In EMC work, “filtering” almost always means low-pass filtering. The purpose is normally to attenuate high-frequency components while passing low-frequency ones. Various simple low-pass configurations are shown below, and filter circuits are normally made up from one or a combination of these.

The effectiveness of the filter configuration depends on the impedances seen at either end of the filter network.The simple inductor circuit will give good results - better than 40dB attenuation - in a low impedance circuit but will be quite useless at high impedances. The simple capacitor will give good results at high impedances but will be useless at low ones. The multi-component filters will give better results provided that they are configured correctly; the capacitor should face a high impedance and the inductor a low one.

Conventionally, manufactured filter assemblies have their attenuation specified for terminating impedances of 50 at each end, mainly for ease of measurement and comparison. In the real application, Z_S and Z_L are complex and perhaps unknown at the frequencies of interest for suppression, which makes accurate design of filter properties largely academic. There is usually a limitation on maximum component values as a result of size or circuit performance, and the actual L and C components will be chosen to be some figure less than this.

You can also download a practical filter guide from here.

Common versus differential mode filter configuration

A filter to attenuate common mode must be configured differently from that to attenuate differential mode. The figure below shows a π-filter for each mode.

The differential mode filter will attenuate interference which appears between terminals 1 and 2, that is, differentially. It will have no effect on interference which appears in common mode between terminals 1 or 2 and ground, since there is no parallel capacitance to ground, and there is a straight-through path via terminal 2.

The common mode filter will attenuate interference appearing between terminals 1 + 2 together, and ground. It may also have a lesser effect on differential mode interference; the differential circuit effectively sees a balanced -filter with capacitance values of 0.5C and an inductance value equal to the leakage inductance of the common mode choke. This may be replaced by two separate chokes in order to increase differential attenuation, at the expense of limiting the bandwidth of the differential circuit.

These basic configurations form the backbone of EMC interface filters.

More detailed interface design practices follow:

Filter components

Power interface filters

Signal interface filters

Balanced interfaces

Isolated interfaces

Transient protection