| 摘要 |
A digital synthesizer generates an electrical digital carrier that drives probe coil to generate an electromagnetic wave propagated into a test material proximate t he probe coil. A return electromagnetic wave generated by eddy currents in the materi al includes signatures of material defects modulated on the return carrier electromagnet ic wave. The return wave is detected by one or more probe coils, amplified, converted fro m an analog signal to a digital signal and then digitally mixed with digital sine and cosine functions also generated by the digital synthesizer to yield sum and difference values , mathematically expressing various eddy current signals received by the probe in a complete set of orthogonal functions. A low pass filter then removes all but the difference values. A direct current reference component is subtracted from t he mixed digital signal, which translates the signal to center about a zero axis for ease of display and analysis.
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