Improvement in Ultrasonic Velocity Measurement Techniques

摘要

A Pulsed Echo ultrasonic velocity measurement system with the design objective of resolving changes of time of 10 to the -11th power seconds (10 picoseconds) has been developed. Performance data detailed later in this report will show that, while the design objective has not yet been fully realized, the system is capable of resolving 40 picosecond changes and is therefore already an extremely valuable tool. The approach that has been utilized in developing the system can best be described as a 'brute force' time interval measurement. 'Time Interval' is measured from a D.C. 'Turn On' pulse (which triggers an R.F. Pulsed Oscillator) to a shaped, selected R.F. cycle from an acoustic echo later in the echo train (where the echo train is generated in an acoustic medium when an appropriate transducer coupled to the medium is driven by the R.F. Pulse Oscillator). The selected R.F. cycle is used as a 'turn off' pulse. The time interval is measured by means of a digital delay in steps of 100 nanoseconds (10 to the -7th power seconds) (to a maximum of 100 microseconds) and an analog vernier of 100 nanoseconds. The vernier can be expanded to a maximum sensitivity of 1 nanosecond full scale with a resolution of 10 picoseconds per division on a calibrated taut-band meter. (Author)