Transponder, especially for a contactless chip card

A transponder comprising an internal digital circuit operated by a clock generator arranged to generate a clock signal having a clock frequency, and an internal energy supply arranged to be supplied contactlessly by an external alternating magnetic field, said clock generator comprising a clock frequency adjusting device arranged to adjust the clock frequency of the clock signal in dependence on a field strength of an external magnetic field on the transponder, said transponder further including a receiving antenna for receiving the external magnetic field and a rectifier connected to the receiving antenna for rectifying a voltage induced in said antenna by said external magnetic field, wherein the clock frequency of the clock signal is adjusted based on a difference signal representative of a difference between an unregulated voltage provided at the output of the rectifier and an internally generated reference voltage.