Automatic Impedance Matching for 13.56 MHz NFC

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

Roland, Witschnig, Merlin, Saminger

Automatic Impedance Matching

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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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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


2


3


4

Summary and Outlook



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Motivation



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


2


3


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 Tuning System

Outline 1


2


3


4

Summary and Outlook



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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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System Design Automatic tuning system design split into its three components: measuring circuit, matching circuit and controller component. waveform generator

phase


measuring bridge


antenna

voltage follower

voltage follower amplitude

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Measuring Circuit

Outline 1


2


3


4

Summary and Outlook



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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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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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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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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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Matching Circuit

Outline 1


2


3


4

Summary and Outlook



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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


2*C

4*C

Sn

S3

S2

S1

S0

Capacitors switched with low-capacitance DMOS switches

8*C


(2^n) * C

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Controller

Outline 1


2


3


4

Summary and Outlook



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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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Results

Outline 1


2


3


4

Summary and Outlook



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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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