The digital four frequency eddy current instrument is integrated in a computer. Its eddy current measuring portion is composed of two integrated circuit boards inserted in two slots of the computer, but it has powerful functions to carry out the multi- frequency eddy current examination in data acquisition and data analysis. Four frequencies of 10 kHz to 2 MHZ produced by the digital four frequency instrument can sequently be applied to two differential probes for differential and absolute eddy current detection caused by a defect in a tube. Sixteen components (x and y) of four frequency differential and absolute eddy current signals can be digitized into the computer. Two components (x and y) and four kinds of eddy current signal impedance planes can be set up from them and be displayed on the screen of the computer terminal according user's selections. The frequency, the gain, and the phase angle of each frequency differential and absolute eddy current signals can be changed by the user during the eddy current data signals acquisition.

The software of the digital four frequency eddy current instrument has been designed for user's convenience. Mixing of eddy current signals of two frequency differential or two frequency absolute channels can be automatically made for restricting some effects on the defect detection and an accurate mixing can be achieved to better understand the eddy current signals of a defect affected by a specific factor, for example to restrict the tube supporting plate effect on defect detection in the tube under it. In order to analyse the strange eddy current signal from a defect software can study the signals point by point. The software will be improved further according to user's experience feedback.