Immunity Testing: Practical Aspects of Basic Standards
This article deals with some practical aspects
of testing according to basic standards in the series
IEC 61000-4-N/EN 61000-4-N, as referred to in generic,
general and product family standards. Although a
number of other disturbance phenomena and methods
for simulating different electromagnetic threats
as well as alternative test methods are under development,
bringing today's total in this series up to around
30, only those standards commonly referred to (N
= 2...6, 8, & 11) are dealt with here. Note,
however, that basic standards are used only as references
in application standards (basic standards are not
published as harmonized standards), and therefore
not all of them will be referred to in other standards.
Electrostatic Discharge (ESD) Testing
(EN 61000-4-2/IEC 61000-4-2/ IEC 801-2)
EMC testing of immunity against electrostatic discharges
as defined in the "old" standard, IEC 801-2, edition
1:1984, concerns air discharges occurring when the
charged tip of an ESD generator is brought close
to a test object. The rise time from the generator
is nominally 5 ns, but due to influences on the
air gap during the flashover, the actual rise time
can vary from 5 ns up to 30 ns. It may also happen
that a high voltage level like 8 kV might create
a series of partial discharges, all with much lower
amplitude. These kinds of variations can elicit
quite nonreproducible results. This is why the discharge
principle was modified by IEC 801-2, edition 2:1991
to include an air gap inside the generator, making
contact discharges possible. To ensure that the
discharges will be insensitive to atmospheric influences,
a "discharge switch" is mounted in a closed space
with air of a given composition. The test generator
is nowadays commonly called an ESD pistol.
The discharge voltage is referred to a reference
ground plane (RGP) on the floor, to which the handheld
ESD pistol is grounded by a wire. When testing tabletop
equipment, this should be placed on a wooden or
plastic table covered with a metallic coupling plane.
This in turn is to be connected to the RGP by a
discharge return cable, with a resistance of about
A thin layer of insulation is used to isolate the
EUT from the coupling plane. Due to the fast rise
time, it is very important that the test setup is
done very carefully to obtain reproducible test
results; for example, the pistol shall be held perpendicular
to the tested surface with the return cable kept
at least 0.2 m from the EUT. Note that two discharge
resistors (2 x 470k) should be used for the
discharge return cable, one at each end of the cable.
The cables, in between each resistor and its contact
point at the reference plane and the coupling plane
respectively, should be as short as possible.
According to the new standard IEC 61000-4-2 (IEC
801-2:1991), testing shall start with contact discharges
toward reachable metal surfaces, then as air discharges
toward insulating surfaces. Note that when testing
an insulated test object (e.g., plastic chassis)
and objects without safety earth connection (Class
2) this has to be manually discharged between successive
tests. Besides contact discharges to metal surfaces
on the EUT itself, the revised standard also describes
how to generate indirect pulsed fields by having
the ESD pistol discharged against a vertical coupling
plane placed close to the EUT (see
The revised standard defines a very short rise
time, about 1 ns, when discharging the pistol to
a target with low resistance (noninsulated parts
of the EUT). An oscilloscope is used for verification
of the pulse waveform; the rise time should be within
0.71 ns and peak current as defined in the
standard within +10%. Note that for measuring the
0.7-ns test pulse, the oscilloscope used must have
at least 1 GHz real-time bandwidth. To realize how
short this rise time is, compare it to the speed
of light propagating one foot during one ns.
The test level is often 48 kV contact or
8 kV air discharge, which is fairly realistic for
most practical situations (a contact discharge of
6 kV corresponds to about an 8-kV air discharge).
Tests should be carried out with the amplitude increased
in binary steps.
The standard prescribes that the pistol has to
be fired at least 10 times toward each test point.
Experience shows, however, that interference in
a digital system may show up on the 50th firing
when the ESD pulse happens to coincide with a specific
clock pulse! So when trying to find weak points
during the development stage, it is recommended
that one perform a lot of discharges at each point.
Immunity Testing with Radiated RF Fields
(EN 61000-4-3/IEC 61000-4-3/IEC 801-3)
The basic instruments required for this test are
an RF signal source, a power meter, broadband power
amplifier(s), broadband antenna(s), and measuring
probe(s). The most commonly used antenna types are
biconical antennas or strip lines for frequencies
up to about 200 MHz, log-periodic antennas for higher
frequencies, or combined antennas of the bilog type
to cover 20 MHz1 GHz. Horn antennas are used
at GHz frequencies. Dipoles are mainly used for
calibration and control.
