Prerequisites for Efficient Programming of RFID Inlays in Reactive Near Field

University dissertation from Chalmers University of Technology

Abstract: The electromagnetically isolated programming of unique user IDs, into the transponder memory of UHF RFID inlays, is essential for successful operation in the RFID inventory process. Bar code printers enabled for RFID represent a typical environment of such programming. They are usually equipped with an RFID interrogator and a reactive near field (RNF) coupler. Bar code printers, in particular those for industrial use, consist of compact robust metallic media housings. This environment fundamentally changes the electromagnetical properties of RFID inlays, nominally designed for operation in the far field. One fundamental requirement put on the coupler design is the strong mutual electromagnetic interaction towards RFID inlays with arbitrary antenna geometries, yet still completely isolated from neighboring inlays, being as close as a couple of millimeters apart. Another requirement is efficient coupling despite strong non-linearity of the transponder chip for increased drive power levels. This thesis treats the prerequisites for an adaptive solution of geometry independent RFID inlay programming in reactive near field. The work is divided into two distinct parts. The first part addresses the non-linear behavior of RFID transponder chips under source impedance and drive level shift. A measurement method with modulated stimuli is developed from which a figure of merit (FOM) is extracted, including the |ΔΓ|-value, a common performance parameter found in the RFID community. A dual probe connection method is introduced, enabling the characterization of RFID transponder chips from several vendors having different size parameters. An approximate ideal clipping model is developed, enabling power swept measurements with a linear vector network analyzer (VNA). In the second part the differential transmission line loop (DTLL) is presented as novel coupling element for high efficient programming of RFID inlays in reactive near field (RNF). Results for simple super elliptic loop geometries show strong magnetic coupling towards a number of different inlay geometries, utilizing the current inductive loop of the inlay. The inductive loop is common for virtually all inlay types, making the DTLL a strong candidate as coupling element in future adaptive coupling technologies. In particular the DTLL offers design flexibility both in component choice and geometry, suitable for further element optimization.