54 / 84
To improve the efficiency of a tag,
these matching circuits can be tuned
using the built-in optimizers in CST
STUDIO SUITE. These find the set of
parameters for the circuit elements
which best fi t a given goal - for
example, it can find the component
values that minimize the S1,1 of the
antenna-chip combination at the
desired resonant frequency.
Optimization is not limited to circuit
elements, however. The dimensions of
the 3D antenna model and its material
properties can also be parameterized
and optimized (Figure 4) in order
to improve its performance. When
dealing with very compact RFID tags,
this approach has the advantage that
it can reduce the number of additional
circuit elements which have to be
added to the tag during construction.
For example, a parameter sweep or
an optimization over the substrate
thickness can be used to adjust the
parameterized, and a parameter
sweep offers a straightforward way to
analyze the effect of misalignment on
the tag.
Figure 6 shows how different
alignment problems affect the
behavior of an RFID-based NFC
system. These planar coils turn out
to be very sensitive to small changes
in the position of the tag relative to
the reader, but are more resilient to
angular changes. Moving the tag by
10 mm either perpendicular or parallel
to reader causes the output power to
drop almost to 0mW, but the effect
of rotating the tag on the reader is
relatively small.
Improving the efficiency of the link
between the reader and the tag
requires a multi-port matching circuit
optimization. The chips used in RFID
tags often have frequency-dependent
impedance profiles with both real and
imaginary parts, which means that a
Figure 5: (top) A bent RFID tag model. (bottom)
The S-parameters from a parameter sweep over
the radius of curvature, taking into account the
complex chip impedance.
Figure 4: (top) A parameterized antenna model. (bottom) The
S-parameters from a parameter sweep over the arm length r,
not taking complex chip impedance into account.
capacitance and inductance of the
antenna and improve its effi ciency
without adding an additional matching
circuit. Alternatively, a parameter
sweep can be used to investigate
how well a design performs when
manufacturing
tolerances
and
deformations are taken into effect
(Figure 5).
Tag and Reader Simulation
The tag is only half of the RFID system.
The reader also needs to be carefully
designed to allow efficient, reliable
scanning. Because RFID readers can
be very sensitive to the distance,
position and angle of the tag, it is
often useful to be able to calculate
the system’s behavior for numerous
different positions and orientations
quickly. With CST STUDIO SUITE, the
tag and the reader can be modeled
together in the same simulation.
The tag’s coordinates can be easily
54 l New-Tech Magazine Europe