3.5 Crude filter rules-of-thumb

So we can now define our crude filter rules-of-thumb:

· If unwanted signal is high impedance and DM, attenuate with a shunt capacitor (connected between its send and return conductors)

· If the unwanted signal is low impedance and DM, attenuate with a series inductor (good high-frequency performance requires a symmetrical layout of identical inductors, one in the send conductor, and one in the return)

· If the unwanted signal is high impedance and CM, attenuate with identical shunt capacitors from each conductor to the local earth reference (usually the chassis)

· If the unwanted signal is low impedance and CM, attenuate with a common-mode inductor applied to all the signal conductors at the same time.

These rules are very crude, because the terms “low” and “high” impedances are relatively ill-defined, and depend upon the impedances of the suppression components available for handling the wanted signal, (and also on the components’ costs and availability, the quality of earth reference and ease of bonding to it, etc.).

3.6 Inductance variation with current

All inductors suffer a reduction in their inductance as current increases, up until they saturate (where they have no inductance at all). This is a common cause of the differences between simple calculations or simulations and real-life filter performance.

As a rule of thumb, this effect should be taken into account whenever the wanted current exceeds 20% of the rated current. Take account of the fact that power supplies that do not meet EN61000-3-2 draw their input currents as peaks many times higher than their rated RMS supply current.

3.7 Determining filter specifications

For controlling emissions: the filter performance required can be estimated by comparing the spectrum of the product’s emissions with the limits in the relevant EMC standard. The  emissions may be either predicted or measured, and the limits are most often related to EN 55022 or EN 55011.

For controlling immunity: the filter performance required can be estimated by comparing the specification of the threats in the intended electromagnetic environment with the susceptibility of the electronic circuits to be protected. The functional performance degradation allowed should also need to be taken into account. The environment specification is usually taken from an EMC standard, often derived from EN 50082-1 (preferably the 1997 version) or EN 50082-2. But even EN 50082-2 may be inadequate in some industrial, scientific, or medical environments where high levels of 50Hz or radio-frequency (RF) power are being used, or when the user expects to use a portable radio transmitter whilst operating the product.

Where safety-critical systems are concerned no functional degradation is permitted during interference events, and a Safety Integrity Level (SIL) should be determined (for e.g. using the new IEC 61508) and used to increase the immunity test level accordingly to achieve the desired level of risk.

This all sounds very organised, but EMC should be designed-in from the start of any project for the greatest cost-effectiveness and we usually don’t know what the actual emissions or susceptibility are until we have built something and tested it, by which time it is fairly late in the project.

The answer to this is to assume that all conductors will need filtering to some degree. But we still need to know: what frequencies? and to what degree?

Sadly, most actual emissions are caused by unwanted CM voltages and currents. Immunity is a similar story: we can specify the frequency range and threat levels, but most problems are caused by CM interference being converted to DM and polluting the signal. Since the conversion from DM to CM, or CM to DM, is caused by imperfections we can’t easily predict filter specifications. (Most of the design techniques discussed in this series will reduce these imperfections and hence reduce the conversion between DM/CM and CM/DM.)

Murphy’s Law ensures that when you have thought of everything, the expensive options will not be needed and you will be damned for over-engineering. But if you overlook any possibility Murphy will expose it and you will be damned for that. Since we are bound to be damned whatever we do, we may as well make our lives a lot easier by including a number of filter options in our initial designs.

When a product is first tested for EMC (long before production drawings are produced) some/all of the filters may be linked out at first, or simple inexpensive filters fitted. Anti-Murphy precautions then require having a wide range of alternative filter types and complexity handy, as well as the tools to fit them quickly. This is why all EMC engineers and test labs have stacks of sample boxes from filter manufacturers, overflowing toolboxes, with soldering and de-soldering irons already warmed-up.

Happily, experience with filtering various electronic technologies to meet various EMC standards is soon gained, and most engineers soon learn which filters usually work best for the different types of conductors in their products. Be aware, though, that every new product has its quirks (even related to its mechanical assembly) and a filter that worked on Model 1 might not be adequate for Model 2. So always make provision (at least on the early prototypes) for more expensive and larger filters than you hope to use, and only remove this insurance when everyone else’s design is complete (including software, although this is probably a vain hope) and the product passes its EMC tests with a suitable margin.

Don’t forget that there are inevitable unit-to-unit variations, so for a serially-manufactured product a prudent designer will aim for a 6dB “engineering margin” on emissions and immunity tests, at least.

