Design of Orthogonal Lock-In Amplifier for Weak Eddy Current Signal Detection

Eddy current test is a kind of general non-destructive test technique which is widely adopted in a range of industrial applications and the detection of the output weak signal of eddy current sensors is a key element in test system. An analogy orthogonal lock-in amplifier was designed based on cross correlation theory. It applied direct digital synthesizer AD9854 to generate orthogonal reference signals, analogy multiplier AD734 to conduct cross correlation operation and low pass filter MAX291 to get the in-phase direct current component and the orthogonal direct current component. By processing these two direct current components, variations of eddy current signal amplitude and phase could be gotten, then, the character of UUT could be judged according to these variations. The eddy current test experiments showed that this amplifier could be applied in eddy current test system conveniently and reliably, and the work frequency could approach to 5 MHz.