EMC Awareness
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Radiocommunications Agency EMC Awareness |
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Household appliances and electronic equipment
Baby alarm interferes with aircraft communications
Domestic appliances interfere with amateur radio reception
Thermostats interfering with TV and radio reception
Numerous other interference problems with household equipment
| Baby alarm interferes with aircraft communications |  |
Description
A well-known make of wireless baby alarm is known to cause occasional interference to aircraft communication when approaching UK airports. It is not the wireless technology in the baby alarm that is the problem, it is their plug-top power supply, which uses a switch-mode converter. A faulty batch of power supplies was shipped with the baby alarms, and although they function well enough they emit on VHF radio channels used by National Air Traffic Services (NATS).
The interference is particularly difficult to detect on the ground but when NATS is informed of problems of this sort, they are able to overfly the troubled area with a specially equipped aircraft, partly funded by the Radiocommunications Agency (RA). When the aircraft has located the source of the interference, NATS will send in a specially equipped road vehicle which identifies the house concerned.
Officers from the RA then exchange the faulty plug-top power supply and send it back to the baby alarm manufacturer, who ship a (non-VHF-transmitting) replacement. It is a lot of trouble to go to for a low-cost electronic item, but flight safety requires us to do it.
Commentary
This source of interference was found to be due to a spurious oscillation that occurred when the internal cables were in a certain position. Designers should ensure that their products’ performance is not significantly affected by the minor variations that will inevitably occur during production.
Even assuming the product had been tested for radiated emissions, it is possible that the test was performed on a sample that did not exhibit the spurious oscillation, though this does not absolve the manufacturer from responsibility. It is also possible that the plug top power supply was only tested for conducted emissions up to 30MHz, in the mistaken belief that it had no mechanism to generate disturbances above this frequency.
Most plug-top power supplies these days use switch-mode power conversion technology, which involves power devices switching at between 50 and 500KHz. However, because they must switch at very high rates to maximise efficiency and minimise heat losses it is easy for them to emit significant amounts of emissions at up to 1000 times their basic switching rate. So a 100kHz switcher might give high levels of emissions at 100kHz intervals all the way from 100kHz to 100MHz.
Such plug-top power supplies must employ filters and other techniques to prevent their emissions exceeding statutory limits. Sometimes a batch is made with faulty or missing filter components, or with a design change that has not been tested for emissions. If the manufacturer is not employing Quality Assurance (QA) methods that ensure that EMC compliance is maintained in volume manufacture, the ‘noisy’ power supplies can find their way onto the market.
References and links
From Tom Perry, U.K. Civil Aviation Authority, private correspondence, 2003; and information supplied by NATS and RA.
Links to Mitigation Techniques
| Installation | Design & Development | Resources | |
| Spurious suppression techniques |  |
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| Filtering |  |
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| Shielding of areas and volumes | |
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| Shielding of cables | |
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| Domestic appliances interfere with amateur radio reception | |
Description
The RSGB EMC Committee receives many enquiries from members about interference to reception of amateur radio signals.
Accordingly, the RSGB (Radio Society of Great Britain) has produced a leaflet that gives advice about identifying and locating sources of Radio Frequency Interference (RFI, also called electromagnetic interference or EMI). Issues covered include…
TVs, set-top boxes, Cable TV
Switch-mode power supplies (e.g. ‘lump in a cord’ or ‘plug-top’ devices)
Lighting
Electric motors
Thermostats
Computers
Intruder alarm systems
Telephones and fax machines
Commentary
Radio amateurs often receive weak signals which are only just detectable, and may therefore suffer interference from sources which would not normally affect domestic radio and TV receivers (except in areas where the broadcast signals are very weak).
References and links
“Interference to Amateur Radio Reception”, RSGB leaflet number: EMC 04, downloadable from http://www.qsl.net/rsgb_emc/emcleaflets4.html.
The above leaflet and a number of other useful information leaflets are indexed at: http://www.qsl.net/rsgb_emc/emc.html#emcaid. Even more information is available from http://www.qsl.net/rsgb_emc/ and http://www.rsgb.org.
“RFI/EMI RadioFrequency Interference/ElectroMagnetic Interference” is the title of a web page on the ARRL site that has links to a number of useful articles and other resources on preventing interference from (or to) household appliances and electrical equipment. Go to: http://www.arrl.org/tis/info/rfigen.html.
Links to Mitigation Techniques
| Installation | Design & Development | Resources | |
| PCB layout to reduce emissions | |
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| Suppressing arcs and sparks | |
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| Shielding of areas and volumes | |
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| Shielding of cables | |
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| Filtering | |
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| Thermostats interfering with TV and radio reception | |
Description
Faulty thermostats can cause annoying interference to television and radio broadcast reception – normally in short bursts, which may recur at intervals. Thermostats switching on and off in central heating systems, refrigerators or freezers can all cause interference problems but, generally, thermostats installed in central heating systems are the most common cause.
Sometimes the offending thermostat is found in the house that is receiving the interference, although there have been cases where the source of the interference has been found some distance away.
