An ultrasound generator comprising: a signal generator arranged to receive, generate or calculate when instructed a modulating signal with a magnitude that varies within a first range and to generate a pulsed drive signal having a predefined first relationship to the modulating signal, the pulsed drive signal having at least a zero output level, a positive output level and a negative output level, wherein the position and width of pulses are defined by at least first and second switching angles (δ 1, δ 2) per half cycle of the modulating signal; and a transducer arranged to generate ultrasound in response to the pulsed drive signal; wherein the first relationship is selected such that within at least part of the range of magnitude of the modulating signal the first and second switching angles (δ 1, δ 2) are adjusted simultaneously to provide for an increase or decrease in the magnitude of the fundamental frequency of the pulsed drive signal corresponding to an increase or decrease in the magnitude of the modulating signal; wherein the first relationship is selected such that a selected harmonic component of the generated pulsed drive signal is maintained below a first threshold; and wherein the first relationship is selected such that throughout part of the range of magnitude of the modulating signal one switching angle (δ 1, δ 2) increases while the other switching angle (δ 1, δ 2) decreases simultaneously to provide for an increase or decrease in the magnitude of the fundamental frequency of the pulsed drive signal corresponding to an increase or decrease in the magnitude of the modulating signal. a signal generator arranged to receive, generate or calculate when instructed a modulating signal with a magnitude that varies within a first range and to generate a pulsed drive signal having a predefined first relationship to the modulating signal, the pulsed drive signal having at least a zero output level, a positive output level and a negative output level, wherein the position and width of pulses are defined by at least first and second switching angles (δ 1, δ 2) per half cycle of the modulating signal; and a transducer arranged to generate ultrasound in response to the pulsed drive signal; wherein the first relationship is selected such that within at least part of the range of magnitude of the modulating signal the first and second switching angles (δ 1, δ 2) are adjusted simultaneously to provide for an increase or decrease in the magnitude of the fundamental frequency of the pulsed drive signal corresponding to an increase or decrease in the magnitude of the modulating signal; wherein the first relationship is selected such that a selected harmonic component of the generated pulsed drive signal is maintained below a first threshold; and wherein the first relationship is selected such that throughout part of the range of magnitude of the modulating signal one switching angle (δ 1, δ 2) increases while the other switching angle (δ 1, δ 2) decreases simultaneously to provide for an increase or decrease in the magnitude of the fundamental frequency of the pulsed drive signal corresponding to an increase or decrease in the magnitude of the modulating signal.