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Applying Standards for EMC to Multi-function Equipment
By Stephen Colclough, SEQAL
Since the introduction of the EMC Directive, there has been a lot of work done on the standards, so that now there are over 80 listed as harmonized under the Directive1. For manufacturers, knowing which standard(s) to apply can prove tricky, especially if the equipment to be tested has multiple functions. This is becoming more prevalent in the Audio/Visual, IT and white goods markets.
For instance TV’s now come with vga inputs so that they can be used as monitors, surround sound Hi-Fi may soon have radio links to the rear speakers, TFT monitors can have optional antenna ports turning them into TV’s and Fridge/Freezers can now be bought with modems, flat screen monitors, AV ports and LAN ports. Making sure that compliance with the essential requirements of EMC Directive means applying more than one product standard. In this article, examples of the application of standards to these types of products shall be given. This will highlight the problems that manufacturers face in trying to show that their products comply with the EMC directive and the potential for making EMC testing very onerous.
A Simple Example Experience gained in applying standards and developing test procedures for some products can help when new products come along. For instance, take the application of standards for combination microwave/grill/convection ovens.
The emissions generated when functioning as a microwave oven is covered by EN 55011, the standard for Industrial, Scientific and Medical Equipment (ISM). ISM equipment is classified into two groups:
Group 1 is equipment which intentionally generates rf energy which is necessary for its own operation.
Group 2 is equipment which generates rf energy for the treatment of material. A microwave oven is a group 2 device, treating material by heating it.
The emissions generated when operating as a grill or convection oven are covered by EN 55014-1. The immunity for all functions is covered by EN 55014-2 and the standards EN 61000-3-2 for mains harmonics and EN 61000-3-3 for voltage fluctuations and flicker will apply to all functions.
Microwave ovens are also a good example to take, because of the challenges of measuring emissions over a wide frequency range and in the presence of a high level of rf energy at the fundamental frequency. The latest amendment of EN 55011 gives specific emission requirements for microwave ovens covering the 1 to 18GHz band, whereas pre-amendment measurements were only made at 11.7 to 12.7GHz. At the same time the amendment changes the measurement from one for radiated power to one for field strength. This amendment becomes mandatory on the 1st August of this year, so laboratories have had to get there test procedures sorted out. It is interesting to note that FCC part 18, which also covers ISM equipment, has required testing to 10 times the fundamental for several years, ie to 24.5GHz for a microwave oven.
Microwave ovens pose particular problems when making emissions measurements. Intrinsic to their function is the generation of large amounts of both frequency and amplitude modulated rf energy. When carrying out pre-scanning, sweeps need to be made at different rates to ensure that the maximum emissions are detected. Then when making final measurements, it is necessary to use at least a 15 second period. The emission can vary considerably over this period due to the effect of the loading of the magnetron output by the water placed in the oven. As the turntable rotates, the emission can be seen to vary in phase with the position of the load on the turntable.
For measurements above 1GHz, care has to be taken with the test set up because the oven generates very high emissions around the nominal magnetron frequency of 2.45GHz. This is allowed as an ISM band of 2.4 to 2.5GHz is defined by the ITU and there is an unrestricted radiation limit. The high energy in this band can cause problems when trying to measure emissions at other frequencies and steps need to be taken to minimize the effect of the high level of fundamental frequency present. The main problem that will be encountered is that the fundamental will swamp the front end of any pre-amplifier that is used or the front end of the spectrum analyzer. This will result in high levels of emissions being measured at other frequencies even though they are not really there. A typical solution is to use band stop filters and horn antennas to provide some rejection of the fundamental.
This is an simple example to start with because clear guidance is given in the relevant standards. A clear test plan can be drawn up and the philosophy behind the plan is that functions of a product are tested against applicable parts of harmonized standards.
Mixing Audio/Visual with IT Equipment Moving on to other products, TV’s with vga inputs and monitors with tuners present a more complicated problem because of the wording of the standards and the interpretation manufacturers can make. If these products could be classified as televisions, then EN 55013 and EN 55020 would apply. If they could be classified as monitors, they would be IT equipment and EN 55022 and EN 55024 would apply.
The test requirements for each set of standards are quite different. The guidance given in the standards is different as well:
EN 550132 states “Multifunction equipment which is subjected simultaneously to different clauses of this standard and/or other standards shall be tested with each function operated in isolation… . The equipment thus tested shall be deemed to have complied with the requirements of all clauses/standards when each function has satisfied the requirements of the relevant clause/standard.”
