Electromagnetic compatibility is a subject most designers did not have to worry about a few years ago. Today, every designer putting a product on the global market has to consider this. There are two main reasons for this:
High-frequency radio transmitters, like mobile telephones, are found everywhere. More and more systems are using switching power supplies in the power circuit, and the overall number of electronic appliances is increasing every year.
Power supply voltages are decreasing, reducing the noise margin of input pins. Circuit geometries get smaller and smaller, reducing the amount of energy required to change a logic level, and at the same time reducing the amount of noise required to alter the logic values of signals.
From a designer's point of view, EMC phenomena have to be considered in two different ways:
Traditionally, the only government regulations have been on the emission side: An electronic device is not allowed to emit more than a certain amount of radio frequency energy to avoid disturbing radio communication or operation of other electronic equipment. Most countries in the world have regulations on this topic.
Additional demands on noise immunity earlier were found only for special applications, like medical equipment, avionics and military applications.
From 1995, Europe introduced regulations on immunity for all electronic products, known as the EMC directive. The purpose of this directive is:
At the same time, enforcement of EMC requirements was strengthened: every product made in or imported to Europe must prove to fulfill both emission and immunity requirements before it can be put on the market.
Countries in other parts of the world also introduce similar legal requirements.
The limits for acceptable emission and immunity levels for different product classes and environments are given in various international standards. A more detailed description of these is found in the reference literature.
The EMC directive applies to finished products, but not to components. As a component will not work without being put into a system, the demands are put on the finished system. How the problems are solved internally is left to the designer.
As a result of this, the test procedures required for CE-marking are well suited for testing finished products, but they cannot be used directly for testing components like microcontrollers. The same applies for the test procedures required for FCC approval. The test boards the components are mounted on during test will influence EMC test data for components. These results should therefore only be regarded as informative.
On the other hand, there are test standards (military, automotive, and others) that are made to test components directly. These standards specify standardized test boards to make sure that measurements on different manufacturer's components can be compared. These tests are not a requirement according to the EMC directive.