New-Tech Europe | March 2016 | Digital edition

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.

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.

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

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

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

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