Automatic Impedance Matching for 13.56 MHzNFC Antennas
Michael Roland1, Harald Witschnig2, Erich Merlin2, Christian Saminger1
1Upper Austria University of Applied Sciences, Hagenberg, Austria2NXP Semiconductors, Gratkorn, Austria
6th Symposium onCommunication Systems, Networks and Digital Signal Processing, 2008
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Outline
1 Overview of the NFC Research Lab
2 MotivationNFC Equipment and its AntennasImpedance Matching
3 Automatic Impedance MatchingAutomatic Tuning SystemMeasuring CircuitMatching CircuitControllerResults
4 Summary and Outlook
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Overview of the NFC Research Lab
NFC Research Lab
Part of the R&D department of the Upper Austria University ofApplied SciencesFunded by the BMVIT (Federal Ministry for Transport, Innovationand Technology) and FFG (Austrian Research Promotion Agency)We are focusing on
NFC hardware developmentinteroperability and performance testing for NFC systemsimplementation of NFC applicationssecure aspects for NFC systems and applications
1st Austrian NFC TrialAnnual NFC Congress in Hagenberg
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Motivation NFC Equipment and its Antennas
Outline
1 Overview of the NFC Research Lab
2 MotivationNFC Equipment and its AntennasImpedance Matching
3 Automatic Impedance MatchingAutomatic Tuning SystemMeasuring CircuitMatching CircuitControllerResults
4 Summary and Outlook
Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 4 / 24
Motivation NFC Equipment and its Antennas
NFC Equipment and its Antennas
NFC transmission module IC has external antennaMobile devices have though space-saving requirementsNon-standard antennas necessary to fit casings
antennas have varying characteristics (shape, impedance)each antenna has a certain impedance measured at its clamps
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Motivation Impedance Matching
Outline
1 Overview of the NFC Research Lab
2 MotivationNFC Equipment and its AntennasImpedance Matching
3 Automatic Impedance MatchingAutomatic Tuning SystemMeasuring CircuitMatching CircuitControllerResults
4 Summary and Outlook
Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 6 / 24
Motivation Impedance Matching
What is Impedance Matching?
Antenna’s impedance is the NFC IC’s load resistance2 parameters depend on the NFC IC’s load resistance
the transmitter supply current andthe emitted RF power
Both parameters decrease with increasing load resistance
020406080
100120140
0 10 20 30 40 50 60 70load resistance [Ohm]
trans
mitt
er s
uppl
y cu
rren
t[m
A]
020406080
100120140
0 10 20 30 40 50 60 70load resistance [Ohm]
emitt
ed R
F po
wer
[mW
]
Good trade-off between both parameters around 40 to 50 ohms
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Motivation Impedance Matching
NFC Antenna Topology
NFC
IC
to R
X
TX1
TX2
RQC1
C2
C1
C0
C0L0
L0
C2RQ
GND
NFC antenna topology with EMC filter, matching circuit, qualityfactor damping resistors and antenna coilC1 and C2 are adjusted until the whole topology circuit matches50 ohms at its clamps (TX1, TX2)
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Motivation Impedance Matching
Disadvantages of Manual Impedance Matching
Expensive equipment (network analyzer, impedance analyzer)Time-consuming procedureGood sense of choosing the right capacitor values necessary
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Automatic Impedance Matching Automatic Tuning System
Outline
1 Overview of the NFC Research Lab
2 MotivationNFC Equipment and its AntennasImpedance Matching
3 Automatic Impedance MatchingAutomatic Tuning SystemMeasuring CircuitMatching CircuitControllerResults
4 Summary and Outlook
Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 10 / 24
Automatic Impedance Matching Automatic Tuning System
Requirements
Most NFC devices are mobile handsetsAutomatic tuning system must be small in shapeand sizeSingle chip solution preferred
⇒ ideal solution has only integrable parts
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Automatic Impedance Matching Automatic Tuning System
System Design
Automatic tuning system design split into its three components:measuring circuit, matching circuit and controller component.
