| Radiocommunications Agency
|Filtering with cable-mounted CM chokes|
What this technique is used for
Filtering in general is described elsewhere on this site.
That section mentioned that cables and other conductors are a route by which interfering signals can enter (immunity) or leave (emissions) an item of equipment. It also mentioned that a great deal of interference propagates in common-mode, and this can be attenuated using common-mode (CM) ferrite chokes.
Standard PCB-mounted CM chokes are available with two, three, four, six and eight circuits. But many types of cables have many more than eight conductors, and it is often required to retro-fit suppression onto an existing product or system. So a number of manufacturers of ferrite RF suppression products offer cable-mounted CM chokes, which are easy and quick to fit to cables with any number of internal conductors.
Cable-mounted CM chokes provide a quick and easy means to attenuate CM noise currents, and hence reduce the emissions and improve the immunity of equipment at RF.
How this technique is used
A very simple 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.
The effectiveness can be increased by looping the cable several times through the core, but the benefit is limited at higher frequencies by the stray capacitance between the turns of the cable. At frequencies above 500MHz this technique provides little or no benefit and the increased stray capacitance could even lead to reduced attenuation. The ferrite is at its most effective when using several cores in series, if it fits snugly over the wire or cable and if it is long and narrow rather than short and fat.
Key issues in employing this technique
Ease of use during fault-finding and problem fixing
A great many products have been made to pass their EMC compliance tests by the judicious application of clip-on cable-mounted ferrite CM chokes. It is quick and easy to try out a variety of CM chokes when failing a test in an EMC test lab, and discover which (if any) solve the problems.
Of course, it is important to ensure that the production versions of the equipment have exactly the same types of CM chokes fitted in exactly the same way as the one that was made to pass in the test lab, so keeping a good record of the chokes’ part numbers and where they were fitted is very important.
Unfortunately, both the cable-mounted CM choke and its assembly are high-cost items. So where a high-volume design needs to use a number of cable-mounted chokes to pass its EMC tests, it is usually necessary for commercial reasons eventually to eliminate the chokes and reduce the overall cost of manufacture.
The cable-mounted chokes allow the product to be sold whilst these cost improvements are going on. Even though the profit margin may be less than was hoped, it is much more profitable than not selling at all.
Ferrite effectiveness increases with frequency. The impedance of a ferrite choke is typically around 50ohm at 30MHz, rising to hundreds of ohms above 100MHz (the actual value depends on shape, size and material composition). Usually a ferrite has little effect 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. Most circuits, and especially cables, show impedances that vary with frequency in a complex fashion but normally stay within the bounds of 10 - 1000ohms, 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 resonances in cable impedance, some noise frequencies may experience dramatic reductions in level, whilst others may be hardly affected.
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 both of their ends. Sometimes two or even three chokes are needed in series at one end.
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.
Ferrites are conductive
All ferrites are conductive, some more so than others, so it is important that conductors passing through them are sufficiently well insulated.
Suitable assembly techniques and mounting methods may be required to deal with the weight and fragility of some types of ferrite sleeve.
Of course, it is very important to ensure that the choke is always mounted in just the same way on all units. Some assemblers might think 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.