PURPOSE: To collect a defect wave irrespective of the change of the surface state by holding a peak value of an echo detecting signal corresponding to the measuring cycle, attenuating the peak value thereby generating a threshold value, and detecting a detecting signal of the next measuring cycle with using the threshold value.
CONSTITUTION: An input signal upon receipt of an echo is extracted only for a certain period by a gate circuit 16 in accordance with a gate signal from an activated gate pulse generating circuit 14a, which is held at a peak hold circuit 17. The output of the circuit 16 is input also to a comparator 18. A threshold value VTH which is a divided voltage of an output of a sample hold circuit 19a by a level setting circuit 19b as a variable resistor is input to the other input terminal of the comparator 18. A detecting signal P output from the comparator 18 triggers a gate pulse generating circuit 14b and enters a time-width variable pulse generating circuit 20, with generating a sample hold signal H. Since a next threshold value VTH is set by a peak value of the one earlier measuring cycle held by the circuit 17, it allows the detecting signal P to follow the detecting level even when it is decreased, thus realizing the detection at the level.
TOKUDA YASUSHI
ONO MASANOBU
