RFID and NFC antenna design Cartes Asia 2014

Jürgen Schröder / Steven Chang March 2014

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NFC Key Points

13,56MHz RF Link

• Wireless Short Range Communication Technology • Based on RFID technology at 13,56 MHz • Operating distance requires close proximity of devices • Data exchange rate today up to 424kbit/s • Compatible with today’s field proven contactless reader infrastructure • Passive communication more allows battery saving implementations

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NFC modes

1 - Card emulation (payment, transport & event

ticketing, access, …)

2 - Card reader (smart posters, tagged

promotions, authentication, …)

3 – P2P (Easy BT/Wifi

pairing, games, data exchange,

…)

Like ISO/IEC 14443 PCD

Like ISO/IEC 14443 PICC

Passive P2P: „Extended ISO/IEC 14443 PCD/PICC“

Active P2P: Both devices generate their own RF field

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How the RFID/NFC Antenna works

energy

data k : coupling coefficient

1

2kφφ

=

PCD

k

PICC

R2 L2 L1 C2

PCD Antenna Coil k

PICC Coil

Φ

Index 1: PCD antenna = design parameters Index 2: PICC antenna = typ. fixed parameters

PCD: Proximity Coupling Device („Reader“) PICC: Proximity Chip Card („Card“)

Transformer principle !

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A general equivalent antenna circuit

Ra

La

Ca

L - Inductance, mainly defined by the number of turns of the antenna R - Resistance, mainly defined by the diameter and length of antenna wires C - Capacitance, mainly defined by the distance of antenna wires from each other and number of turns

Free air condition

NFC-IC L / R / C

Network

I

U Z

La

Ra

Ca

Example values of an existing antenna L - 1,45uH R – 0,4 Ohm C – 3,5pF

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Antenna matching / tuning

A RFID or NFC antenna with the typical sizes has a impedance that is rather low and resonance frequency which is higher than 13,56MHz

Therefore a antenna matching and tuning is required.

The antenna tuning is adjusting the antenna resonance frequency by the addition of parallel capacitors (typically) to bring the resonance frequency close to 13,56 MHz.

The antenna matching is adjusting the impledance of the antenna seen by the NFC IC, so that a optimal RF power efficiency can be achieved and the maximum current of the NFC-IC is not exceeded.

A typical impedance for an antenna is ranging from 30…60 Ohms, depending on the antenna

A typical inductance is in the range of 1…3 µH

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Antenna performance, integration, optimization

NFC Antenna performance depends on: – Coil outline area – Number of turns – Track width and pitch – Track material – Environmental impact

How is it made: – Wire (copper) in a loop and embedded in phone. – Ferrite layer is used in designs to avoid interferences.

Location is critical for usability: – Different approaches depending on OEM.

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Considerations for Antenna sizes Mobile phones typically implement a rather small antenna for the NFC function

A large antenna of a typical RFID reader works well for SmartCard communication, but might have a weak coupling to a mobile phone NFC antenna and by this a reduced operating distance might occur.

NFC Device (Primary coil)

NFC Phone (Secondary coil)

The number of antenna turns has no impact on coupling. The size (geometry) of an RFID/NFC antenna defines the operating distance („performance“):

Small size = small operating distance Large size = large operating distance

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Typical Antenna Matching network

EMC Filter: Square wave output driversignal is shaped to a sinus singal Macthing Cirquit: Lowers the self resonance frequency of the antenna to approx 13,56Mhz

Typical NFC development board and components as described in the schematics

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Determining the equivalent antenna components Equivalent antenna components are Ra, La, Ca , the damping resistance Ra is defining the Q

factor of the antenna

The inductance and resistance should be measured a frequency of 1MHz.

A VNA (Vector Network Analyzer) can be used to perform this measurement.

The self resonance frequency (fra) and the parallel resistance can be obtained at the resonant point of the system where the imaginary part is zero.

The antenna capacitance Ca can be calculated with:

The quality factor of the antenna is calculated with:

A proposed value for Q is 30. If the calculated Q is higher an external damping resistor RQ has to be inserted on each antenna side to reduce the Q-factor to the target value.

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Determining the self resonance frequency

So far, we have determined the equivalent components Ra,and La a

In order to calculate the parameter Ca the self resonant frequency of the loop is needed.

The self resoncant frequency is typically high, therefore configure the VNA for a frequency range 1-100MHz

->The frequency point where Xs (imaginery part) is sharply going from inductive to capacitive (negative) value is the self resonant frequency

The self resonance frequency measured = 70.75MHz.

Then the capacitance Ca of equivalent circuit can the be calculated using this frequency.

Knowing now the equivalent components of the antenna, we can calculate the required parallel capacitor values for C2 to bring the resonance frequency down to the desired 13,56MHz

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Calculation of EMC filter and matching capacitors

The remaining parts of the matching network are the capacitors C1 which is used to match the impedance and the capacitor C0 and inductivity L0 which are components for the EMC filter.

There is a calculation excel table available to ease the calculation of this components. The table can be downloaded using the following link:

– http://www.nxp.com/documents/application_note/AN1425_AN166510.zip

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Verifying the correct tuning and matching of the antenna

The VNA can be used to display a smith chart.

Measuring needs to be done before the EMC filter inductors

Target is to achieve a real impedance @13,56Mhz

NFC Forum compliant transceiver like PN512 device operates in card and reader mode and a tradeoff for the optimal Z matching must be evaluated.

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Which tools do I need?

Network or Impedance analyzer – Rohde & Schwarz – Agilent – miniVNA Pro (cheap alternative) – etc…

Oscilloscope – Like for every electronic design

RFID / NFC Test tools – Calibrated Reference PICCs – NFC Listener – Test software – …

Design itself

Test & Qualification

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Conclusion RFID and NFC antennas are „transformers in resonance“

NFC mobile devices must cover 2 (or 3) operation modes: – Card Reader Mode (like RFID reader) – Card Emulation Mode (like RFID smart card): optional – P2P mode (between two NFC devices)

The size (geometry) of an RFID/NFC antenna defines the operating distance („performance“) in principle:

– Small size = small operating distance – Large size = large operating distance

Metal – around or behind the RFID/NFC antenna „kills“ the magnetic field. – behind the antenna can be shielded with ferrite.

The final design of an RFID/NFC antenna is quite straight forward with the right tools.

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