Real-life filter performance is totally dependant on how they are installed, especially on the impedance of the RF Reference and the impedance of the method used to electrically bond the filter to its RF Reference. Not only should these impedances be much lower than that of the shunt capacitors in the filters, they should also allow the internal and external CM surface currents to find their optimum return paths. This series of articles discusses these issues, and the practical installation guidance that results.
Stray RF coupling between the conductors associated with their unfiltered and filtered sides easily degrades filter attenuation. This problem is generally worse at higher frequencies, because the impedances of stray capacitances and stray mutual inductances reduce as frequencies increase, increasing the amount of stray coupling bypassing the filter. Many engineers have been very surprised by the ease with which high frequencies will bypass (‘leak around’) a filter, given half a chance.
In an unshielded enclosure, filters should be positioned as near to the point of entry of the cable as possible. The maximum possible separation distances should be maintained between the filter’s external and internal cables, and between all of the conductors associated with the circuits on either side of the filter. Conductors in air should be spaced at least 100mm apart, more if they are routed in parallel for more than a few centimetres. Closer spacings might be acceptable for PCB traces and components – but only if they are much closer to the PCB’s RF Reference Plane than the spacing between them. Filter input and output conductors should never, ever, be bundled together, or share the same cable or cable route, unless they are each well shielded. See [3] for how to shield conductors effectively.
Where the enclosure is shielded, it is essential to mount the filter in the wall of the enclosure, with the filter’s body electrically bonded directly to the shielding surface of the wall, otherwise both the filtering and shielding performances will be degraded by stray coupling around the filter. The type of filter required is often called a bulkhead-mounting, or through-bulkhead filter, because it fits through the metal wall (bulkhead) that it is mounted upon. The shielded enclosure considerably reduces the stray coupling between the filter’s input and output. It may even be necessary to fit a conductive EMC gasket around the aperture in the shield where the filter is mounted, for the maximum possible filtering and shielding. Issues of filtering with shielded enclosures are covered in more detail below.
Skin Effect and the Flow of Surface Currents
Where the frequencies to be attenuated are not very high, it could be acceptable to use a few direct bonds, or a few millimetres of wire or braid to provide the electrical bonding to the RF Reference, providing the impedance of the bonding method is much less than that of the filter’s shunt capacitors at the highest frequency of concern. But to understand how to assemble/install filters correctly for good RF performance at high frequencies, we need to understand ‘skin effect’.
All RF currents travel as surface currents, because all conductors have a skin effect that effectively causes them to shield their inner depths from RF currents.
Figure 1 gives the formula for calculating one skin-depth δ, where μ0 is the permeability of free space (4π.10-7 Henries per metre); μR is the (dimensionless) relative permeability of the conductor material (most common conductors, such as copper, aluminium and tin, have a μR of 1.0) and σ is the conductivity of the conductor material in mho/metre. Copper has a nominal volume resistivity ρv of 1.72.10-8 Ω-m, giving it a nominal conductivity of 58.106, so one skin-depth in nominal copper is given by δ = 66/√ƒ (δ is given in millimetres when ƒ is in Hz). For example, at 160MHz: one skin-depth is 0.005mm, so 0.05mm below the surface of a copper conductor, the RF current density is 0.0025 of the density at the surface, an attenuation of 52dB.
Figure 2 shows graphs of skin-depth versus frequency for some common materials, to save having to find out the values of their conductivity and calculate δ. Mild steel is shown as an example of a ferromagnetic material (nickel is another), and to show that their high values of μR result in smaller skin-depths, but also that their permeability is frequency-sensitive and disappears above some critical frequency.
[1] contains information on the material properties of a wide range of conductors, for calculating skin depth, and also a great deal of other useful information for designers. [2] is a useful source for information on skin-depth.
Above a few tens of MHz most conductors and metal items (such as the cases of filters) are several skin-depths thick, so RF currents travel as surface currents in them. Taking this phenomenon into account in the design of a filter’s assembly/installation is essential for the achievement of good emissions and/or immunity performance.
Figure 3 shows how providing a continuous metal bond between a filter and the shielding enclosure of a product ensures that the external CM noise currents do not enter the enclosure and cause interference and immunity problems, and the internal CM noise currents remain inside the enclosure and do not escape to cause emissions problems. Figure 3 shows a simple capacitor filter, but the principle applies to all types of filters.
As a result, the optimum way to bond a filter to its RF Reference Plane, for the best performance at the highest frequencies, is what is often called ‘360° direct metal-metal contact’ – meaning that the filter’s metalwork and the RF Reference Plane are in direct contact with each other all around the periphery of the filter (hence the term 360°).
Commercial and industrial conducted emissions standards generally only measure up to 30MHz, and at such low frequencies it is often sufficient to bond a filter to an enclosure with a single direct metal-to-metal connection between the filter’s case and the enclosure. Where the filter is only required for low frequencies, e.g. below 1MHz, it may even be possible to use a very short length of wire or braid to connect its metal case to the enclosure metalwork, plus of course the enclosure will not need to be a proper shield either. But there is a synergistic relationship between filtering and shielding, discussed in more detail in the following section.
Filters that employ capacitors connected between power or signal conductors and the RF Reference depend upon the RF Reference – and their connection to it – having a much lower impedance than the filter capacitors, at all of the frequencies to be attenuated. The connection between the capacitors and RF Reference should be very short and direct, less than one-hundredth of a wavelength long at the highest frequency to be attenuated, and should also have a very low inductance. This usually means that wires or even braid straps cannot be used to electrically bond filters to the RF Reference Plane, except for low frequencies (say, below 1MHz).
Figure 4, which is taken from [4], shows the sorts of bad effects that even a short length of interconnecting wire can have on a standard single-stage mains filter even when measured with 50Ω/50Ω source and load impedances – its best possible case. If the 10mm wire were replaced with at least one direct metal-to-metal bond, performance at 30MHz and above would improve dramatically.
It is acceptable to fit green/yellow wires of any length to mains filters, for safety reasons, as long as there is also at least one direct metal-to-metal electrical bond between the filter’s metal case and the product’s RF Reference. When a mains filter’s metal-to-metal bonds have been designed to maintain a very low impedance over the lifecycle of the product, there is no need for a green/yellow ‘safety earth’ wire as well – but safety inspectors are generally much more reassured when they can see a green/yellow bonding wire with anti-vibration anti-corrosion connections at both ends. (But, as discussed above, it would be a mistake to assume that the green/yellow safety wire was adequate for achieving the filter’s EMC performance.)
- John R Barnes, Robust Electronic Design Reference Book, Volume II, Appendices, Kluwer Academic Publishers, 2004, ISBN 1-4020-7738-6
- RF Café, Skin Depth, www.rfcafe.com/references/ electrical/skin_depth.htm
- Keith Armstrong, “Design Techniques for EMC, Part 2 – Cables and Connectors”, The EMC Journal, May and July 2006, available from www.compliance-club.com.
- Tim Williams and Keith Armstrong, “EMC for Systems and Installations”, Newnes 2000, ISBN 0 7506 4167 3, especially chapter 8, www.newnespress.com, RS Components Part No. 377-6463
