Download 3c-emissions compliance testing LISN 2008-2009

EMC and COMPLIANCE ENGINEERING
EMISSIONS 3c: CONDUCTED EMI EMISSIONS
COMPLIANCE TESTING - LISN
1. LISN OBJECTIVE
Conducted disturbances, as per CISPR22ed.2, are measured between the phase lead and the
reference ground and between the neutral lead and the reference ground, and both measured
values are required to be within the limits. No measurement is made with respect to the
ground lead.
The typical line (ac mains) source impedance of utility networks varies widely hence
repeatability and comparative performance of systems and measurements requires some form
of standardisation of measurements.
Test measuring instrumentation tend to present a 50  impedance to the EUT interference
currents which is much greater than low source impedance however this is acceptable (at
least for comparative purposes) as the measured interference is dependent on source
impedance which is not constant!
Line Impedance Stabilising Networks (LISNs), sometimes called Artificial Mains Networks
(AMNs), are transducers that interface between the AC source and the EUT input power lines
to provide a source of known and 'stabilised' impedance to conducted emissions without
affecting normal 50Hz or 60 Hz performance.
Standard’s documentation specifies the use of a LISN as the transducer for measuring
conducted emissions, without the need for a current transducer, and provides full details of
the LISN and the appropriate test set up including the requirements of the ground plane.
LISN
L
AC
INPUT
TRIPORT

EUT
L
emc
analyser
N
EUT
N
LISN
ANALYSER
gnd
gnd
FIGURE 1a
FIGURE 1b
The LISN, as shown in Figure 1a, is effectively a 2 channel (L and N) triport; mains ac input
at port 1, the EUT ac input at port 2 and the emission voltage to be measured outputted at port
3.
Measurement at either L-G or N-G is the sum of DM and CM emissions with the nonmonitored input terminal internally 50 loaded. The LISN therefore incorporates a switch,
as shown in Figure 1b, to enable the connection of the measuring receiver to the conductor
under test and providing the correct termination to the other mains conductor.
The primary objective of the LISN is to make the line appear to be 50 ohm.
EET422 EMC & Compliance Eng
EMISSIONS-3c COMPLIANCE TESTING
LISN
1
Prof R T Kennedy
2008-2009
2. LISN STRUCTURES
A variety of LISNs are commercially available having different structures dependent on the
particular Standard’s measurement frequency bands; however all are required to present 50 
impedance to the EUT.
L presents low impedance

L or N
F
AC
SOURCE
F
k
analyser
or 50 load
F
EUT
G
FIGURE 2
The single cell structure, shown in Figure 2, referred to as a 50 50H LISN, is basically
a low pass  filter designed to isolate the EUT from the mains with respect to rfi by steering
(diverting) the interference to a 50 loadedport (analyser input).
The 50 input impedance of the test receiver together with the 50H inductor determine the
rf impedance presented by the LISN with respect to the ground terminal of the LISN. The
ground terminal, the ‘zero reference’ for the test set-up a must therefore be a low rf
impedance ground plane.
L
L
2 F
L
50
7.5 F


2 F

7.5 F
k
E
U
T
analyser
50 
k k
G
N
0.47 F
0.47 F
G

k

0.47 F 0.47 F
50
N
L


G
N
FIGURE 3a
FIGURE 3b
Figure 3a is an alternative unit, referred to as a 50 (50H+5) LISN that includes both
the Line and Neutral networks. The single phase LISN is sometimes referred to as a ‘V’
network as shown in Figure 3b.
The levels of conducted emissions are measured as a voltage at the output of the LISN and
are a function of the EUT impedance (source impedance) and the LISN impedance (load
impedance). The 1 kresistors provide a discharge path for the 0.25 F coupling capacitors
to avoid damaging the sensitive and expensive analyser’s RF input circuitry.
In the case of a three phase supply each of the phase lines would have an associated single
cell unit.
EET422 EMC & Compliance Eng
EMISSIONS-3c COMPLIANCE TESTING
LISN
2
Prof R T Kennedy
2008-2009
L
L
250 
L
50
0.47 F
7.5 F
2 F