A homogenous electromagnetic field can also be
generated in a guided-wave TEM cell, which has two
flat conducting surfaces to generate a traveling
electromagnetic wave in transfer electromagnetic
mode. Such a TEM cell can be used for very low frequencies
up to about 200 MHz. One type of open TEM strip
line is described in CISPR 20/EN 55020, where it
is used for immunity testing of audio and broadcast
receiving equipment (radio & TV) up to 150 MHz.
A GTEM cell can be seen as an alternative to the
use of antennas in an anechoic chamber. The GTEM
cell is a special kind of enclosed transmission
line device. It is a pyramid-shaped line terminated
with absorbers and resistive loads in the far end.
It can be used up to the GHz region (GHz TEM).
|Figure 2. Testing of immunity against radiated
fields for large test objects requires a big
anechoic or semianechoic chamber (SEMKO EMC
Immunity testing by illuminating the EUT with radio
frequency electromagnetic fields usually has to
be performed in a shielded enclosure in order not
to disturb radio communications in the surroundings.
Tests can be performed in a cell, a shielded tent,
or in a shielded chamber. Any metallic enclosure,
however, gives rise to reflections that would make
it impossible to control the field. It is therefore
necessary to use some kind of lining with absorbers
(see Figure 2). A fully anechoic room is preferred,
but a semianechoic room (with reflecting ground
plane) may also be used if, during the test, the
floor between the antenna and the test object is
covered by absorbing material (ferrites, foam absorbers,
or a combination). There is one exception, the stirred-mode
reverberation chamber. This is a shielded enclosure
with moving reflectors, where the RF field is scattered
around the room in order to achieve the worst possible
situation for the EUT. (Refer to the draft basic
standard IEC 61000-4-21.)
The test object has to be illuminated from several
sides. In a shielded room, testing is therefore
usually performed by placing the EUT on a turntable,
manually or remotely controlled. Using a turntable
in a GTEM cell might, however, cause some problems.
In light of that difficulty, another interesting
approach might be to keep the EUT at a fixed position
and circulate the electromagnetic field.
The antenna is placed in one or several positions
in front of the EUT in such a way that the whole
test object can be illuminated by the field, usually
of 3, 10, or 30 V/m amplitude.
The power needed to generate a certain field strength,
E [V/m], depends on the power at the antenna input,
Pa[W], the distance to the
EUT, r [m], and the antenna gain (factor) G:
Note that the gain G of a broadband antenna varies
with frequency; this explains why the power has
to be adapted for each frequency step. Broadband
antennas, like the biconical type, usually have
a high VSWR at low frequencies and reflect much
of the power back to the amplifier, which has to
be taken into account.
Testing as defined in the "old" standard, IEC 801-3:1988,
can be performed in a nonanechoic shielded room,
by closed-loop control from a measurement probe
placed 2040 cm beside the EUT. Two probes
may be needed in case of large equipment (mean value
or the lowest value is then used for the control).
This method, however, is ambiguous. Modern testing
according to IEC 61000-4-3:1996 refers to a substitution
method, based on calibration in an empty test chamber
(without the EUT). Calibration is usually performed
by measuring, at each frequency, the feed-forward
power needed to generate the defined field strength.
The results from the empty-chamber calibration can
then be used during testing to control the amplifier
output power or the forward power (with correction
for the reflected power).
The electromagnetic field is measured with a field
strength meter (probe), which has a typical accuracy
of about +1 dB. A triaxial dipole probe is preferred,
to make it possible to measure fields of any polarization
without changing the probe direction. A power meter
(typical accuracy about +0.1 dB) is usually connected
to the power amplifier output via a probe. The probe
includes a measuring head with a direction coupler,
making it possible to measure the feed-forward power
as well as the reflected power.
The antenna shall be placed at a fixed height above
the floor, at a suitable distance from the EUT.
A distance of 3 m is recommended, but a shorter
distance is often used when illuminating small test
objects. Tests shall be performed with the antenna
in a vertical and horizontal position respectively.
Tabletop equipment is placed 0.8 m above the floor.
