A hands-on guide to finding the sources of electromagnetic interference and then fixing the problems. Includes basic theory of EMI as well as detailed explanations of why this problem is becoming more serious as the international scope of the communications and electronics industries grow. This book is not a textbook, but rather a handbook that will become a constant source of reference for anyone who runs into trouble with EMI. Includes chapters on grounding, circuit shielding and filtering, preventing EMI in circuit design, as well as EMI sources such as power lines, transmitters, television, consumer electronics, telephones, automobiles, and the ever-frustrating mystery EMI.
There are very few other books available even though EMI is constantly discussed and cursed. Most of the books on the market are about how to prevent EMI in circuit design or approaches to understanding the theory behind EMI. Though this information is important, especially to an engineering audience, these books hold no value at all to the technicians and hands-on practitioners in the fields of communications and servicing.These savvy professionals know that the book they are looking for and need is just not on the market. To get the information they need, this group is forced to read every magazine article they can find on the subject and rely on the advice of other professionals whether through technician groups or newsgroups. This book fills a void in the telecommunications and electronics industries by providing practical troubleshooting information.
Addresses the technician's needs and interests
Written by an eminent authority in the field
Covers correction and prevention of problems with EMI
Introduction to the
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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Typical sources of noise pollution in the environment.
any of the connections in the electrical cir-
cuit are loose or corroded, then the possibil-
ity of higher order harmonics increases
Once you get inside the building, the
electromagnetic interference (EMI) situation
deteriorates rapidly from room to room.
Computers send out very large signals, espe-
cially if one is so unwise as to buy unshielded
interconnection cables. Light dimmers, mi-
crowave ovens, motors on appliances and
heating equipment, appliances, and electric
blankets all have the potential for creating EMI.
Television sets and VCRs are particularly
troublesome in populated areas. In my neigh-
borhood, the housing density is moderately
high. I can tell by listening to a high-quality
shortwave receiver when a popular television
show is being aired. How? Try listening to the
low bands! In the United States and Canada,
television receivers follow the NTSC color TV
standard (which some claim means "Never
Twice Same Color"). This means that the hor-
izontal deflection system operates at 15,734
Hz. It is a high-powered pulse with a moder-
ately fast rise time. The harmonics of 15,734
Hz are found up and down the radio dial all
the way up to about 20-MHz.
To make the situation worse, the NTSC
color subcarrier operates at 3.58 MHz and of-
ten has enough power to radiate through
poorly shielded television and VCRs. The sit-
Introduction to the EMI Problem 3
HARMONICS OF 60 Hz
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Fig,. 1.2 Harmonics of 60-Hz power line currents can extend well into the RF spectrum.