Chirp signal matching and signal power optimization in pulse-echo mode ultrasonic nondestructive testing

Chirp pulse compression is a signal correlation technique that uses frequency modulated pulses as transmitted signals. Usually, signals with linear frequency modulation are applied. They can be generated rather easily, but their spectra are not totally matched to the transfer function of ultrasonic systems. In pulse-echo mode operation, with signal duration and consequently the time-bandwidth product being critical parameters, waveforms should be applied which make full use of the available power and bandwidth resources. We report here two methods to improve the overall efficiency of an ultrasonic pulse-echo system. Transmitter signals with constant amplitude level and nonlinear frequency modulation can be generated in such a way that they are spectrally matched to the system. A formula for the calculation of such a matched nonlinear chirp signal is presented. This modulation scheme also leads to a side-lobe level reduction of the compressed pulses. The application of square wave chirps derived from sine type chirps yields an additional gain of echo signal amplitude. Moreover, the complexity of the signal generation hardware is reduced. The methods are illustrated by an example.<
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