Design of RFID Tags and Systems
Transcript of Design of RFID Tags and Systems
Design of RFID Tags and SystemsDesign of RFID Tags and Systems
Prof. Raj MittraPennsylvania State University
University Park , PA, USA
Dr. KVS RaoIntermec Technologies
Everett, WA, USA
June 10, 2007
APS 2007Honolulu, Hawaii
ContentsContents1. Introduction to RFID2. RFID link budget and backscattering fundamentals3. RF front end considerations of ASIC.4. RF matching from ASIC to antenna5. Complete design, simulation and test of RFID tag,
including antenna, ASIC and application examples6. Near-field antenna considerations for RFID tags7. RFID applications, examples, and latest developments8. Conclusions
1. Introduction to RFID
Operating principlesHistoryRFID systems
FrequenciesTags ReadersProtocols
RFID TechnologyRFID TechnologyRFID (Radio Frequency Identification) uses radio frequencies for sending data from the tagged object to the reader.
RFID system components
TagsReadersMiddleware
Operating Principle: LF/HF RFIDOperating Principle: LF/HF RFID
Analogy: loaded transformer
Reader coil antenna creates RF magnetic field Passive tag gets power from the energy of the field responds by loading its own coil antenna with different impedances
LF/HF – low and high frequency
Operating Principle: UHF RFIDOperating Principle: UHF RFID
Analogy: using flashlight and mirror to communicate with Morse codeA
nten
na
Reader antenna transmits RF signal for the tagPassive tag gets power from the energy of the received signal and reflects it, modulating with its own impedance and thus presenting different RCS values
UHF – ultra high frequency
History of RFID History of RFID World War II
German pilots rocked their planes to signal back to the radar that they are comingBritish aviation used “identify friend or foe” system with simple transponder in the nose of the plane
1945 – Soviet eavesdropping device “Buran” (Leo Theremin)1948 - paper “Communication by means of reflected power
(Harry Stockman)1973 – first RFID patent 1999 – Auto-ID center is formed at MIT2004 – EPC Gen2 standard is ratified
RFID vs. Bar CodeRFID vs. Bar CodeRFID tag Bar code
Rewriteable Yes No
Read distance Up to 30 ft Up to 3 ft
Line of sight Not necessary Necessary
Read speed Up to 1500 tags/sec(Gen2)
2 bar codes/sec(trained operator)
Reliability Difficult to damage and counterfeit
Easy to damage and counterfeit
Cost 0.1-1 $ 0.001-0.01$
Equipment cost Approximately the same
RFID Frequency BandsRFID Frequency BandsLF HF UHF
Frequency 125-134 KHz 13.56 MHz
ISO 15693ISO 14443ISO 18000-3up to 1m
up to 64 kbps
860-960 MHz
Standards ISO 11784/5ISO 14223ISO 18000-2
ISO 18000-6
Read range up to 1 m up to 15 m
Data rate up to 9600 bps up to 640 kbps
Applications Logistics, manufacturing, access control
100 kHz 1 MHz 10 MHz 1 GHz 10 GHz100 MHz
13.56 MHz
915 MHz
125-134 kHz
2.45 GHz
LF HFMF UHFVHF
UHF Tag ClassificationUHF Tag ClassificationBy power
Passive Semi-passive Active
By memoryChipless (read only)With chip
• Class 0• Class 1• ISO (18000-6B), • Gen2 (ISO 18000-6C)
1976
1987
1999
Tag evolution
Flexible Passive UHF TagsFlexible Passive UHF TagsUsed in smart labels
Rigid Passive UHF TagsRigid Passive UHF TagsUsed for tagging pallets, machine parts, metal objects
RFID Readers and PrintersRFID Readers and PrintersReaders
Direct conversion transceivers, simultaneous Tx/RxAntennas: separate (switching) or singlePower: up to 1 W (30 dBm)Sensitivity up to -80 dBmDynamic range up to 100 dBWireless or wired database interface
PrintersReader integrated into the printerPrint bar code/text/images on the label and encode RFID inlay
UHF RFID ProtocolsUHF RFID Protocols
Class 0 Class 1 ISO-6B Gen2 (ISO-6C)
Anticollisionmethod
Binary Binary Binary Aloha
Read rate Low Low Low High
Password No No No Write/read/kill
Group select No No No Yes
Read/write No Limited Yes YesEncryption No No No Reader
Gen2 Electronic Product Code (EPC)Gen2 Electronic Product Code (EPC)
Header (8 bit): version number256 combinations
EPC manager number (28 bit): manufacturer268 million combinations
Object class number (24 bit): type of product16 million combinations
Serial number (36 bit): unique68 billion combinations
User data (32 bit).