In one example, the interference signal is generated from a boiler gas control valve and its associated thermostat switching from stand-by to ON and vice versa. These systems have a typical power consumption of 3kW and can generate a low power single-phase arc when switched. This causes a short burst of radiation. When the thermostat is malfunctioning this burst of radiation can be heard as a rough rasping noise which typically lasts for a few seconds but may last for 20 seconds or more. It repeats typically every 10 minutes but in some cases, a faulty thermostat may arc several times per minute. In Figure 1, the stand-by to ON case is shown, while in Figure 2 the ON to OFF case is presented.
Figure 1
Figure 2
This kind of interference which is intermittent in nature is mostly noticed in relation to the reception of analogue TV signal at 500 to 850 MHz and sometimes on FM radio at 88-108 MHz. The effect of the interfering signal created by a faulty thermostat on the UHF analogue TV signal can be seen in Figure 3.
Figure 3
Commentary
The Radiocommunications Agency deals with many cases of interference caused by thermostats or the radio-suppression components fitted to them. In about 90% of these cases, the interference is attributable to thermostats in gas boilers. It seems that, as these operate in a heat-stressed environment, they are prone to more rapid deterioration than other domestic thermostats such as room thermostats, cylinder thermostats and diverter switching valves.
Thermostats and other automatic switching contacts of all sorts are a major source of noise complaints, particularly when they are faulty as in the above example. New domestic appliances are required to pass tests for “discontinuous disturbance” emissions (the current harmonised standard is EN 55014-1), but this does not guarantee that such products will remain noise-free after many years of operation. The limits for RF emissions are related in a complex way to the repetition rate and duration of the automatic switching event.
Replacing the faulty thermostat will normally resolve the problem, but a better solution is to fit suppression to all such switching contacts. This prevents the arc forming at the instant of switching and if properly designed has the side effect of lengthening the contact life, but the added cost is usually viewed unfavourably by manufacturers.
References and links
“Problems Thermostats Can Cause to Television and Radio Reception”, leaflet RA 272 from the Radiocommunications Agency, download from: http://www.radio.gov.uk/publication/ra_info/ra272.htm
Other information sheets, including RA 179 'Television and Radio Interference', are downloadable from http://www.radio.gov.uk, or from: Library and Information Service, Radiocommunications Agency, Wyndham House, 189 Marsh Wall, London, E14 9SX, U.K., Tel: 020 7211 0502/0505, Fax: 020 7211 0507, Email: library@ra.gsi.gov.uk
Example given in RA report “Design and Development of a Methodology for Efficiently Tracing the Source of Intermittent Wideband EMC Disturbances to Radio Reception”, University of Surrey, May 2000, Project no AY 3639. http://www.radio.gov.uk/topics/research/topics.htm#emc
Links to Mitigation Techniques
| Installation | Design & Development | Resources | |
| Suppressing arcs and sparks | |
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| Filtering with CM cable-mounted chokes | |
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| Filtering | |
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| Numerous other interference problems with household equipment | |
Description
As our lives and our homes become filled with more and more technology, the likelihood of unwanted electromagnetic interference increases.
This
graphic links to an audio file (download size 460KB) that plays an example of
radio interference from a portable power tool. (If you can't hear this audio
file you may not have a suitable audio player on your computer. Click
here to download a free copy of Realplayer.)
Every lamp dimmer, low-energy light bulb, hair dryer, electric drill, garage-door opener, TV, computer, microprocessor-controlled appliance, cellphone or wireless remote control contributes to the electromagnetic noise around us.
Many of these devices also accidentally ‘pick-up’ the electromagnetic noise in their environment, and may suffer reduced performance or react unpredictably to their ‘electronic neighbours’.
Commentary
Complex electronic circuitry is found in many devices used in the home. This creates a vast interference potential that didn't exist in previously.
Your own consumer electronics equipment can be a source of interference, or can be susceptible to interference from a nearby source of electromagnetic noise.
References and links
“Guidelines for Improving Television and Radio Reception” is a resource list giving advice to TV and radio dealers on dealing with interference problems. It is provided by the Radiocommunications Agency via: http://www.radio.gov.uk/publication/ra_info/ra323/ra323.htm.
“What To Do if You Have an Electronic Interference Problem”, a self-help guide for the consumer published jointly by the American Radio Relay League (ARRL), an organization representing Amateur Radio operators, and the Consumer Electronics Association (CEA), at: http://www.arrl.org/news/rfi/neighbors.html.
“Interference”, advice from the FCC’s Consumer & Governmental Affairs Bureau, is at: http://www.fcc.gov/cgb/consumerfacts/interference.html.
“Interference Handbook”, FCC Compliance & Information Bureau, covers a very wide range of interference problems and their sources, and how to track them down and stop them. Now provided by the ARRL, not the FCC, at: http://www.arrl.org/fcc/tvibook.html.
“RFI/EMI RadioFrequency Interference/ElectroMagnetic Interference” is the title of a web page on the ARRL site that has links to a number of useful articles and other resources on preventing interference from (or to) household appliances and electrical equipment. Go to: http://www.arrl.org/tis/info/rfigen.html.
“Reception Advice” is a BBC webpage with links to related pages and other information, including interference, at: http://www.bbc.co.uk/reception. Click on the ‘Factsheets’ button, or go to: http://www.bbc.co.uk/reception/factsheets/index.shtml, for a list of downloadable factsheets including some on interference.
Links to Mitigation Techniques
| Installation | Design & Development | Resources | |
| Any/all EMC mitigation techniques could be relevant, depending on the application | |
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