High voltage arbitrary waveform generator (1) comprising: - a voltage-controlled oscillator (3) arranged to output a clock signal (CLK) and an inverted clock signal (Formula (I)) at a frequency determined by at least one control voltage (Formula (II)); - a first charge pump (7) arranged to receive said clock signal (CLK) and said inverted clock signal (Formula (I)) and to increase an output voltage (Vout) at an output node (10) in response to said clock signal (CLK) and said inverted clock signal (Formula (I)); - a second charge pump (9) arranged to receive said clock signal (CLK) and said inverted clock signal (Formula (I)) and to decrease said output voltage (Vout) at said output node (10) in response to said clock signal (CLK) and said inverted clock signal (Formula (I)); - a selector adapted transmit said clock signal (CLK) and said inverted clock signal (Formula (I)) to either said first charge pump (7) or said second charge (9) pump at any given time in response to a select signal (Select); - a load (CL) connected between said output node (10) and ground.