[pcbforum] FUNDAMENTALS OF EMC
- From: "vishwa" <vishwa@xxxxxxxxxxxx>
- To: <pcbforum@xxxxxxxxxxxxx>
- Date: Sat, 14 Sep 2002 11:49:28 +0530
The Evolution of EMC
During a job interview, a young candidate when asked what he understood by EMI
replied: "Whenever something goes wrong with a piece of electronic equipment
and you cannot explain why - that's EMI!" A good explanation, but perhaps a
little more depth is needed.
Radio Frequency Interference (RFI) was the precursor to Electromagnetic
Interference (EMI). Regulations concerning RFI have existed since the early
50's. These regulations were primarily concerned with interference to radio and
TV, hence the name Radio Frequency Interference. Due to increasing problems of
RFI in a wide range of environments, more specific requirements were produced.
The International Electrotechnical Committee (IEC) through CISPR (International
Special Committee on Radio Frequency Interference) developed recommendations to
test and measure interference. As RFI and its propagation became better
understood and with the expansion of the usable frequency spectrum, EMI
(Electromagnetic Interference) became the new buzzword.
Military disciplines were concerned, not only with EMI emissions emanating from
their equipment, but also with the susceptibility of their sensitive electronic
equipment to EMI in the environment. Electromagnetic Compatibility (EMC)
encompasses both the emissions and the immunity (or susceptibility) portion of
EMI. The proliferation of sensitive electronic equipment within the commercial
environment made EMC the new concern.
Electromagnetic Compatibility
EMI can be described as the degradation of a device or system caused by an
electromagnetic disturbance. An electromagnetic disturbance is any
electromagnetic phenomena which may degrade the performance of a device,
equipment or system, or adversely affect living or inert matter. An example of
EMI affecting living matter is the current controversy regarding portable
cellular telephones causing brain tumors. Therefore, an electromagnetic
disturbance can be an unwanted signal or even a change in the propagation
medium itself. A change in the propagation medium can attenuate the signal and
have a direct effect on the level of disturbance. EMC, on the other hand, can
be described as the ability of different pieces of electrically operated
equipment to work in close proximity to each other without causing any mutual
interference. EMC therefore implies the ability of equipment to function
satisfactorily in its electromagnetic environment without introducing
intolerable electromagnetic disturbances to any other equipment in that
environment. EMC is a twofold occurrence and consists of emissions and immunity.
First, EMC implies that the equipment will not generate unacceptable
interference emission levels which could cause interference (the emissions
portion); and second, EMC implies that the equipment's intrinsic immunity
levels are such that it can tolerate ambient levels of interference without
degradation of performance (the Immunity portion). Therefore, EMC means that a
device must be capable of operating in all modes in the environment for which
it was designed without degrading its own performance or that of any nearby
equipment.
Sources of Electromagnetic Interference
An Electromagnetic Environment can be described as the electromagnetic
conditions existing at a given location. The EMI environment includes
interference emanating from natural sources like lightning and atmospheric
static to the various man-made sources of interference such as vacuum cleaners,
washing machines, power tools, computers, cellular phones, mobile radios and
even electronic toys. Natural sources can be either terrestrial or
extraterrestrial in nature. Man-made sources include intentional or
unintentional radiators (see figure 1 below). Within the scope of man-made
noise sources we can break it down even further into Intersystem interference
and Intra-system interference. Inter-system interference is EMI in a system
caused by an electromagnetic disturbance generated by another system; whereas
Intra-system interference is self-generated EMI present in a system.
There is very little that can be done to prevent electromagnetic energy
generated from natural interference sources. However, natural sources do not
create that much of a problem except for perhaps, surges and spikes on power
lines induced by lightning strikes. It is also very difficult to prevent EMI
from intentional sources of electromagnetic energy. Cellular telephones and
two-way radios are a major problem and can create havoc for example in hospital
environments. It is therefore crucial that electronic equipment be made immune
or less susceptible to environmental interference.
However, the major source of all interference is generated from unintentional
manmade sources. This is due to the vast amount of electrical and electronic
equipment in use.
The Three Elements of an EMI Problem
There are three essential elements to any EMC problem. There must be an EMI
source or an electromagnetic disturbance, a receptor or "victim" that cannot
function properly due to the electromagnetic phenomenon, and a path between
them that allows the source to interfere with the receptor. This is shown in
figure 2 below. Each of these three elements must be present (although they may
not readily be identified) at the same time in order to have an electromagnetic
disturbance or EMI. EMC problems can be solved by identifying at least two of
these elements and eliminating or attenuating the interference from one of them.
Characteristics of an EMI Source
Interference signals are established whenever electrons move. Therefore, any
current flow may cause either direct coupling to other circuits or radiated
fields, which may in turn couple unwanted signals into other circuits.
