Automatic system for dynamic control of resonance in high power and high Q ultrasonic transducers

This paper describes the fundamentals, design criteria and electronic structure of a new frequency control system to keep permanently at resonance high Q ultrasonic transducers whose mechanical resonance band may vary within wide limits under normal operating conditions. The procedure developed is based on keeping constant at its zero value the phase of the motional admittance of the transducer by automatically and instantaneously adjusting the frequency of the driving signal provided by a voltage controlled oscillator. The characteristics of this system, especially the fact that the transducer is not an integral part of the feed-back loop of the oscillatory circuit and the frequency tracking mechanism does not depend directly on the magnitude of the motional variables of the transducer, offer some advantages in construction and performance with respect to the conventional motional positive feed-back systems.