Pulse Compression in Nondestructive Testing Applications: Reduction of Near Sidelobes Exploiting Reactance Transformation

Pulse compression (PuC) is exploited in several applications, where the impulse response of a linear system must be estimated in a noisy environment. In nondestructive testing (NDT), PuC based on frequency-modulated signals is applied with sensors of different types. The presence of sidelobes in the impulse response retrieved after the PuC is a major drawback of the method as it degrades the quality of the estimation. To limit this effect, the matched filter is usually shaped by means of windows. Here, a windowing function based on reactance transformation is proposed that outperforms the most used ones in terms of near-sidelobes level reduction and sidelobes oscillations suppression. Numerical and experimental data were used to validate the technique. It is shown that the proposed approach is particularly suitable for those NDT techniques, such as eddy current and thermography that use very broadband excitations. The proposed window function is first introduced and applied to linear frequency-modulated signals and then extended to nonlinear frequency-modulated signals, which are needed in some applications. The quite general framework introduced in this paper for designing frequency-modulated signals and optimizing sidelobes is of general validity and can be successfully exploited in any application relying on chirp PuC.