The above implies that the TV function of showing broadcast pictures and sound, input from any of the rf or audio/visual inputs should be tested to one standard and the monitor function of displaying data input from the vga input should be tested to another standard. This would mean that these products could be tested with the same philosophy as combination ovens, which would arguably be the best approach to take.
But when one reads EN 55022, a different interpretation could be applied by those wishing to reduce the burden of testing. The definition of IT equipment is given as:
“equipment which has a primary function of either (or a combination of) entry, storage, display …. of data …”3
Further, “Any equipment (or part of the IT equipment) which has a primary function of radio transmission and/or reception…. are excluded from the scope of this publication.”4
Manufacturers on the look out to cut corners may read the above in the standard and think that they could declare that the primary function of a product as a monitor and not bother testing against the Audio/Visual standards for instance. However, a consumer could take a monitor with an RF input and use it just as a television, which would mean that in the market place its primary function is different to that declared by the manufacturer.
Choosing Standards by Colour DVD and CD players seem to be classified as different types of equipment, based on the colour of the plastic used for them. If the equipment is silver and stand alone, it is classified as Audio/Visual and EN 55013/EN55020 are applied by manufacturers. If the colour matches the ubiquitous beige colour of a PC and the device slots in the hole provided for drives in a PC, then manufacturers call them IT equipment and apply EN55022/EN55024.
Adding Radio With the advent of surround sound Hi-Fi and television, the nation’s living rooms could soon become festooned with cables linking the rear speakers to the amplifier. So, thought is now being given to the use of a low power, short range radio link to provide wireless rear speakers (albeit still requiring a mains power supply to the speakers). To provide the consumer with the simplest solution, the radio transmitter would be incorporated in the Hi-Fi. This automatically means that the Hi-Fi now falls under the RTTE directive, with its own list of harmonized standards and EMC requirements. A product using an audio link at 863 to 865MHz would have to meet the requirements of EN 301 489-9. This standard calls up emission and immunity tests for the radio equipment which are based on EN 55022 product standard and the EN 61000-4 basic standards. Effectively this means the Hi-Fi would need to be tested as a piece of IT equipment for the functions using the radio link.
The Ultimate Household Product? Large fridge/freezers are now available with all sorts of extras added on to make them the control center of the home. This takes home integration and automation to a level which perhaps most of us can live without, but the products are being developed and they will require testing. If you can get one of these beasts through the front door of your house, it will provide internet access, cable TV interconnectivity, PDA like operation, portable (within the home) sound and light, mains signaling to all your other domestic appliances and still keep things cool. The flat screen monitor parks in the front of the fridge door and contains a battery pack, loudspeakers, a camera, a microphone, touch screen and a radio link to the “main unit”. When you feel like it, you can take the monitor anywhere in the house or garden so that you can maintain second by second control over your house, or have a video telephone call with your loved ones from the comfort of your favourite armchair.
All of this sounds wonderful for the next generation of householder, but from the point of view of testing, it becomes very time consuming. Following the test philosophy developed earlier in the article, this fridge/freezer/entertainment center/communications hub would require EMC testing against:
EN 50065-1 for the mains signaling aspects, EN 55013 and EN 55020 for the audio/visual functions, EN 55014-1 and EN 55014-2 for the fridge/freezer functions, EN 55022 and EN 55024 for the IT equipment functions, EN 301 489 for the radio function.
Some of the tests may be duplicated and so some time could be saved, but the multiple functions contained in the above example means that great care would have to be taken in generating the EMC test plan.
In Conclusion... The convergence of technologies and increasing desire for communication is continuing at a greater pace than standards are being developed. It is unlikely that standards will be developed to cover all the possible combinations of functions that could be developed by a manufacturer. For multi-function equipment, irrespective of the declared primary function of the product, probably the best policy for a manufacturer to take is to try to cover as many test scenarios as possible. By working closely with their chosen test laboratory, time can be saved if several functions can be tested simultaneously, but the conclusion must be that the more functions there are, the more tests will be required.
References 1. Commission communication in the framework of the implementation of the Council Directive 89/336/EEC. 9 March 2002. 2. BS EN 55013:2001, clause 4.1 3. BS EN 55022:1998 clause 3.1 4. ibid.
Stephen Colclough can be contacted on +44 (0)1252 863820 or email shcol@samsung.co.kr |
