Chapter I
Introduction to the
EMI Problem
All forms of electronic equipment, particularly
radio frequency communications equipment,
suffer from electrical and electromagnetic in-
terference. To electronic systems such signals
constitute a serious form of pollution. The ef-
fects may range from merely annoying (e.g., a
minor interfering buzz on a radio) to cata-
strophic (e.g., the crash of an airliner). There
are a number of sources (Figure 1.1) of this
noise. Some of these can be dealt with at the
source; others are beyond our ability to affect
at the source and so must be dealt with at the
receiver end. The collective term for this pol-
lution is electromagnetic interference (EMI);
the ability to withstand such assaults is called
electromagnetic compatibility (EMC).
We are all familiar with lightning bolts.
The lightning oscillates back and forth be-
tween positive and negative ends very rapid-
ly, and is effectively a fast rise-time pulse. As
a result, it will have significant harmonics
well into the low-band, although the peak is
below 500 kHz. Short blasts of static charac-
terize this noise.
The 60-Hz alternating current (AC) pow-
er lines are a significant source of noise, espe-
cially in the lower frequency bands (including
the AM broadcast band). This may seem coun-
terintuitive because those bands are so low in
frequency compared with the bands we are
discussing. The problem is that the harmonics
of 60 Hz extend well into the low-band region
of the spectrum. Although they may be down
many dozens of decibels from the fundamen-
tal, the high voltage and high power levels of
the fundamental mean that those "way down"
harmonics are still significant to radio receivers
at short distances. This situation is seen in the
spectrum chart of Figure 1.2.
Several mechanisms are found in 60-
Hz AC interference. First, of course, is radia-
tion from the high-voltage distribution lines
and the local lower voltage residential feed-
ers. The transformers also radiate signals. If
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