An apparatus for driving an end effector coupled to an ultrasonic drive system of a surgical instrument, the apparatus comprising: a generator configured to couple to an ultrasonic drive system of an ultrasonic instrument, the ultrasonic drive system comprising an ultrasonic transducer coupled to a waveguide and an end effector coupled to the waveguide; wherein the generator is configured to: generate a first ultrasonic drive signal; actuate the ultrasonic transducer with the first ultrasonic drive signal for a first period; generate a second ultrasonic drive signal by the generator; and actuate the ultrasonic transducer with the second ultrasonic drive signal for a second period, subsequent to the first period; wherein the first drive signal is different from the second drive signal over the respective first and second periods; and wherein the first and second drive signals define a step function waveform over the first and second periods, the apparatus further comprising: a measurement module to monitor a measurable characteristic of the ultrasonic drive system, wherein the generator is configured to generate any one of the first and second drive signals based on a measured characteristic, wherein the measurement module comprises: a radio frequency generator coupled to the end effector, the radio frequency generator configured to generate a subtherapeutic RF signal; and a clamp arm assembly coupled to a distal end of the ultrasonic drive system, the clamp arm assembly comprising an electrically conductive portion forming a return electrode coupled to the RF generator, the clamp arm assembly being operatively coupled to the end effector to grasp tissue therebetween. a generator configured to couple to an ultrasonic drive system of an ultrasonic instrument, the ultrasonic drive system comprising an ultrasonic transducer coupled to a waveguide and an end effector coupled to the waveguide; wherein the generator is configured to: generate a first ultrasonic drive signal; actuate the ultrasonic transducer with the first ultrasonic drive signal for a first period; generate a second ultrasonic drive signal by the generator; and actuate the ultrasonic transducer with the second ultrasonic drive signal for a second period, subsequent to the first period; wherein the first drive signal is different from the second drive signal over the respective first and second periods; and wherein the first and second drive signals define a step function waveform over the first and second periods, the apparatus further comprising: a measurement module to monitor a measurable characteristic of the ultrasonic drive system, wherein the generator is configured to generate any one of the first and second drive signals based on a measured characteristic, wherein the measurement module comprises: a radio frequency generator coupled to the end effector, the radio frequency generator configured to generate a subtherapeutic RF signal; and a clamp arm assembly coupled to a distal end of the ultrasonic drive system, the clamp arm assembly comprising an electrically conductive portion forming a return electrode coupled to the RF generator, the clamp arm assembly being operatively coupled to the end effector to grasp tissue therebetween. generate a first ultrasonic drive signal; actuate the ultrasonic transducer with the first ultrasonic drive signal for a first period; generate a second ultrasonic drive signal by the generator; and actuate the ultrasonic transducer with the second ultrasonic drive signal for a second period, subsequent to the first period; wherein the first drive signal is different from the second drive signal over the respective first and second periods; and wherein the first and second drive signals define a step function waveform over the first and second periods, the apparatus further comprising: a measurement module to monitor a measurable characteristic of the ultrasonic drive system, wherein the generator is configured to generate any one of the first and second drive signals based on a measured characteristic, wherein the measurement module comprises: a radio frequency generator coupled to the end effector, the radio frequency generator configured to generate a subtherapeutic RF signal; and a clamp arm assembly coupled to a distal end of the ultrasonic drive system, the clamp arm assembly comprising an electrically conductive portion forming a return electrode coupled to the RF generator, the clamp arm assembly being operatively coupled to the end effector to grasp tissue therebetween. a radio frequency generator coupled to the end effector, the radio frequency generator configured to generate a subtherapeutic RF signal; and a clamp arm assembly coupled to a distal end of the ultrasonic drive system, the clamp arm assembly comprising an electrically conductive portion forming a return electrode coupled to the RF generator, the clamp arm assembly being operatively coupled to the end effector to grasp tissue therebetween.