An ultrasonic surgical generator, comprising: a variable frequency oscillator configured to generate an oscillator output signal having a frequency determined according to an oscillator control signal; a variable gain amplifier operably coupled to the oscillator and configured to amplify the oscillator output signal by an amount determined according to an amplifier control signal to generate a driving signal; a controller operably coupled to the oscillator and the amplifier, and configured to provide an oscillator control signal and an amplifier control signal in response to a user input; a user interface operably coupled to the controller and adapted to convey a user input thereto; an actuator input operably coupled to the controller and adapted to received an actuation signal from an actuator, and to convey the actuation signal to the controller; and an ultrasonic instrument output adapted to deliver the ultrasonic driving signal to an ultrasonic instrument, further comprising at least one sensor (70, 72) configured to sense an operating parameter of the variable frequency oscillator or the variable gain amplifier, and to provide a sensor signal corresponding to the sensed parameter to the controller, characterised in that the sensor is a zero crossing detector, and wherein the controller is configured to provide an oscillator control signal or an amplifier control signal in response to the sensor signal, wherein the ultrasonic surgical generator is further configured so that the variable frequency oscillator generates the oscillator output signal having a first carrier frequency, and the variable gain amplifier modulates the amplitude of the oscillator output signal to generate the driving signal including a plurality of carrier frequency bursts, and configured so that the variable frequency oscillator generates the oscillator output signal having a second carrier frequency, wherein successive carrier frequency bursts alternate between the first carrier frequency and the second carrier frequency. a variable frequency oscillator configured to generate an oscillator output signal having a frequency determined according to an oscillator control signal; a variable gain amplifier operably coupled to the oscillator and configured to amplify the oscillator output signal by an amount determined according to an amplifier control signal to generate a driving signal; a controller operably coupled to the oscillator and the amplifier, and configured to provide an oscillator control signal and an amplifier control signal in response to a user input; a user interface operably coupled to the controller and adapted to convey a user input thereto; an actuator input operably coupled to the controller and adapted to received an actuation signal from an actuator, and to convey the actuation signal to the controller; and an ultrasonic instrument output adapted to deliver the ultrasonic driving signal to an ultrasonic instrument, further comprising at least one sensor (70, 72) configured to sense an operating parameter of the variable frequency oscillator or the variable gain amplifier, and to provide a sensor signal corresponding to the sensed parameter to the controller, characterised in that the sensor is a zero crossing detector, and wherein the controller is configured to provide an oscillator control signal or an amplifier control signal in response to the sensor signal, wherein the ultrasonic surgical generator is further configured so that the variable frequency oscillator generates the oscillator output signal having a first carrier frequency, and the variable gain amplifier modulates the amplitude of the oscillator output signal to generate the driving signal including a plurality of carrier frequency bursts, and configured so that the variable frequency oscillator generates the oscillator output signal having a second carrier frequency, wherein successive carrier frequency bursts alternate between the first carrier frequency and the second carrier frequency.