voltage follower
ante
nna
bridge
volta
ge fo
llow
er
generator
amplitude
phase
waveform
measuring
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Automatic Impedance Matching Measuring Circuit
Outline
1 Overview of the NFC Research Lab
2 MotivationNFC Equipment and its AntennasImpedance Matching
3 Automatic Impedance MatchingAutomatic Tuning SystemMeasuring CircuitMatching CircuitControllerResults
4 Summary and Outlook
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Automatic Impedance Matching Measuring Circuit
Measuring Bridge
Wheatstone bridge with 13.56 MHz sinusoidal supplyBridge is balanced (Vdiff = I2 · R2 − IZ · Z = 0 V)
amplitude &
tuningcircuit
phasemeasurement
R1
R2
R3
5050
50⇒ Tuning circuit is matched to 50 ohms (at the operating frequency)
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Automatic Impedance Matching Measuring Circuit
Balancing the Amplitude of Vdiff
Measuring rectifiers are used to rectifyVSINE1 = I2 · R2 andVSINE2 = IZ · Z
Resulting signals are low pass filteredVdiff is calculated in software after analog-digital conversion
2
31
U1A84
D1
D2
R1
R3
R2
R4
C1
VSINEVRECT
GN
D
GND
V+V-
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Automatic Impedance Matching Measuring Circuit
Balancing the Phase of Vdiff
VSINE1 and VSINE2 shaped into rectangular signalsPhase detector: phase-offset is converted into a pulse-widthmodulated signalPWM signal is low pass filtered and fed into an analog-digitalconverter
SETRESET
/Q
Q
1
24
U1A
16
1513
U1D
R1
R3
R2
R4
VSINE1
VSINE2
PHASE
GN
DG
ND
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Automatic Impedance Matching Measuring Circuit
Verification of the Measuring Circuit
Verified with a manually tunable antenna circuitThe antenna is connected to the measuring circuit and tunedTuning is verified with a network analyzerResult is approximately the same with several tested antennas:
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Automatic Impedance Matching Matching Circuit
Outline
1 Overview of the NFC Research Lab
2 MotivationNFC Equipment and its AntennasImpedance Matching
3 Automatic Impedance MatchingAutomatic Tuning SystemMeasuring CircuitMatching CircuitControllerResults
4 Summary and Outlook
Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 18 / 24
Automatic Impedance Matching Matching Circuit
Matching Circuit
C1 and C2 in the antenna topology are replaced with capacitancearraysCapacitance array: network of parallel switched capacitorsCapacitors switched with low-capacitance DMOS switches
1 * C 2 * C 4 * C 8 * C
S0 S1 S2 S3 Sn
(2^n) * C
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Automatic Impedance Matching Controller
Outline
1 Overview of the NFC Research Lab
2 MotivationNFC Equipment and its AntennasImpedance Matching
3 Automatic Impedance MatchingAutomatic Tuning SystemMeasuring CircuitMatching CircuitControllerResults
4 Summary and Outlook
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Automatic Impedance Matching Controller
Controller
Analyses the measurands by reading the values fromanalog-digital converterControls the capacitance arraysTuning algorithm to minimizes amplitude and phase of Vdiff byoptimal adjustment of C1 and C2
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Automatic Impedance Matching Results
Outline
1 Overview of the NFC Research Lab
2 MotivationNFC Equipment and its AntennasImpedance Matching
3 Automatic Impedance MatchingAutomatic Tuning SystemMeasuring CircuitMatching CircuitControllerResults
4 Summary and Outlook
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Automatic Impedance Matching Results
Automatic Tuning Results
Antenna 1 Antenna 2
Antenna 3 Antenna 4
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Summary and Outlook
Summary and Outlook
Automatic impedance matching significantly simplifies theintegration of NFC ICs into applications.This automatic tuning system successfully finds the optimumvalues for the antenna circuit’s tuning capacitors.Only integrable components are used.
OutlookParasitic effects of the DMOS switches need to be investigated.The tuning algorithm may be optimized.The whole system should be integrated into a single IC.
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