Automatic Impedance Matching for 13.56 MHz. NFC Antennas. Michael Roland1, Harald Witschnig2, Erich Merlin2, Christian Saminger1. 1Upper Austria ...
Automatic Impedance Matching for 13.56 MHz NFC Antennas Michael Roland1 , Harald Witschnig2 , Erich Merlin2 , Christian Saminger1 1 Upper
Austria University of Applied Sciences, Hagenberg, Austria 2 NXP Semiconductors, Gratkorn, Austria
6th Symposium on Communication Systems, Networks and Digital Signal Processing, 2008
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This document has been downloaded from http://www.michael.x0.ath.cx/.
Outline 1
Overview of the NFC Research Lab
2
Motivation NFC Equipment and its Antennas Impedance Matching
3
Automatic Impedance Matching Automatic Tuning System Measuring Circuit Matching Circuit Controller Results
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 of Applied Sciences Funded by the BMVIT (Federal Ministry for Transport, Innovation and Technology) and FFG (Austrian Research Promotion Agency) We are focusing on NFC hardware development interoperability and performance testing for NFC systems implementation of NFC applications secure aspects for NFC systems and applications
1st Austrian NFC Trial Annual NFC Congress in Hagenberg
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Motivation
NFC Equipment and its Antennas
Outline 1
Overview of the NFC Research Lab
2
Motivation NFC Equipment and its Antennas Impedance Matching
3
Automatic Impedance Matching Automatic Tuning System Measuring Circuit Matching Circuit Controller Results
4
Summary and Outlook
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Motivation
NFC Equipment and its Antennas
NFC Equipment and its Antennas
NFC transmission module IC has external antenna Mobile devices have though space-saving requirements Non-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
Motivation NFC Equipment and its Antennas Impedance Matching
3
Automatic Impedance Matching Automatic Tuning System Measuring Circuit Matching Circuit Controller Results
4
Summary and Outlook
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Motivation
Impedance Matching
What is Impedance Matching? Antenna’s impedance is the NFC IC’s load resistance 2 parameters depend on the NFC IC’s load resistance the transmitter supply current and the emitted RF power 140
140
120
120
emitted RF power [mW]
transmitter supply current [mA]
Both parameters decrease with increasing load resistance 100 80 60 40 20 0
100 80 60 40 20 0
0
10
20
30
40
50
60
70
0
load resistance [Ohm]
10
20
30
40
50
60
70
load resistance [Ohm]
Good trade-off between both parameters around 40 to 50 ohms
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Motivation
Impedance Matching
TX1
L0
RQ C2
NFC IC
C0
C1
to RX
NFC Antenna Topology
C0 L0
C1
GND TX2
C2 RQ
NFC antenna topology with EMC filter, matching circuit, quality factor damping resistors and antenna coil C1 and C2 are adjusted until the whole topology circuit matches 50 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 procedure Good 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
Motivation NFC Equipment and its Antennas Impedance Matching
3
Automatic Impedance Matching Automatic Tuning System Measuring Circuit Matching Circuit Controller Results
4
Summary and Outlook
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Automatic Impedance Matching
Automatic Tuning System
Requirements
Most NFC devices are mobile handsets Automatic tuning system must be small in shape and size Single 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. waveform generator
phase
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measuring bridge
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antenna
voltage follower
voltage follower amplitude
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Automatic Impedance Matching
Measuring Circuit
Outline 1
Overview of the NFC Research Lab
2
Motivation NFC Equipment and its Antennas Impedance Matching
3
Automatic Impedance Matching Automatic Tuning System Measuring Circuit Matching Circuit Controller Results
4
Summary and Outlook
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Automatic Impedance Matching
Measuring Circuit
Measuring Bridge Wheatstone bridge with 13.56 MHz sinusoidal supply
50
50 50
R3
R1 R2
Bridge is balanced (Vdiff = I2 · R2 − IZ · Z = 0 V)
amplitude & phase measurement tuning circuit
⇒ 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 rectify VSINE1 = I2 · R2 and VSINE2 = IZ · Z
Resulting signals are low pass filtered Vdiff is calculated in software after analog-digital conversion R3
R1
2
R4
1
D2
R2
8
3
U1A
VRECT C1
V+
GND
VSINE
4
V-
D1
GND
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Automatic Impedance Matching
Measuring Circuit
Balancing the Phase of Vdiff VSINE1 and VSINE2 shaped into rectangular signals Phase detector: phase-offset is converted into a pulse-width modulated signal PWM signal is low pass filtered and fed into an analog-digital converter R1
2
GND
GND
VSINE1
VSINE2
U1A 4
1
15
R2
U1D
Q
13 16
/Q SET RESET
PHASE
R4
R3
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Automatic Impedance Matching
Measuring Circuit
Verification of the Measuring Circuit Verified with a manually tunable antenna circuit The antenna is connected to the measuring circuit and tuned Tuning is verified with a network analyzer Result 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
Motivation NFC Equipment and its Antennas Impedance Matching
3
Automatic Impedance Matching Automatic Tuning System Measuring Circuit Matching Circuit Controller Results
4
Summary and Outlook
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Automatic Impedance Matching
Matching Circuit
Matching Circuit
C1 and C2 in the antenna topology are replaced with capacitance arrays Capacitance array: network of parallel switched capacitors
1*C
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2*C
4*C
Sn
S3
S2
S1
S0
Capacitors switched with low-capacitance DMOS switches
8*C
Automatic Impedance Matching
(2^n) * C
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Automatic Impedance Matching
Controller
Outline 1
Overview of the NFC Research Lab
2
Motivation NFC Equipment and its Antennas Impedance Matching
3
Automatic Impedance Matching Automatic Tuning System Measuring Circuit Matching Circuit Controller Results
4
Summary and Outlook
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Automatic Impedance Matching
Controller
Controller
Analyses the measurands by reading the values from analog-digital converter Controls the capacitance arrays Tuning algorithm to minimizes amplitude and phase of Vdiff by optimal adjustment of C1 and C2
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Automatic Impedance Matching
Results
Outline 1
Overview of the NFC Research Lab
2
Motivation NFC Equipment and its Antennas Impedance Matching
3
Automatic Impedance Matching Automatic Tuning System Measuring Circuit Matching Circuit Controller Results
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 the integration of NFC ICs into applications. This automatic tuning system successfully finds the optimum values for the antenna circuit’s tuning capacitors. Only integrable components are used. Outlook Parasitic 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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