The platform used for this is usually a wooden or
a plastic table without metallic parts (plastic
usually shows better transparency than wood). Floor-standing
equipment should be placed on a nonconducting support
of 0.1 m thickness. If a turntable is used when
testing a system, special arrangements may be needed
to turn around all the cabling.
The RF frequency is usually varied from 27 or 80
MHz up to 1 GHz or more. The dwell time, the time
during which the EUT is continuously illuminated
at each frequency, must be long enough for the EUT
to complete a given working sequence and for monitoring
eventual reaction. If the frequency is swept over
the frequency range, the maximum sweep rate should
not exceed 0.0015 decades a second. When stepping
the frequency, each step should not exceed 1% from
the previous frequency. This involves 231 frequency
steps [1/log (1+1%)] per decade. To step over the
frequency range 80 MHz to 1 GHz with a dwell time
of 3 s thus takes about 20 minutes (including amplifier
settling time). A complete testing with illumination
of vertical and horizontal fields against four sides
of the EUT usually takes a couple of hours, depending
on the software used.
Modern standards require tests to be performed
with a modulated field. This is usually performed
by adding a 1-kHz sinusoidal modulation signal (amplitude
modulation) or by keying the carrier (pulse modulation),
as shown in Figures 3a, 3b, and 3c. Modulation with
keyed carrier is defined in the temporary European
standard ENV 50 204.
|Figure 3a. The unmodulated carrier.
|Figure 3b. Illustrates amplitude modulation
of the RF carrier by adding a 1-kHz sinusoidal
at 80% modulation depth.
|Figure 3c. Modulation with a keyed carrier
simulating a typical GSM, DECT, TD-CDMA, or
other digital mobile communication systems.
The power requirements usually in-crease significantly
at the low end of the fre-quency spectrum, due to
the fact that the effective antenna gain usually
decreases significantly for frequencies under 80
MHz. For generation of homogenous fields in a smaller
room, 150 W might be needed up to 80 MHz, but 30
W might be sufficient for higher frequencies. For
a large anechoic chamber, 1 kW might be needed for
illuminating a large test object, as the antenna
has to be placed at some distance from the EUT.
Since broadband, high-power amplifiers tend to become
rather costly, two amplifiers are often used (each
can then be matched to the frequency range and power
requirements for the antenna used for that range).
Modulating the RF carrier implies that the amplifier
must be linear and capable of producing enough peak
capacity. Modulation at 80% increases the instantaneous
power requirement by a factor of 5.2 dB as to an
The same antenna that is used for emissions measurements
can be used for immunity testing, usually a biconical
and a log-periodic or a combined bilog antenna.
Since the power handling capacity of the antenna
is limited due to the fact that some power will
end up in heat, care must be taken not to blow the
balun (a transformer built into the antenna to adapt
the balanced antenna to the unbalanced coax cable
Distance to the Test Object
The new standard requires that the field is homogenous
within a certain vertical area in which the EUT
is to be placed. Since it is impossible to establish
a uniform field above a reference ground plane,
this has to be covered by absorbing material in
between the antenna and the EUT. For this purpose,
ferrite tiles mounted on plates on wheels are handy
to use in conjunction with a semi-anechoic chamber
used for emission testing (when a RGP is needed).
In the case of tabletop equipment, the uniform area
is required at a height of 0.82.3 m above
the floor. The required vertical area of the uniform
field represents a grid of 16 points within 1.5
x 1.5 m.
Control of the uniformity shall be accomplished
as an empty-chamber calibration without the EUT.
Calibration shall be repeated in frequency steps
no greater than 1% of the start frequency for both
horizontal and vertical polarization. From knowledge
of the input power and the field strength, the necessary
forward power for the required test field strength
can be calculated. The recorded "empty-chamber file"
can therefore be used for controlling the amplifiers
during the test. A probe is, however, normally used
during the tests for supervision. In the case of
a large EUT, illumination may have to be performed
with different antenna positions covering part of
the EUT. In the case of a small EUT, the uniform
area might be decreased (1 x 1 m may be used).