3.8 Problems with real-life impedances

Most filter data comes from tests done with 50W source and load impedances, which leads us to a very important point – filter specifications are always hopelessly optimistic when compared with their performance in real life.

Consider a typical supply filter, installed at the AC power input to the DC power supply of an electronic apparatus. The CM and DM impedances of the AC supply can vary from 2 to 2,000W during the day depending on the loads that are connected to it and the frequency of interest. The DM impedance of the AC-DC converter circuitry looks like a short-circuit when the rectifiers are turned on at the peaks of the waveform, but otherwise looks like an open-circuit. The CM impedance of the DC power supply’s AC input is very high indeed, due its isolation from earth for safety reasons (this is why most mains filters connect Y capacitors from line to earth on the equipment side of a mains filter: to create the maximum impedance discontinuity). This is clearly very far from being a matched 50W / 50W situation.

Because filters are made from inductors and capacitors they are resonant circuits, and their performance and resonance can depend critically on their source and load impedances. An expensive filter with excellent 50/50W performance may actually give worse results in practice than a cheaper one with a mediocre 50/50W specification.

Filters with a single stage (such as those in Figure 3D) are very sensitive to source and load impedances. Such filters can easily give gain, rather than attenuation, when operated with source and load impedances other than 50W. This filter gain usually pops up in the 150kHz to 10MHz region and can be as bad as 10 or 20dB, so it is possible that fitting an unsuitable mains filter can increase emissions and/or worsen susceptibility.

Filters with two or more stages, such as those in Figure 3E, maintain an internal circuit node at an impedance which does not depend very much on source or load impedances, so provide a performance at least vaguely in line with their 50/50W specifications. Of course, they are larger and cost more.

The best way to deal with the source/load impedance problem is to only purchase filters whose manufacturers specify both CM (sometimes called “asymmetrical”) and DM (sometimes called “symmetrical”) performance, for both matched 50/50W and mismatched sources and loads.

Mismatched figures are taken with 0.1W source and 100W load, and vice versa, using the CISPR17 test standard that is also used for 50/50W tests. Combining all the worst-cases of all the different curves results in a filter specification that may be relied upon, providing the filter is not overloaded with current (as discussed above), and earthed properly (as discussed below). An example of extracting the worst-case filter curve is sketched in figure 3F.

3.9 Earth leakage currents, and safety

Most supply filters use Y-rated capacitors between phases and earth, with values around a few nF not to exceed the earth leakage limits in the relevant safety standard. Fixed equipment permanently wired-in is allowed higher earth leakage currents, up to 5% of phase current in some cases (when appropriate warning labels are fitted). Industrial power conversion equipment can have very high levels of emissions, and often requires large filter capacitors and hence large earth leakage currents. This is one area where EMC and Safety considerations are unavoidably intertwined, and of course safety wins, so the relevant safety standards must always be referred to when designing mains filters, remembering that most filter capacitors have tolerances of ±20%.

For medical apparatus which may be connected to patients, earth-leakage currents may be limited by safety standards to such low levels that the use of any reasonable size of Y capacitor is impossible. Such filters tend to use better CM chokes to achieve the same performance without Y capacitors, and/or more stages, so tend to be larger and more expensive.

In systems, the earth leakages from numbers of Y capacitors (even small ones) can create large earth currents. These can cause earth voltage differences which impose hum and high levels of transients on cables between different equipments. Modern best-EMC-practices require equipotential three-dimensional meshed earth bonding, but many older installations do not have this so apparatus intended for systems in older buildings may benefit from the use of low leakage filters.

It is always best to use mains filters (or components) for which third-party safety approval certificates have been obtained and checked for authenticity, filter model and variant, temperature range, voltage and current ratings, and the application of the correct safety standard.

Filters sold for use on 50/60Hz may generally be used on power ranging from DC to 400Hz with the same performance, but it is best to check with the manufacturers beforehand.  Also remember that earth leakage currents will increase as the supply frequency increases, so filters which just meet safety standards at 50Hz may not meet them at 60Hz, and may be decidedly dangerous on 400Hz.

3.10 Issues of frequency and/or sensitivity of wanted signals

Most EMC filters are low-pass. Power supply filters have their design difficulties, but do benefit from their wanted signal (DC, or 50/60Hz) being very much lower than the frequencies of most types of interference. Where signals are digital or high-level analogue, and not very high frequency or very sensitive, simple R, L, C, RC, LC, tee, or p filters are often used, as shown by figure 3A.

