Electronic crosstalk and linearity in the acoustic travel-time current meter, MAVS

Differential travel-time measurements of bidirectional transmission of sound between two fixed transducers underwater is the principle of the travel-time acoustic current meter. Sound is transmitted simultaneously in opposite directions and the difference in arrival time is a measure of the component of flow velocity along the axis of propagation. Electronically, the detection of this difference in arrival time involves voltage comparators and fast switching. Leakage of switching current into the detection circuit may bias the measurement of arrival time when the delay between the two directions of acoustic propagation is comparable to the propagation delay in the detection circuit. In 1980 such an effect was detected and removed in the Benthic Acoustic Stress Sensor, BASS, by careful separation of power and signal grounds. In 2003 a slight relaxation of this ground separation appeared to have caused another instance of this effect in the Modular Acoustic Velocity Sensor, MAVS. The detection circuit, a voltage comparator with a TTL output, has a 17 ns propagation delay causing a voltage ´glitch´ to appear on the ground line 17 ns after the first signal crosses zero. This ´glitch´ distorts the second signal to arrive at the moment of its zero crossing when the speed of current measured along the acoustic axis is 20 cm/s in MAVS. Electronically simulated errors in speed have been as much as 12% near 20 cm/s, returning to 0.5% by 60 cm/s. The search for a cure revealed that a totally different cause was responsible for this nonlinearity and the story of this search bears repeating.