IEC TR 61917:1998
(Main)Cables, cable assemblies and connectors - Introduction to electromagnetic (EMC) screening measurements
Cables, cable assemblies and connectors - Introduction to electromagnetic (EMC) screening measurements
Gives a brief introduction to basic concepts and terms trying to reveal the common features of apparently different test methods. Should assist in correct interpretation of test data, and in the better understanding of screening (or shielding) and related specifications and standards.
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Standards Content (Sample)
TECHNICAL
IEC
REPORT
First edition
1998-06
Cables, cable assemblies and connectors –
Introduction to electromagnetic (EMC)
screening measurements
Câbles, cordons et connecteurs –
Introduction aux mesures de blindage électromagnétique
Reference number
IEC 61917:1998(E)
Numbering
As from 1 January 1997 all IEC publications are issued with a designation in the
60000 series.
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available. For example, edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the
base publication, the base publication incorporating amendment 1 and the base
publication incorporating amendments 1 and 2.
Validity of this publication
The technical content of IEC publications is kept under constant review by the IEC,
thus ensuring that the content reflects current technology.
Information relating to the date of the reconfirmation of the publication is available
in the IEC catalogue.
Information on the subjects under consideration and work in progress undertaken by
the technical committee which has prepared this publication, as well as the list of
publications issued, is to be found at the following IEC sources:
• IEC web site*
• Catalogue of IEC publications
Published yearly with regular updates
(On-line catalogue)*
• IEC Bulletin
Available both at the IEC web site* and as a printed periodical
Terminology, graphical and letter symbols
For general terminology, readers are referred to IEC 60050: International
Electrotechnical Vocabulary (IEV).
For graphical symbols, and letter symbols and signs approved by the IEC for
general use, readers are referred to publications IEC 60027: Letter symbols to be
used in electrical technology, IEC 60417: Graphical symbols for use on equipment.
Index, survey and compilation of the single sheets and IEC 60617: Graphical symbols
for diagrams.
* See web site address on title page.
TECHNICAL
IEC
REPORT – TYPE 3
First edition
1998-06
Cables, cable assemblies and connectors –
Introduction to electromagnetic (EMC)
screening measurements
Câbles, cordons et connecteurs –
Introduction aux mesures de blindage électromagnétique
IEC 1998 Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or
mechanical, including photocopying and microfilm, without permission in writing from the publisher.
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For price, see current catalogue
– 2 – 61917 © IEC:1998(E)
CONTENTS
Page
FOREWORD . 3
Clause
1 Scope and object . 5
2 Reference documents. 5
3 Electromagnetic phenomena. 5
4 The intrinsic screening parameters of short cables . 7
4.1 Surface transfer impedance, Z . 7
T
4.2 Capacitive coupling admittance, Y . 7
c
4.3 Injecting with arbitrary cross-sections . 9
4.4 Reciprocity and symmetry . 9
4.5 Arbitrary load conditions. 9
5 Long cables – coupled transmission lines. 9
6 Transfer impedance of a braided-wire outer conductor or screen . 16
7 Test possibilities . 21
7.1 Measuring the transfer impedance of coaxial cables . 21
7.2 Measuring the transfer impedance of cable assemblies . 22
7.3 Measuring the transfer impedance of connectors. 22
Annex A List of symbols . 25
Annex B Bibliography . 27
Annex C Additional reading. 29
61917 IEC:1998(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
___________
–
CABLES, CABLE ASSEMBLIES AND CONNECTORS
INTRODUCTION TO ELECTROMAGNETIC (EMC)
SCREENING MEASUREMENTS
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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The main task of IEC technical committees is to prepare International Standards. In
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longer valid or useful.
IEC 61917 which is a technical report type 3 has been prepared by subcommittee 46A: Coaxial
cables, of IEC technical committee 46: Cables, wires, waveguides, r.f. connectors, and
accessories for communication and signalling.
– 4 – 61917 © IEC:1998(E)
The text of this technical report is based on the following documents:
Committee draft Report on voting
46A/267/CDV 46A/284/RVC
Full information on the voting for the approval of this technical report can be found in the report
on voting indicated in the above table.
A bilingual version of this technical report may be issued at a later date.
61917 IEC:1998(E) – 5 –
CABLES, CABLE ASSEMBLIES AND CONNECTORS –
INTRODUCTION TO ELECTROMAGNETIC (EMC)
SCREENING MEASUREMENTS
1 Scope and object
Screening (or shielding) is one basic way of achieving electromagnetic compatibility (EMC).
However, a confusingly large number of methods and concepts is available to test for the
screening quality of cables and related components, and for defining their quality. This
technical report gives a brief introduction to basic concepts and terms trying to reveal the
common features of apparently different test methods. It should assist in correct interpretation
of test data, and in the better understanding of screening (or shielding) and related
specifications and standards.
