Automatic Impedance Matching for 13.56 MHz NFC Antennas Michael - - PowerPoint PPT Presentation

Automatic Impedance Matching for 13.56 MHz NFC Antennas

Michael Roland1, Harald Witschnig2, Erich Merlin2, Christian Saminger1

1Upper Austria University of Applied Sciences, Hagenberg, Austria 2NXP Semiconductors, Gratkorn, Austria

6th Symposium on Communication Systems, Networks and Digital Signal Processing, 2008

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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

Both parameters decrease with increasing load resistance

20 40 60 80 100 120 140 10 20 30 40 50 60 70 load resistance [Ohm] transmitter supply current [mA] 20 40 60 80 100 120 140 10 20 30 40 50 60 70 load resistance [Ohm] emitted RF power [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 RX

TX1 TX2

RQ C1 C2 C1 C0 C0 L0 L0 C2 RQ GND

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.

voltage follower antenna bridge voltage follower generator amplitude phase waveform measuring

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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 Bridge is balanced (Vdiff = I2 · R2 − IZ · Z = 0 V)

amplitude & tuning circuit phase measurement R1 R2 R3 50 50 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 rectify

VSINE1 = I2 · R2 and VSINE2 = IZ · Z

Resulting signals are low pass filtered Vdiff is calculated in software after analog-digital conversion

2 3 1

U1A

8 4

D1 D2 R1 R3 R2 R4 C1

VSINE VRECT

GND GND V+ V- Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 15 / 24

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

SET RESET /Q Q

1 2 4

U1A

16 15 13

U1D R1 R3 R2 R4

VSINE1 VSINE2 PHASE

GND GND Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 16 / 24

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 Capacitors 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

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

• ptimal 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 Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 23 / 24

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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