In this paper we investigate the frequency resolution of empirical mode decomposition (EMD) and explore means to improve it through a series of numerical experiments. Using a compound signal composed of two pure tones, we find the marginal frequency ratio at which EMD can no longer separate them into different intrinsic mode functions (IMFs). Based on this observation we determine the optimal frequency for a masking signal, the application of which can not only alleviate mode mixing but also enhance frequency resolution of EMD.
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