Download EMI EMC unit 2

Definitions
• Electromagnetic Compatibility : (EMC)
The capability of electrical and electronic
systems, equipment, and devices to operate in
their intended electromagnetic environment
within a defined margin of safety and at design
levels or performance without suffering or
causing unacceptable degradation as a result of
electromagnetic interference [American National
Standards Institute (ANSI) C64.14-1992)].
Electromagnetic Interference : (EMI) :
• The process by which disruptive
electromagnetic (EM) energy is
transmitted from one electronic device to
another via radiated or conducted paths
(or both).
• In common usage, the term refers
particularly to RF signals; however, EMI is
observed throughout the EM spectrum.
• Radio Frequency. (RF)
• A frequency range containing coherent EM
radiation of energy useful for communication
purposes—roughly the range from 9 kHz to 300
GHz.
• This energy may be emitted as a by-product of
an electronic device’s operation.
• Radio frequency is emitted through two basic
mechanisms
• Radiated Emissions.
• The component of RF energy that is emitted through a
medium as an EM field. Although RF energy is usually
emitted through free space, other modes of field
transmission may be present.
• Conducted Emissions.
• The component of RF energy that is emitted through a
medium as a propagating wave generally through a wire
or interconnect cables. Line-conducted interference (LCI)
refers to RF energy in a power cord or alternatingcurrent (AC) mains input cable. Conducted signals
propagate as conducted waves.
• Susceptibility.
• A relative measure of a device or a system’s
propensity to be disrupted or damaged by EMI
exposure to an incident field. It is the lack of
immunity.
• Immunity.
• A relative measure of a device or system’s ability
to withstand EMI exposure while maintaining a
predefined performance level.
• Electrostatic Discharge (ESD).
• A transfer of electric charge between
bodies of different electrostatic potential in
proximity or through direct contact.
• This definition is observed as a highvoltage pulse that may cause damage or
loss of functionality to susceptible devices.
• Radiated Immunity.
• A product’s relative ability to withstand EM
energy that arrives via free-space
propagation.
• Conducted Immunity.
• A product’s relative ability to withstand EM
energy that penetrates through external
cables, power cords, and input–output
(I/O) interconnects.
• Containment. A process whereby RF energy is
prevented from exiting an enclosure, generally by
shielding a product within a metal enclosure (Faraday
cage or Gaussian structure) or by using a plastic housing
with RF conductive coating.
• Reciprocally, we can also speak of containment in the
inverse, as exclusion—preventing RF energy from
entering the enclosure.
• Suppression. The process of reducing or eliminating RF
energy that exists without relying on a secondary
method, such as a metal housing or chassis.
• Suppression may include shielding and filtering as well.
•
There are five major considerations when performing EMC analysis on a product
or design [1]:
•
1. Frequency. Where in the frequency spectrum is the problem observed?
•
2. Amplitude. How strong is the source energy level and how great is its
potential
to cause harmful interference?
•
3. Time. Is the problem continuous (periodic signals) or does it exist only during
certain cycles of operation (e.g., disk drive write operation or network burst
transmission)?
•
4. Impedance. What is the impedance of both the source and receptor units and
the impedance of the transfer mechanism (related to separation distance, which
affects wave impedance) between the two
•
5. Dimensions. What are the physical dimensions of the emitting device (or
device groups) that cause emissions to be observed? The RF currents will
produce EM fields that will exit an enclosure through chassis leaks that equal
significant fractions of a wavelength or significant fractions of a “rise time
distance.”
Problems during emission testing
• Equipment Setup and Environment.
• Most products require use of support (auxiliary)
systems to ensure functionality. Auxiliary
equipment may be located directly adjacent to
the EUT or be remote. If remote, routing cables
between systems play an important part in the
setup. Cables may be routed under the floor or
overhead.
• The following problems commonly happen
during equipment setup:
• 1. Ambient Assessment.
• 2. Mismatch and VSWR Errors.
• 3. Background Noise within
Instrumentation Setup.
TIME-DOMAIN VERSUS
FREQUENCY-DOMAIN
ANALYSIS
• It is common for digital design engineers to think in terms
of a time frame or in the time domain.
• Electromagnetic interference is generally viewed as a
frequency spectrum or in the frequency domain. Radiofrequency energy is typically a periodic wave front that
propagates through various media.
• Different wavelengths of a sine wave are recorded as
EMI for those products that are not designed to be
intentional radiators. It is difficult to understand an EMI
problem in the time domain alone.
• All digital transitions (when viewed in the time domain)
produce a spectral distribution of RF energy (frequency
domain).
• Conversely, a series of fast slew rate sine
waves appear as a digital transition pulse.
• In other words, a time-domain waveform
may be defined as a set of sine waves and
may be combined graphically or
mathematically, but not physically, into a
time waveform.
• Coupling is a significant aspect of signal
propagation. When performing testing and
troubleshooting, we need to understand
the types of coupling we deal with and
how they affect overall system operation.
. Understanding the 2 modes of current
flow.
• Differential and Common-mode.
• For almost every type of communication,
differential mode is desired; however,
common mode is generated due to various
reasons.
