rfid design on zeland

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RFID design

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    RFID Antenna Design Using Zeland ToolsRFID Antenna Design Using Zeland Tools

    Zeland Software, Inc.Zeland Software, Inc.

    48834 Kato Road, 103A48834 Kato Road, 103A

    Fremont, CA 94538, U.S.A.Fremont, CA 94538, U.S.A.

    EE--mail:mail: [email protected]@zeland.com

    www.zeland.comwww.zeland.com

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    RFID Antenna Design Using Zeland Tools

    IntroductionIntroduction

    IE3D and FIDELITY are powerful full-wave EM

    tools good for simulation, tuning, optimization and

    synthesis. They can handle general 3D and planar structures.

    IE3D can perform mixed EM and circuit co-

    simulation,

    Special implementation in IE3D and FIDELITY to

    help RFID designers. This presentation provides some general guide

    lines for RFID antenna designs using Zeland tools.

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    RFID Antenna Design Using Zeland Tools

    Important Definitions in IE3D and FIDELITYImportant Definitions in IE3D and FIDELITY

    Incident Wave, a: The propagating wave from the

    source to the antenna with specified Zc.

    Reflected Wave, b: The propagating wave fromthe antenna to the source with specified Zc.

    Incident Power, Pinc: The power from the incidentwave

    Reflected Power, Pref: the power from the

    reflected wave. Input Power, Pin: The net power going into the

    antenna or Pin = Pinc - Pref

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    RFID Antenna Design Using Zeland Tools

    Important Definitions in IE3D and FIDELITYImportant Definitions in IE3D and FIDELITY

    Radiated Power, Prad: It is the power radiated into

    the space from the antenna.

    Radiation Efficiency, Effrad: It is the ratiobetween Prad and Pin, or Effrad = Prad / Pin

    Antenna Efficiency, Effant: It is the ratio betweenthe Prad and Pinc, or Effant = Prad / Pinc.

    Source Impedance, Zs: The impedance of the

    excitation source. Antenna Impedance, Za: The input impedance of

    the antenna.

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    RFID Antenna Design Using Zeland Tools

    Important Definitions in IE3D and FIDELITYImportant Definitions in IE3D and FIDELITY

    Conjugate Match: When Za is conjugate of Zs or

    Za = Zs*, it is called conjugate match.

    Conjugate Match Factor, CMF: CMF is the ratiobetween antenna input power with given Zs and

    Za and the antenna input power with given Zs and

    assuming Za = Zs*. CMF is not defined in

    textbook but in IE3D 12.12 and FIDELITY 5.20

    only.

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    Typical RFID StructureTypical RFID Structure

    An RFID is a chip connected to an antenna. It may work

    at different frequency ranges such as 13.56 and 900 MHz.

    The design principle is about the same.

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    Working Principles and Design GoalsWorking Principles and Design Goals

    Normally, the chip has an impedance with a largecapacitive impedance value. For example, a typical 13.56MHz RFID may have an impedance of 5.8 j 250 ohms.

    There are two working modes: (1) The RFID is working inreceiving mode. The RFID antenna is receiving signal froma readers antenna and the signal is powering the chip in the

    RFID; (2) The chip is serving as a source and it is sendingout signal thru the RFID antenna.

    The goals are to design the antenna to receive the maximumpower at the chip from the readers antenna and to allowthe RFID antenna to send out the strongest signal.

    The chip internal impedance Zs is given. We need to tunethe antenna impedance to achieve the goals.

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    Equivalent Circuit at Receiving ModeEquivalent Circuit at Receiving Mode

    The readers antenna is creating the EM field at where the

    RFID is located. The RFID receives the radiation from the

    readers antenna and it is powering the chip.

    Chip

    Zs

    Za

    Va

    Zs Chip Impedance

    Za Antenna Impedance

    Va The equivalent voltage

    source from receiving

    radiation from the reader

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    RFID Antenna Design Using Zeland Tools

    Equivalent Circuit at Transmitting ModeEquivalent Circuit at Transmitting Mode

    The received energy is powering the chip. The chip is

    driving the antenna to send out radiation into the space.

    Chip

    Zs

    ZaVs

    Zs Chip or Source Impedance

    Za Antenna Impedance

    Vs The equivalent voltage

    source of the chip from

    received power.

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    RFID Antenna Design Using Zeland Tools

    Ultimate GoalsUltimate Goals

    In receiving mode, we would

    like to chip impedance Zs to

    receive the maximum powerfrom the equivalent voltage

    source Va.

    In transmitting mode, wewould like to deliver the

    maximum power from the

    equivalent voltage source Vs

    to the antenna impedance Za.

    Chip

    Zs

    Za

    Va

    Chip

    Zs

    ZaVs

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    RFID Antenna Design Using Zeland Tools

    Ultimate GoalsUltimate Goals

    The system is transposable. We can just consider

    the transmitting mode. If we can achieve the best

    results in transmitting mode, we can achieve thebest results in receiving mode.

    In transmitting mode, we would like to deliver

    the maximum power from Vs to Za. Therefore,

    we need to achieve conjugate match or Za = Zs*.