k
G
k

E
U
T
analyser
50 
G

2 F
N
0.47 F
k
7.5 F
250
250 


0.47 F
50
N
FIGURE 4
The single cell structure is generally not suitable for measurements at frequencies below 150
kHz and is replaced by a double cell structure, as shown in Figure 4, that includes the
increased inductance (250 H) required to maintain the defined 50  impedance.
3. SAFETY FACTORS and EARTHING
FIGURE 5
Capacitors in the LISN circuit (10 F) result in a ground current ( 0.75 A) and if the
LISN’s earth connection is not reliably bonded to the safety earth of the incoming mains
supply then the LISN casing and anything connected to it will be ‘live’ and will present a
serious risk of electric shock. The LISN case must therefore be properly bonded to the mains
safety earth before connecting the mains supply, as shown in Figure 5.
LISNs cannot be used directly on a mains supply circuit that is protected by a Residual
Current Device (RCD) or Earth Leakage Circuit Breaker (ELCB) as the continuous earth
current will ensure that the circuit breaker stays permanently tripped; 30mA is a typical safety
trip level.
A residual current device (RCD), similar to that of a residual current circuit breaker (RCCB), is an
electrical wiring device that disconnects a circuit whenever it detects that the electric current is not
balanced between the phase and neutral conductors e.g. imbalance caused by current leakage
through a grounded human body that accidentally touches the energised part of a circuit. RCDs are
designed to disconnect quickly enough to mitigate the harm and avoid lethal shocks.
EET422 EMC & Compliance Eng
EMISSIONS-3c COMPLIANCE TESTING
LISN
3
Prof R T Kennedy
2008-2009
isolating
transformer
L
N
G
E
FIGURE 6
If an unprotected supply circuit is not available a suitably rated isolation transformer between
the supply and the LISN can be used, as shown in Figure 6.
Alternatively an Earth Line Choke can be introduced to isolate the EUT from rf ground while
maintaining a safety ground.
EUT
LISN
LISN
L
L
E
E
vcm
vdm
N
vcm
E
N
N
EUT
L
E
N
vcm
vdm
CS1
vcm
CS2
CS
G
G
FIGURE 7a
FIGURE 7b
Repeatable measurements require EUT stray capacitance to be controlled and EUTs that are
intended to be separately earthed should use an earth strap.
EUTs that are not intended to be separately earthed, or if the case is non-conductive
(insulating), then the EUT must be positioned a fixed distance from the ground plane and at a
fixed orientation to it.
Stray capacitance between the EUT and the ground plane is a critical component for
measurement repeatability. Non-conductive casing EUTs have stray capacitance CS between
the internal circuits and the ground plane, as shown in Figure 7a. Conductive casing EUTs
have CS1 from the internal circuits to the case and CS2 from the case to the ground plane, as
shown in Figure 7b.
EET422 EMC & Compliance Eng
EMISSIONS-3c COMPLIANCE TESTING
LISN
4
Prof R T Kennedy
2008-2009
LISN
L
E
N
EUT
LISN
L
E
N
L
vcm
vdm
E
CS1
vcm
N
EUT
L
E
N
vcm
vdm
CS2
CS2
G
G
FIGURE 9a
FIGURE 9b
Connecting the case to the ground plane via the mains lead, as shown in Figure 9a does not
eliminate the stray capacitive coupling path; it merely modifies the path and can introduce
resonance between the stray capacitance and the mains lead inductance that can increase the
coupling at certain frequencies. A ground strap, as shown in Figure 9b, bypasses the stray
capacitance and is the preferred approach.
FIGURE 10
The shape and size of the LISN ground strap and its proximity to the ground plane has
negligible effect below 15 MHz but the ground strap inductance and the natural parasitic
capacitance between LISN case and ground can result in resonance effects in the 10 MHz
z range as shown in Figure 10.
EET422 EMC & Compliance Eng
EMISSIONS-3c COMPLIANCE TESTING
LISN
CS1
vcm
5
Prof R T Kennedy
2008-2009
4. LISN OPERATIONAL PERFORMANCE
Line Impedance Stabilisation Networks (LISNs), specialised low pass filter networks used to
measure conducted emissions on the mains power lines, perform three functions

provide, in conjunction with the analyser, a defined rf impedance to the emissions
between L-G and N-G; (as per sec. 3 )

isolate the 50 Hz (or 60Hz) ac power mains from the EUT such that any noise on the line
will NOT be coupled to the EMC analyser and compromise the measured emissions by
being interpreted as EUT generated noise.

couple rf emissions generated by the EUT to the EMC analyser
4.1 LISN – AC SOURCE INTERACTION
4.1.1 POWER FREQUENCY OPERATION
L or N
L presents low impedance