• Electromagnetic compatibility problems
are mainly common mode.
• The Poynting vector is a convenient method for
expressing the direction and the power of the
EM wave with units of watts per square meter
(W/m2).
• In the far field, both electric and magnetic field
components are at right angles to each other
and perpendicular to the direction of
propagation.
• There is no such thing as an electric wave or a
magnetic wave by itself.
• 1. Current Amplitude in Loop.
• 2. Orientation of Source Loop Antenna
Relative to Device Under Test.
• 3. Size of Loop.
• 4. Distance.
The fields created by an electric source are a function of four variables:
1. Current Amplitude in Dipole. The fields created are proportional to the amount
of current flowing in the dipole.
2. Orientation of Dipole Relative to Measuring Device. This is equivalent to the
magnetic source variable described earlier.
3. Size of Dipole. Fields created are proportional to the length of the current
element.
This is true if the length of the trace is a small fraction of a wavelength.
4. Distance. Electric and magnetic fields are related to each other. Both field
strengths fall off with increasing distance. In the far field, the behavior is similar
to that of the loop source. When we move in close to the point source, both
magnetic and electric fields have a greater dependence on distance from the
source.
Propagation of RF energy, both electric and magnetic fields, can be represented
as equivalent component models that help describe their propagation mode with
an antenna structure as the visual display.
A time-varying electric field between two conductors can be represented as a
capacitor configuration (dipole antenna).
A time-varying magnetic field between these same two conductors is represented
by mutual inductance (loop antenna)
Figures 2.3a,b illustrate these two coupling configurations
Common Impedance coupling
• Common-impedance coupling develops when
both source and victim share a transmission
path through a common impedance. The most
frequent example of common-impedance
coupling is found in a shared reference
connection between modules.
• This shared reference is usually a return wire
within a cable assembly or in the earth ground
reference of the power distribution system.
• Figure 2.5 illustrates two means of commonimpedance coupling in a simplified manner
• Electromagnetic Field Coupling
• Electromagnetic field coupling is a combination
of both magnetic and electric fields affecting a
circuit simultaneously. Depending on the
distance between source and receptor, the
electric field (E) and magnetic field (H) may be
operatively dominant, depending on whether we
are in the near field or far field.
• This is the most common transfer mechanism
observed by measurement with an antenna.
•
Conductive Coupling
• The process of conduction between a source and
receptor involves transference of an EM field through a
metallic interconnect.
• Interference energy can be carried between power
supply lines and signal transmission cables.
• For example, interference between systems plugged into
the same electrical outlet may share undesired RF
energy, causing harmful interference or disruption of
functionality.
• Conductive transfer can occur through commonimpedance coupling.
• This happens when both the noise source and
susceptible circuits are connected by mutual impedance.
Radiated & Conducted coupling
combined :
Conducted Emissions
This is the first objective of the LISN—to present a constant impedance
to the product’s power cord outlet over the frequency range of the
conducted emission test.
second objective of the LISN—to block conducted emissions that are
not due to the product being tested so that only the conducted emissions of
the product are measured.
Power supply filters
• Filters are typically characterized by their
insertion loss (IL), which is typically stated in dB.
• Consider the problem of supplying a signal to a
load as shown in Fig. 6.6a.
• A filter is inserted between the source and the
load in order to prevent certain frequency
components of the source from reaching the
load, as shown in Fig. 6.6b.
RF & Transient Immunity
• Conducted rf immunity standards for commercial
products typically require that the product
operate properly without degradation
(performance criteria A) when an rf voltage of 3
V (for residential/commercial products) or 10 V
(for industrial equipment), 80% amplitude
modulation (AM) from 150 kHz to 80 MHz is
coupled common-mode into the alternating
current (ac) power cables (EN 61000-6-1, 2007).
• The test also must be applied to signal cables, to
direct current (dc) power cables and ground
conductors if they are over 3 m in length.
• The voltage is applied as a common-mode
voltage to the cable conductors.
• Radiated rf immunity standards for
commercial products typically require that
the product operate properly without
degradation (performance criteria A) when
exposed to an electric field of either 3 V/m
(for residential/commercial products) or 10
V/m (for industrial equipment), 80% AM
modulated from 80 to 1000 MHz.
• Higher field strengths, up to 200 V/m, are
applicable for automotive and military
products.
RE
• Can be done in OATS.
• Can be carried out in a shielded anechoic
chamber.
• Dipole antennas & other broad band
antennas like log-periodic, biconical horn
can be used.
• Vertical and horizontal polarization.
• All test equipment in ante-room.
RS
• Use of dipole and other broad band
antennas.
• Test instrumentation is ante room.
CE
• CE…..Evaluation.
• Always with LISN.
CS
• Injecting CM disturbance.
• Injecting DM disturbance.
• Backfilter to prevent injected EMI from
reaching the power source.
Pulsed Interference Immunity
• Transients.
• ESD………On the equipment.
• EFT ………On all A.C.& D.C.power lines,
signal lines, control lines, I/O ports.
• Lightning Surges…..On A.C and D.C
power lines.