AXIEM®
Application Note
INTRODUCTION
Near 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 AXIEM
The 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 responsible for developing related NFC specifications and technology. To begin, the design is
imported into AWR’s Microwave Office™/AXIEM environment by means of a Gerber file.
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)
Design Of A Near
Field Communication
Antenna System
Figure1: Rectified DC voltage vs
polling/listening distances (in mm)
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 finite 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),
— LSSnm(PORT_1,PORT_1,1,1)[1,1X]
Listener_and_poller_circuit_Z
— VSWR_CIR(2)
Listener_and_poller_circuit_Z
the resulting swept model is interpolated
such that steps of 1mm are computed.
An interesting observation to note is
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
that if the RF matching of the polling
antenna is determined in the absence of
the listening antenna, the matching
deteriorates significantly when the
listener is brought nearer. This is clearly
seen in the nonlinear matching plot on
the Smith chart in Figure 2.
CONCLUSION
The 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 Office/AXIEM software
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combination is making it even easier for designers of NFC antenna systems to realize
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AX-NFC-2012.9.14