Abstract: A drive network for an ultrasonic probe includes a network operative to apply a pulse to a feedback terminal of a phase comparator substantially simultaneously with the application at the probe of an excitation signal from a source at a predetermined frequency. This insures that at the start-up of the drive network the frequency of the feedback signal to the phase comparator leads the excitation signal also being applied thereto, to cause the excitation signal from the source to increase toward the resonant frequency of the probe, thereby to search for the resonant frequency of the probe.