LV Signal Qualification Using Complex Tranform Results

Much effort has been applied to using the Fourier transform to determine the Doppler frequency contained in an unknown burst. This process consists of digitizing the time signal, performing a fast Fourier transform, and looking for the highest spectral peak. Since there is not sufficient resolution in the line spectra to provide an adequate estimate of the Doppler frequency, interpolation techniques are applied to measure the peak in the spectra, when, theoretically, it lies between the frequencies defined by the transform. This interpolation process is normally based on the magnitude of the spectra near the peak, and the shape of the curve realized by the transform. Even though, in many cases, a high-resolution complex transform has been performed, the phase information has been discarded. The phase information, however, is of significant value. In fact, it is possible to estimate the frequency of an under-sampled signal after aliasing, by evaluating the phase relationships of the transform data (I). This paper examines the phase produced by the fast Fourier transform for different frequencies between the line spectra. The phase measurement itself is compliceted by the fact that the sampling is not synchronized to the Doppler signal. Nevertheless, the variation in phase is used to predict the actual Doppler frequency, when its approximate location is known by the peak in the spectra. For this work the parabolic estimator is used to interpolate the Doppler frequency from the transform signal, and the error is plotted as a function of the position between the Spectra. With the phase estimator applied also, it is shown that the error in the estimate is reduced. Phase information used to estimate Doppler frequency provides the opportunity not only to improve the accurracy of LV signal processing instrumentation, but also offers the possibility of improving the signal processing rate for these instruments. Estimate of the error when noise is present in the signal has not yet been performed. Further work will concentrate on research in this area. This manuscript would not be completed on time. If you are interested in it, please contact : ICIASF '93 Mr. Jay R. MURPHY Macrodyne Incorporated P.O. Box 376 CLifton Park New York, 12065, USA Tel. (518) 383 3800 FAX (518) 383 0049