In practice – when performing eddy current tests – a series of interfering or unwanted signals can manifest themselves. Examples include:

1) variations in conductivity, thermal drift, mechanical vibrations, changes in geometry or lift-off signals. Usually these signals can appear over a longer period of time as defined reference defects (= low-frequency signals),

2) electromagnetic interferences or electronic noise from the test instrument, which usually appear for a shorter time than a defined reference defect (= high-frequency signals).

In a worst case scenario the different interference signals can occur simultaneously, so they overlap in such a way that it is impossible to detect and assess the signals of interest (e.g. crack indications) at all anymore.

By filtering, it is possible to weaken or eliminate certain frequency components contained in the demodulated signal.

To be able to suppress specific interference signals, the following conditions must be met:

* Firstly, the frequency spectrum of the signals of interest and that of the interference signals to be suppressed must be known.

* Secondly they must differ from one another sufficiently.

* Moreover, the testing speed must be constant (time-based filter).

This way, pseudo-indications or misinterpretations can be avoided and consequently the reliability of test conclusions increased.

Available for filtering are the filter types: high-pass, low-pass and band pass.