Function: The filter is a circuit or operation processing system with frequency selection function. It has the function of filtering noise and separating different signals. 2. Type: According to the form of signal processing: analog filter and digital filter are classified by function: low-pass, high-pass, band-pass, band-stop according to circuit components: LC passive, RC passive, passive filter composed of special components, RC active filter according to the order of differential equation of transfer function: first, second, second, second and mode. Transfer function and frequency characteristics of Quasi-filters (1) The characteristics of analog filter's transfer function analog filter circuit can be described by transfer function. The transfer function is the ratio of output to input voltage or current Laplace transform. After cascading any isolated linear network, the total transfer function is equal to the product of each network transfer function.