    Only a fraction of the power delivered to Za willbe radiated out. We need to achieve highest

    radiation efficiency Effrad.

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    RFID Antenna Design Using Zeland Tools

    Ultimate GoalsUltimate Goals

    We need to implement some good RFID antenna

    configurations with high radiation efficiency.

    With a given antenna configuration, we need totune the dimensions of the antenna to achieve Za

    = Zs* at frequency range of interests.

    When the antenna basic configuration is given,

    the radiation efficiency normally may not be

    very sensitive to different dimensions. Weshould focus on tuning the dimensions for

    conjugate match or Za = Zs*.

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    RFID Antenna Design Using Zeland Tools

    Incorrect Concepts and GoalsIncorrect Concepts and Goals

    There have been many incorrect concepts in thedesign of RFID.

    Is it the best design of the RFID antenna if wecan achieve maximum gain and maximumefficiency?

    Is it the best design of the RFID antenna if weachieve minimum S(1,1) normalized to thecomplex impedance of Zs (or Zs*)?

    Neither achieving maximum gain and maximumefficiency nor achieving minimum S(1,1)normalized to complex Zs (or Zs*) is the correct

    goal.

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    RFID Antenna Design Using Zeland Tools

    Maximum EfficiencyMaximum Efficiency

    To achieve maximum efficiency with given

    voltage source Vs and source impedance Zs, we

    can increase the antenna resistance Ra and thereactance Xa, where Ra and Xa are defined as Za

    = Ra + j Xa.

    Larger Ra will increase the efficiency but reducethe maximum received power and radiated

    power.

    Maximum efficiency (or maximum gain) of the

    antenna is not the best design.

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    RFID Antenna Design Using Zeland Tools

    Complex Normalization Impedance ZsComplex Normalization Impedance Zs

    It is incorrect to use complex normalization impedance Zc.

    It is proven in the Appendix of IE3D Users Manual that

    complex Zc is an incorrect concept. RF designers are

    suggested to avoid using complex Zc.

    Multiple definitions of reflection coefficient:

    = ( Za Zs ) / ( Za + Zs )

    = ( Za Zs* ) / ( Za + Zs )

    No definition is precisely correct. The 1st definition may

    yield || > 1 for a passive system. The 2nd definition will

    not predict || > 1 for a passive system. However, it alsoloses meaning. The fundamental reason for invalid is

    from the fact that incident and reflected waves are no

    longer precisely valid with complex Zc.

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    RFID Antenna Design Using Zeland Tools

    IE3D Modeling of RFID AntennaIE3D Modeling of RFID Antenna

    There are many good RFID antenna designs. We will not

    try to develop some new configuration here. We will

    demonstrate how to use IE3D to optimize the 900 MHzRFID design published by K. V. Seshagiri Rao, et al. on

    IEEE AP-T Dec. 2007. The IE3D example file is privided

    in .\zeland\ie3d\samples\LoadedMeanderTag.geo.

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    RFID Antenna Design Using Zeland Tools

    IE3D Matching Measured ResultsIE3D Matching Measured Results

    IE3D results compare very

    well with the measure

    results from literature.

    Assume the chip

    impedance for the RFID is

    Zs = 17.5 j 350 ohms at

    875 MHz. Our goal is

    optimize the antenna to

    achieve Za = 17.5 + j 350ohms.

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    RFID Antenna Design Using Zeland Tools

    IE3D Simulation Setup for RFIDIE3D Simulation Setup for RFID

    To check how good the pattern and the conjugate

    matching is, please make sure you setup the simulation

    properly.

    Select Voltage Source excitationand define source impedance Zsas 17.5 j350. You can choose Zc= 50. It is not critical.

    Enable pattern calculation

    Check f = 875MHz to makesure it runs atthe frequencyeven with AIFenabled.

    (Note: frequency dependent Zs can be defined for pattern calculation in post-processing)

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    RFID Antenna Design Using Zeland Tools

    Check Radiation PatternsCheck Radiation Patterns

    After simulation, PATTERNVIEW is invoked to display theradiation pattern. You can display the 3D pattern to see the

    radiation distribution. Please select Edit->Pattern Properties

    dialog to check the radiation parameters.

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    RFID Antenna Design Using Zeland Tools

    Important Parameters for RFIDImportant Parameters for RFID

    For most microwave antennas, we should check theRadiation Efficiency, Antenna Efficiency and Gain. They

    are important for wave sources. For voltage and current

    sources, we should try to check the conjugate matching andInput Power with given voltage source.

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    RFID Antenna Design Using Zeland Tools

    Important Parameters for RFIDImportant Parameters for RFID

    An important parameter introduced in PATTERNVIEW 12.12or later for RFID antenna is the Conjugate Match Factor

    (CMF). Its definition can be found from the Definitions button

    of Pattern Properties of PATTERNVIEW. CMF ranges from 0 to 1. When CMF = 1, it means the Za is

    conjugate-matching the Zs perfectly and the RFID will be

    working in the best condition in both modes.

