Electronically Scanned and Focused Receiving Array

Abstract

A ninety element electronically scanned and focused receiving array has been built and tested. The discrete transducer elements are formed by partial dicing of PZT-5 ceramic bars. Focused beamforming is accomplished by a hybrid analog-digital electronic system. Correct phasing of the transducer elements is achieved by multiplying the received 500 kHz acoustic signals by a low frequency FM sweep or “chirp” signal. The chirp, whose frequency range determines the focal distance, is generated by a VCO, digitized, and entered into a four-level digital delay line ninety units in length. An output tap on the delay line is provided for each hydrophone. Multiplication of the chirp and the acoustic carrier is accomplished by an integrated circuit digital-to-analog multiplier. The current outputs of the multipliers are added by a network of summing amplifiers. After high pass filtering, an amplitude modulated 500 kHz carrier remains. Single sideband demodulation is used to produce the scanning beam output. This signal is then rectified and low-pass filtered and can be used to intensity modulate a CRT display. The system can focus down to a range of less than one meter and produces a resolution of the order of one centimeter at that distance.