Of the 16 measured points, at least 12 should represent
a uniform field within 06 dB of the nominal
value (see Figure 4). This is sometimes done by
first eliminating four extreme values and keeping
12 reference points to represent a field strength
within 06 dB. However, other methods are also
used. Note that first calculating the mean value
and then eliminating extreme points, which differ
more than ±3 dB, may not lead to the same remaining
reference points. Algorithms for empty-chamber calibration
are usually included in commercially available software
packages; note, however, that these might be based
on different methods for defining the homogeneity.
|Figure 4. Field uniformity is measured
at 16 points of a vertical grid (an empty-chamber
calibration without EUT). At least 75% of these
points („12) must be within a 6 dB spread.
|Figure 5. Testing of immunity against an
induced RF current according to IEC 61000-4-6.
Immunity Testing with Induced RF Disturbances
(EN 61000-4-6/IEC 61000-4-6)
This standard defines RF immunity testing by inducing
radio frequency disturbances into mains, cables,
and into cables connecting the EUT to auxiliary
equipment (AE) (see Figure 5). Testing is usually
performed by applying a voltage via a coupling device
to one cable at a time while keeping the remaining
cables terminated. In case of a screened cable with
multiple lines, the RF disturbance may be applied
directly on the screen as bulk current injection.
The disturbance signal is usually applied to the
cable connected to the input or output side of the
EUT through a coupling network or a clamp. In order
not to allow the disturbance signal to interfere
with the AE, there is also a need for a decoupling
device, consisting of filters with series inductors
or a separate ferrite tube. Coupling/decoupling
impedances will of course affect the induced currents,
depending on all other impedances in the system
(input/output impedances as well as the balanced
impedances to the reference ground plane). An extra
ferrite tube (absorber) can be used to enhance the
decoupling at the auxiliary end to improve reproducibility.
Injection can be performed by three different coupling
methods. Voltage injection via a combined coupling/decoupling
network is the recommended test method. Note that
isolated cables may have to be deisolated or cut,
which, of course, might be a disadvantage. Unscreened
power supply lines are injected through a resistor
and a capacitor in series (Figure 6a) (the capacitor
is needed to prevent the line voltage from affecting
the disturbance source). For screened and coaxial
cables, the disturbance signal is directly injected
via a resistor (Figure 6b). In case CDNs cannot
be used, injection by clamp may be used as an alternative.
The first choice is then an EM clamp, which is a
ferrite clamp providing both E- and H-field coupling
to the cable. A ferrite tube on the auxiliary side
is recommended as mentioned above. The second choice
is a clamp-on current probe, which, like the EM
clamp, is easy to use (nonintrusive method) but
requires more power for the same stimulus due to
higher insertion losses. When using a current clamp,
a separate current transformer is usually used for
measuring the current.
|Figure 6a. Injection by a couple of different
coupling/decoupling networks, CDNs. Unscreened
power supply lines are injected through a resistor
and a capacitor in series.
|Figure 6b. For screened and coaxial cables
the disturbance signal is directly injected
via a resistor.
The different injection methods do not produce
identical results. In order to make tests reproducible,
it is important to stick to the injection method
recommended. Furthermore, it is important to keep
track of the impedances. The power applied to the
EUT will depend on the common-mode impedance at
the EUT port. A low impedance may imply over-testing
if a voltage source is used and under-testing if
a current source is used (vice versa for a high
The output power from the amplifier, and the voltage
across the 50-()
termination connected to the output of the coupling/decoupling
network, is measured with a power meter and a couple
of probes. Calibration is performed by recording
the power needed to give the defined voltage over
the whole frequency range. In order to reduce reflections
due to mismatched impedances, it is recommended
to use a power attenuator (6 dB), placed as close
to the coupling network as possible.
The injection technique requires that the common-mode
impedance at the far end of the cable from the EUT
is defined. Each cable therefore requires a common-mode
decoupling network (including an isolating inductor
or common-mode choke) at its far end, to ensure
the impedance and to isolate any ancillary equipment.
The coupling and decoupling devices can be combined
into one box, the so-called coupling/decoupling
network (CDN). Injection on different cables therefore
re-quires a set of different CDNs. The standard
specifies different CDNs for specific unscreened
cables, for example CDN-M1/-M2/-M3 for unscreened
supply (mains) lines, CDN-AF2 for unscreened nonbalanced
lines, and CDN-T2/-T4/-T8 for unscreened balanced
pair(s). CDN-S1 is used for screened cable (see
The test object is to be placed 0.1 m above an
RGP. All cables must have relevant CDNs placed in
contact with the RGP at a given length. Cables connected
to the AE, at the far end from the EUT (that is,
they are not connected to the EUT) should also be
terminated at a short distance from the AE. This
can be done with CDNs.