Sources of interference can be characterized by their frequency, bandwidth and
amplitude. The frequency spectrum line chart shown in figure 3 depicts the
frequency range with respect to EMC. The frequency range of concern for EMI
from electrical apparatus is the harmonic range from 50 Hz (fundamental of the
mains frequency) to 2 kHz (the 40th harmonic of the mains frequency), the
conducted range from 150 kHz to 30 MHz and the radiated emission range from 30
MHz to 300 MHz and higher. Although we now only measure conducted emissions for
the EU from 150 kHz, there is discussion of increasing the spectrum and
starting from as low as 9 kHz. This could make it more difficult for
manufacturers of SMPS, Inverters and SCR circuits to comply with the
regulations.
The propagation medium of EMI below 30 MHz tends to be mains-borne or
conducted. The interference travels along the power cord or signal lines from
the source to the receptor or victim circuit. The conducted interference is not
easily attenuated over distance.
The radiated portion of EMI emissions is borne as an electromagnetic wave,
propagating through the air or any other non-conducting media. Generally, the
higher the EMI in the frequency spectrum, the more easily it will radiate. EMI
and EMC are becoming more of a problem due to the trend to produce equipment in
smaller packages operating at very high speeds and processing rates.
The use of higher speed switching logic increases emissions from printed
circuit boards. Also the use of devices with low operating voltages and
currents, packaged more closely together, increases the potential for
intra-system interference and reduced immunity (increased susceptibility).
Bandwidth
One cannot sufficiently classify EMI disturbances in terms of frequency content
only. The character of the signal also needs to be examined. The character of
the interference signal can be described as either narrowband or broadband in
nature. Classification according to bandwidth is determined from the ratio of
the EMI signal to a reference bandwidth. This ratio can be given in a manner
derived from both the measuring receiver bandwidth and the characteristics of
the disturbance signal. Therefore, an electromagnetic emission is classified as
broadband if while tuning the measuring bandwidth over a range of two impulse
bandwidths (IBW) around its center frequency, a change in peak response of 3 dB
or less is detected. If a change of 3 dB or more is depicted the signal would
be determined to be narrowband. Therefore, bandwidth is a function of the
measuring receiver. Unintentional radiating sources usually exhibit wide
bandwidths (broadband noise) and usually have high magnitudes. Deliberate noise
sources like cellular telephone,
Radio and TV transmitters, etc., will generate narrowband signals. Examples of
broadband noise sources are typically brush motors, inverter circuits, SCR
circuits and more. Single frequency signals can be described as narrowband. A
sine wave is a pure tone and contains only one single frequency. However, a
square wave such as produced by a digital switching circuit or pulse, contains
more than one frequency, comprising of the fundamental frequency and harmonics.
Each harmonic, therefore, represents a narrowband source. Military Standards
and some of the older CISPR standards referred to test procedures and limits
using Narrowband and Broadband detectors. However, the EU European Norm (EN)
standards specify the use of Quasi-peak and Average detectors for measuring EMI
levels.
A Quasi-peak detector has specified electrical time constants which, when
regularly repeated identical pulses are applied to it, delivers an output
voltage which is a fraction of the peak value of the pulses, this fraction
increasing towards unity as the pulse repetition rate is increased. The
quasi-peak EMI detection system is probably the fairest way of assessing
interference as it is based on the annoyance factor of the interfering signal.
The higher the repetition rate of the interfering signal (i.e. the higher the
annoyance factor) the longer and higher the detector stays charged, therefore,
the higher the level recorded. As the repetition rate of the interfering signal
increases, the quasi-peak level approximates the peak level.
The Average detector gives the average value of the envelope of an applied
signal. The average value must be taken over a specified time interval. A
constant signal like a clock signal will be measured with an average detector.
Typically, a broadband signal will be detected as quasi-peak and a narrowband
signal as average. Common Mode and Differential Mode Interference
Electromagnetic disturbances can appear in the form of Common-Mode (CM) and
Differential-Mode (DM) voltage and current components. Differential mode also
called symmetrical noise or interference occurs when noise currents travel
between live and neutral (or return). The differential mode voltage components
are measured between the phase conductors. Differential mode signals are
usually used to convey the desired information and do not usually cause that
much interference as the EMI fields generated by differential currents oppose
each other (180o out of phase)causing a cancellation effect. Common mode
signals, on the other hand are usually the major source of EMI from power &
transmission (all I/O) cables. They cause the cables to behave as monopole
antenna. These currents flow from the phase and neutral conductors to ground
(earth). The circuit for the common mode component is completed by the stray
impedance (capacitance) to ground.
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