我们知道,在对信号或系统作频域分析时,富里叶变换起着关键性的作用.频谱分析的例子有:信号频率的精确测定、稳定弱信号的检测、频谱精细结构的分析、随机振动的测量等等.但是,富里叶变换只能用于稳定信号和线性非时变系统的场合.对于非稳定信号和线性时变系统,富里叶变换就显得无能为力了,这是因为富里叶变换在实际应用时和在概念上都存在着某些限制.例如,为了要确定信号频谱在某一频率(仅仅是一个频率)上的强度,必须要知道此信号在整个时间 We know that Fourier transform plays a key role in frequency-domain analysis of signals or systems. Examples of spectrum analysis include accurate measurement of signal frequency, detection of stable weak signals, analysis of fine structure of the spectrum, random vibration , Etc. However, the Fourier transform can only be used in the case of stable signals and linear time-invariant systems, and the Fourier transform becomes powerless for unstable signals and linear time-varying systems because the Fourier transform For practical purposes and conceptually, there are some limitations, for example, in order to determine the intensity of a signal spectrum at a certain frequency (just one frequency), it must be known that this signal is transmitted over the entire time