The test system should be connected to the power
line by an isolation transformer. This is necessary
for keeping the impedance to the reference ground
correct. If the EUT has a part that is to be manually
controlled by hand, an artificial hand should be
used (refer to CISPR 16).
The test method illustrates the situation where
long cables pick up electromagnetic energy via antenna
effects at lower frequencies. Subunits that are
part of the equipment to be tested, which are connected
by short cables < 1 m, may be considered part
of the EUT (no separate testing on these cables
Injection is often used as an alternative to radiated
field testing in the lower part of the RF frequency
range, usually 150 kHz80 MHz, sometimes up
to 230 MHz. The RF signal is varied over the frequency
range. Frequency step, modulation, and dwell time
are the same as for IEC 61000-4-3/EN 61000-4-3.
Immunity Testing with Electrical Fast Transients,
Bursts (EN 61000-4-4/IEC 61000-4-4/IEC 801-4)
The standard defines an idealized burst (15 ms)
of short pulses repeated every 300 ms. Pulse rise-time
and width are typically 5 and 50 ns respectively,
and the pulse repetition frequency a few kHz (5
or 2.5 kHz). Real transients, occurring when breaking
inductive loads, might have another shape and cause
higher repetition frequencies.
The fact that a modern test generator gives reproducible
bursts is of primary concern when testing for legal
purposes. There have been discussions on the use
of higher repetition frequencies, up to 100 kHz,
but since the test method is well established by
now, it will not be popular to have to exchange
(or rebuild) all burst generators in use. Old generators
with a spark gap, however, are not recommended for
use since they give less-reproducible test pulses
than modern generators.
Control of the pulse shape is easily performed
with an oscilloscope (bandwidth >=400 MHz) connected
by an attenuator to the output of the generator.
The repetition frequency shall lie within 5 kHz
± 20%, and the burst length shall be 15 ms
The test signal is to be applied to power supply
ports, protective earth, and signal and control
ports. I/O lines shorter than 3 m, however, need
not be tested according to the standard. The transients
are injected through a coupling/decoupling network
or a capacitive clamp, and the length of the signal
and power lines between the coupling device and
the EUT shall be 1 m or less. The test setup is
designed for 50-()
loads. For power lines, injection is to be performed
in both common and differential mode, with the signal
applied using a capacitor (33 nF) from a 50-()
source. For I/O cables, the injection is performed
using a distributed capacitance along the cable.
This is usually achieved through a 1-m-long capacitive
Tests have to be performed with the EUT placed
on a reference ground plane. As the rise time is
very fast, it is important that the test generator
is connected with a short, wide strap to the reference
ground. The test setup, with necessary insulating
supports and ground connections as described in
the standard, should be followed in detail to get
a reproducible result.
Immunity Testing with Surges
(EN 61000-4-5/IEC 61000-4-5/IEC 801-5)
|Figure 7a. Common-mode (Ua-c and Ub-c)
and differential-mode (Ua-b) disturbances, respectively.
The standard surge is defined as a "single pulse
with a double exponential waveform." This is usually
simulated with a hybrid generator dimensioned to
generate a voltage surge having a front time and
pulse width of 1.2 and 50 µs, respectively,
at open circuit conditions and a current surge of
8/20 µs into a short circuit. The generator
has an output impedance of 2 W, which means that
a 2-kV open-circuit voltage injection generates
a short-circuit current of 1 kA max. By adapting
the impedance of the injection circuit, tests can
be performed in different modes on different types
of lines. The surge is injected through a coupling/decoupling
network. When testing ac/dc lines, line-to-line
(symmetrical/differential mode) coupling is performed
via a capacitor, and in case of line to ground (asymmetrical/common
mode), via a coupling capacitor in series with a
resistor (see Figure 7a). When testing unshielded
symmetrically or unsymmetrically operated lines,
e.g., telecommunication lines, arrestors are placed
in parallel with the coupling capacitors.
|Figure 7b. True common mode in a three-phase
system can be seen as injection on all wires
with reference to the ground wire or injection
on all wires, including the ground wire with
respect to the cabinet or reference ground plane.
