Low-pass filters are used to suppress interfering high-frequency signal components of the frequency spectrum, while frequencies below a lower cut-off frequency (i.e. the signals of interest) remain unaffected (see also bandwidth). This type of filter is also known as static filter, because the frequency spectrum of the demodulated signal also contains a constant (static) component.
In eddy current testing low-pass filters are used to suppress high-frequency electromagnetic interference, for example, but also electronic device noise.
It should be noted that the frequency spectrum of the interference signals and the signals of interest depend on both, the actual testing speed as well as on the type and geometry of the probe used:
1) the faster the testing speed and the smaller the effective coil width, the higher the frequency of the signals (-> “shorter pulses”),
2) for lower testing speeds and for relatively large coil dimensions the frequencies of the signals are correspondingly lower (-> “longer pulses”).
The lower cut-off frequency of the low-pass filter is correct (i.e. set high enough), if the unwanted signals are suppressed effectively, but the signals of interest are still displayed at maximum signal level.
The minimum lower cut-off frequency fTPmin for the low-pass filter can be approximately determined with the following formula:
fTPmin > vtest / Bw (with: vtest = test speed and BW = effective coil width).