Active transponders for short-range communications between on-board units and roadside units

An RF transponder is a system allowing contactless identification or communication in applications ranging from mountain/sea rescue to automatic waste management. Such a system is woken by an interrogating signal and sends back a coded response. A potential application in Intelligent Transport Systems concerns dedicated short-range communications (DSRC), in which information is exchanged between roadside units and on-board transponders. The European pre-standard defining the framework for DSRC recommends a 5.8 GHz carrier frequency within a 10 MHz band. Since the communication distance extends from a few meters (vehicle tolling applications) to several tens of meters (traffic information), a small primary battery must be inserted in the transponder. The purpose of this paper is to present two transponders suitable for this specific application, both based on the locking of a local oscillator by a pure interrogating RF frequency.