A robotic work tool system (200) comprising a signal generator (240), a boundary wire (250) and a robotic work tool (100), said boundary wire being adapted to demark a work area (205), said signal generator (240) being arranged to transmit a control signal (255) through said boundary wire (250), and said robotic work tool (100) comprising at least one sensor (170) for detecting said control signal (255) being transmitted through the boundary wire (250). The robotic work tool system (200) is characterized in that said pulse signal (255) comprises at least one first pulse train (256) being transmitted at a first amplitude (C1) and at least one second pulse train (257) being transmitted at a second amplitude (C2), wherein said first amplitude (C1) and second amplitude (C2) being arranged so that said first amplitude (C1) is greater than said amplitude level (C2) and a sum of the amplitudes (C1, C2) for all pulse trains (256, 257) over time falls below a threshold value.
