General information on EMC
and filter design using discrete chokes
(dBµV)
EMC compliance is now a
73
79
Quasi-peak
fundamental element of the
Average
66
60
electrical/electronics equipment
design process, with legislation in
Europe to make compliance
INDUSTRIAL
obligatory. This section provides
an introduction to interference and
noise limits - using the influential
30
0.15
0.5
5
Frequency (MHz)
European standards as an example
- with an introduction to the three
Figure 2.
Permissible interference limits
main forms of choke components
for EN 50081-2
and their application.
Level
Interference propagation
EMI can propagate by two means:
• by radiation - where the energy can be
coupled either through magnetic or
electric fields, or as an electro-magnetic
wave between the source and victim.
• by conduction - where the EMI energy
will propagate along power supply and
data cables.
Radiated and conducted EMI cannot be
thought of as totally separate problems
because noise conducted along a cable
will, to some extent, be radiated because
the cable will act as an antenna. The
radiation will increase as the cable length
becomes comparable to the wavelength of
the noise. Also, the cable will act as a
receiving antenna and pick up radiated
interference.
Below about 100-200MHz, the most
efficient radiators in a system are usually
the power supply and data cables. Proper
filtering of these cables will reduce
radiation due to the cables as well as
conducted interference.
Above about 100-200MHz, PCB tracks and
Permissible noise limits
The various standards set down limits for
conducted EMI emissions. These limits are
measured in voltage and given in dBµV
where 0dB is 1µV. The interference is
measured using a measurement receiver
which has defined bandwidths and
receivers. The two receivers used are a
quasi-peak detector, and an average
detector. To ensure repeatability of the
measurements, the impedance of the
mains supply must be constant. The
standards calls for a defined artificial mains
network - sometimes called a line
impedance stabilization network (LISN) -
which gives a defined impedance to the
noise and also helps filter any noise on the
mains which may affect the measurements.
Figure 1 shows the limits for EN 50081-1,
which is the European generic standard for
residential, commercial and light industrial
environments, and Figure 2 shows the
limits for EN 50081-2, which is the
European generic standard for the
industrial environment.
Interference sources and
spectrums
The most common sources of conducted
EMI are power electronic products such as
switched mode power supplies (SMPS),
pulse width modulated (PWM) frequency
inverter motor drives and phase angle
controllers.
The emissions spectrum typically starts off
very large at low frequency and rolls off as
frequency increases. The point at which
the noise falls below the permitted limits
depends on several factors, the most
important being the frequency of operation
and the switching time of the
semiconductor devices.
Interference spectrums generated can be
either continuous, as in the case of phase
angle controllers (see Figure 3), or discrete
(see Figure 4), which is typical of an SMPS.
Level
Pegel
dB
dB
short internal cables will start to become
efficient radiators. To reduce this radiation
PCBs should be laid out to reduce track
length and loop areas; ground planes
should be used if possible. Decoupling of
digital ICs is very important and shielding
may be necessary.
Interference types
To understand the problems associated
Above 30MHz, radiated noise interference
is measured as radiated noise instead of
conducted noise. This takes place on an
open field test site using defined antennas.
Level
(dBµV)
66
56
46
56
46
60 Quasi-peak
50 Average
with conducted EMI it is first necessary to
understand the two modes of conducted
Frequency Hz
Frequenz Hz
propagation: differential mode
(symmetrical mode) and common mode
(asymmetrical mode). Differential mode
interference appears as a voltage between
the phases of the system and is
independent of earth; the differential mode
currents flow along one phase and return
along another phase (see Figure 5).
Common mode noise appears as a voltage
between each phase and earth. The
Figure 3.
Continuous spectrum
Level
dB
LIGHT INDUSTRIAL & DOMESTIC
0.15
0.5
30
5
Frequency (MHz)
Frequency Hz
Figure 1.
Permissible interference limits
for EN 50081-1
common mode currents flow from the
noise source to earth (usually via a
Figure 4.
Discrete spectrum
3
ETC [ ETC ]
