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Frequently Asked Questions

Blue lineEnd of Section/Next sectionThis page answers some popular FAQs (frequently asked questions) on EMC. It's divided into two sections. The first will be of interest to someone who has just come across EMC for the first time and wants some initial idea of the subject. The second is aimed at designers and installers of electronic equipment, who know about the issues but want some technical answers.

In each case, the Further Details link takes you to a relevant page on this site.


Have you got a glossary for all those acronyms?

What is EMC?

What is EMI?

What is RFI?

Are there regulations in the UK and Europe to control EMC?

What about the rest of the world?

Why must I turn off my mobile phone in hospitals and aircraft?

My computer’s always crashing at odd times. Is this EMI?

I’ve heard that broadband internet might cause EMC problems. Why?

I get white flecks (like snow) on my TV screen whenever someone is using a vacuum cleaner/hairdryer/power drill in the house. Why?

My digital TV picture goes “blocky” or freezes whenever someone is using a vacuum cleaner/hairdryer/power drill in the house. Why?

There’s a buzzing or crackling sound from my transistor radio sometimes when I’m listening to Radio 5. What is it?

If I leave my mobile near to the TV set, the TV makes a noise just before I get a call. Is that EMI?

What techniques are available to cure EMI?

What’s the relationship between EMI and the human health effects of electromagnetic fields?

When is the best time to think about designing in EMC to a product?

Should I specify screened cables for my product to improve EMC?

Do I need to worry about the layout and routing of my cables?

I have a simple analogue circuit design that doesn’t use microprocessors. Can I ignore EMC?

I have a simple microprocessor circuit design that doesn’t use analogue inputs. Can I ignore EMC?

Are video signals a particular EMC problem?

What’s a ferrite choke and how is it used?

Do I always need to use twisted pair cable?

Do I need a ground plane on my printed circuit board?

How do I suppress electromechanical switches and motors?

What techniques can be used to improve adjacent channel rejection of radio receivers?

I’ve put my circuit into a metal box to shield it. Is this OK for EMC?

Blue lineEnd of Section/Next sectionGeneral

Have you got a glossary for all those acronyms? Click to go to top of current page

Yes, click here to go to the Glossary.

What is EMC? Click to go to top of current page

EMC stands for ElectroMagnetic Compatibility. It is defined as the ability of a device, unit of equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment. It can be regarded as the absence of effects due to electromagnetic interference.

What is EMI? Click to go to top of current page

EMI stands for ElectroMagnetic Interference. It includes any phenomenon which causes a disturbance to the correct operation of electrical or electronic systems, including but not limited to radio receivers, in a particular environment. Some environments (for instance factories) have a greater inherent level of EMI than others (for instance people’s homes, or the open countryside).

What is RFI? Click to go to top of current page

RFI stands for Radio Frequency Interference. It is an older term than EMI but means a sub-set of all types of electromagnetic interference, limited to disturbances created by radio transmitters or which affect the operation of radio receivers.

Are there regulations in the UK and Europe to control EMC? Click to go to top of current page Click for further details

Yes. Various European Directives, chief among them being the EMC Directive (89/336/EEC), are now law throughout the European Union. These make it illegal to place on the market, or take into service, equipment which may create unacceptable disturbances or which does not have adequate immunity.

What about the rest of the world? Click to go to top of current page Click for further details

Many other countries have similar legislation, for instance the US has the FCC rules, Australia has the C-tick mark and so on. These requirements are almost without exception not as comprehensive as the European regulations.

Why must I turn off my mobile phone in hospitals and aircraft? Click to go to top of current page Click for further details

Any mobile transmitter – mobile phones are simply the most universal – can create levels of disturbance that are unacceptable to some safety-critical systems. Air transport systems and medical electronics are particularly safety-critical and, rather than ensure that all such apparatus is immune to the emissions from mobile phones, which would take a great deal of effort, it is simpler and safer just to ban their use.

My computer’s always crashing at odd times. Is this EMI? Click to go to top of current page

Perhaps. It could also be software problems, or an intermittent hardware failure. But it is possible for nearby interference to upset a computer’s operation and cause a crash, even though modern machines should be more immune than those built, say, 10 years ago. EMI leaves no fingerprints – the best way to trace it is to see if anything electrical, such as a thermostat or a radio transmitter, is operating nearby at the time of the fault.

I’ve heard that broadband internet might cause EMC problems. Why? Click to go to top of current page Click for further details

Delivering broadband data to the home involves transmitting high frequencies down telephone wires or, in the worst case, down power lines. These were not designed for the purpose and, though they work, they “leak” a lot of energy into the environment. This can cause interference to radio reception nearby and the cumulative addition of many such emissions could cause difficulties on a much wider scale.

I get white flecks (like snow) on my TV screen whenever someone is using a vacuum cleaner/hairdryer/power drill in the house. Why? Click to go to top of current page Click for further details

Motor-driven electric apparatus is very good at creating radio noise – including in the TV bands – and needs careful treatment to keep this to a minimum. New products should have this suppression, but older or sub-standard products may not, or may be worn or the suppression may be failing. Motor noise has a characteristic effect on the analogue TV picture.

My digital TV picture goes “blocky” or freezes whenever someone is using a vacuum cleaner/hairdryer/power drill in the house. Why? Click to go to top of current page Click for further details

Digital transmissions respond differently to interference than older analogue transmissions. Although the interference source is the same, its effect is to corrupt the digital data used to construct the picture. Depending on the nature or severity of the interference, this can result in part or all of the picture data being unusable. It’s probably not the set that is at fault.

