A very simple, inexpensive and easily-fitted filter is obtained by slipping a ferrite sleeve around a wire or cable. Any wire carrying a current has a magnetic field around it. The effect of the ferrite is to concentrate this field and hence to increase the wire’s inductance by several hundred times.

The attractiveness of the ferrite choke is that it involves no circuit redesign, and often no mechanical redesign either. It is therefore very popular for retro-fit applications. Several manufacturers offer kits which include halved ferrites, which can be applied to cable looms immediately to check for improvement.

If a ferrite is put over a cable which includes both signal and return lines, it will have no effect on the signal (differential-mode) current but it will increase the impedance to common-mode currents. This is because the differential currents, by definition, sum to zero in each wire pair and therefore there is no net magnetic field. If there is no field, the ferrite is invisible. But the common mode currents do produce a net magnetic flux and this flux is concentrated in the bulk of the ferrite, leading to an increased impedance for these currents only.

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; you need to ensure that current imbalance is minimised.

Ferrite impedance

Ferrite effectiveness depends on frequency. The impedance of a ferrite choke is typically around 50Ω at 30MHz, rising to hundreds of ohms above 100MHz (the actual value depends on shape, size and material composition - some ferrites are available optimised for higher or lower frequency ranges). Usually a ferrite is ineffective at frequencies lower than 30MHz, becomes most effective above 100MHz and falls off in performance as the frequency approaches 1GHz. A useful property of ferrites is that their impedance becomes resistive at the higher frequencies, so that interference energy tends to be absorbed rather than reflected. This property is deliberately enhanced in parts that are designed for suppression purposes, whereas for other applications it is usually undesirable. Therefore ferrite components that are not intended for suppression should not be used for this.

Because a ferrite choke is effectively a lossy inductor, it only functions usefully between low impedances. A ferrite included in a high-impedance line will offer little or no attenuation. Think of the ferrite in circuit terms: it works as a resistive divider in series with the cable common mode impedance at the point at which it is fitted. Most circuits, and especially cables, show impedances that vary with frequency in a complex fashion but normally stay within the bounds of 10 – 1000Ω, so a ferrite will give modest attenuation factors averaging around 10dB and rarely better than 20dB.

In practice, it is often hard to predict the effect of applying a CM choke to a cable – because of these resonances in cable impedance, some noise frequencies may experience dramatic reductions in level, whilst other may be hardly affected.

An interesting case is demonstrated here

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 circuit which is to be prevented from emitting or protected from interference. Some cables may need chokes on each end, particularly if each end exhibits a low common mode impedance. The impedance can be increased by applying two or even three chokes in series, or by looping the cable several times through the core. The ferrite is at its most effective when it fits snugly over the wire or cable and if it is long and narrow rather than short and fat.

Ferrite has a high dielectric constant, and placing the core very close to a metal chassis or enclosure gives some distributed capacitance, making the simple choke behave like an L-filter. Sometimes this technique can provide a useful improvement in attenuation.

All ferrites are conductive, some more so than others, so it is important that conductors passing through them are sufficiently well insulated.

Mounting

Suitable assembly techniques and mounting methods - typically a clamp or moulded-in holder - may be required to deal with the weight and fragility of a ferrite sleeve. Cable-mounted chokes are quite heavy and their ferrite is a ceramic and very hard and brittle. If the total assembly is subject to strict vibration requirements the use of a ferrite may be limited.

It is very important to ensure that the choke is always mounted in just the same way on all units. A mechanical designer might conclude that the plastic clip holding the choke in place is only there for mechanical reasons, and might move the choke to a different place if this makes it easier to assemble. Quality control of build standard is very important for EMC compliance.

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