Double reference pulsed phase locked loop

摘要

A double reference pulse phase locked loop for measuring the phase shift between tone burst signals initially derived from the same periodic signal source (voltage controlled oscillator 16) and delayed by different amounts because of two different paths. A first path is from transducer 12 to surface 14 of sample 11 and back, and a second path is from transducer 12 to surface 15 and back. A first pulse phase locked loop including phase detector 26 and phase shifter 22 forces the tone burst signals delayed by the second path in phase quadrature with the periodic signal source. A second pulse phase locked loop including phase detector 21 forces the tone burst signals delayed by the first path into phase quadrature with the phase shifted periodic signal source.