Abstract: It is proved by theory and experiments in this paper that (a) the results of step-progressive accumulation (SPA) is essentially a narrow band-pass filter. The line-shape is a like-sinc function. In the single pulse case, the amplitude of the selected peak will approach to a maximum value with the increment of the accumulation number N. For two pulses case, the maximum amplitude appears only in the optimum N, which depends on the step-length of SPA. (b) The realization of so-called chemical shifts filter (CSF) depends on the offset used in experiment and the coupling of spin system. For weakly coupling spin system, CSF can be observed only if the offset is put on the chemical shift of this system. When the offset is set on one peak of the spectrum, only this peak will be observed. For the strong coupling spin system, CSF is no longer valid. At last, the choice of parameters in SPA experiments and some applications are discussed.

Key words: Step-progressive accumulation, Narrow Band-pass filter, Chemical shifts filter