TYPES AND CAUSES OF EMC PROBLEMS
traceable to either "thermal agitation" or "shot effect."
Thermal-agitation noise is caused by the unequal flow of
a. Electromagnetic interference (EMI), commonly
electrons across a circuit element from the effects of
called "interference," is defined herein as any
heat, resulting in a net voltage of random variation.
Shot effect is caused by the random emission of
performance of C-E equipment.
electrons from an electron tube cathode. Noise in a
b. EMC problems resulting from interference are
semiconductor device is primarily caused by the random
directly related to three factors: (1) interference sources,
motion of majority carriers crossing a junction and by
(2) transfer media, and (3) equipment susceptibility.
fluctuations in the relative amounts of current flowing
from the emitter to the collector and base of a transistor.
2-2. Interference Sources
The noise power level of a semiconductor device is
a. General. Interference may result from either
greater at the lower frequencies and depends upon the
natural or manmade causes (fig 2-1). While the effects
material of the semiconductors used. The types of
of interference from natural sources are geographically
noise described are strictly-speaking man-made,
widespread, manmade interference (intentional or
because the equipment is man-made. However, they
unintentional) usually affects only limited areas near the
are described here as "natural" to make a point. They
specific sources. An exception is high frequency (HF)
do-not arise from fault conditions or from equipment
maladjustment or any other conditions which can be
which have been refracted from the ionosphere.
remedied by operating personnel.
Nor are they
b. Natural Interference Sources. These sources
intentionally generated. The types of noise described
may be as close to the operator as his own equipment
are "natural" in the sense that they are always present to
and the earth's atmosphere, or as distant as the sun and
varying degrees in all energized electronic equipment
the outer reaches of the galaxy.
and the extent to which they can be minimized in
practice, is dependent on the funds available during the
design and construction stages of the equipment.
(1) Equipment noise.
This noise, self-
generated within electronic equipment, is generally