A filter is a two-port network that selectively passes signals of certain frequencies (usually within a specific frequency range) while attenuating or suppressing signals of other frequencies. When interference signals are not within the same frequency range as the useful signal, a filter can effectively suppress the interference. These networks can be passive filters composed of RLC or RC components, or active filters composed of RC components and active devices. In practical applications, selecting the appropriate filter type and parameters according to specific needs can effectively improve signal quality, enhance circuit performance, and ensure the normal operation of the circuit.