ST AN898 Application note
AN898
APPLICATION NOTE
EMC GENERAL INFORMATION
by Microcontroller Division Applications
INTRODUCTION
Because many electronic circuits are in proximity to each other, it is essential that their design
is not aff ected by e xterna l noi se so urce s and t hat the circ uit its elf is not a no ise s ourc e affecting other circuits. This relationship is known as electromagnetic compatibility or EMC.
Sources of electromagneti c noise are numerous and have both natural and man-made ori-
gins.
Natural sources below 10 MHz are dom inated by the atmospher ic noise genera ted by electrical storms. Above 10 MHz, natural sources consist primarily of cosmic noise and solar radiation.
Man-made noise include AM, FM, TV and other broadcast transmitters, mobiles radios but
also computing devices, mo tors, appliances, power line s, auto ignition systems and m any
others. W ith the p rolife ration of t hese sour ces (p arti cula rly in u rban area s ), e lect romag net ic
noise has reached important levels.
For detailed information regarding EMC guidelines for microcontroller - based applications,
please refer to AN901.
Figure 1. Electromagnetic Radiation Sources
ELECTROMAGNETIC RADIATION
NATURAL ORIGIN
M A N-M A DE ORIGIN
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EMC GENERAL INFORMATION
1 EMI AND EMS
Almost every electronic device emits some electromagnetic interferen ce (EMI). These
emissions can be transm itted as electromagnetic radiation or conducted through cables
such as power cords. At the s ame time, most el ectronic devi ces are s usceptible to emissions
generated either internally or by other devices.
Receptors susceptible to electromagnetic radiation include communication receivers such as
radio and television, radar and navigation devices or computing devices. The effect of the interference on the receptor depends on several parameters: strength of the source, transmission medium, distance from the source, coupling mechanisms and degree of susceptibility of
the medium.
Figure 2. EMC Contents: EMI and EMS
EMC
Electromagnetic Compatibility
EMS
Electromagnetic
S u s c eptibility
Conducted
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Electromagnetic
Radiated
EMI
Int e rfe re n c e
Condu cted
Radiated
Electromagnetic interference (EMI) tests involve measuring the frequency and amplitude of
undesirable signals emitted by the tested equipment. Signal radiated into free space are
called radiated emissions, whereas signa ls trave lling a long p ower cords or other int erconnecting cables are called conducted emissions.
Electromagnetic susceptibility ( EM S) testing is a way to determine the ability of the device
to operate properly in an undesirable electromagnetic environment. These tests use signal
sources and power am plifiers to gene rate high level fi elds aroun d a dev ice. Co nducted s usceptibility me as urem ent s are p erfo rmed by co upling an o ffe nding si gnal o f a s pe cified le vel
onto cables to try to induce a malfunction into the tested device. Other forms of susceptibility
tests include electrostatic discharge (ESD), transient burst and voltage surge testing.
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