New-Tech Europe | March 2016 | Digital edition

Figure 1: H-field (left) and surface current density (right) for a typical LF RFID tag.

Figure 2: A high-frequency RFID tag (top) and its farfi eld pattern at its

best position and orientation for the RFID tags relative to the reader and, using the built-in “Calculate RFID Read Distance” macro in CST STUDIO SUITE, the readable range of the tag can be calculated over the range of possible angles (Figure 3) given the output power and sensitivity of the reader antenna. Most RFID tags include an integrated circuit, which contains the data associated with that tag. The chip itself will have a characteristic inductance and capacitance which will affect the tuning of the antenna, and may also include a matching circuit to improve antenna efficiency. To allow these to be taken into account by the simulation, CST STUDIO SUITE also includes a schematic circuit simulation tool which is integrated into the 3D design environment. The 3D model can be treated as a block and included in a circuit simulation or, using true transient-circuit co-simulation, the chip can be inserted into the 3D model as a SPICE or IBIS fi le. A simulation involving the complex chip impedance is shown in Figure 5.

RFID tags are best simulated using the frequency domain solver in CST STUDIO SUITE. For these tags, simulation can be used to calculate the H-field and surface currents induced in the coil (Figure 1), and to extract an equivalent circuit for the tag. HF RFID systems on the other hand offer higher data rates and longer ranges, making them suitable for applications such as inventory tracking and electronic toll collection. In HF RFID tags, the coil acts as a normal antenna, usually tightly folded

to reduce its area. This means that the impedance matching in HF RFID tags needs to be carefully optimized to allow the small antenna to operate efficiently. HF RFID tags can be simulated using the time domain solver or the frequency domain solver, depending on the antenna geometry and model size (including the environment). Useful results when dealing with HF RFID tags include their S-parameters and their farfields (Figure 2). The farfields can be used to identify the

Figure 3: The read range of the RFID tag in Figure 2.

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