Radiocommunications Agency  EMC Awareness

Filtering

What this technique is used for

As we all know, electrical energy can travel in a conducted mode along any conductor, and we measure it using volts and amps. However, electrical energy can also travel in a radiated mode through the air and other insulators such as wood and plastic, and now we measure it using volts/meter and amps/metre (i.e. electric and magnetic field strengths).

Filtering is used to prevent internal energy from ‘leaking out' of cables and causing interference with other equipment. In this case the filtering would be said to be reducing emissions.

The filtering technique is also used to prevent external energy from ‘leaking into’ cables and causing interference. In this case the filtering would be said to be increasing immunity.

Because all metallic cables behave as ‘unintentional antennas’ and to some degree convert conducted energy to radiated energy (and vice-versa), the technique known as filtering helps to control both conducted and radiated emissions and immunity.

How this technique is used

Filtering reduces the amount of unwanted electrical energy travelling along a cable by creating a significant change in the circuit’s impedance. Filters themselves are electrical circuits comprising components such as resistors, inductors, chokes and capacitors, connected to the cable at the point where it is to be filtered.

A choke is a type of inductor which has a core which is lossy (resistive) at very high frequencies, which usually helps make more effective filters at those frequencies. The so-called ‘soft ferrites’ (they are actually hard and brittle) used in RF suppressor beads and chokes are good examples of such lossy core materials.

Filtering can be used…

• Between an item of equipment and its external environment, to reduce its emissions or improve its immunity;

• Between individual items of equipment or zones in an installation to prevent one from interfering with another.

Key issues in employing this technique

Purchasing filters

Few installation engineers would design their own filters – instead they would buy suitable models from one of the large number of EMC filter manufacturers.

There is usually a standard part that would suffice, but in some situations custom filters are required, and most filter manufacturers will be able to design and manufacture them.

Some filter manufacturers have mobile engineers equipped with portable spectrum analysers and a range of filter products. They can measure a site, then recommend and/or install a filter that will deal with a specific cable noise problem.

Common Mode (CM) chokes

Above about 1MHz, most interference is caused by CM noise, so filter design can use CM chokes to attenuate the CM noise without attenuating wanted signals (which are always differential-mode, or DM). Refer to the section on filtering with cable-mounted CM chokes.

RF Reference Plane

Other than cable filtering with CM chokes all other filters must be bonded to an RF Reference Plane, and the filter performance at a frequency will depend upon the impedance of the RF Reference Plane at that frequency.

The RF Reference Plane for an item of equipment is its metal enclosure, chassis, rack or frame, so this is a suitable place for bonding a filter intended to suppress or protect that item of equipment. Where it is not desired to modify an item of equipment (maybe for warranty concerns) the filter should be bonded to the RF reference plane for the appropriate zone. It may be only the RF reference plane for that zone, or it might be a plane shared by a number of zones – or even the whole installation.

A seamless metal floor or seamless shielded volume (e.g. a shielded room) makes the best RF reference plane, but more often the RF reference plane is a meshed common bonding network of some description. In the computer and telecomm’s industries some installation standards call it the ‘system reference potential plane’ (SRPP).

Most installations that have an RF reference plane use a meshed common bonding network (CBN) made from existing metalwork plus other ‘meshing’ conductors as needed to get the mesh size as small as is required.

Meshed bonding systems are described in IEC 61000-5-2, where they are called MESH-CBNs or MESH-IBNs. (An IBN is an insulated bonding network, not generally recommended.)

The diagonal size of a mesh governs its RF performance, with the highest useful frequency of a meshed RF reference plane given by the formula: F (in MHz) = 30/d (where d is the size of the largest diagonal in metres).

Without a meshed CBN (or IBN) or sheet steel floor there is no RF reference plane for a zone. If the filter cannot be connected to the enclosure, chassis, rack or frame of the equipment concerned, then achieving respectable filtering at frequencies above a few hundred kHz will not be possible. In such situations only cable-mounted CM chokes will function as advertised by their manufacturers, and expensive filters will be wasted.

Bonding to the RF Reference Plane

The impedance of the electrical bond between a 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.

Where the RF reference plane is a mesh, a filter plate should be used to provide improved (lower impedance) bonding to the mesh.

Location

It is important that filters be located at the boundaries between two segregated zones to reduce the stray electromagnetic coupling between one side of the filter and the other.

The synergy of filtering and shielding

High-performance filtering at any frequency, or any useful degree of filtering at frequencies above 200MHz, requires both filtering and shielding and also requires careful location and bonding of the filters to the shield’s surface.

It also requires careful location and bonding of the filters to the RF reference plane and careful control of cable routing to prevent ‘RF leakage’ between the filter’s input and output cables.

Leakage currents and safety

Most filters leak current into the protective 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, so filters may need to be designed/chosen to meet safety requirements too.

In a cabinet or rack unit containing a number of individual items of equipment, each meeting its own safety standards for leakage current, the total leakage current in its protective earthing conductor can be at hazardous levels. Special ‘earthing’ requirements may apply as a result. Such equipment may need special precautions to be taken to prevent electric shock hazards from its dangerously high leakage currents.