Cutting the cost of EMC compliance

Eric Tucker of Feedback Instruments Limited, reveals a DIY solution to the problem of EMC testing.

A hundred years ago the whole radio spectrum was as it had been for millions of years - quiet except for the crashing of distant thunderstorms. Since then, the pace of change has been almost cataclysmic. Increasing congestion of the radio spectrum during the eighties and nineties made it essential that something was done to limit interference. In Europe, that something was the EMC Directive.

At the outset, some feared that the Directive was little more than an opportunity for the test houses, many of whom sat on the standards committees, to make money. The DTI did a very creditable job in pointing out that this was not the case, but some non-DTI material did attempt to frighten the less well informed into almost crippling costs. Over the last three years, some people have done very nicely out of EMC.

Enforcement difficulties

It is interesting to note which equipment has the most difficulty complying with the Directive. It is an open secret that many PCs are still not compliant, but at least some effort to reduce emissions is being made - effort which would not have happened without the Directive. Interestingly, the products with the worst radiated immunity record are low frequency high precision instruments. Most systems which actually operate at high frequencies are designed with RF in mind and are therefore better at keeping it out. The real problems come with precision analogue systems with an operating upper frequency limit of a few tens of Hertz. These often come in nice plastic boxes containing single-sided printed circuit boards and were, quite understandably, not designed with RF in mind. Before surgery, such systems commonly malfunction badly in only the smallest RF field. Sadly, design engineers who are skilled in making this type of product are usually the least familiar with high frequency engineering.

With some of these considerations in mind, Feedback Instruments decided during 1995 to set up its own 'pre-compliance' test facility, a term which wrongly implies an obligation to put all products through a 'compliance' test house. As we all know, this is not the case. The obligation is for manufacturers to satisfy themselves that products comply with the Directive. Use of a compliance test house is probably the only absolutely certain way of doing this, but it is not compulsory.

Performance/cost compromise

An antenna being tested in the EMC test room at Feedback Instruments Ltd

To carry out radiated immunity testing and comply with the EMC Directive it is essential to use a screened room. An 'unlined' room which has no absorber gives very pronounced field variations due to standing waves. This makes it quite difficult to use. When absorber is added these variations are reduced for a cost which is modest by comparison with that of good test equipment. Opinions differ on the amount of absorber needed but excellent results can be obtained when about half the chamber is covered. Various materials are available but for small rooms some form of tile is almost the only option. Once you have a reasonably anechoic chamber, emission measurements can be carried out without the problems of outside interference.

For emissions testing, Feedback Instruments uses a combination of antenna, preselector, spectrum analyser and software, supplied as a turn-key solution. The argument about spectrum analysers versus receivers for emission has raged for several years, and it has to be said that both have their advantages. The trick is to match your system. There is no point in buying an expensive receiver to do rough and ready measurements in the company car park.

The immunity system is made up of an antenna, an RF generator, an amplifier, a field probe and GPIB software for automatic field levelling. Other measurements of transients and static discharge are carried out using readily available systems.

The system has been in use since mid-1995 both by Feedback Instruments staff and by an increasing number of companies which use it on a commercial basis, hiring the room by the day. Most people who have used the test room have come to the conclusion that, like most science and engineering, making EMC measurements is simply applied common sense.

So, where are we after a year or so of the EMC Directive? The level of EMC-related prosecutions will doubtless increase, but expectations of the end of electronic manufacture as we know it remain unfulfilled. The Directive has had the effect of removing from the market some of the worst offending products, whose emissions could be heard several miles away, providing an easier remedy than the Wireless Telegraphy Act at the expense of some loss of sovereignty. Companies' attitudes to the requirements of the Directive and the alternative means of meeting them are maturing fast, and many businesses now have their own test facility or hire one like that at Feedback, thereby saving a considerable amount of money.

If anyone reading this article would like some no-strings advice based on experience, they are welcome to telephone the author on 01892 653322.