But where emission/immunity frequencies overlap with, or are close to, the wanted signals, it is no good fitting DM filters such as those in Figure 3A – filtering out the unwanted signals will eliminate the interference, but will also eliminate the wanted signals. Screened cables and connectors will be required instead of/as well as filtering.

At high data rates single-ended signals can prove very difficult for EMC, even with very expensive (thick and inflexible) cables. The use of balanced drive/receive circuits (described in Part 1.4) with balanced cabling makes the filtering and screening of high-rate signals much easier, reduces cable costs (and thicknesses), and makes EMC compliance much easier. CM filters, rather than DM, may then be used within the spectrum of the wanted signals. This is an example of good thinking at an early design stage to minimise overall project timescales and manufacturing cost, even though the component cost of the functional circuit may not be minimised. Examples abound of cheap functional circuit designs that incur huge costs and delays when the time comes to make them EMC compliant or to fix an interference problem in a system.

Low-frequency instrumentation, audio, and other sensitive analogue signals may need to use multi-stage filters to achieve the desired immunity, unless adequate screening has been applied over the entire length of the conductor (unfortunately, good RF screening is not “traditional” in industries that still think cable shields should be bonded to earth at only one end, see Part 2.6.6).

Where an electronics module has a sensitive input, high-performance filters are often needed on all its inputs, outputs, and power conductors (unless the sensitive internal circuitry has been adequately protected with internal filtering and shielding from the rest of its circuits, discussed in Part 5).

3.11 Filter earthing

One of the secrets of RF filters which use capacitors connected to earth, is that they can never be a lot better than the RF performance of the reference (almost always earth or 0V) they are connected to. Most earths in domestic, commercial, and industrial applications have poor RF performance and are nothing like an ideal “infinite RF sink”.

The best place for mounting a filter for the purposes of this article is at the boundary between the product’s “inside world” and the cables in its “outside world”. For a shielded enclosure, filters should be RF-bonded (i.e. metal-to-metal) to an external surfaces, preferably using a through-bulkhead style of filter. For an unshielded enclosure, filters are generally best bonded to the printed-circuit board’s ground plane, at one edge of the PCB.

The connection between the capacitors in the filters and whatever is being used for its 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 means that wires cannot be used as filter grounds except for low frequencies (say, below 1MHz), even if they do have green/yellow insulation (electricity is colour-blind). For example, if a supply filter with 2.2nF Y capacitors is earthed solely by a 100mm long wire, its Y capacitors will be rendered completely ineffective at frequencies above 20MHz by the inductance of that wire.

1nH per millimetre is a good rule-of-thumb when calculating the effects of wired connections to earth. The only correct bonding for filters is at least one (preferably more) direct metal-metal connection(s) from the filter’s metal body to the earth reference. It is acceptable to fit green/yellow wires to filters for safety reasons, as long as they are in parallel with at least one good RF earth bond.

Military signal filters tend to rely on C-only and p types, apparently because most traditional military equipment has a substantial and well-engineered RF earth reference (die-cast alochromed boxes bolted firmly to metal bulkheads in all-metal bodied machines and vehicles), so their earthed capacitors do not suffer from poor earth RF integrity.

Unfortunately, RF earth integrity is often a serious problem for domestic, commercial, and industrial products, which are constructed with low-cost in mind. The most predictable filters in these applications tend to be RC, LC, or tee types (using soft ferrites for the L components). These impose lower levels of RF currents on the earth reference than C-only or p filters. As military vehicles use new materials such as carbon fibre, their earth reference becomes less effective and they may find R, L, RC, LC, and tee filters more cost-effective than C or p.

D-type and some other connectors are available with a wide range of filter and shielding options, and because they are so easy to apply remedial EMC improvements to they are a good choice for low-current and signal cables. Most of these use capacitors from each signal/power pin to their earthed metal bodies. There are some D-types available with a soft-ferrite tube around each pin, and even some with LC, tee, or p filters for each pin. Pin filters treat CM and DM currents identically, so may not be suitable where wanted signals have a high frequency.

RJ45 and similar telecommunication or Ethernet connectors are available with built-in common-mode chokes. The baluns and pulse transformers often used in high-speed LANs to reject low-frequency common-mode noise and/or provide galvanic isolation, are sometimes available combined with common-mode chokes for better rejection of high frequency noise.