Resonant Frequency and Impedance Determination for Backscattered Signals in CSC System

Proximity and vicinity contactless systems are currently normalized by ISO/IEC 14443 and ISO/IEC 15693. They have to operate at 13.56 MHz in a range of 0 to 10 centimeters for proximity cards and 0 to 20 centimeters for vicinity cards. Depending on applications, contactless systems can sometimes be tuned for a fixed distance. It means that the position of the deported element (or contactless smart card) is invariant during the communications. Most of the systems use a batteryless card. In such systems, RF designers have to particularly pay attention to the tele supply voltage and to the data rate which are straightly constrained. In this paper we explain and demonstrate that it is possible to tune the RF system in order to achieve the best compromise between tele supply and modulation degree of the backscattered signal coming from the deported element. The best modulation degree is reached for a given minimum voltage supply value (V _SUPPLY-MIN). Complete contactless system is simulated with a method which allows computing the exact value of the coupling coefficient between reader and card antenna (A. Tramoni et al., 2005). Theoretical and simulation results are explained and compared to measurements