96 bit (24 hexadecimal symbols)
ReferencesReferencesH. Stockman, “Communication by means of reflected power,”Proc. IRE, pp. 1196–1204, Oct. 1948.A. R. Koelle, S. W. Depp, and R. W. Freyman, “Short-range radio-telemetry for electronic identification, using modulated RF backscatter”, Proceedings of the IEEE, vol. 63, no. 8, Aug. 1975, pp. 1260-1261R. Glidden, et al., “Design of ultra-low-cost UHF RFID tags for supply chain applications”, IEEE Communications Magazine, vol. 42, no. 8, Aug. 2004, pp. 140-151J. Landt, “The History of RFID”, IEEE Potentials, vol. 24, no. 4, Oct.-Nov. 2005, pp. 8-11R. Want, “An Introduction to RFID Technology”, IEEE Pervasive Computing, vol. 5, no. 1, Jan.-Mar. 2006, pp. 25-33
2. RFID Link Budget Calculations and Backscattering Fundamentals
Link budgetTag read range
DefinitionLimitationsMeasurement
RCS (Radar Cross Section)Scalar RCSDifferential RCS
• Scalar • Vector
Link BudgetLink Budget
( ) ( ) pd
GGPP rtta
2
4,, ⎟
⎠⎞
⎜⎝⎛⋅⋅⋅=πλϕθϕθ
Power received by tag antenna in free space (from Friis equation)
tag the todistancewavelength
lossmismatch on polarizati antenna tag theofgain
antennareader theofgain poweroutput eader
antenna by tag receivedpower
−−−−−−−
d
pGG
rPP
r
r
t
a
λ
Tag Read Range
4 th
rtt
PpGGPr τ
πλ
=
IC of sholdpower thre t)coefficien matching impedance (a.k.a.
t coefficiensmission power tran
−
−
thP
τ
Maximum read range
Chip must receive enough power to turn on:
thc PP ≥
Power absorbed by the chip is determined by impedance matching
τ⋅= ac PP
Tag Read Range and Read RateTag Read Range and Read Rate
Read rate in a typical RFID system depends on distance and frequency (figure above is for the case where a tag is tuned to 900 MHz in a specific environment)
90 %
Tag Read RangeTag Read Range
TagChip sensitivityAntenna gainImpedance matchingPolarization matching
ChannelPropagation lossReflectionsInterference
ReaderTransmitter powerReceiver sensitivity
Depends on the following factors:
Tag antenna impedance and gain depend on the properties of tagged object
Ideally, RFID reader detects the tag as soon as it responds.
Interference from other wireless sources can significantly degrade tag range.
Tag LimitationsTag Limitations
Tag 1Antenna gain = 0 dBiChip sensitivity = -10 dBmMaximum range = 16 ft
(900 MHz, 4 W EIRP)
Tag must provide strong modulated backscattered signal for the reader
Tag 2Antenna gain = 10 dBiChip sensitivity = 0 dBmMaximum range = 16 ft
(900 MHz, 4 W EIRP)
!2012 dBPPredbackscatteredbackscatte=−
Two tags may have the same maximum range but signals seen by thereader from these tags may be different
lossmodulation antenna tag theofgain
−−
KGr2
rredbackscatte GP ∝
Propagation Environment LimitationsPropagation Environment Limitations
Path lossFree spaceMulti-path environmentWaveguide environment
Tag detuning on materialsAntenna impedanceAntenna gain
Reader LimitationsReader LimitationsEIRP (US: 4 W, Europe: 3.3 W)Sensitivity (equivalent)
Reader must be able to detect and decode noisy tag signal whose SNR proportional to tag differential RCS
0.059
0.0595
0.06
0.0605
0 10 20 30 40 50 60 70 80 90 100
Time (us)
Vol
tage
(V)
Gen2
ExampleExample
4 W EIRP915 MHz-80 dBm equivalent sensitivity
Free space
-10 dBm chip sensitivity2 dBi antennaperfect impedance match3 dB modulation loss
ExampleExample
Received power vs. distance for tag and reader in example RFID system
Transmitted EIRP (36 dBm)
Typical tag chip sensitivity (-10 dBm)
Typical reader sensitivity (-80 dBm)120 ft
20 ft
