Abstract: The invention relates to a circuit for determining the arrival time of an ultrasonic signal. The circuit includes a differentiator for transforming an ultrasonic signal to a substantially linearly increasing signal, and a threshold value detector for transforming the linearly increasing signal to an arrival delay signal. A phase detector in the form of a D-type flip-flop determines the phase position of the arrival delay signal relative to a reference signal. The phase detector generates early signals on early arrival of the delay signal and late signals on late arrival of the delay signal. A regulator integrator is provided between the phase detector and the differentiator for producing an output voltage for altering the delay period of the arrival delay signal responsive to the phase position of the delay signal.