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Antennas / Subsystems and Systems

Tag Designs and Techniques Used in HF RFID Item Level Tracking

October 8, 2010
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The choice and placement of a RFID tag on a product requires an investigation to determine optimal performance. Tags come in many sizes and shapes and their placement is dictated by the product size and the tags orientation with respect to the interrogator antenna. The tag size will often be a performance issue with use on small products and may also require reorientation of the product and the interrogator antenna which may not be an available option. The current technology of HF tags is focused on the mass production of low cost paper thin tags that have maximized read range performance perpendicular to the broad plane of the tag. These tags are 2D in nature and vary in size dependant upon layout and the chip capacitance of the RFID chip chosen. An alternative tag that incorporates a ferrite core offers other options in that it allows for maximum read range normal to the small aperture of its geometry. Incorporating both these tag designs for use within the RFID architecture allows for flexibility of product placement and orientation while maintaining optimum performance.

A RFID system consists of a host system and RF components (Figure 12). The RF components consist of an interrogator (reader and antenna) and tags. The purpose of the interrogator is to communicate to the tags in the field and to also (for passive systems) power the tag through the transmitted RF signal. The interrogator is responsible for;

1. Protocol
2. Tag power
3. Reading tag information
4. Writing tag information
5. Ensure message delivery and validity to host system

The tags are placed on products which the user desires to track. The tag has the capability of storing a unique ID number or user programmed data and can communicate this information to the interrogator. The tag design dictates the limiting parameters of the RFID system such as read range and product orientation.

Two tags currently available are the ferrite core tag (FC) and the planar tag (PC). The FC has been outshined today by the lower cost and paper thin profile of the PT. The FC design incorporates a wire coil that is wound around a ferrite material as shown in Figure 1. This type of design resembles that of the common ferrite core inductor and has a 3D footprint. The inductance of either the PT or the FC is designed to resonate with the internal capacitance of the RFID IC. The ICs available for HF RFID are most commonly offered with a capacitance of 23pF and models with 97pF are also available. The Q of the inductors/coils in these designs must be adequate to provide both efficiency and functionality within the system. The Q is directly related to read range performance but should not be so high as to limit the required system bandwidth or manufacturability of the product.

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