Tag Range Measurement ProcedureDistance d to the tag is fixed, transmitted power is variableTest equipment sends queries and analyzes backscattered signalMinimum power Pmin at which tag response is detected allows one to determine tag range at different frequencies for any given EIRP value
minP
EIRPdr =
Range Measurement Facility at IntermecAnechoic chamber
4 ft x 4 ft x 6 ft6 dBi linearly polarized antenna
Measurement EquipmentMeasurement EquipmentNational Instruments PXI RF hardware platform controlled by LabVIEWMeasures the minimum power needed for tag to respondMulti-protocol (ISO, Gen2, etc.)Broadband (800 -1000 MHz)
RFID Tag Scalar RCSRFID Tag Scalar RCSScalar RCS of an RFID tag determines the backscattered power when the tag antenna loaded with constant impedance loadBackscattered power = Power scattered from the open-circuited antenna + Power re-radiated by the loaded antennaOpen-circuited thin wire antennas scatter little power compared to when they are loaded (minimum scattering)For minimum scattering loaded antennas, re-radiated power can be calculated from the simple equivalent circuit
radiatedreradiatedrecircuitedopenredbackscatte PPPP −−− ≈+=
ginco
redbackscatte
SP
min
=σ density power power
−−
SP
Scalar RCS DerivationScalar RCS Derivation
Power collected by the loaded antenna
impedance antennaimpedance chip
wavelengthEIRP ed transmitt
area effective antennaantenna tag theofgain
−+=−+=
−−
−−
aaa
ccc
tt
e
jXRZjXRZ
GPAG
λ
Gr
GPASP ttea π
λπ 44
2
2 ⋅==
Power re-radiated by the loaded antenna(power dissipated in Za) G
ZZRPP
ca
aaradiatedre 2
24+
=−
2
222
ca
aradiatedre
ZZRG
SP
+== −
πλσScalar radar cross-section
K-factorK-factorK-factor shows how much power is re-radiated
2
24
ca
a
ZZR
K+
=22
24
aa
a
XRR+
0cZ *aZ ∞
K 1 0
Power re-radiated by complex conjugate loaded antenna is normalized by the power re-radiated by the short circuit loaded antenna
Measurement Methodology
Antenna gain is 6 dBi, distance to the tag is 0.5 m (to power up tag IC)
Anechoic chamber, antenna, network analyzer, tag tester Return loss is measured without a tag (for calibration) and then with the tag present inside the chamber
RFID tag
Antenna
Tags used in Measurements Tags used in Measurements
16 m
m
Return Loss Calibration (without Tag)Return Loss Calibration (without Tag)
Measured Return Loss (with Tag)Measured Return Loss (with Tag)
( )22
432
114
tGrS
λπσ =RFID tag scalar RCS
Comparison of Theory and DataComparison of Theory and Data
RFID Tag Differential RCSRFID Tag Differential RCS
Differential RCS of an RFID tag is an important parameter which determines the power of the modulated tag signal received by the reader
Non-coherent receiver can only register a magnitude difference between two scalar RCS values (scalar differential RCS)
Coherent receiver can detect both amplitude and phase of the signal and hence can register a vector difference between two RCS values (vector differential RCS
Vector Differential RCS DerivationVector Differential RCS Derivation
ac
ac
ZZZZ
+
−=
2,1
*2,1ρ
GRIIP absdif2
21.. 21
−=
( ) ( )2,12,1
.2,1 12
ρ−=+
=a
o
ca
o
RV
ZZVI
221
22..
4ρρ
πλσ −==Δ
GS
P bsdifRFID tag differential RCS
Currents in the antenna due to two different loads
Differential backscattered power
Load reflection coefficient
Vector Differential RCS on Smith ChartVector Differential RCS on Smith Chart
ac
ac
ZZZZ
+
−=
2,1
*2,1ρ
Measurement SetupMeasurement Setup
( )2
43
24λπσ d
GPP
tt
received=Δ
Received power is measured directly in the signal analyzer
Comparison of Theory and DataComparison of Theory and Data
-40
-35
-30
-25
-20
870 880 890 900 910 920 930Frequency (MHz)
diffe
rent
ial R
CS
(dB
sqm
)
Theory
Data
