说明：  这类本征模态函数的瞬时频率(Instantaneous Frequency ,IF)有着明确的物理意义。因此,经验模态分解后, 对每一个IMF作希尔伯特变换( Hilbert Transform,HT)，继而可求取每一个IMF的瞬时频率。
(The instantaneous frequency (if) of this kind of intrinsic mode function has a clear physical meaning. Therefore, after empirical mode decomposition, the instantaneous frequency of each IMF can be calculated by Hilbert transform (HT).)