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?
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.
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.
minor differences in the partitioning of multi-board designs can
have dramatic effects on EMC performance.
minor differences in cabling can have effects on EMC performance.
- The choice
of components or the method of their interconnection can effect
- 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