1, discreteness: the spectral line is discrete.

2. Harmony: The spectral line only appears at the integer multiple frequency of the fundamental frequency.

3. Convergence: The general trend of harmonic amplitude is to decrease with the increase of harmonic times.

When the pulse width of the signal is constant and the signal period increases, the interval between adjacent spectral lines becomes smaller and the spectrum becomes denser. If the period grows indefinitely, the interval between adjacent spectral lines will approach zero, and the discrete spectrum of periodic signals will be filtered into the continuous spectrum of aperiodic signals.

In addition, the spectral line amplitude of each frequency point in the spectrum is also related to the pulse width τ. When the signal pulse width is constant, the longer the signal period, the smaller the spectral line amplitude of each frequency point, and vice versa.

Extended data:

Common periodic signals include sinusoidal signals, pulse signals and their rectification, differentiation and integration. This signal can be called a simple signal. Their characteristic is that there will not be more than two extreme points in a period, and the periodicity is obvious.

For this kind of signal with periodic characteristics, it is relatively simple to judge whether it is periodic, and the methods of periodic measurement are very mature and perfect, such as zero-crossing detection and pulse shaping.

The signal can be analog or digital. If it is continuous time and continuous value, then it is an analog signal. If it is discrete time and discrete value, then it is a digital signal.

In addition to this distinction, signals can also be divided into periodic or aperiodic. Periodic signals will repeat themselves after a certain period of time, while non-periodic signals will not repeat themselves. Analog and digital signals can be periodic or aperiodic.

Methods of distinguishing periodic signals from aperiodic signals;

The frequency spectrum of 1. periodic signal is discrete, while the frequency spectrum of quasi-periodic signal is continuous.

2. Because the periodic signal can be expressed by a set of trigonometric functions with integer times frequency, it is a discrete frequency point in the frequency domain. After Fourier transform, the quasi-periodic signal tends to infinity, so it becomes continuous in frequency spectrum.

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