Automatic Impedance Matching for 13.56 MHz NFC · PDF fileAutomatic Impedance Matching for...

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

Roland, Witschnig, Merlin, Saminger Automatic Impedance Matching CSNDSP08 1 / 24

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


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


Motivation NFC Equipment and its Antennas

Outline






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


Motivation Impedance Matching

Outline







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



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)



Disadvantages of Manual Impedance Matching

Expensive equipment (network analyzer, impedance analyzer)Time-consuming procedureGood sense of choosing the right capacitor values necessary


Automatic Impedance Matching Automatic Tuning System

Outline







Requirements

Most NFC devices are mobile handsetsAutomatic tuning system must be small in shapeand sizeSingle chip solution preferred

⇒ ideal solution has only integrable parts



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


Automatic Impedance Matching Measuring Circuit

Outline







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)



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-



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



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:


Automatic Impedance Matching Matching Circuit

Outline






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


Automatic Impedance Matching Controller

Outline






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


Automatic Impedance Matching Results

Outline






Automatic Impedance Matching Results

Automatic Tuning Results

Antenna 1 Antenna 2

Antenna 3 Antenna 4


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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