ReferencesReferencesD. D. King, “The measurement and interpretation of antenna scattering”, Proc. IRE, 1949, 37, (7), pp. 770 – 777R. Harrington, “Electromagnetic scattering by antennas”, IEEE Transactions on Antennas and Propagation, 1963, 11, (5), pp. 595-596R. C. Hansen, “Relationships between antennas as scatterers and as radiators”, Proceedings of the IEEE, 1989, 77 (5), pp. 659-662K. Schneider, “A re-look at antenna in-band RCSR via load mismatching”, Proc. of IEEE Ant. and Prop. Soc. Int. Symposium, June 1996, pp. 1398-1401L. Penttila, M. Keskilammi, L. Sydanheimo, and M. Kivikoski, “Radar cross-section analysis for passive RFID systems”, IEE Proceedings on Microwaves, Antennas and Propagation, 2006, 153, (1), pp. 103-109P. V. Nikitin and K. V. S. Rao, “Theory and measurement of backscattering from RFID tags”, IEEE Antennas and Propagation Magazine, vol. 48, no. 6, pp. 212-218, December 2006P. V. Nikitin, K. V. S. Rao, and R. Martinez, “Differential RCS of RFID tag”, Electronics Letters, vol. 43, no. 8, pp. 431-432, April 2007
3. RF Front End Considerations of ASIC
Tag Block DiagramRectifier and Voltage MultiplierModulator and DemodulatorTag Equivalent Circuit ExampleDesign Criteria
Tag Block DiagramTag Block Diagram
Supplies DC voltage
Changes tag input impedance
Detects signal envelope for data decoding
Rectifier and Voltage MultiplierRectifier and Voltage Multiplier
RFV
( )DRFOUT VVNV −≈
OUTV
- number of diodes - amplitude of RF input signal from tag antenna- forward voltage of Schottky diodes (~0.2V)
RFVDV
N
Modulator and DemodulatorModulator and Demodulator
Modulator
DemodulatorAntenna terminals
Antenna terminals
Tag Equivalent Circuit Example
Front End Impedance Calculation
Impedance calculation of the front-end (ohms) :
Rpad 102:= Cpad .0991:= Zpad Rpad103− i⋅
2 π⋅ fr⋅ Cpad⋅+:= Rs 123:= Rj 300000:= Cj .02:=
Zdiode Rs1
1Rj
1
10−( )3 i⋅2 π⋅ fr⋅ Cj⋅
+⎡⎢⎢⎣
⎤⎥⎥⎦
+:=Rnwell 100:= Cnwell .074:= Znwell Rnwell103− i⋅
2 π⋅ fr⋅ Cnwell⋅+:=
Zpwrpt Zdiode1
1300000
1
10−( )3 i⋅2 π⋅ fr⋅ 15⋅
+⎡⎢⎢⎣
⎤⎥⎥⎦
+:= Zsigpt Zdiode1
1300000
1
10−( )3 i⋅2 π⋅ fr⋅ 1.5⋅
+⎡⎢⎢⎣
⎤⎥⎥⎦
+:=
Zsp 5000103− i⋅
2 π⋅ fr⋅ 1.⋅+:=
Zchip1
1Zpad
1Znwell
+1
Zdiode+
1Zpwrpt
+1
Zsigpt+⎛
⎜⎝
⎞⎟⎠
:= Zchip 36.802 747.014i−=
Package Impedance Calculation and Matching
ML 0.954−=ML 10 log τ( )⋅:=Mismatch loss in dB is :
τ 0.803=τ1 τtag( )2−⎡⎣ ⎤⎦ 1 τpkg( )2
−⎡⎣ ⎤⎦⋅
1 τtag τpkg⋅−( )2:=τ
4 Re Zpkg( )⋅ Re Ztag( )⋅
Zpkg Ztag+( )2:=
This mismatch can be calculated from the transmission coefficient which can be expressed as
Zpkg 19.869 534.241i−=τtag 0.994=τpkg 0.993=τtagZtag 50−
Ztag 50+:=τpkg
Zpkg 50−
Zpkg 50+:=
Ratio 1.028=RatioZchip( )2
Re Zchip( )Re Zpkg( )
Zpkg( )2⋅:=Zpkg 19.869 534.241i−=Zpkg Z11
Z12 Z21⋅
Z22 Zchip+( )−:=
Z22 Z11:=Z21 Z12:=Z12 Zt4:=Z11 Zt4 XL+:=
Zcfl Zchip XL+:=Zt4 0103−
2 π⋅ fr⋅ Cpkg⋅i⋅+:=XL 2 π⋅ fr⋅ Lpkg⋅ i⋅:=Lpkg 1.5:=Cpkg .085:=
Parameters of the package :
Z tag 20 554 1i⋅+:=
Front End Design ConsiderationsFront End Design Considerations
Rectifer/voltage multiplerHaving a high RF to DC power conversion efficiency Providing sufficiently high output voltage (~2 V) to power up digital logic
ModulatorChoosing the modulation typeSelecting the impedance states to achieve high modulated backscattered power
Forward link limitation on tag range
Return link limitation on tag range
ReferencesReferencesU. Karthaus and M. Fischer, “Fully integrated passive UHF RFID transponder IC with 16.7 u/W minimum RF input power”, IEEE Journal of Solid-State Circuits, Volume 38, Issue 10, Oct. 2003, pp.1602 - 1608
