A pulse compression procedure for the measurement and characterization of non-linear systems based on exponential chirp signals

Ultrasonics diagnostic and non-destructive evaluation techniques exploiting non-linear phenomena such as harmonics and sub-harmonics generation, non-linear propagation, etc., are more and more diffused due to the benefits they assure in terms of spatial resolution and SNR for specific noise components. Nonetheless some issues still hamper a widespread diffusion of non-linear ultrasonics applications mainly due to the low energy associated to the harmonics or sub-harmonics generation or to the high excitation power needed to trigger the non-linearities. Despite the better resolution, the resulting inspection range of such techniques is typically lower than that of common procedure relying on the assumption of “linearity” of the system. Strategies able to increase the inspection capability are therefore desirable. At the same time it is worth trying to exploit all the information enclosed in the linear and non-linear response instead of using only a part of it. For this reason, in the present paper we review a pulse-compression procedure based on exponential chirp developed for the characterization of the acoustic distortion that could be extremely useful in non-linear ultrasonics applications. The measurement procedure provides the benefit of Pulse Compression in term of SNR enhancement and contextually allows the responses associated to the different harmonics to be easily separated. A detailed theoretical discussion of the technique is reported, and it is also explained how to extend the procedure to deal with sub-harmonic generation.