Radiocommunications Agency  EMC Awareness

Filtering

What this technique is used for

Noise being conducted out of or into equipment along connecting leads will bypass and render ineffective any attempts at screening the circuit. It is unfortunately impossible to completely eliminate it. The purpose of filtering at interfaces is to attenuate such noise to a level either at which it meets a given specification, for exported noise, or at which it does not result in malfunction of the system, for imported noise.

If a filter contains lossy elements, such as a resistor, a ferrite component or a transient suppressor, then the noise energy may be absorbed and dissipated within the filter. If it does not, then the energy is reflected and must be dissipated elsewhere in the system, typically in the impedance of the source or as re-radiation from the incoming cables. Absorptive filters are generally to be preferred for EMC applications.

Filtering is not confined just to interfaces. It is often helpful to limit the bandwidth or the induced voltages at particular circuit nodes. Suppression of motors and inductive loads is one example where interference is prevented at source.

How this technique is used

In EMC work, “filtering” usually (but not always – see bandpass filtering) means low-pass filtering. The purpose is to attenuate high-frequency noise components while passing low-frequency wanted ones. Filter circuits are normally made up from a combination of simple low-pass configurations.

Filtering can be used at every level of assembly: between segregated areas of circuitry on a PCB; between PCBs within a module or sub-assembly; between modules or sub-assemblies within an equipment; and between an item of equipment and its external environment and other items of equipment.

Key issues in employing this technique

Circuit design

The design of the filter circuit itself – to be capable of providing the required degree of attenuation (or insertion loss) for the unwanted signals – must take into account the source and load impedances as well as the spectra of the wanted and unwanted signals in the conductor. The effectiveness of the filter configuration depends on the impedances seen at either end of the filter network. A simple inductor circuit will give good results - better than 40dB attenuation - in a low impedance circuit but will be quite useless at high impedances. A simple capacitor will give good results at high impedances but will be useless at low ones. 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 50ohms at each end. In the real application, source and load impedances are complex and perhaps unknown at the frequencies of interest for suppression, which makes accurate design of filter properties largely academic.

Common versus differential modes

The distinction between the two modes of interference coupling is crucial for filter design. Filters to attenuate these modes must be configured appropriately.

A differential mode filter will attenuate interference which appears between its signal terminals. It will have no effect on interference which appears in common mode between these terminals and ground, since there is no parallel capacitance to ground. The common mode filter will attenuate interference appearing between the signal terminals together, and ground. It may also have a lesser effect on differential mode interference.

A common mode choke is most easily described when it has two windings, one in each leg of a two wire circuit. The two windings are nominally identical and on the same core. The sense of the windings is such that differential currents, in which the “go” current in one wire is equal and opposite to the “return” current in the other, each create a magnetic flux in the core, but being equal and opposite the two fluxes cancel, leaving no net magnetic flux. Thus since the core is invisible the differential mode inductance is very small, being dominated by the residual difference between the windings, known as the leakage inductance.

By contrast the flux from common mode currents in the wires adds in the core, and therefore the full inductance of the choke is presented to common mode signals.

Common mode chokes can be constructed with multiple windings for filtering interfaces with many lines. As long as the fluxes due to the differential mode currents sum to zero in the core, the choke will operate correctly. Thus a winding must be inserted in series even with the 0V rail, if this carries differential return currents.

Ferrite sleeves give a simple method of creating common mode chokes on cables. Refer to the section on cable-mounted ferrite CM chokes.

RF Reference Plane

At radio frequencies, simple filters can employ resistors inductors or chokes in series with the conductor to achieve a degree of attenuation. Such filters do not depend on an RF Reference Plane to function correctly.

However, filters which employ capacitors require an RF Reference Plane, and the filter performance depends upon the RF Reference Plane maintaining a low impedance at the frequencies to be filtered.

The RF Reference plane for a shielded enclosure is actually the enclosure’s shield. For printed circuit boards (PCBs) in products which do not use a shielded enclosure it is the usually a 0V plane or mesh on one or more layers of the item’s PCB. For an equipment rack or cabinet it is its metal framework.

Higher performance filters need RF Reference Planes that have higher integrity at the frequencies of concern. The RF integrity of a plane is related to its shielding effectiveness, the number and size of the apertures in it, and its overall size. A seamless solid metal enclosure makes the best RF Reference Plane for the electrical/electronic circuits it encloses.

Bonding to the RF Reference Plane

The impedance of the electrical bond between the filter and its RF reference plane should be lower than the impedance of the RF Reference Plane, over the frequency range of interest – otherwise, the benefits of the plane are wasted.

Leakage currents and safety

Any filter with common mode Y capacitors to earth will pass current into this earth when used on a.c. power (e.g. mains) and a.c. signals. The amount of leakage current that is permitted in an equipment’s protective earthing conductor is limited by the relevant safety standards. Mains filters must therefore be designed with this limit in mind, so that there is a practical ceiling to the value of the common mode capacitors.

There are particular requirements for the voltage withstand performance of the capacitors that are used in mains filters; these are laid out in EN 132400/IEC 60384-14. Failure of “X class” capacitors would result in a fire hazard, while failure of “Y class” capacitors would result in both a fire and potential electric shock hazard. The properly rated components must be used in the appropriate positions.

Many safety standards require a bleed resistor to discharge the capacitors when the connection to the supply is broken. In general, this is needed if the total capacitance value is more than 0.1µF.