An ultrasonic signal generator, comprising: a drive module to generate a modulated drive signal, the drive module comprising: means for providing a modulation waveform; a current set point module configured to generate a current set point signal; a modulator comprising a first input to receive the current set point signal and a second input to receive the modulation waveform, the modulator configured to generate a modulated signal by modulating the modulation waveform onto the current set point signal; and a current control loop coupled to the modulator to receive the modulated signal and to control the modulated drive signal; wherein the modulation waveform is a time varying signal comprising: a time varying modulation amplitude A m component, or a time varying modulation frequency f m component, or a time varying modulation amplitude A m component and a time varying modulation frequency f m component; wherein the modulation waveform is defined by the following relationship: S m = 1 + A m ⋅ sin 2 π ⋅ f m ⋅ t wherein a distal end of an end effector coupled to the modulator via a transducer is movable in response to the modulated drive signal. a drive module to generate a modulated drive signal, the drive module comprising: means for providing a modulation waveform; a current set point module configured to generate a current set point signal; a modulator comprising a first input to receive the current set point signal and a second input to receive the modulation waveform, the modulator configured to generate a modulated signal by modulating the modulation waveform onto the current set point signal; and a current control loop coupled to the modulator to receive the modulated signal and to control the modulated drive signal; means for providing a modulation waveform; a current set point module configured to generate a current set point signal; a modulator comprising a first input to receive the current set point signal and a second input to receive the modulation waveform, the modulator configured to generate a modulated signal by modulating the modulation waveform onto the current set point signal; and a current control loop coupled to the modulator to receive the modulated signal and to control the modulated drive signal; wherein the modulation waveform is a time varying signal comprising: a time varying modulation amplitude A m component, or a time varying modulation frequency f m component, or a time varying modulation amplitude A m component and a time varying modulation frequency f m component; wherein the modulation waveform is defined by the following relationship: S m = 1 + A m ⋅ sin 2 π ⋅ f m ⋅ t wherein a distal end of an end effector coupled to the modulator via a transducer is movable in response to the modulated drive signal. a time varying modulation amplitude A m component, or a time varying modulation frequency f m component, or a time varying modulation amplitude A m component and a time varying modulation frequency f m component; wherein the modulation waveform is defined by the following relationship: S m = 1 + A m ⋅ sin 2 π ⋅ f m ⋅ t wherein a distal end of an end effector coupled to the modulator via a transducer is movable in response to the modulated drive signal.