A digitally-controlled instrument for conducting single-frequency and swept-frequency ultrasonic phase measurements has been developed based on a constant-frequency pulsed phase-locked-loop (CFPPLL) design. This instrument uses a pair of direct digital synthesizers to generate an ultrasonically-transceived tone-burst and an internal reference wave for phase comparison. Real-time, constant-frequency phase tracking in an interrogated specimen is possible with a resolution of 0.00038 radians (0.022°), and swept-frequency phase measurements can be obtained. Using phase measurements, absolute thickness in borosilicate glass is presented to show the instrument’s efficacy, and these results are compared to conventional ultrasonic pulse-echo time-of-flight (ToF) measurements. The newly-developed instrument predicted thickness with a mean error of −0.04 μm and a standard deviation of error of 1.35 μm. By showing higher accuracy and precision than conventional pulse-echo ToF measurements, the new digitally-controlled CFPPLL instrument provides high-resolution absolute ultrasonic velocity or path-length measurements in solids or liquids, as well as tracking of material property changes with high sensitivity. In addition to improved resolution, swept-frequency phase measurements add useful capability in measuring properties of layered structures, such as bonded joints, or materials which exhibit non-linear frequency-dependent behavior, such as dispersive media.
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