Time base measurement circuit

Claim 1

A time base measurement circuit for providing a time measurement electrical signal proportional to the time required for an ultrasonic energy signal to pass into and then out of an object being inspected, comprising, in combination: a transmitting transducer providing said ultrasonic energy signal; drive means for periodically driving said transmitting means; delay means connected to said drive means and responding thereto to provide a start time period signal at a time substantially equal to the time when said transmitted acoustical energy signal enters the object being inspected; a receiving transducer responding to receipt of acoustical energy from such an object being inspected to provide an echo signal; echo signal detection means responding to the amplitude and phase of said echo signal to provide a stop time period signal only upon occurrence of a predetermined phase condition of said echo signal, said echo signal detection means including a level detector and a zero crossover detector, and means for combining the outputs of said level detector and said zero crossover detector to provide said stop time period signal, said combining means including an OR circuit connected at one input to said level detector; means responding to said start and stop time period signals to provide said time measurement electrical signal, the duration of which is substantially equal to the time being measured; analog means for converting said time measurement electrical signal to an analog signal proportional to the duration of the time period being measured, said analog means including voltage storage means responding to said start time period signal and increasing in voltage at a substantially linear rate from a low voltage value at the time of receipt of said start time period signal to a higher voltage to provide said analog signal at the time of receipt of said stop time period signal, said voltage storage means comprising a capacitor and further including a constant current source for charging said capacitor at a linear rate, said constant current source turning on in response to receipt of said start time period signal and turning off in response to the receipt of said stop time period signal, and said voltage storage means being connected to said drive means to respond to the excitation of Said transmitting transducer to discharge said capacitor to said low voltage value prior to receipt of said start time period signal; and means for comparing the level of said analog signal with a plurality of similar signals to provide an output signal proportional to the smallest of the compared signals.