All inductors (L) suffer from RF resonances, and are only effective in filters at frequencies not far above their first (parallel) resonance (see section 1.8.1 of [7]). But so-called ‘soft ferrites’ behave resistively at RF, and the resulting lack of RF resonances helps make filters that use them have better and more predictable performance at RF. For example, a typical small ‘soft ferrite’ bead a few millimeters in diameter will have around 1μH of inductance and 0.1Ω of resistance at DC, but around 80Ω of real resistance (not inductive reactance) at frequencies from 30MHz to 1GHz or more. Some leaded soft ferrites are available with resistances of over 1kΩ at 100MHz, but a much wider range of surface mounted device (SMD) soft ferrites is available with resistances up to 1kΩor more at selectable frequencies from 30MHz to 2GHz.
Soft ferrite components are known by a variety of names, including ‘RF suppressers’, ‘Interference suppressers’, ‘Suppression chokes’, and ‘Shield Beads’. Figures 1 and 2 show some of the cable-mounted soft ferrite parts available. A very wide range of PCB-mounted soft ferrite components is also available, but not shown in these figures. Figure 2 includes a standard VGA cable, showing the standard soft-ferrite CM choke that all VGA cables are required to have at each end, for the products they interconnect to meet emissions regulations in Europe and the USA (at least).
(these examples of toroidal and cylindrical types are from Philips)
As will be described below, the attenuation of a filter at the highest frequencies is governed by the stray coupling between its input and output conductors – so it is important that the input and output conductors of a CM choke, such as the one in this photograph, are as far apart from each other as possible. This results in the winding format that can clearly be seen in the figure – only half the core is wound and the input and output cables are on opposite sides from each other, the perhaps odd description of it as having ‘4½ turns’ is a common way of making clear that input and output conductors are on opposite sides.
A useful soft ferrite component is a cylinder split lengthways and held in a plastic clip-on housing, and some examples of this are also included in Figure 2, for round cables as well as for flat cable styles. Such split ferrites are very easy to apply to cables (and to remove if found to be ineffective), and EMC engineers tend to carry many of these around with them, using them for the diagnosis, isolation and curing of EMC problems, both DM and CM. A ferrite cylinder clipped around an entire cable or cable bundle, including all the send and return conductors, is a CM choke, but if clipped over just a send or return conductor it is a DM choke.
Choosing soft ferrites involves checking that their impedance is as high as required over the frequency range for which significant attenuation is required. Soft ferrite components always have impedance versus frequency curves that are smooth and not discontinuous, whereas the curves for inductors will show one or more discontinuities (changes in slope from positive to negative, or vice-versa, that occur at a point) that reveal the presence of resonances.
Some data sheets only provide impedance data for a portion of the frequency range you are concerned with. But it would be a mistake to assume anything about the impedance they will achieve in the frequency range for which no data is provided – always make sure you have manufacturers data on the impedance over the entire frequency range that you wish to control.
An aspect of choosing soft ferrites that is often overlooked, is that their impedance versus frequency curves vary with their DC and/or LF current. Typical data sheet curves assume zero current in the device, but as the current increases the frequency at which the peak impedance occurs will also increase, and may not be as high as it was with no current. Often, when an emissions or immunity test is failing at some frequency (e.g. a clock harmonic at 228MHz), a soft ferrite will be chosen that has a very high impedance close to this frequency, and it will be added in series with traces on the PCB that are thought to be the cause of the problem.
But the DC or LF current in those traces could make the frequency of the peak impedance increase by enough that the actual impedance achieved at the problem frequency is not high enough to provide significant attenuation and pass the test. Instead, the currents in the traces and the frequency/current variation of the type of devices to be used should have been taken into account, to select a device that would have its peak impedance at the problem frequency when the trace current is passing through it.
Several manufacturers offer wide ranges of soft ferrite RF suppression components, and are continually adding to them. Recent additions include SMD parts rated at 3A continuous current at DC and low frequencies; yet provide 1kΩ or more around 100MHz. Other recent additions include parts that provide impedances of 1kΩ or more over the range 100MHz to 2GHz.
Curves such as those in the two top graphs in Figure 4 are most suitable for filtering low-frequency signals, whereas the two bottom curves show devices that have been tailored for filtering unwanted harmonics from digital waveforms whilst leaving sufficient lower-frequency harmonics to create reasonable digital waveforms that have rise and fall-times fast enough to reliably meet the maximum skew requirements of the circuit. CM ferrites tend to have impedance versus frequency curves similar to the top left-hand graph, since there is no need for any CM currents at any frequencies.
(these examples from the Murata BLM21 series, 0805, typically rated 100 – 200mA)
When simulating filters or other circuits using soft ferrite components, a simple device model cannot be used – the parameters are frequency-dependant and current-dependant, and may also be temperature dependant, and should be modelled as such to achieve any accuracy in the simulation over a range of frequencies. Some circuit simulators may be unable to handle models with such complex parameters.
