•Increased Multiprocessing •Performing the solution at a very small bandwidth ( e.g. 914 MHz… 916 MHz )

Simulation of a realistic UHF RFID transmission setup:

Typical transmission losses between reader antenna and transponders tags (S-parameters).

Variation of transmission-lossat 915 MHz between -35dB and-52 dB

Simulation of a realistic UHF RFID system :

Integration of the transmission circuit into an overall simulation with nonlinear input circuits at the transponders

.

.

.

3D Simulationof the distance between reader antenna and transponder

TX Reader

P_tag1

P_tag2

P_tagn

The direct link of the simulated transmission setup with nonlinear input circuits of the tags allows a very realistic estimations of the receiving conditions ( input DC power at the different transponders: P_tagn )

verticalpolarisation

horizontal polarisation

magΦ

Simulation of a realistic UHF RFID system :

An equivalent circuit of an RFID input section ( under non-disclosure agreement ) was provided by Philips AG / (Gratkorn , Austria) to study the effects of the power dependent receiving conditions of an ensemble of RFID tags. The model has been transferred to an encrypted format.

Equivalent circuit of the flip chip mounting was accomplished by measurementsand simulations with HFSS. The modulation input for FSK transmission is not used here.

encrypted

data

Simulation of a realistic UHF RFID system EM - cosimulation:

Link between circuit simulator ( NEXXIM ) and 3D field simulator (HFSS)

"dynamic link": i.e. use of the parameters of the 3D model and the according results within the circuit simulator

Circular polarisation accomplishedby two splitted signals with 90°phase offset

Simulation of a realistic UHF RFID system-polarization effects

Feed of the antenna in horizontal polarization ( equivalent with orientation of the tag antennas ). As no over-voltage protection is included and the distance is quite short the voltages are somehow higher than in real cases.

Reader operating frequency

Simulation of a realistic UHF RFID system-polarization effects

Circular polarisation vertical polarization

This scenario clearly shows the advantage of a circular polarized reader antenna:While linear polarized reader antennas may provide a higher power level in case of polarisation- alignment is may also cause low power level if polarisation of readerand tag are perpendicular.The effects of polarisazion studies can be accomplished within the circuit simulatorby a combining the two linear polarisation contents of the reader

Simulation of a realistic UHF RFID system-reader-power sweep & frequency sweep :

DC input power at three transponder chips at varying transmitting power of the reader ( a) and varying frequency ( b) :

Simulation of a realistic UHF RFID system variance of geometry:

Parametric change of geometry in the vicinity of the tag-antennas:

Simulation of a realistic UHF RFID system time domain analysis :

Time domain simulation of ASK modulated reader signal ( Cosimulation System & NEXXIM )

0

SP

output

CCONST

SP

tag_in

AWGN

SP

tag_out

1234567

891011121314

ref

Input_PAD RChip

U2tag_2p2

868MHz RFID ASK test ( one arbitrary tag )

power splitter90° shift

ASK modulator 90% modulation,50 % duty cycle

NEXXIM subcircuitof transponder inputcircuit

Simulation of a realistic UHF RFID system–time domain analysis:

Time domain simulation of ASK modulated reader signal ( Cosimulation System & NEXXIM )Charging of the buffer C of the passive transponder

Simulation of a realistic UHF RFID system proposed approach:

Most recent approaches for UHF-RFID systems are based on trial and error or on the simulation of some single parts within the system.Supposed the reader antenna geometry and equivalent transponder circuit are available the simulation of a whole scenario is feasible with Ansoft simulation tools:

3D Simulation of readerAntenna (HFSS )

Optimisation of transponderAntenna (HFSS +Optimetrics )

Parametric studiesof environmentalchanges ( HFSS,Optimetrics )

Extraction of Packaging para-sitics ( Q3D, HFSS)

Simulation of complete 3Dsystem setup includingpiece goods in the environ-ment( HFSS, parametrized com-ponents transferred by copyand paste )

Cosimulation EM and circuit ( HFSS,NEXXIM, DESIGNER)

Equivalent circuitof transponder input( chip vendor)

Simulation of a realistic UHF RFID transmission setup:

Conclusions:

• Todays solver efficiency of HFSS and circuit cosimulation with DESIGNER allow the simulation of a realistic UHF-RFID system within a few hours.

• Semi-analytical statements based on “ideal” antenna parameters ( like directivity, polarisation ) are not always reliable if many other parts are in the vicinity of the antennas.

• Even transient simulations of such scenarios and the integration of a system simulation for the characterisation of ASK/PSK modulation, BER etc. can be made.

• The availability of 64 bit solver technology on LINUX and WINDOWS significantly shifts the complexity limit for such simulation scenarios.

radio frequency identification (rfid) antenna and … and syst… · rfid antenna and system design...

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