What is involved in Designing for EMC? -- does it mean we have to go to deep multi-layer boards? Does it mean we have to keep our clock speeds low? Does it mean we have to shield our product?

There really is no single answer. EMC concerns alone generally do not push for deep multi-layer board designs, they generally do not push for additional shielding (except in very rare situations) and they generally do not restrict clock speeds.

If appropriate EMC attention is applied, as a continual focus throughout the development process, to the design of the entire system then EMC compliance is substantially inherent in the end product.

Some examples:

• Seemingly minor differences in the partitioning of multi-board designs can have dramatic effects on EMC performance.
• Seemingly minor differences in cabling can have effects on EMC performance.
• The choice of components or the method of their interconnection can effect EMC performance.
• The positioning of circuit traces on a printed circuit board is a notorious contributor to EMC performance (both positive and negative).

Each and every product presents its' own challenges and they all require individual evaluation for EMC performance. There are many books that have "Dos and Don'ts" lists purporting to tabulate the paths and pitfalls to success. They tend to oversimplify the issue and imply that if you follow their suggestions closely you will not have EMC problems; don't bet anything important on it! There are some situations when one or more of the Don'ts can't be avoided; then what do you do? There are ways to deal with these situations, but without an understanding of the WHY behind the Dos and Don'ts success is not so easily achieved.

All too often people will apply the Dos and Don'ts only to most of the product and omit some small portion of the product from the EMC evaluation. After all this small piece doesn't have any chips on it. It is just an LED and a photo-transistor on a board that is used to sense the edge of a moving item. The LED and the photo-transistor themselves are probably not too much of an issue but the circuitry that drives the LED could be an important source of emissions if the LED is pulsed at a constant rate and the driver is designed to drive highly capacitive loads. This driver will probably have very fast edge rates and that 22 inch cable connecting the sensor to the main board just might make a very effective antenna for the frequency associated with the pulse repetition rate or those fast edges.

An EMC analysis or audit of the total product during the early phases of product development can help avoid costly remedial work at the end of the development.