An advanced signal processor has been developed to enable high-resolution measurement of the frequency and phase shifts of noisy laser Doppler velocimeter (LDV) and phase Doppler particle analyzer (PDPA) signals. The purpose of the measurements is to exploit the phase Doppler principle to determine the sizes and velocities of droplets entrained in airflows. By enabling such measurements, this signal processor is making it possible to perform ground-breaking research in icing of airplanes and combustion of liquid fuels, involving (1) water-droplet-laden, high-speed airflows that simulate clouds flowing past airplanes and (2) high-speed, dense sprays of liquid fuels, respectively.
The phase Doppler signals of interest are bursts with transit times
The present signal processor operates in real time. It includes two analog-to-digital converters that operate at a sampling frequency of 160 MHz and at a phase shift of 90° relative to each other, so that they offer an effective sampling frequency of 320 MHz. Signals at frequencies up to 150 MHz can be measured. Processing involves the use of Fourier-transform techniques to detect bursts and to retrieve signals from a noisy background (signal-to-noise ratio as low as -5 dB). Furthermore, accurate measurements can be taken in limited numbers of samples, making it possible to extract useful information in transit times as small as 0.25 µs.
This work was done by Larry Berkner of TSI Inc. and Khalid M. Ibrahim and William D. Bachalo formerly of TSI Inc. for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4—8
21000 Brookpark Road
Refer to LEW-16993.