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    RFID Antenna Design Using Zeland Tools

    CMF Vs. FrequencyCMF Vs. Frequency

    Display CMF Vs. Frequency on PATTERNVIEW andcheck the trends. For this particular antenna with original

    dimensions, the CMF is about -4.2 dB at 875 MHz. There is

    still much room to improve.

    Smooth curve obtained without AIF but longer time.

    Selected points simulated with AIF for fast speed

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    RFID Antenna Design Using Zeland Tools

    CMF Vs. FrequencyCMF Vs. Frequency

    CMF Vs. Frequency can also be created from s-parameterson MODUA (Process->General Lumped Equivalent Circuit)

    and MGRID (Process->S-Parameters and Lumped

    Equivalent Circuit).

    Linear Scale dB Scale

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    RFID Antenna Design Using Zeland Tools

    Defining Optimization VariablesDefining Optimization Variables

    The reason for CMF obvious below 0 dB at 875 MHz is thatZs = 17.5 j 350 and Za = 50.4 + j 418.1. They do not

    differ much while we can optimize the antenna for better

    result. There are many dimensions we can optimize theantenna. We will demonstrate the concept with two

    variables shown below. FastEM data is prepared on

    .\zeland\ie3d\samples\LoadedMeanderTag_for_optim.geo.We can perform real-time EM tuning and optimization on it.

    Change the Y of this group of vertices to adjust the length of the traces of the antenna

    Change the Y of this group of vertices to adjust the coupling gap

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    RFID Antenna Design Using Zeland Tools

    FastEMFastEM RealReal--Time Tuning and OptimizationTime Tuning and Optimization

    Open the file and select Process->Full Wave EM Design

    Manual tuningon the bars

    AutomaticOptimization

    Tuned geometry

    Tuned Z(1,1)

    Define displaygraph

    Define goalsfor tuning andoptimization

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    RFID Antenna Design Using Zeland Tools

    Goals for Tuning and OptimizationGoals for Tuning and Optimization

    We define the goals as Re[Z(1,1)] = 17.5 ohms andIm[Z(1,1)] = 350 ohms at 875 MHz

    (Finding CMF vs. Frequency will be available based upon s-parameters on MODUA)

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    RFID Antenna Design Using Zeland Tools

    FullFull--Wave EM OptimizationWave EM Optimization

    Two ways of EM optimizations on IE3D

    Full-blown IE3D optimization: Highest accuracy;

    Possibly lower efficiency; Intermediate results

    discarded for each individual optimization.

    Real-Time Full-Wave Optimization on FastEM: Needs

    preparation (possibly long time); Reasonable

    accuracy; Real-time tuning and optimization allowingyou to see the change in geometry and results

    interactively; Re-usable results; Extremely efficient

    for tuning and optimization of batch designs withsimilar structure but slightly different goals; Allowing

    you to create a big design library.

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    RFID Antenna Design Using Zeland Tools

    FastEMFastEM RealReal--Time OptimizationTime Optimization

    We can achieve the best conjugate matching with the given

    dimensions in seconds on FastEM Design Kit (saved in

    .\zeland\ie3d\samples\LoadedTag_for_optim_fastem.geo).

    Slide bars for manual tuning Select Optimize for Automatic Optimization

    Save the optimized geometry for full-blown IE3D simulation to check results.

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    RFID Antenna Design Using Zeland Tools

    Check Optimized ZCheck Optimized Z--ImpedanceImpedance

    As it is shown, Xa is tuned from 418.1 to 348 ohmswhile Ra is tuned from 50.4 to 41. Xa is perfectly

    optimized while Ra still differs from 17.5 ohms.

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    RFID Antenna Design Using Zeland Tools

    Check Conjugate Match FactorCheck Conjugate Match Factor

    The CMF is improved from -4.2 dB to -0.8 dB. If we cantune Ra close to 17.5 ohms, we should be able to bring

    CMF closer to 0 dB. It has to be done by tuning other

    dimensions.

    (Note: On MODUA, we can compare CMF of different files and we can define frequency dependent Zs)

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    RFID Antenna Design Using Zeland Tools

    Check Radiation ParametersCheck Radiation Parameters

    The following table compares the original and optimizedradiation properties on PATTENRVIEW with Vs = 1 (v) &

    Zs = 17.5 j 350 ohms at 875 MHz. As you can see, the

    Effrad is almost unchanged while the Prad is more thandoubled after the optimization.

    0.8380.381Conj. Match Factor

    88.8%89.0%Radiation Efficiency

    5.32 mW2.43 mWRadiated Power

    5.99 mW2.72 mWInput Power

    OptimizedOriginalParameter

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    RFID Antenna Design Using Zeland Tools

    SummarySummary

    IE3D and FIDELITY yield high accuracy results on

    RFID antennas.

    Conjugate Match Factor (CMF) is introduced in

    IE3D 12.12 and FIDELITY 5.20 for the designs of

    RFID antennas. CMF is the most important factorneeds to consider in RFID antenna designs.

    IE3D FastEM Design Kit allows designers to tune

    and optimize RFID antenna efficiently.