The three analog signal types are the DC signal, the time domain (or AC) signal, and the frequency domain signal.

三种模拟信号分别为DC信号，时域(或ac)信号，以及频域信号。

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It is normal to use the FFT method to analyze signals. This method is one kind of mathematical method that changes the time domain signals to frequency domain signal.

对信号进行分析时通常采用传统的傅立叶变换方法，傅立叶变换是时域和频域相互转换的数学工具，从物理意义上讲其实质是将信号分解成许多不同频率的正弦波的叠加。

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However, the information extracted from a frequency domain signal is based on the signal frequency content, as opposed to the shape or time-varying characteristics of the waveform.

然而，从频域信号中提取出的信息是以信号频率成分而非信号形状为基础。

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