IEC 62783-1-2:2025

IEC 62783-1-2 ®
Edition 1.0 2025-09
INTERNATIONAL
STANDARD
Twinax cables for digital communications -
Part 1-2: Time domain test method for twinax cables for digital communications -
Impedance
ICS 33.120.20  ISBN 978-2-8327-0706-7

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CONTENTS
FOREWORD . 2
INTRODUCTION . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Principle of differential time domain measurement . 6
5 Test equipment and accessories . 9
6 Measurement method . 9
6.1 Calibration . 9
6.1.1 General . 9
6.1.2 Method A: coaxial calibration kits . 10
6.1.3 Method B: air line and standard terminations . 10
6.2 Rise time of measurement system . 11
6.3 DUT length . 11
6.4 Measurement . 11
7 Requirement . 12
8 Test report . 12
Bibliography . 13

Figure 1 – Fall time . 6
Figure 2 – Illustration of signal flow and reflected impedance in time domain . 7
Figure 3 – Illustration of three typical reflection coefficients in time domain . 8
Figure 4 – PI model and T model of a symmetrical transmission line impedance . 9
Figure 5 – Calibration method B . 10

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Twinax cables for digital communications -
Part 1-2: Time domain test method for twinax cables for digital
communications - Impedance
FOREWORD
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IEC 62783-1-2 has been prepared by subcommittee 46C: Wires and symmetric cables, of IEC
technical committee 46: Cables, wires, waveguides, RF connectors, RF and microwave passive
components and accessories. It is an International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
46C/1306/CDV 46C/1320/RVC
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 62783 series, published under the general title Twinax cables for
digital communications, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
INTRODUCTION
This document gives the introductions of differential-mode impedance and common-mode
impedance, as well as odd-mode impedance and even-mode impedance. These four types of
impedance are used in the design and measurements of differential devices. This document
also specifies the test methods of time domain impedance for twinax cables used in
high-performance information technology systems and data interface interconnection systems.
Although frequency domain analysis has powerful functions, time domain analysis is still an
effective tool, especially in the short-reach and high frequency data transmission applications,
for example fault localization, identifying impedance variations in connectors, PCBs, and
transmission lines, selectively eliminating redundant responses (e.g. gate function in VNA-
based time domain analysis), etc.
Time domain analysis includes time domain transmission and time domain reflection. This
document concerns the latter. Impedance measurement is the most typical application of time
domain reflectometry. This measurement technology is based on the transmission line theory,
which works similarly to the principle of radar. When the signal travels along the transmission
line or DUT, there are no reflections if the transmission line is homogeneous and continuous,
but if impedance varies, a part of or all incident signal is reflected back, while the rest keeps
travelling forward.
Time domain analysis can be achieved either by traditional time domain reflectometer or
modern vector analyzer equipped with IFFT function.
When measuring the reflection signals, reflection coefficient ρ at a given point along a
transmission line or device under test depends on the deviation between the impedance of this
point and its nominal impedance; the position of reflection (i.e. the distance to the source) can
be determined according to the travelling delay of the reflected signal and propagation velocity
in the transmission line or DUT. A traditional time domain reflectometer (TDR) instrument
displays this information directly.

1 Scope
This part of IEC 62783 gives the definitions of different types of impedance in time domain and
specifies the test method of these kinds of impedance. It is applicable for determining the values
of time domain impedance of twinax cables and twinax cable assemblies.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 62783-1:2023, Twinax cables for digital communications - Part 1: Generic specification
IEC 62783-1-1:2022, Twinax cables for digital communications - Part 1-1: Time-domain test
methods for twinax cables for digital communications - General requirements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 62783-1,
IEC 62783-1-1 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
3.1
reflection coefficient
ρ
ratio of the reflected voltage to incident voltage along the transmission line at any given point
3.2
single-ended
measured with respect to a fixed potential, usually ground
3.3
fall time
time interval between the instants at which t
...