T.O. 33B-1-14-35Figure 4-32. Effect of Material Variables on Magnitude of Alternating Current in Test Coil with ConstantScanning Speed.4.3.6.2 Filters.Filtering is often used to improve the signal-to-noise ratio in the eddy current signal display as illustrated in Figure 4-33. Three types of filters can be used; the high-pass, the low-pass, and the band-pass. High pass filtering removes thelow frequency components of the eddy current signal from the bridge. This type of filtering can eliminate the effect ofgradual variations in conductivity or dimensions on the eddy current inspection response. Low pass filtering removesrapid (high frequency) response from electronic noise and from harmonic frequencies related to variations in magneticpermeability. Band pass filters allow a response over a specific range of frequencies and suppress frequencies aboveand below this range.
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