F
AC
SOURCE
F
k
analyser
or 50 load
F
EUT
G
FIGURE 11
Representative LISN component values are used in Figure 11 for performance analysis.
At 50 Hz

the 50 H inductor presents a low impedance;
X L  2    50  50 106  0.015 7 

the 2 F capacitor presents a high impedance;
XC 
1
 1.592 k 
2    50  2 106
low impedance path @ mains frequency
L OR N
SOURCE
LISN
LOAD
G
FIGURE 12
The LISN provides a low impedance to the 50 Hz voltage and current delivered to the EUT
and, as shown in Figure 12, the 50 H inductor and 2 F capacitor can be approximated
respectively as a short circuit and an open circuit.
EET422 EMC & Compliance Eng
EMISSIONS-3c COMPLIANCE TESTING
LISN
6
Prof R T Kennedy
2008-2009
4.1.2 AC SOURCE RF EMISSIONS
The 50 H inductor’s impedance increases significantly, compared to the 50 Hz value, in the
150 kHz - 30 MHz EN55022 conducted emissions frequency range.
X L  2    150  103  50  10 6  47.12 
X L  2    30 106  50 106  9.425 k 
high impedance path to RF
L OR N
SOURCE
LISN
LOAD
G
FIGURE 13
The LISN inductor provides a high impedance to incoming rf disturbances from the AC
source (effectively preventing incoming emissions reaching the EUT), and the 50 H
inductor can be approximated by an open circuit, as shown in Figure 13.
50 
50 
low impedance path
for mains rf noise
2 F
FIGURE 14a
XC 
XC 
1
2   150 10 3  2 10 6
FIGURE 14b
 0.53 
1
 0.0027 
2    30 106  2 106
The 2 F capacitor, Figure 14a, presents low impedance in the 150 kHz - 30 MHz EN55022
conducted emissions frequency range, and can be approximated as a short circuit as shown in
Figure 14b.
EET422 EMC & Compliance Eng
EMISSIONS-3c COMPLIANCE TESTING
LISN
7
Prof R T Kennedy
2008-2009
4.1.3 EUT RF EMISSIONS
low impedance path;
EUT noise to analyser
via high pass filter
L presents high impedance to EUT noise
L or N

F
AC
SOURCE
F
F
analyser
EUT
or 50 load
G
F
AC
SOURCE
F
k
E
U
analyser T
FIGURE 15
Conducted rf emissions from the EUT go via the lower impedance high pass filter to the
measuring instrument, as shown in Figure 15, and not to the AC supply.
X L  2    150  103  50  10 6  47.12 
X L  2    30 106  50 106  9.425 k 
XC 
1
 4.51 
2   150 103  0.235 106
XC 
1
 22.55 
2    30 106  2 106
The 50 H inductor’s impedance in the 150 kHz - 30 MHz EN55022 conducted emissions
can be approximated as an open circuit, and the 0.47 F capacitors can be approximated as a
short circuit.
4.1.4 SUMMARY
AC
rfi
F
rfi
F
F
2 F
source
F
analyser
E
U
T
FIGURE 16
Figure 16 summarises the performance of the LISN at the AC mains port and shows that the
LISN effectively filters the supply voltage and prevents disturbances passing to the EUT, and
effectively isolates the source and EUT with respect to rf signals. The LISN presents a high
impedance to rf signals in both directions (source  EUT and vice versa).
The 2 F capacitor at the input provides a low impedance path to mains rf and has negligible
effect on the 50 Hz power transfer from the source to the EUT.
The inductor effectively isolates the EUT and 50 Hz ac source ensuring that conducted
emissions from the EUT go via a high pass filter to the measuring instrument and not to the
AC supply.
SOURCE NOISE DOES NOT GO TO EUT OR ANALYSER
EUT NOISE DOES NOT GO TO SOURCE BUT GOES TO ANALYSER
EET422 EMC & Compliance Eng
EMISSIONS-3c COMPLIANCE TESTING
LISN
8
Prof R T Kennedy
2008-2009
4. 1.5 LISN APPROXIMATION
L
L
50
L
0.47 F
7.5 F
2 F
analyser
50 
k k


