A detector unit apparatus and method for operating and producing a detector unit to be adapted to varying environmental influences or to its own, in particular mechanical, electrical or magnetic parameters of the detector unit, which are dependent on measuring lengths. An inventive positional sensor operating according to the running time principle of a mechanically elastic shaft and comprising a waveguide, a positional element, e.g. a positional magnet, which can be displaced in particular along the waveguide, in addition to a detector assembly that is located on the waveguide and comprises a detector coil wherein the detector assembly has a current source so that a current can flow through the detector coil.