microsoft powerpoint - rfid_design_on_zeland

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

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.


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 from the antenna to the source with specified Zc.

• Incident Power, Pinc: The power from the incident wave

• Reflected Power, Pref: the power from the reflected wave.

• Input Power, Pin: The net power going into the antenna or Pin = Pinc - Pref



• Radiated Power, Prad: It is the power radiated into the space from the antenna.

• Radiation Efficiency, Effrad: It is the ratio between Prad and Pin, or Effrad = Prad / Pin

• Antenna Efficiency, Effant: It is the ratio between the 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.



• Conjugate Match: When Za is conjugate of Zs or Za = Zs*, it is called conjugate match.

• Conjugate Match Factor, CMF: CMF is the ratio between 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.


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.


Working Principles and Design GoalsWorking Principles and Design Goals

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

• There are two working modes: (1) The RFID is working in receiving mode. The RFID antenna is receiving signal from a reader’s antenna and the signal is powering the chip in the RFID; (2) The chip is serving as a source and it is sending out signal thru the RFID antenna.

• The goals are to design the antenna to receive the maximum power at the chip from the reader’s antenna and to allow the RFID antenna to send out the strongest signal.

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


Equivalent Circuit at Receiving ModeEquivalent Circuit at Receiving Mode• The reader’s antenna is creating the EM field at where the

RFID is located. The RFID receives the radiation from the reader’s antenna and it is powering the chip.

ChipZs

Za

Va

Zs – Chip ImpedanceZa – Antenna ImpedanceVa – The equivalent voltage

source from receivingradiation from the reader


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

ZsZaVs

Zs – Chip or Source ImpedanceZa – Antenna ImpedanceVs – The equivalent voltage

source of the chip fromreceived power.


Ultimate GoalsUltimate Goals

• In receiving mode, we would like to chip impedance Zs to receive the maximum power from the equivalent voltage source Va.

• In transmitting mode, we would like to deliver the maximum power from the equivalent voltage source Vs to the antenna impedance Za.

ChipZs

Za

Va

Chip

ZsZaVs



• The system is transposable. We can just consider the transmitting mode. If we can achieve the best results in transmitting mode, we can achieve the best 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 will be radiated out. We need to achieve highest radiation efficiency Effrad.



• We need to implement some good RFID antenna configurations with high radiation efficiency.

• With a given antenna configuration, we need to tune 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. We should focus on tuning the dimensions for conjugate match or Za = Zs*.


Incorrect Concepts and GoalsIncorrect Concepts and Goals

• There have been many incorrect concepts in the design of RFID.

• Is it the best design of the RFID antenna if we can achieve maximum gain and maximum efficiency?

• Is it the best design of the RFID antenna if we achieve minimum S(1,1) normalized to the complex impedance of Zs (or Zs*)?

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


Maximum EfficiencyMaximum Efficiency

• To achieve maximum efficiency with given voltage source Vs and source impedance Zs, we can increase the antenna resistance Ra and the reactance Xa, where Ra and Xa are defined as Za = Ra + j Xa.

• Larger Ra will increase the efficiency but reduce the maximum received power and radiated power.

• Maximum efficiency (or maximum gain) of the antenna is not the best design.


Complex Normalization Impedance ZsComplex Normalization Impedance Zs• It is incorrect to use complex normalization impedance Zc.

It is proven in the Appendix of IE3D User’s Manual that complex Zc is an incorrect concept. RF designers are suggested to avoid using complex Zc.

• Multiple definitions of reflection coefficient:• Γ = ( Za – Zs ) / ( Za + Zs )• Γ = ( Za – Zs* ) / ( Za + Zs ) …

• No definition is precisely correct. The 1st definition may yield |Γ| > 1 for a passive system. The 2nd definition will not predict |Γ| > 1 for a passive system. However, it also loses meaning. The fundamental reason for invalid Γ is from the fact that incident and reflected waves are no longer precisely valid with complex Zc.


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 MHz RFID design published by K. V. Seshagiri Rao, et al. on IEEE AP-T Dec. 2007. The IE3D example file is prividedin .\zeland\ie3d\samples\LoadedMeanderTag.geo.


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 350 ohms.


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 excitation and define source impedance Zs as 17.5 – j350. You can choose Zc = 50. It is not critical.

Enable pattern calculation

Check f = 875 MHz to make sure it runs at the frequency even with AIF enabled.

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


Check Radiation PatternsCheck Radiation Patterns• After simulation, PATTERNVIEW is invoked to display the

radiation pattern. You can display the 3D pattern to see the radiation distribution. Please select Edit->Pattern Properties dialog to check the radiation parameters.


Important Parameters for RFIDImportant Parameters for RFID• For most microwave antennas, we should check the

Radiation Efficiency, Antenna Efficiency and Gain. They are important for wave sources. For voltage and current sources, we should try to check the conjugate matching and Input Power with given voltage source.


Important Parameters for RFIDImportant Parameters for RFID• An important parameter introduced in PATTERNVIEW 12.12

or 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.


CMF Vs. FrequencyCMF Vs. Frequency• Display CMF Vs. Frequency on PATTERNVIEW and

check 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


CMF Vs. FrequencyCMF Vs. Frequency• CMF Vs. Frequency can also be created from s-parameters

on MODUA (Process->General Lumped Equivalent Circuit) and MGRID (Process->S-Parameters and Lumped Equivalent Circuit).

Linear Scale dB Scale


Defining Optimization VariablesDefining Optimization Variables• The reason for CMF obvious below 0 dB at 875 MHz is that

Zs = 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 the antenna. 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


FastEMFastEM RealReal--Time Tuning and OptimizationTime Tuning and Optimization• Open the file and select Process->Full Wave EM Design…

Manual tuning on the bars

Automatic Optimization

Tuned geometry

Tuned Z(1,1)

Define display graph

Define goals for tuning and optimization


Goals for Tuning and OptimizationGoals for Tuning and Optimization• We define the goals as Re[Z(1,1)] = 17.5 ohms and

Im[Z(1,1)] = 350 ohms at 875 MHz

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


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 allowing you to see the change in geometry and results interactively; Re-usable results; Extremely efficient for tuning and optimization of batch designs with similar structure but slightly different goals; Allowing you to create a big design library.


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.


Check Optimized ZCheck Optimized Z--ImpedanceImpedance• As it is shown, Xa is tuned from 418.1 to 348 ohms

while Ra is tuned from 50.4 to 41. Xa is perfectly optimized while Ra still differs from 17.5 ohms.


Check Conjugate Match FactorCheck Conjugate Match Factor• The CMF is improved from -4.2 dB to -0.8 dB. If we can

tune 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)


Check Radiation ParametersCheck Radiation Parameters• The following table compares the original and optimized

radiation 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 than doubled 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


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 factor needs to consider in RFID antenna designs.

• IE3D FastEM Design Kit allows designers to tune and optimize RFID antenna efficiently.

microsoft powerpoint - rfid_design_on_zeland

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