Chip cards, or smart cards, are used as servers in association with a digital data transmission network terminal. A coil is formed in a form of continuum in which a first winding unit wound clockwise around a bottom pole and a second winding unit wound counterclockwise around a top pole are alternatively coupled. A switching circuit is arranged between the power supply outputs and the amplifier power-supply input. The ratio of the oxide composition of the underlying aluminum oxide film is smaller than that of the upper aluminum oxide film. A method for filtering gaseous hydrocarbons from an air flow is also provided.