Quadrature Demodulation Circuit for Two-directional Eddy Current Test of Defects in Steel Block

To solve the problem of two-directional eddy current signal demodulation, a quadrature demodulation circuit for low frequency range application was designed. The circuit includes a phase shifter, an analog multiplier and a low-pass filter. Their performances were tested in experiments. The circuit can shift the input signal (in the frequency range of 10-30 kHz) phase by 90° by adjusting the slide rheostat to modify the input and output impedance of the two-stage amplifier. The DC bias characteristics of the analog multiplier were also tested. Finally, the quadrature demodulation circuit together with the two-directional eddy current probe was applied for detecting notches with various depths in steel block. Results show that the DC bias voltage slightly fluctuates in its operation frequency range. The mutual correlation coefficient of the calculated and tested signal is about 0.955 1 after amplitude compensation being applied to the tested signal. The relative error in amplitude and phase demodulation results obtained from the circuit is estimated as 13% and 8%, respectively. A notch with a depth of 7 mm can be detected, and both the demodulated amplitude and phase of the eddy current signal tend to approximation increase when the depth of the notches increases.