rfid design on zeland
DESCRIPTION
RFID designTRANSCRIPT
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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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.