Three Rules for EMC Tests: Plan, Plan, Plan

If you could spend a few minutes at several EMC testing laboratories before starting your next product design, what would you learn? How could the specialists there help you save time and money as you qualify your product for the marketplace?

We asked several experts what they would like to tell you if you visited them, and here are their answers. Their recommendations range from planning EMC compliance, designing compatibility into your product, and pretesting to scheduling the lab-test phase and rework time.

Find an EMC Class

One of the top-priority, up-front steps before you get into designing a product is to participate in a training class led by an EMC expert at one of the testing laboratories. “We recommend that all design engineers become educated in EMC technology,” Brian Mattson, general manager at DLS Electronic Systems, said. “This can be accomplished through continuing education courses.

“Education gives designers the knowledge of EMC test technology and practical design techniques to help them pass future precompliance and compliance tests. Follow this up by procuring the services of an EMC laboratory early in the design phase to discuss test plans and the way to make the tests successful,” he said.

Prepare for Testing

EMC test laboratories want your products to pass their tests with flying colors. That isn’t going to happen every time, of course, but you can improve your chances of success.

Bruce Curry, senior EMC engineer at Trace Laboratories, recommends that you think about EMC requirements even before the PCBs and chassis are laid out. Visit the lab before tests are scheduled, and be prepared to define failure modes and determine the best way to monitor performance.

Let the laboratory be your partner in test planning, noted Tim Dwyer, EMC program manager at TÜV Rheinland. “The design should have hooks for incorporating filters, shields, and grounds to help achieve compliance. Bring these components to the test lab as insurance. Even if they prove to be unnecessary, they still are a good investment,” he explained.

Most testing labs have a standard preparation checklist. Use it as a reminder about test modes, running time, configurations, accessories, support equipment, and cables.

“Know where you intend to market your product,” Thomas Parker, chief technical officer at Elliott Laboratories, reminded us, “and understand the standards that apply in those marketplaces. Plan for bureaucratic hurdles, and determine the tests that will be most difficult to pass. Over-design for EMC at first, recognizing that you can remove unnecessary components later.”

As an example of complex standards and their potential impact on the lab, Dean Ghizzone, president of Northwest EMC, referred to the closely regulated performance of radio transmitters. “The laboratory needs a very comprehensive understanding of the regulatory requirements before it can develop a test program. This involves knowing all the countries where you plan to make the product and researching the requirements relating to each of them,” he said.

Work with a test laboratory to configure your product to operate in its intended mode at its location, and provide all the peripherals and cables. Eric Nguyen, EMC laboratory supervisor at Garwood Laboratories, explained, “Make sure that your equipment operates properly before the laboratory gets it, and tell the lab personnel how it is configured and what to look for in operation. And leave yourself some time to correct problems that show up during test.”

There are no magic tricks at the test site. A compliant product will pass every time, and a noncompliant product will fail. The laws of electronics are in effect 24 hours every day. “You must design EMC compliance into your product because you certainly can’t test it in,” according to Scott Proffitt, director of marketing and sales at U.S. Technologies. He and our other experts have seen the agony of designers who forgot this reality.

Get a Head Start

You will increase the probability that your product will pass the EMC tests at an independent laboratory if you run abbreviated precompliance tests at your facility while the design is underway. These tests are not terribly expensive and generally save time, money, and headaches in the long run.

But don’t ignore the EMC test laboratory that you have selected. Your precompliance testing should be done with the guidance of that laboratory, according to Mr. Proffitt. “You can test the product there and at your place and compare the test results. Sure, reflections and ambient noise are different at the two locations, and the results are not expected to correlate completely. In general, though, properly planned precom-pliance testing can tilt the odds of success in your favor.”

Sometimes you may be tempted to eliminate precompliance testing to reduce the time to market. This is not a good idea. “Generally, it has the opposite effect. Problems that would have been uncovered during precompliance testing may require redesign, retrofit engineering, and retest,” noted Mr. Dwyer of TÜV Rheinland. “We encourage clients to consult us at the design phase and perform precompliance testing as early as possible, using a prototype or even a similar product. Where prolonged setup is not necessary, we can perform a very meaningful immunity and emissions pretest up front for a modest charge.”

Mr. Proffitt of U.S. Technologies continued this theme: “Sure, you will be making changes later that will invalidate the data from these tests, but it gives you something to work with before you pass the point of no return.”

Challenging the Facilities

Remember that your product testing requirements are one of a kind. Every test sequence at an EMC testing laboratory is unique. Some are simple and uneventful, but others present interesting challenges—even problems sometimes.

For example, an extremely large system may tax the ingenuity of engineers at the testing laboratory. Mr. Mattson explained some of the ways that DLS Electronic Systems has handled such challenges.

“We built two very large buildings with attached extensions for testing at our open field site. We also constructed a large turntable that handles up to 16,000 lb. For susceptibility tests, we bought a building with nine shielded rooms. We also enlarged our anechoic chamber. In each case, we incorporated large doors and loading docks.”

And then there are the extremes. “When the equipment to be tested is so big or unwieldy that even large facilities are not sufficient,” Mr. Mattson continued, “we may be able to break down the unit and test pieces as long as all the electrical components can be put in the test chamber and a typical mode of operation can be simulated.

“In the worst cases, even these techniques are not sufficient. Then we develop a unique test plan and a written rationalization of our testing philosophy. In one case, we tested a product that took up the client’s entire warehouse. We ran tests in situ, after getting approval from the competent body as we generated our test plan,” he explained.

At U.S. Technologies, one product to be tested had to be maneuvered around the OATS with multiple forklifts working together. Still another, an earth-station antenna pedestal, required a unique test arrangement. And National Technical Systems has gone so far as to use overhead cranes to handle a product or run a test at the customer’s manufacturing plant when size prohibited more convenient testing.

For the large and complex products, laboratory personnel are glad to visit your facility and plan the test program with you. They may even suggest doing some of the tests in situ.

The complexity of your product can provide another type of challenge. Bellcore tests, for example, require custom fixtures.

Mr. Parker of Elliott Laboratories said, “Often considerable advance preparation is required. Complex laser systems involve high power plus unique debugging equipment and techniques. Wireless product specifications are not as well harmonized as other testing requirements. Also, there are more standards every year.”

High-frequency products, especially those using spread-spectrum technology, require special laboratory equipment to perform the tests. High-powered transmitters are a challenge because harmonic measurements must be done very carefully to guard against nonlinearities.

Plan ahead. There is no substitute for reviewing a product early in its design phase. Potential problems in performance or in test-lab requirements can be considered and resolved at that stage without excessive costs and frayed nerves. Save the surprises for your birthday.

Acknowledgements

The following companies provided information for this article.

Published by EE-Evaluation Engineering
All contents © 2000 Nelson Publishing Inc.
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without the express written consent of the publisher.

May 2000