Circuit for use in a surgical operation including a phase lock loop having a voltage controlled oscillator, and a phase comparator for comparing a phase of a voltage of a driving signal and a phase of a signal representing the vibration phase of the ultrasonic transducer to derive a phase difference...http://www.google.es/patents/US4965532?utm_source=gb-gplus-sharePatente US4965532 - Circuit for driving ultrasonic transducer