G. De Vita and G. Iannaccone, “Design Criteria for the RF Section of UHF and Microwave Passive RFID Transponders”, IEEE Transactions on Microwave Theory and Techniques, vol. 53, no. 9, Sept. 2005, pp. 2978-2990
J.-P. Curty, N. Joehl, C. Dehollain, and M. J. Declercq, “Remotely powered addressable UHF RFID integrated system”, IEEE Journal of Solid-State Circuits, Volume 40, Issue 11, Nov. 2005, pp. 2193 - 2202
4. RF matching from ASIC to antenna
Equivalent CircuitPower transmission coefficient
Impedance contoursNormalized contours
Reflection coefficientSmith chart
Tag Equivalent CircuitTag Equivalent CircuitPower absorbed by the chip is determined by impedance matching
impedance antennaimpedance chip
t coefficiensmission power tran antenna thefrom availabepower chip by the absorbedpower
−+=−+=
−−−
aaa
ccc
a
c
jXRZjXRZ
PP
τ
24
ac
ac
ZZRR
+=τ
Antenna
Chip
τ⋅= ac PP
Impedance Matching Contour ChartImpedance Matching Contour Chart
( ) ( )2
22
21412τ
ττ
−=++⎟⎟
⎠
⎞⎜⎜⎝
⎛⎥⎦⎤
⎢⎣⎡ −− c
cacaRXXRR
24
ac
ac
ZZRR
+=τ
( ) ( )τ
acacac
RRXXRR 422 =+++
Contours of constant transmission coefficient are circles on (Ra,Xa) planeCenter of circles corresponds to perfect matchCircle radius depends on chip impedance
⎟⎠⎞
⎜⎝⎛ −12τcR
aX−
ττ
−12 cR
1=τ
Normalized Impedance Matching Contour ChartNormalized Impedance Matching Contour Chart
( ) ( )2
22
1412ττ
τ−
=−+⎟⎟⎠
⎞⎜⎜⎝
⎛⎥⎦⎤
⎢⎣⎡ −− Qxr aa
⎟⎠⎞
⎜⎝⎛ −12τ
( ) ( )τ
aaa
rQxr 41 22 =−++
Contours of constant transmission coefficient are circles on (ra,xa) planeCenter of circles corresponds to perfect matchCircle radius depends only on transmission coefficient
Q
ττ
−12 ( ) ( )14
22aa
a
xQrr
+++=τ
c
c
c
a
c
a
RX
RX
RR
−=
=
=
Q
x
r
a
a
1=τ
Complex Impedance Matching on Smith ChartComplex Impedance Matching on Smith Chart
ca
ca
ZZZZ
+−
=*
ρ
( )( ) ccaa
ccaa
RXXjRRXXjR
+++−++
=ρ
Smith chart is normalized to real impedance RcQuantity plotted is Ra+j(Xa+Xc)Origin corresponds to perfect match
21 ρτ −=
Range vs. Transmission CoefficientRange vs. Transmission Coefficient
Shows the effect of better matching on relative range improvementMaximum possible range is when the tag is perfectly matched
τ∝r
ReferencesReferencesK. Kurokawa, “Power waves and the scattering matrix”, IEEE Transactions On Microwave Theory and Techniques, 1965, MTT-13, (3), pp. 194–202
K. V. S. Rao, H. Heinrich, R. Martinez, “On the analysis and design of high-performance RFID tags," IEEE Workshop on Automatic Identification Technologies, pp. 68-69, March 2002
P. V. Nikitin, K. V. S. Rao, S. Lam, V. Pillai, R. Martinez, and H. Heinrich, “Power Reflection Coefficient Analysis for Complex Impedances in RFID Tag Design”, IEEE Transactions on Microwave Theory and Techniques, vol. 53, no. 9, pp. 2721-2725, September 2005
5. Complete Design, Simulation and Test of RFID Tag, including Antenna, ASIC and
Application Examples
Tag design processTag characteristicsPerformance chartDesign examples
Most flexible tags are dipoles (folded, loaded, stubbed, meandered, fed with slotline, crossed, combined with loops, etc.)
RFID Tag Design: Art or Science?RFID Tag Design: Art or Science?
RFID Tag MaterialsRFID Tag Materials
Dielectric SubstrateFR4 (rigid)PET (flexible)High permittivity dielectrics Paper
Antenna TraceCopper (most expensive)Aluminum (less expensive)Silver ink (cheapest)
Chip packagingFlip-chipWire-bondedSMD packages (TSSOP, MSOP)
RFID Tag Design ProcessRFID Tag Design Process
Select the application and define tag requirements
Determine the materials for antenna construction
Determine RF impedance of packaged ASIC
Identify the type of antenna and its parameters
Perform parametric study and optimization
Build and measure prototypes
Design requirements
met?
