EMC Design Practices: Defining the Problem Part 1

Some people believe that EMC-related problems are rare and that EMC engineers can exorcise the demons causing malfunctions in various electronic systems. It’s not that way at all.

Any electronic system can be thought of as a collection of EMC problems waiting to happen. These may occur from within the system, the system may cause another to malfunction, or the system may malfunction because of the surrounding electromagnetic ambient. These emissions or susceptibility problems result from unwanted radiated RF energy or from RF energy conducted on power, control, and signal lines (Figure 1).

A typical scenario—much like one that the FCC tries to control—might be the interference that occurs on your television when the microwave oven is on or when your kids are doing homework on their computer. RF energy is being radiated from the culprit source and picked up by the TV antenna, by direct penetration into the systems PCBs, and by the power, control, and signal lines (Figure 2).

Often, RF energy is conducted along the incoming power line to the receiver through shared common impedances. In both cases, it is the unwanted RF energy reaching the TV that causes it to malfunction. These malfunctions generally are temporary in nature. When the RF source is removed, the problem goes away.

As a result, the FCC requirements state that you cannot design, build, and sell equipment that jams the public broadcast service. The FCC does not address hardening equipment to prevent susceptibility or the problems of interference between devices that are not used to receive public broadcast programming. Nor does the commission consider health and safety issues. The European requirements go beyond those of the FCC, addressing many of these additional situations.

Interfering conditions may exist that are more than just a nuisance. For example, we find prohibitions against using certain types of electronic devices on aircraft, especially during the first and last 10 minutes of a flight when the operation of flight-control systems is critical.

One of the worst problems is emissions from FM receivers, because their local oscillator radiation falls in the same frequency range as the instrument landing system on the aircraft. Cell phones, remote-control toys, and computer systems all rank high on the list of potential RF hazards. As a result, aircraft electronics undergo very severe RTCA DO-160 EMC tests.

EMC-related problems have been steadily increasing since the early to mid 1970s when we began to see a large increase in the use of microprocessor systems. This increase occurred, in part, because microprocessors were becoming cheaper, mechanical timers could be replaced with microprocessors that were reliable, and marketing people discovered that the general public was very enamored with microprocessor-based systems.

During that time, the FCC developed the early requirements for computer-based systems that were released in Docket 20780 and ultimately included in FCC Part 15. Today, the FCC still only controls systems-level radiated emissions and power-line conducted emissions in their rules and regulations. The FCC has no susceptibility requirements.

EU Directive Requirements

The European EMC requirements released in 1989 ended the era of weak commercial EMC regulations and their associated lack of commercial immunity requirements. Many industries, such as medical and industrial-control manufacturers, had little experience meeting EMC regulations of any kind. They were caught way behind on the learning curve, with few, if any, EMC engineers available to satisfy the need.

The European Union’s (EU) new directives are configured so that systems and equipment are classified by function and must meet specific requirements for each classification if product-specific requirements exist. Otherwise, depending upon the equipment, the following generic requirements must be met:

EU Emissions Requirements

Radiated—almost the same as the FCC requirements.

Conducted—almost the same as the FCC requirements.

Line Harmonics.

Line Fluctuations.

EU Immunity Requirements

Electrostatic Discharge (ESD).

Radiated RF Fields.

Electrical Fast Transient (EFT)/Burst.

Surge.

Conducted RF.

Others—such as magnetic field susceptibility.

The EU radiated and conducted emissions requirements are based on CISPR 11 and 22 and essentially are the same as our current FCC requirements. The EU immunity requirements test the susceptibility of the system or equipment to an adverse electromagnetic environment.

This represents a major change in commercial EMC requirements. In the United States, only the military previously had susceptibility requirements. Now, the FDA places susceptibility requirements on medical devices such as powered wheel chairs and sleep apnea devices. These new FDA requirements are similar to and, in some cases, based on the EU requirements.

In spite of the multitude of EMC tests that exist to verify compliance to commercial and military EMC requirements, there basically are only four coupling categories: radiated emissions, radiated immunity, conducted emissions, and conducted immunity. When these are compared, designing to meet the immunity requirements tends to be more difficult than designing to meet the emissions requirements. Designing to meet the radiated requirements tends to be more difficult than designing to meet the conducted requirements.

Testing Issues

Bearing this in mind, when designing to meet the EU requirements, there are two distinct problems to deal with:

EU conducted immunity, notably the EFT/burst test applied to power lines, and control and signal leads at levels up to 4 kV (5 ns × 50 ns).

EU radiated immunity, notably the ESD test that can be as high as 15 kV (<1 ns) for air discharge.

The conducted immunity EFT/burst test is very severe. The test signal is a very fast transient and has significant energy levels existing at frequencies greater than 100 MHz. Likewise, the radiated immunity ESD test generally is more severe than the 1/3/10 V/m radiated RF test. Many EMC test labs are reporting that most of the equipment being tested fails to meet these immunity requirements.

If these two problems can be solved, then the remaining requirements normally will be resolved in the process. Immunity hardening certainly is achievable, but it should be implemented early in the design, not after everything else is complete.

For example, if the system or equipment is a low-powered, digitally based device, it will need approximately 20 to 40 dB more hardening than required for the FCC emission requirements to pass the EU conducted immunity tests. It also will need the same level of EMC hardening as required for FCC emissions to meet either the radiated emissions or immunity requirements.

However, if the system or equipment combines both low-level analog circuits and digital circuits, it still will need 20 to 40 dB more hardening for the EU conducted immunity requirements. In addition, it now will need about 40 dB more hardening to meet the EU radiated immunity requirements.

In new designs, the most cost-effective approach to controlling EMC problems requires understanding the synergistic relationship of PCB design, cable hardening, and PCB/enclosure shielding. These and other EMC design practices will be discussed in future articles in EE. In August, the topic will be the illusive RF radiated coupling loop.

About the Author

Ron Brewer is vice president of EMC technical services at Instrument Specialties. He is a NARTE-certified EMC/ESD engineer with more than 25 years in EMC/ESD/Tempest engineering. Mr. Brewer serves on three technical committees and, as an internationally recognized EMC authority, has made more than 185 technical presentations in North America, Europe, Asia, and the Pacific. He also has been named a Distinguished Lecturer by the IEEE EMC Society. Instrument Specialties, P.O. Box 650, Delaware Water Gap, PA 18327, (570) 424-8510.

July 1999