A CCD chirp-Z FFT Doppler signal processor for laser velocimetry

Abstract

A charge-coupled Chirp-Z fast Fourier transform device, which has been configured to obtain the frequency of peak spectral amplitude in signals from laser velocimeters, is described for use in oceanographic and other low laser power field applications where weak optical signals and frequent dropout are encountered. With sufficient bandwidth to resolve photon arrivals, and with a pulse-height discriminator, the processor is equivalent to a photon-correlator. The resulting signal processor gives 8-bit resolution in the velocity word, with a cycle time of approximately 4 ms while scanning a spectral range of 0-125 kHz; higher frequency ranges are possible at proportionately faster spectral scan rates, up to a maximum of 2 MHz. Error estimates are presented which indicate 2-bit accuracy at a signal-to-noise power ratio of -2 dB. Intercomparison data with discontinuous signals fed to this processor and a commercial frequency tracker are presented. The results demonstrate, conclusively, the superior accuracy of this processor.