Design is ready
EM Software used at IntermecEM Software used at Intermec
Ansoft HFSSThick (rigid) tags, chip packaging modeling
DesignerThin (flexible) tags
Pallet tag
Rigid tag
Meander tag
Main Tag CharacteristicsMain Tag Characteristics
RangeBandwidthOmni-directionality
Tag Antenna Performance ChartTag Antenna Performance Chart
or - range of the tag with 0 dBi antenna and perfect impedance match
RFID Tag Design ExampleRFID Tag Design Example
RFID tag for smart label applications on variety of materialsRange: at least 15 ft in 860-960 MHz band (free space, 4W EIRP)Size limitation: 4 x 4 inMaterials: 1 mil copper on 2 mil polyester substrate Chip: ISO 18000-6B Attachment: flip-chip, packaging variations possible
Requirements
Tag antennaS-structure
WidebandTunable to accommodate for:
• Chip packaging variations• Different tagged materials
Packaged Chip ImpedancePackaged Chip ImpedanceTheoretical
Modeling/simulation with HFSS
PracticalMeasure packaged chip impedance at different power levels with a network analyzer
TSSOP Flip-chip
Chip Impedance vs. Frequency and PowerChip Impedance vs. Frequency and Power
Impedance of RFID Gen2 Chip (Freq 900)
-10
-5
0
5
10
15
20
25
30
-20 -15 -10 -5 0 5
Power Delivered to Chip (dBm)
R
-150
-130
-110
-90
-70
-50
-30
-10
Xj
R(Ohms)
Impedance of RFID Gen2 Chip (-10 dBm)
-10
-5
0
5
10
15
20
25
30
800 820 840 860 880 900 920 940 960 980 1000
Frequency
R
-150
-130
-110
-90
-70
-50
-30
-10
Xj
R(Ohms)
Frequency dependence Power dependence
Tag Design Process with DesignerTag Design Process with DesignerCreate parametric geometry
Specify excitation
Write post-processing variables
Parametric StudyParametric StudyIdentify key parameters and fix othersRun parametric analysis, plot and examine the rangeIdentify optimal parameter combinations
Parametric Study (S-antenna)Parametric Study (S-antenna)Fixed parameters
L=H=80 mm W1=0.5 mm
Variable parametersW2=W3=W L3
Optimal parameter combinationW=10 mmL3=80 mm (untrimmed), 40 mm (trimmed for 915 MHz band)
L3 varied W varied
Experimental VerificationExperimental Verification
Make several prototypes (use X-acto knife)Compare with experimental dataIdentify sources of discrepancy
0
5
10
15
20
25
30
800 820 840 860 880 900 920 940 960 980 1000Frequency (MHz)
Ran
ge (f
t)
DataSimulation
L3 = 40 mm
Free space, 4 W EIRP
Designer simulation has frequency offset compared to data (HFSS agrees with data well)
Final Design of S-antenna*Final Design of S-antenna*Tuning by trimming (punching)
Prototype
Gain pattern
* patent pending
80 mm
80 mm
0.5 mm
10 mm10 mm
Performance on Different MaterialsPerformance on Different Materials
0
5
10
15
20
25
30
820 840 860 880 900 920 940 960 980
Frequency (MHz)
Ran
ge (f
t)
Free spaceCardboardPlastic
L3 = 40 mm Free space, 4 W EIRP
S-antenna tag can be easily tuned for any material
Minimum required range
Another Design ExampleAnother Design Example
Intellitag ID card915 MHz bandISO 18000-6B
Antenna was modeled and simulated with Ansoft Designer
70 mm x 22 mm4 mil FR4 substrate with copper traceFairchild RFID ASIC chip
Experimental and Simulation ResultsExperimental and Simulation Results
RFID tag characteristics depend on surrounding material
EIRP=4 W
Free space resonant frequency
Resonant frequency inside plastic card
ReferencesReferencesP. R. Foster and R. A. Burberry, “Antenna problems in RFID systems”, IEE Colloquium on RFID Technology, October 1999, pp. 3/1-3/5
K. V. S. Rao, P. V. Nikitin and S. Lam, “Antenna Design for UHF RFID Tags: a Review and a Practical Application”, IEEE Transactions on Antennas and Propagation, vol. 53, no. 12, pp. 3870-3876, December 2005
D. M. Dobkin and S. M. Weigand, “UHF RFID and tag antenna scattering (Part I: Theory, Part II: Experimental Results)”, Microwave Journal, vol. 49, no. 5-6, May-June 2006
T. C. Chau, B. A. Welt, and W. R. Eisentadt, “Analysis and Characterization of Transponder Antennae for Radio Frequency Identification (RFID) Systems”, Packaging Technology and Science, no. 19, pp. 33-44, 2006
6. Near-field Antenna Considerations for RFID Tags
Antenna field regions Reader-tag antenna coupling Material penetration and antenna sizeNear Field UHF RFID OptionsMeasurements
Antenna Field RegionsAntenna Field Regions
DRFID reader
Antenna
D
r
RFID reader
Reactive
πλ 2/=r
λ/2 2Dr <
λ/62.0 3Dr =
Electrically small antenna
Electrically large antenna
πλ 2/≈rRadiating near field
Reactive near field
Near-field theory is the same for LF, HF, and UHF RFID(comes from Maxwell’s equations)
Reader-Tag Antenna CouplingReader-Tag Antenna Coupling
τρ CPP readerchip =2
4
tr
tr
ZZRR
+=ρ 2
4
ac
ac
ZZRR
+=τ
Reader impedance matching coefficient
Tag impedance matching coefficient
Coupling coefficient C depends on:Reader and tag antenna geometries;Relative position of antennas (distance and orientation);Environment, including any objects near antennas.
