A monolithic time of arrival detector for acoustic signals

This paper describes the design and implementation of a time of arrival detector for long-range underwater acoustic signals. The intended application of this device is a tag for acoustically tracking aquatic animals, but it is suited for computing general low-frequency signal arrival times in an underwater environment. This device is optimized for low SNR as well as Doppler insensitivity. The two main components of the detector are an analog multi-rate filter, and a digital correlator. The analog filtering is realized by three second order switched-capacitor filter stages. The digital correlation consists of 2048 bits of static RAM (SRAM): 1024 bits to store the expected pattern and 1024 as a virtual shift register. Due to the nature of the intended application, power consumption must be kept to a minimum. Two prototype dies have been fabricated by a 0.5 μm double-poly CMOS process. The filters were shown to operate at under 1.5 μA per stage and the SRAM to operate correctly.