G

2 F
G

k
k
7.5 F

E
U
T
0.47 F 0.47 F
50
N
L
0.47 F
N
L
L
L

analyser
50 
k k

G

k
E
U
T
analyser
50 
G
G

k

analyser
50 
N
N
FIGURE 17
N
Evaluating conducted emission pathways can be approximated by replacing the inductors as
open circuits and the capacitors as shorts circuits resulting in the simplified circuit shown in
Figure 17.
EET422 EMC & Compliance Eng
EMISSIONS-3c COMPLIANCE TESTING
LISN
9
Prof R T Kennedy
2008-2009
E
U
T
5. LISN IMPEDANCE
The impedance versus frequency of an LISN must match the requirements of the test
specification being applied to the EUT. The most widely used LISN's present a 50 Ω
impedance to the EUT; selected because theoretical and empirical data (mean value of power
line impedances) have shown that the power circuitry statistically looks like a 50 
impedance to standard electronic equipment, and rf test equipment is typically designed for
50  input. (LISN is used to make the line appear to be 50 ohm).
Impedance, voltage rating, current rating and insertion loss are key parameters in LISN
selection.
L: 250H 50 mH 5 H
50 
Z
f
9 kHz
150 kHz
30 MHz
FIGURE 18
Figure 18 shows that increased LISN inductance can extend the low frequency range over
which a 50   L LISN provides the required 50 impedance to the EUT emissions.
50 
50 
50 
Z
9 kHz
150 kHz
30 MHz
FIGURE 19
Figure 19 represents the impedance that a 50   (50  + 5) LISN presents to the EUT
emissions, and includes the CISPR allowable ± 20% tolerance.
10
EET422 EMC & Compliance Eng
EMISSIONS-3c COMPLIANCE TESTING
LISN
Prof R T Kennedy
2008-2009
6. CM and DM EMISSION EXTRACTION
LISN
L
L
L
Cn
CS
R
G
G
Cm Rm
L
R
Rm
Cn
I CM
2
L
ICM
 I DM
2
ANALYSER
G
CS
N
N
I DM
LISN
EUT
I CM
 I DM
2
N
I DM
I CM
2
G
G
I CM
FIGURE 20
Figure 20 shows that the analyser receives a different combination of CM and DM noise from
the L and N lines. Compliance requires the highest level to be below the limit i.e both L an N
emissions must pass!
NOTE: ground current is not measured.
Although the conducted emission limits refer to the total emission (CM+DM) separation of
the CM and DM noise can be useful when designing to reduce noise at source or select
appropriate filters to minimise / reduce conducted noise.
6.1 OSCILLOSCOPE SEPARATION
ICM
 I DM
2
ICM
 I DM
2
S
U
B
T
R
A
C
T
A
D
D
I CM
 I DM
2
I CM
 I DM
2
 ICM
  ICM

 2  I DM    2  I Dm   ICM

 

 ICM
  ICM

 2  I DM    2  I Dm   2 I DM

 

FIGURE 21
A dual channel oscilloscope with add and subtract functions, as shown in Figure 21, can
provide time displays of CM and DM noise. Add provides the CM noise and subtract
provides twice the DM noise.
The PDS2000 can then apply the MATHS FFT to display the spectrum.
11
EET422 EMC & Compliance Eng
EMISSIONS-3c COMPLIANCE TESTING
LISN
Prof R T Kennedy
2008-2009
6.2 MAGNETIC SEPARATION
FIGURE 22
Figure 22 shows the use of a current transformer and appropriate L and N wire orientation to
provide separation.
6.3 SWITCHABLE SEPARATOR-COMBINER INTERFACE
LISN
L
L
L
LISN
Cn
CS
R
I DM
Cm Rm
ANALYSER
G
CS
N
N
L
ICM
 I DM
2
interface
G
G
I CM
2
L
R
Rm
Cn
EUT
I CM
 I DM
2
N
I DM
I CM
2
G
FIGURE 23
G
I CM
Figure 23 shows an alternative approach in which an interface unit’s combiner part does the
addition and the separator part does the subtraction. The actual interface is a very basic
magnetic component shown in Figure 24.
FIGURE 24
12
EET422 EMC & Compliance Eng
EMISSIONS-3c COMPLIANCE TESTING
LISN
Prof R T Kennedy
2008-2009
L
ICM
 I DM
2
N
I CM
G
I CM
 I DM
2
N
Ranalyser
N
G
FIGURE 25
Figure 25 shows the combiner effect resulting in the CM emission extraction.
L
I DM
ICM
 I DM
2
N
N
G
I CM
 I DM
2
Ranalyser
N
N
G
FIGURE 26
The 2 IDM primary current in Figure 26 is transformed by the turns ratio to provide IDM to the
analyser.
6.4 BALUNS
The system shown in Figure 26 is a special type of ‘matching’ transformer sometimes termed
as transmission line transformers as they transmit the energy from input to output by a
transmission line mode instead of by flux linkages as in the case of conventional
transformers.
An alternative and more common terminology is BALUN
BALanced to UNbalanced converter
A balanced circuit has its electrical midpoint grounded whereas an unbalanced circuit has one
side grounded. Balanced circuits are typically used in communications equipment due to
better spurious noise suppression however unbalanced measuring equipment needs a
BALUN.
13
EET422 EMC & Compliance Eng
EMISSIONS-3c COMPLIANCE TESTING
LISN
Prof R T Kennedy
2008-2009