Types of CouplingTypes of CouplingRadiative (far field)
Different regions of reader antenna impedance (shown for a dipole antenna).
Non-radiative (near field)Inductive (magnetic)Capacitive (electric)
Hertzian Dipole FIeldsHertzian Dipole FIelds
θββπ
βη β cos)(
1)(
12 32
2rj
r erjrj
lIE −⎟⎟⎠
⎞⎜⎜⎝
⎛+Δ=
θβββπ
βη βθ sin
)(1
)(11
4 32
2rje
rjrjrjlIE −
⎟⎟⎠
⎞⎜⎜⎝
⎛++Δ=
θββπ
β βφ sin
)(11
4 2
2rje
rjrjlIH −
⎟⎟⎠
⎞⎜⎜⎝
⎛+Δ=
Near field decays as 1/r^2 and 1/r^3Far field decays as 1/r
x
E
z
Energy radiation
Energy storage
r
r EH
Near field Far field
Far Field Coupling (Radiative)Far Field Coupling (Radiative)Mutual effect of antennas is minimalAntenna impedance and gain can be specified independently of each
other and of distance between the antennasLong range, associated with electromagnetic plane wavesReader transmits a modulated signal which decays in free space as 1/rTag responds by modulating the backscattered signal from the tag
pGLGC rpatht=
2
4 ⎟⎟⎠
⎞⎜⎜⎝
⎛=
dLpath π
λ
In free space, path loss is:
RFID reader
RFID tag
Near Field Coupling (Non-radiative)Near Field Coupling (Non-radiative)
Inductive Coupling (magnetic): most energy is stored in magnetic fieldExample: multiple turn coil, axial magnetic field decays as 1/r^3RFID application: HF RFID systems Field is mostly affected by the presence of magnetics and conductors
Capacitive Coupling (electric): most energy is stored in electric fieldExample: parallel plate capacitorRFID application: UHF RFID printer couplerField is mostly affected by the presence of dielectrics and conductors
Mutual effect of antennas cannot be ignoredAntenna impedance and gain become dependent on mutual antenna position and orientation
Magnetic Coupling Example (HF RFID)Magnetic Coupling Example (HF RFID)Basic theory: Faraday’s law (induced voltage is proportional to the magnetic flux rate of change)
If the tag antenna is small, the magnetic field created by the reader antenna is not perturbed by the tag and the coupling coefficient is:
α2222 BSNfC ∝
dtdV Φ
−=
RFID reader antenna RFID transponder
Material Penetration and Antenna SizeMaterial Penetration and Antenna Size
fσμπδ 1=
Frequency 125 KHz (LF) 13.56 MHz (HF) 900 MHz (UHF)
Wavelength 2400 m 22.1 m 0.33 m
Skin depth(aluminum)
230 um 22.3 um 2.7 um
Field penetration into materials is limited by skin depth For near field operation antenna size can be much smaller than wavelengthTo radiate efficiently into the far field, antenna size needs to be comparable with wavelength
frequencytyconductivitypermeabili
−−−
fσμ
Item Level UHF RFID System OptionsItem Level UHF RFID System OptionsReader Antenna
Reader Power
Tag Read zone
1
2
Standard Full
3
4
Standard Low
LargeStandard
Standard
Short range1.Mistuned 2.Special
Small
Small for short range tagsLarge for standard tags
Special Small
Standard Full
AnySpecial
Standard – RF antennas which radiate well into the far field Special – RF antennas which generate primarily near field
Tag Range for Option 2 (Low Reader Power)Tag Range for Option 2 (Low Reader Power)
048
1216202428
800 820 840 860 880 900 920 940 960 980Frequency (MHz)
Tag
rang
e (ft
)
EIRP=4 WEIRP=0.04 W
RFID tag: TI Dallas
Experimental Test SetupExperimental Test Setup
Antenna (Sinclair LPD) Tag (TI Dallas)
Inside anechoic chamber
Minimum Power vs. DistanceMinimum Power vs. Distance
-6
-2
2
6
10
14
800 830 860 890 920 950 980Frequency (MHz)
Min
imum
pow
er (d
Bm
) 242220181614121086
d (in)
RFID reader antenna: Sinclair, RFID tag: TI Dallas
Far Field BoundaryFar Field Boundary
-6-4-202468
1012
0 2 4 6 8 10 12 14 16 18 20 22 24Distance (in)
Min
imum
pow
er (d
Bm
)
Theory (far field)Data
Reader antenna: Sinclair, RFID tag: TI Dallas
Liquids Demo: UHF RFID Tag in GatoradeLiquids Demo: UHF RFID Tag in Gatorade
Standard UHF RFID reader antennaStandard UHF RFID tag Tag is immersed into the bottle of Gatorade
Liquids Demo: Measurement ResultsLiquids Demo: Measurement Results
