A filter is a frequency selective device that allows a specific frequency component of a signal to pass through and greatly attenuate other frequency components. In the test device, the interference noise can be filtered or the spectrum analysis can be carried out by using the frequency selection function of the filter.

Broadly speaking, any channel (medium) for information transmission can be considered as a filter. Because the response characteristic of any device is a function of the excitation frequency, it can be described by the frequency domain function. Therefore, any link that constitutes the test system, such as mechanical system, electrical network, instrument and even connecting wires, will be transformed and processed according to the characteristics of frequency domain within a certain frequency range.

The content described in this section falls within the scope of analog filtering. The principles, types, mathematical models, main parameters and design of RC filters are introduced. Although digital filtering technology has been widely used, analog filtering is still widely used in automatic detection, automatic control and electronic measuring instruments.