On narrow-band spectrum calculation by direct decimation

The frequency spectrum of a long dsta sequence over a narrow band can be efficiently computed by decimating the sequence using FIR filters, and calculating an FFT on the shortened output. This method offers a low computation rate, a small storage requirement, good frequency resolution, and adjustable accuracy [1]. The number of computations can be decreased for the lowpass case by relaxing the filter constraints [2]. This lowers the order of the filter necessary to attain a given level of accuracy. This paper extends the procedure proposed in [2] to bandpass filters. An algorithm is developed for choosing the best decimation rates and designing appropriate decimating filters for any chosen frequency band. The algorithm is used to derive tables of optimum decimation rates for a range of band positions and widths. The results demonstrate that direct decimation with a minimal passband constraint can improve on the results of [1].