Radiocommunications Agency  EMC Awareness

Filtering with cable-mounted CM chokes

What this technique is used for

Filtering in general is described elsewhere on this site.

All conductors that penetrate a shield must either be filtered or shielded. When using filters it is very important that the metal body of the filter makes metal-to-metal contact with the shield barrier itself. The same applies to the shield when using a shielded conductor.

There are a number of manufacturers of ferrite RF suppression products who offer cable-mounted CM chokes which are easy and quick to fit to cables with any number of internal conductors. These provide a quick and easy means to attenuate CM noise currents on cables, and hence control EMC.

How this technique is used

Quite simply, the ferrite CM choke component is either threaded over the cable, or else split hinged clip-on types are clipped around the cable.

The latter is much easier to do when a cable has connectors fitted to it, as they are usually too large to thread through the centre of a solid component.

This ease of assembly is why all EMC ‘trouble-shooters’ carry around a variety of ‘clip-on’ ferrite CM chokes – split in half with a hinged plastic housing – rectangular types are suitable for a wide range of flat cables and flexible circuits, and tubular or toroidal types for round cables up to about 20mm in diameter. Clip-on chokes up to 25mm internal diameter are available, but expensive. Much larger chokes are available but are almost always unsplit.

Clip-on ferrite cable chokes are often used to help identify the source of EMC problems, by adding large numbers of them to each cable in turn until the culprit cable(s) is (are) found.

Key issues in employing this technique

Unpredictable attenuation

It is often hard to predict the effect of applying a CM choke to a cable – some noise frequencies may experience dramatic reductions in level, whilst other may be hardly affected. However, since they are so easy to apply they are often used as a first resort when RF interference problems arise.

The best locations for the choke

Chokes are usually best fitted within a few tens of millimetres from the end of the cable closest to the equipment or zone which is to be prevented from emitting, or to be protected from interference. Some cables may need chokes on both of their ends. Sometimes two or even three chokes are needed in series at one end. Some cables may need chokes distributed every metre or two along their length, to help suppress resonances.

Saturation

Where a CM choke is fitted to a high-current cable it is possible for small unbalances in the currents in the cable’s conductors (usually at d.c. or power frequencies) to saturate the ferrite material and prevent it from working correctly as an RF choke.

So it is important to ensure that chokes fitted to power cables don’t saturate, and this can mean using chokes which are very thick (in the direction perpendicular to the axis of the cable), or ensuring that current imbalance is minimised. Saturation is sometimes accompanied by a small heating of the choke which can be felt by hand.

Multiple turns of cable

Wrapping a cable around and through the middle of a choke to achieve multiple turns is a way to increase the choke’s impedance and get higher attenuation.

But the benefit is limited at higher frequencies by the stray capacitance coupling between the turns of the cable.

At frequencies above 500MHz this technique provides little or no benefit and the increased stray capacitance from one side to the other could even lead to reduced attenuation.

Clamping the choke close to equipment chassis or RF reference plane

Ferrite has a high dielectric constant, and if the core is placed very close to an equipment’s metal chassis or a zone’s RF reference plane it can provide some distributed capacitance, making the simple choke behave as if it were a distributed RC filter. Sometimes this technique can provide a useful improvement in attenuation.

Weight, chafing and chipping

Cable-mounted chokes are quite heavy and their ferrite is a ceramic and very hard and brittle.

Where vibration or shocks are present the weight of the choke can cause plug-in connectors to come loose or solder joints to crack. The cable insulation can chafe on the ferrite, causing unreliability and even electrical faults. If the ferrite vibrates against another hard surface it can crack or chip (cracked or chipped ferrites can lose a lot of their attenuation).