A method and apparatus for cleaning a surface of a microelectronic substrate. The first control unit and the second control unit have a signal generator and a signal synthesizer circuit. In one disclosed embodiment, a voltage pump generates a pumped voltage and a voltage regulator provides a regulated voltage, where the pumped voltage is characterized as being either more positive than the most positive externally-applied positive voltage supply signal or more negative than the most negative externally-applied negative voltage signal. A magnitude of a discontinuous component in a frequency domain belonging to the block boundary is adjusted based on a magnitude of a corresponding discontinuous component selected from a pixel contained entirely within a block adjacent the block boundary. At least one of the first and second axial positions is located in the downhole end.