How VHF Acoustics can Help Process Underwater Acoustic Signals
Surface Acoustic Wave (SAW) techniques allow to build analog Fourier transformers and convolvers able to process wideband (up to 100 to 500 MHz) limited duration (10–100 µs) signals. The processing speed is equivalent to 109 to 1010 floating point operations per second.
They exhibit 10–12 bit (60–80 dB) dynamic range and a floating point type accuracy of 6 to 8 bits. The major drawback is the lack of memory and this is why it is very often useful to place digital interfacing circuits around the SAW processor to be able to employ the digital memories.
Addition of these circuits limit the useful bandwidth to a few tens of MHz but adds a lot of flexibility: One can process non simultaneous signals and signals with time, bandwidth characteristics very different from those of the analog acoustic devices. Underwater acoustic signalsare a good example.
Several applications belonging to the sonar field are presented. They require high computation speeds like those quoted earlier when real time operation and small volume and power are needed. Fast Fourier analysis can be used to perform beamforming. This requires fast multiplexing of the various input signals. Multi-channel spectrum analysis is another application where input memories perform time scale compression. In both instances, two-dimensional schemes exist and are discussed. Finally correlation techniques are briefly presented. They always require digital compatibility and offer interesting solutions to the problem of signature analysis for instance.
These are not new concepts but SAW devices and compatible digital circuits have only been recently available. Few implementations have been tried in the underwater acoustics field but it seems already clear that interest exists when information rate is large enough: a lower bound can be set around 200 kword/s today.
KeywordsQuartz Microwave Lithium Radar Convolution
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