Polling and listening layouts as depicted in AXIEM with ports shown as well

(Design is courtesy of Rohde & Schwarz)

AXIEM®

Application Note

Design Of A Near Field Communication Antenna System

INTRODUCTIONNear field communication (NFC) is being developed as a form of contactless

communication between wireless devices like smartphones and tablets. This

technology enables users to do things like swipe their devices at the checkout

stand or wave them over another NFC-compatible device to share information

instantly without complicated setups or physical connections.

The antenna systems of these NFC-enabled devices are a critical component. In

general, the successful design of NFC antenna systems operating at 13.56MHz

requires electromagnetic (EM) simulation of both the polling and listening sides of

the antenna system, as well as the incorporation of discrete elements (including

nonlinear diode bridges that convert RF energy into DC signals). The typical analyses

performed include RF matching as a function of polling/listening distance, detection

of DC signal levels under various conditions, polling coil inductance, and generation of

harmonics at the DC port.

EM SIMULATION USING AXIEMThe sample NFC design used in this application note (monitor image below) is courtesy of

Rohde & Schwarz (R&S), a long-standing member of the NFC consortium that is respon-

sible for developing related NFC specifi cations and technology. To begin, the design is

imported into AWR’s Microwave Offi ce™/AXIEM environment by means of a Gerber fi le.

Once within AXIEM, the layout is assigned ports for the relevant discrete elements in the

design. AWR’s symbol generator wizard is then used to create a self-evident symbol.

Polling and listening layouts as depicted in AXIEM with ports shown as well

(Design is courtesy of Rohde & Schwarz)

Corporate Headquarters

1960 E. Grand Avenue

Suite 430

El Segundo, CA 90245

+1 310 726 3000

+1 310 726 3005 (fax)

www.awrcorp.com

www.awr.tvCopyright © 2012 AWR Corporation. All rights reserved. AWR and AXIEM are registered trademarks and the AWR logo and Microwave Office are trademarks of AWR Corporation. Other product and company names listed are trademarks or trade names of their respective companies.

This more convenient and accurate method replaces the typical manual wiring of the

discrete elements using a 60-port generic block element.

Another novel and helpful feature within AXIEM is parameterization. Here, the

distances between polling and listening coils are parameterized such that a single

numerical parameter, Z, is swept over a range of values between 1mm and 100mm

(Figure 1), and, consequently, the thickness of the air layer between the coils is

controlled parametrically.

While the EM structure is for a fi nite set

of discrete Z parameters, in general Z

is continuous. In other words, while the

EM simulation is carried out at discrete

steps of 10mm (starting from 1mm),

the resulting swept model is interpolated

such that steps of 1mm are computed.

An interesting observation to note is

that if the RF matching of the polling

antenna is determined in the absence of

the listening antenna, the matching

deteriorates signifi cantly when the

listener is brought nearer. This is clearly

seen in the nonlinear matching plot on

the Smith chart in Figure 2.

CONCLUSIONThe innovative capabilities of AWR

software are well suited to the design of NFC antenna systems. With the ability to

incorporate EM models easily into nonlinear schematics, as well as support for EM

model parameterization and interpolation, AWR’s Microwave Offi ce/AXIEM software

combination is making it even easier for designers of NFC antenna systems to realize

compact designs and take their designs and productivity to new levels.

the resulting swept model is interpolated

antenna is determined in the absence of

listener is brought nearer. This is clearly

This more convenient and accurate method replaces the typical manual wiring of the

AX-NFC-2012.9.14

Figure1: Rectifi ed DC voltage vs polling/listening distances (in mm)

Figure 2: Proximity of listener circuit deteriorates RF matching at the poller input port. VSWR>2 if the distance between poller/listener is less than 18mm

— LSSnm(PORT_1,PORT_1,1,1)[1,1X] Listener_and_poller_circuit_Z — VSWR_CIR(2) Listener_and_poller_circuit_Z