23
24
25
26
27
28
840 860 880 900 920 940Frequency (MHz)
Pow
er (d
Bm
)
Tag can be read using as little as 250 mW reader power at 890 MHz
Tag response
Near Field UHF RFID TagsNear Field UHF RFID TagsLF and HF RFID Tags can not work in far field
Physical sizes of the antenna ( both reader and tag) need to be large to operate in far field
UHF RFID Tags can work both in near and far fieldRadiation mechanism includes both near and far fields for realizable physical sizes of the antenna (both reader and tag)
Near Field ConsiderationsNear Field ConsiderationsBasic physics and design methodology are the same for RFID at LF, HF, and UHF
Near field UHF RFID can be used for item level tagging
Magnetic or electric coupling can be used for near field UHF RFID depending on application
UHF has the advantage of smaller antenna size for both near and far field operation
ReferencesReferencesC. A. Balanis, “Antenna theory: analysis and design”, John Wiley & Sons, 1997R. Bansal, “Near-field magnetic communication, IEEE Antennas and Propagation Magazine, Vol. 46, No. 2, Apr. 2004, pp. 114 – 115“Item-level visibility in the pharmaceutical supply chain: a comparison of HF and UHF RFID technologies”, white paper by Philips, TAGSYS, and Texas Instruments, available at http://www.tagsysrfid.com/modules/tagsys/upload/news/TAGSYS-TI-Philips-White-Paper.pdfT. Lecklider, “The world of the near field”, Evaluation Engineering, October 2005, available at http://www.evaluationengineering.com/archive/articles/1005/1005the_world.asp P. V. Nikitin and K. V. S. Rao, “An Overview of Near Field UHF RFID”, IEEE RFID Conference, Grapevine, TX, March, 2007, pp. 167-174
7. RFID Applications, Examples, and Latest Developments
ApplicationsExamplesLatest Developments
RFID ApplicationsRFID Applications
LogisticsManufacturingAccess controlAnti theftWireless pay systemsDocuments/cards
Automobile industry
Aviation
Pharmaceuticals
Application: Supply Chain LogisticsApplication: Supply Chain Logistics
RFID portalsDock-systemsDoorsBays
RFID forklifts and loaders
Large retailers (Wal-Mart, Target, Metro) operate on profit of less than 5%Saving only 1% significantly increases their competitiveness
Application: Border ControlApplication: Border Control
US-Canada borderSpecial lane for owners of NEXUS pass (plastic card with photograph and RFID tag)Equipment and tags are supplied by Intermec
Program NEXUSProgram NEXUS
Application: AirportsApplication: AirportsPassenger registration, passport control, passenger trackingBaggage and cargo tagging
Application: AirplaneApplication: Airplane
RFID for part trackingReduces time for:
• Part identifications• Locating spare parts• Service document writing
Prevents usage of counterfeit and used partsAllows to control part assembly procedure
Boeing 787 Dreamliner will have RFID tags on ~2000 critical (expensive or requiring frequent or regular servicing) parts
Application: ID CardsApplication: ID Cards
Badges and ID cardsGen2
• 869 or 915 MHz• Read range up to 10 m
Latest DevelopmentsLatest DevelopmentsUHF Near Field RFID
Tags can work close to metal/liquidsCompanies: Smartcode, RsiID
Multi-protocol tagsSingle tag (multi-protocol IC with special antenna) can work in several bands (LF, HF, UHF)Companies: TwinLinx, Texas Instruments
RFID Reader ChipsetsSeveral chips do most of RFID reader functionsCompanies: Intel, WJ, Anadigm
Organic/polymer electronicsLow cost, printable, biodegradable tagsCompanies: OrganicID, PolyIC, ORFID
8. Conclusions8. Conclusions
RFID tag antenna design is both art and science
Best results are obtained when tag antenna gain is maximized and tag antenna is well matched to ASIC
EM software tools are necessary for RFID tag design modeling and optimization
Accurate wideband tag range measurement capability is crucial for quality tag design implementation and performance verification