There’s a buzzing or crackling sound from my transistor radio sometimes when I’m listening to Radio 5. What is it? Click to go to top of current page Click for further details

Radio 5 is transmitted on medium wave. The buzzing or crackling could come from a number of possible sources, but a very common one is a faulty thermostat that arcs for a long period of time – several seconds – when it switches. During this period it’s creating a lot of radio noise. The cure is to replace the thermostat.

If I leave my mobile near to the TV set, the TV makes a noise just before I get a call. Is that EMI? Click to go to top of current page

Yes. When a call comes through, the mobile responds with an acknowledgement to the base station before it starts to ring. This transmission has a characteristic pulsed format which is picked up by other nearby apparatus – in this case the TV set – and affects its operation. In the case of the TV it’s just a nuisance, but in other situations it could be more serious.

What techniques are available to cure EMI? Click to go to top of current page Click for further details

The simplest is segregation: put some space between the interfering source and its victim. More sophisticated techniques involve cable re-routing, filtering, bonding of metal surfaces, making screened enclosures and circuit modifications. Any EMI problem can normally be cured, but it often involves a lot of effort and cost.

What’s the relationship between EMI and the human health effects of electromagnetic fields? Click to go to top of current page

Not a lot, apart from the fact that we are talking about the same electromagnetic phenomenon. In apparatus suffering EMI, the levels at which it may be affected are often quite low ( for commercial products, anyway) and its response is immediate. For humans, the known dangerous field strengths are a lot higher, and they need to be applied for a long time to have a measurable effect.

Blue lineEnd of Section/Next sectionProduct designers and installers

When is the best time to think about designing in EMC to a product? Click to go to top of current page

At the beginning. EMC requirements should be part of the initial design specification of any product. The later you leave it in the design cycle, the harder and more costly it becomes to implement EMC.

Should I specify screened cables for my product to improve EMC? Click to go to top of current page Click for further details

There’s no one right answer to this question. Screened cables have their merits but are generally more expensive than unscreened and, to be effective, must be applied properly. Careful design of the cable interface may allow you to use unscreened cables and still meet EMC requirements, but this isn’t always so.

Do I need to worry about the layout and routing of my cables? Click to go to top of current page Click for further details

Generally, yes. Proper layout and routing can contribute a great deal to the good EMC of an installation. If you know that your installers have no control over the cables, then it is necessary to be more careful over the cable specification and/or the interface design.

I have a simple analogue circuit design that doesn’t use microprocessors. Can I ignore EMC? Click to go to top of current page Click for further details

No. Two compelling reasons: one, your product will still be subjected to incoming RF and transient interference and so must be designed from the start to cope with and reject this interference. Two, even analogue circuits can suffer from spurious high-frequency oscillations which affect other radio services, and have to be actively designed to avoid this.

I have a simple microprocessor circuit design that doesn’t use analogue inputs. Can I ignore EMC? Click to go to top of current page Click for further details

No again. Digital circuits may be less susceptible to radio frequency interference but they’re not totally immune to it, and they are generally more susceptible to transients than analogue circuits. And, the clock frequencies that the micro generates are a fruitful source of interference to radio services and their emissions must be limited.

Are video signals a particular EMC problem? Click to go to top of current page Click for further details

They certainly can be. High-resolution video has a wide bandwidth – 100MHz or more – and the video clock frequencies can radiate effectively. The cable from a PC to its monitor – and the monitor itself – is inevitably a prime source of radio emissions and has to be carefully treated by screening and often with a ferrite choke.

What’s a ferrite choke and how is it used? Click to go to top of current page Click for further details

It’s a particular component which absorbs the magnetic field around a cable, reduces the interference current flowing in the cable, and hence reduces its radiative coupling. Sleeve-shaped chokes can simply be threaded onto a cable and are most effective in the VHF region; toroids can have a cable wound several turns around them and are most effective in the HF region.

Do I always need to use twisted pair cable? Click to go to top of current page Click for further details

It’s certainly one technique in the EMC armoury. It reduces magnetic coupling to its near environment and if the circuit it carries is balanced, it also reduces the effect of capacitive coupling. Both of these effects can be helpful, and indeed essential in some applications such as unscreened data network cabling, but it is by no means a universal solution.

Do I need a ground plane on my printed circuit board? Click to go to top of current page Click for further details

As a general rule, a ground plane is always a good idea. Even for an analogue circuit, it will help improve its immunity to RF. For high-speed digital circuits, it’s necessary for correct operation, let alone for EMC. But a ground plane needs to be applied carefully and with some thought as to its purpose, or it won’t be effective.

How do I suppress electromechanical switches and motors? Click to go to top of current page Click for further details

The key to this technique is to limit the rate-of-rise of the voltage across the contact (or motor armature) as the current is interrupted. This is normally achieved by a capacitor of the appropriate rating, but this needs to be supplemented by a snubber resistor that controls the peak current in the capacitor. Transient suppressors can also be used to limit the inductively-generated spike voltage.

What techniques can be used to improve adjacent channel rejection of radio receivers? Click to go to top of current page Click for further details

A range of techniques are possible; bandwidth-limiting filters at the antenna input are the most common, but a receiver’s performance can also be improved by designing it for a wide dynamic range, so that out-of-band high level signals don’t cause non-linear operation. Low-cost designs are naturally the hardest to optimise.

I’ve put my circuit into a metal box to shield it. Is this OK for EMC? Click to go to top of current page Click for further details

The metal box needs to be properly designed for shielding before it will be acceptable for EMC. Apertures, gaps and seams in the metal must be minimized or treated with conductive gaskets, and all cable penetrations must be bonded or filtered to the shield. Only a total, integrated approach to EMC design will yield good results.

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