In some applications, instead of common-mode testing
on one wire at a time, "true common mode" testing
is used to better mirror surges as they may appear
in real life. This involves application of the surge
on all wires at the same time. "True common mode"
in a three-phase system can be performed as a surge
injection via parallel coupling devices on all wires
with reference to the ground wire (GND), or injection
on all wires including the ground wire with reference
to the cabinet or reference ground (RGP) (Figure
7b). The latter may be seen as the most realistic
case when simulating a surge caused by a lightning
A regular check is recommended to control the generator
voltage and current, e.g., a daily check of 1 kV
+10% open voltage output, and current 500 A +10%
(with a short circuit plug on the output).
The pulses cannot usually be generated with a repetition
rate of more than 1 pulse per minute due to the
fact that the power otherwise can destroy components
like varistors in the equipment under test. When
testing the power supply inlet of the EUT, the pulse
is to be synchronized to the line frequency and
shall be injected at the following phase angles:
0°, 90°, 180°, and 270°, with
5 positive and 5 negative pulses at each phase angle.
The open circuit test voltage is usually in the
range 0.54 kV +10 %). The level to be used
is usually defined in an applied generic or product
family standard. As an example: +2 kV line-to-earth
and +1 kV line-to-line, respectively, for equipment
intended for use in a residential or light industrial
environment. The test procedure shall also consider
nonlinear current-voltage characteristics of the
EUT. The test voltage therefore has to be increased
by steps up to the defined test level.
If not otherwise specified, the interconnection
line between the EUT and the coupling/decoupling
network should be 2 m or shorter.
Immunity Testing with LF Magnetic Fields
(EN 61000-4-8/IEC 61000-4-8)
This test method describes immersion of the EUT
by a continuous LF magnetic field, obtained by a
current of power frequency flowing in an induction
coil. The test equipment needed for this test includes
a current source, the induction coil, and auxiliary
test instrumentation. The test setup requires a
nonmagnetic ground plane under the EUT of at least
1 x 1 m. The equipment is to be placed on the ground
plane with an insulating support.
Depending on the size of the EUT, induction coils
of different dimensions may be used. The dimensions
recommended are to produce a magnetic field in the
whole volume of the EUT within +3 dB. For small
EUTs, like terminals and instruments, a square-shaped
or circular coil of 1 m side or diameter is recommended.
Double coils of the Helmholtz type could be used
in order to obtain a homogeneous field better than
+3 dB or to extend the volume of testable EUT.
Immunity Testing with Voltage Dips, Short Interruptions,
and Voltage Variations (EN 61000-4-11/IEC 61000-4-11)
Testing is performed by changing the ac voltage
level supplying the EUT during a short period of
"Short interruptions and voltage dips," as defined
in the basic standard, are to be performed with
the following test levels in percentages of the
rated voltage: 0% (100% dip = interrupt), 40% (60%
dip), 70% (30% dip). The change of these values
with the load shall be maintained within 5%.
The dips are applied as abrupt voltage changes
occurring at zero crossings of the mains voltage
+10%), unless otherwise specified in a relevant
product standard (other angles might be considered
more critical for certain products). The duration
of a dip is usually specified as a number of periods
of the mains frequency, ranging from half a period
up to 1 second or more.
Voltage variations are gradual changes of the supply
voltage to a higher or lower value than the rated
mains' voltage. Voltage variation testing is usually
The generator shall have a low output impedance,
predominantly resistive (< 0.5 ).
During an interrupt, the generator shall be able
to act as a low impedance source capable of giving
an inrush current with a peak value up to 500 A
for 220240-V mains, or 250 A for 100120-V
mains. For dip testing, the generator must be capable
of carrying 40 A at 40% of rated voltage for a duration
of up to 5 seconds.
Tests are performed with the test generator connected
to the EUT with the shortest power supply cable,
as specified by the EUT manufacturer. If no cable
length is specified, 1 m should be used.
When referring to one and the same basic standard,
generic or product family standards may define specific
requirements concerning immunity parameters. For
example, voltage interruption according to the generic
standard is defined as a 5-second interval, while
the product family standard for household equipment
defines this interval as 0.5 period (10 ms at 50