Card reader for contactlessly readable cards and method for operating such a card reader, as well as contactlessly readable card

The invention relates to a method of operating a card reader (1), a contactlessly readable card and a card reader (1). A method of operating a card reader (PCD) (1) comprising the steps of: generating a high frequency magnetic field in a vicinity of the card reader (PCD) (1), wherein the radio frequency magnetic field is controlled to perform a communication with a contactless readable card (PICC), and wherein, when an information transmission from the card reading device (PCD) (1) to a located in the close range contactless readable card (PICC) is carried out a modulation of the radio frequency magnetic field; Evaluating the high-frequency magnetic field to detect a variation of the high frequency magnetic field through a located in the vicinity contactlessly readable card (PICC), and to analyze, in a time domain between a correct receipt of an information transmission from the contactlessly readable card to the card reader and a subsequent transmission of information from the card reader (PCD) to the contactlessly readable card (PICC), the magnetic field strength relative to an initial magnetic field strength is lowered to a pre-set with lowered magnetic field strength. The card reader and the contactlessly readable card (PICC) are adapted to perform this method. This can achieve an energy saving in the card reader (PCD), which in particular for mobile card reader (PCD) is advantageous.