2 Reference documents
IEC 60096-1:1986, Radio-frequency cables – Part 1: General requirements and measuring
methods
Amendment 2 (1993)
IEC 60096-2:1961, Radio-frequency cables – Part 2: Relevant cable specifications
Amendment 1 (1990)
IEC 60096-4-1:1990, Radio-frequency cables – Part 4: Specification for superscreened cables
– Section 1: General requirements and test methods
IEC 60169-1:1987, Radio-frequency connectors – Part 1: General requirements and measuring
methods
IEC 60169-1-3:1988, Radio frequency connectors – Part 1: General requirements and
measuring methods – Section 3: Electrical tests and measuring procedures – Screening
effectiveness
IEC 61196-1:1995, Radio-frequency cables – Part 1: Generic specification – General,
definitions, requirements and test methods
IEC 61726:1995, Cable assemblies, cables, connectors and passive microwave components –
Screening attenuation measurement by the reverberation chamber method
3 Electromagnetic phenomena
It is assumed that if an electromagnetic field is incident on a screened cable, there is only weak
coupling between the external field and that inside, and that the cable diameter is very small
compared with both the cable length and the wavelength of the incident field. The superposition
of the external incident field and the field scattered by the cable yields the total electromagnetic
field (E , H , in figure 1). The total field at the screen's surface may be considered as the
t t
source of the coupling: electric field penetrates through apertures by electric or capacitive
coupling; also magnetic fields penetrate through apertures by inductive or magnetic coupling.
Additionally, the induced current in the screen results in conductive or resistive coupling.
(E ,H )
i i
– 6 – 61917 © IEC:1998(E)
E
t
H
t
n
σ
J
X
(E ,H ) = (E ,H ) + (E ,H) (1)
t t i i s s
= n (2)
J • H
t
σ = n • E ε ε (3)
t 0 r
n: unit vector normal to surface
Figure 1 – Incident (i), scattered (s) and resulting total electromagnetic fields (E , H ) with
t t
induced surface current- and surface charge-densities J (A/m) and σ (C/m )
.
As the field at the surface of the screen is directly related to density of surface current and
surface charge, the coupling may be assigned either to the total field (E , H ) or to the surface
t t
current- and charge- densities (J and σ). Consequently, we may simulate the coupling into the
cable by reproducing through any means the surface currents and charges on the screen.
Because we assume a cable of a small diameter, we may neglect higher modes and can use
an additional coaxial conductor as our injection structure, as shown in figure 2.
l Concept of a triaxial set-up
+
1) outer circuit, formed by injection
E
1 cylinder and screen, characteristic
U impedance Z ,
Z
1n
U
1f
Z
1f
2) inner circuit, formed by a screen,
Z U
I
2n 2n U Z
1 (1)
2f and centre conductor, characteristic
2f
impedance Z ; screening at the ends
D
(2)
1 not shown.
Z
Z
Observe the conditions Z , Z , Z and λ in figure 3a and figure 3b.
1f 2n 2f
NOTE 1 – D << l.
NOTE 2 – Both ends of circuit (2) must be well screened.
Figure 2 – Defining and measuring screening parameters – A triaxial set-up
(E ,H )
s s
61917 IEC:1998(E) – 7 –
4 The intrinsic screening parameters of short cables
The intrinsic parameters refer to an infinitesimal length of cable, like the inductance or
capacitance per unit length of transmission lines. Assuming electrically short cables, with l << λ
which will always apply at low frequencies, the intrinsic screening parameters are defined and
can be measured as follows:
4.1 Surface transfer impedance, Z
T
As shown in figure 2 and figure 3a (where Z and Z are zero):
1f 2f
ZU=⋅/(Il) (Ω/ m) (4)
T 21
The dependence of Z on frequency is not simple and is often shown by plotting log Z against
T T
log frequency. Note that the phase of Z may have any value, depending on braid construction
T
and frequency range.
NOTE – In circuit 2 of figure 3a the voltmeter and short circuit can be interchanged.
4.2 Capacitive coupling admittance, Y
c
As shown in figure 2 and figure 3b (where Z and Z are open circuit):
1f 2f
YC==jω I /(U⋅l) (mho / m) (5)
CT
The through capacitance (C ) is a real capacitance and has usually a constant value up to
T
λ
1 GHz and higher (with aperture a << ).
While Z is independent of the characteristics of the coaxial circuits, C is dependent on those
T T
characteristics. There are two ways of overcoming this dependence:
a) The normalized through elastance K derived from C is independent of the size of the
T T
outer coaxial circuit, but it depends on its permittivity:
=⋅ εε+
KC /(CC ) (mF/
...
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