Abstract: A high voltage modulator is shown to be made up of: (a) a square wave generator adapted to produce signals with adjustable duty cycles at different repetition frequencies; (b) a pair of switching transistor assemblies arranged to connect either a high positive voltage or a high negative voltage to an output line; and (c) switch actuating circuitry optically coupled to the square wave generator to produce actuating signals for the pair of switching transistor assemblies in accordance with the signals out of the square wave generator, the switch actuating circuitry further being adapted to reduce to a minimum the time during which the high positive and high negative voltages may simultaneously be connected to the output line.