All electronic equipment is vulnerable to electromagnetic interference. Electromagnetic fields are everywhere. Without safeguards, circuitry may malfunction or may even be damaged if exposed to strong enough field levels.
A robust design applies the right combination of electromagnetic shielding and EMI filtering and operates flawlessly in its intended environment.
Design for Immunity
Users may perceive products that are susceptible to electromagnetic interference as poor quality or as defective.
Many regulatory agencies now require products to pass radiated susceptibility (RS) tests. Military and aviation products have the strictest requirements. Equipment that malfunctions during radiated immunity testing cannot be put on the market or delivered to the end customer.
The Biggest Challenge: Fields Coupled to Cables
Most radiated susceptibility problems occur when fields couple to equipment power and signal lines, not to printed circuit boards.
Why? There are two reasons.
Nearly all circuitry on printed circuit boards is digital. Digital circuits usually have higher noise margins than sensitive analog circuits and are therefore inherently more immune to field-coupled interference.
Second, most of the time, electrical cables are much longer than printed circuit board traces, and they are much higher above ground. For a given field strength, the amount of energy coupled to cabling is much greater than the energy coupled to printed circuits. Often, several orders of magnitude greater. More coupling equates to more interference.
The short video clip below shows the current induced in a five-meter long wire when exposed to a 20 V/m electric field. The wire is 50 cm above ground and terminated at each end by 1 kΩ.
The electromagnetic environment to which electronic equipment is exposed is made up of many transmission sources. Anthony DiBiase, in his Interference Technology article, “Electromagnetic Interference Sources and Their Most Significant Effects,” provides a comprehensive summary of sources and their severity.
Radiated Susceptibility tests, also called Radiated Immunity tests by some agencies, expose the equipment under test and its cables to controlled electric fields levels. Test setups and field levels are standardized.
While exposing the electronic gear and cables to the field, key functions are monitored for signs of susceptibility. Field exposure levels range from 1 V/m for consumer electronics to thousands of volts per meter for aviation and military systems.