Pre-processing methods for parametric array to generate wideband difference frequency signals

One of the advantages of parametric array is the convenience to generate low-frequency wideband signals in difference frequency components. Several kinds of signals usually used in linear active sonar generated by nonlinear self-demodulation of primary waves are studied in this paper. Emphases are on the pre-processing methods and the spectrum characteristics before and after self-demodulation process related to these methods. Analyses and simulations show that the inverse processing, which includes a double integration and a square-root operation, is with no practical value because the integration shifts most of the signal power to the very low frequency components, and the useful signal generated from the nonlinear acoustic interaction whose frequencies are relatively much higher is very weak. For generating LFM signal in the secondary acoustic field of parametric array, a LFM waveform divided by its instantaneous frequency to be the envelope of primary wave is suitable. For generating digital modulated signals from the self-demodulation process, PAM signals with M=2 and zero as one of the two symbols have much better performance than BPSK signals. CFPSK signals are also suitable for digital signal transmission through the nonlinear self-demodulation process of parametric array. Simulation also shows that Ricker wavelet can be generated by simply using half-cosine wave passing through a low pass filter as the envelope of primary wave.