Frequency domain electromagnetic induction and time domain electromagnetic induction, two observation methods.
Time domain (time domain) - the independent variable is time, that is, the horizontal axis is time and the vertical axis is the change of the signal. Its dynamic signal x(t) is a function that describes the value of the signal at different times.
Frequency domain (frequency domain) - the independent variable is frequency, that is, the horizontal axis is frequency, and the vertical axis is the amplitude of the frequency signal, which is usually called a spectrum diagram. A spectrogram describes the frequency structure of a signal and the relationship between frequency and signal amplitude at that frequency.
When performing time domain analysis on signals, sometimes the time domain parameters of some signals are the same, but this does not mean that the signals are exactly the same. Because the signal not only changes with time, but is also related to frequency, phase and other information, it is necessary to further analyze the frequency structure of the signal and describe the signal in the frequency domain.
The transformation of dynamic signals from the time domain to the frequency domain is mainly achieved through Fourier series and Fourier transform. Periodic signals rely on Fourier series, and non-periodic signals rely on Fourier transform.