IEC TR 61156-1-5:2013

Multicore and symmetrical pair/quad cables for digital communications - Part 1-5: Correction procedures for the measurement results of return loss and input impedance

IEC/TR 61156-1-5:2013(E), which is a Technical Report, describes correction procedures for the measurement results of return loss and input impedance.

General Information

• Status: Published
• Publication Date: 24-Jun-2013
• ICS: 03.120.01 - Quality in general
• ICS: 19.020 - Test conditions and procedures in general
• ICS: 21.020 - Characteristics and design of machines, apparatus, equipment
• ICS: 33.120.20 - Wires and symmetrical cables
• Technical Committee: SC 46C - Wires and symmetric cables
• Current Stage: PPUB - Publication issued
• Start Date: 25-Jun-2013
• Completion Date: 15-Oct-2013

Overview

IEC TR 61156-1-5:2013 is a Technical Report in the IEC 61156 family covering correction procedures for return loss and input impedance measurements of multicore and symmetrical pair/quad cables for digital communications. The report explains how to reduce or remove measurement artefacts caused by cable end preparation, test fixtures and multiple reflections so that measured scattering parameters (S11) and derived impedances better represent the cable’s true behaviour-particularly at higher frequencies where stray inductance and capacitance distort results.

Key topics and technical content

• Scope and intent

  • Correction procedures for measured return loss (RL) and input impedance for balanced cables.
  • Guidance intended for laboratory and industrial test situations where end-preparation and fixture effects degrade results.

• Measurement concepts explained

  • Return loss (RL) - operational RL measured with S11 and termination at reference impedance; includes structural and junction reflections.
  • Open/Short return loss (OSRL) - RL derived from open/short characteristic-impedance measurements; avoids multiple reflections but omits round-trip effects.
  • Structural return loss (SRL) - RL component from structural variations along the cable.

• Correction procedures defined

  • Parasitic inductance corrected return loss (PRL) - compensates for stray inductance at cable ends/fixtures.
  • Gated return loss (GRL) - time-domain gating to remove localized reflections.
  • Fitted return loss (FRL) - mathematical fitting of residual impedance to extract cable intrinsic behaviour.
  • Informative annexes compare GRL vs FRL and show influence of corrections on return-loss peaks.

• Practical measurement considerations

  • Effect of sample length on results (IEC 61156 series typically specifies sample lengths ≥ 100 m).
  • Example: avoiding multiple reflections at low frequencies may require very long test lengths (order of 1 000 m for 1 MHz) - motivating correction methods.

Applications and users

• Who uses it

  • Cable manufacturers and design engineers validating transmission performance.
  • Accredited test laboratories performing compliance and quality testing.
  • Test-equipment vendors implementing fixture and post-processing correction algorithms.
  • R&D and QA teams troubleshooting high-frequency anomalies in LAN and data cables.

• Practical uses

  • Improve accuracy of return loss and input impedance measurements for Cat/structured cabling and other balanced data cables.
  • Reduce false failures caused by connector/fixture artefacts in automated test systems.
  • Support clearer comparison between cable designs and compliance to IEC 61156 family specifications.

Related standards

• IEC 61156-1 (generic specification)
• IEC/TR 61156-1-2 (electrical transmission characteristics and test methods)
• IEC/TR 62152 (transmission properties of cascaded two-ports)
• IEC 62153-1-1 (pulse/step return loss measurement using IDFT)
• ASTM D4566

Keywords: IEC TR 61156-1-5, return loss correction, input impedance measurement, multicore and symmetrical pair/quad cables, PRL, GRL, FRL, cable testing, measurement correction procedures.

Standards Content (Sample)

IEC/TR 61156-1-5 ®
Edition 1.0 2013-06
TECHNICAL
REPORT
colour
inside
Multicore and symmetrical pair/quad cables for digital communications –
Part 1-5: Correction procedures for the measurement results of return loss and
input impedance
IEC/TR 61156-1-5:2013(E)
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IEC/TR 61156-1-5 ®
Edition 1.0 2013-06
TECHNICAL
REPORT
colour
inside
Multicore and symmetrical pair/quad cables for digital communications –

Part 1-5: Correction procedures for the measurement results of return loss and

input impedance
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
S
ICS 33.120.20 ISBN 978-2-83220-918-9

– 2 – TR 61156-1-5  IEC:2013(E)
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Acronyms . 5
4 Return loss measurements . 6
4.1 General . 6
4.2 Return loss (RL) . 6
4.3 Open/short return loss (OSRL) . 7
4.4 Structural return loss (SRL) . 7
5 Correction procedures . 8
5.1 General . 8
5.2 Parasitic inductance corrected return loss (PRL) . 9
5.3 Gated return loss (GRL) . 10
5.4 Fitted return loss (FRL) . 12
Annex A (informative) Comparison of gated return loss (GRL) with fitted return loss (FRL) . 18
Annex B (informative) Influence of the correction technique on return loss peaks . 20

Figure 1 – Return loss with and without junction reflections . 6
Figure 2 – Return loss and open/short return loss . 7
Figure 3 – Return loss, open short return loss and structural return loss . 8
Figure 4 – Input impedance (magnitude) and operational return loss up to 2 GHz . 9
Figure 5 – Equivalent circuit for the corrective calculation . 9
Figure 6 – Return loss with gating correction . 12
Figure 7 – Return loss and input impedance before any correction . 15
Figure 8 – Return loss and input impedance (with fitting) after fixture correction . 15
Figure 9 – Residual impedance (after fixture correction) and its fitting . 16
Figure 10 – Return loss and input impedance after complete correction . 16
Figure 11 – Return loss, with fixture correction vs. without fixture correction . 17
Figure 12 – Input impedance (real and imaginary part), with fixture correction vs. without
fixture correction . 17
Figure A.1 – Return loss, fitting vs. gating correction . 18
Figure A.2 – Real part of input impedance, fitting vs. gating correction . 18
Figure A.3 – Imaginary part of input impedance, fitting vs. gating correction . 19
Figure B.1 – Reflection coefficient of a Cat.6 data cable in polar coordinates without and
with PRL-correction . 20
Figure B.2 – Return loss traces corresponding to Figure B.1 . 20
Figure B.3 – Reflection coefficient of a Cat.6 data cable in polar coordinates without and
with PRL-correction . 21
Figure B.4 – Return loss traces corresponding to Figure B.3 . 21

TR 61156-1-5  IEC:2013(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MULTICORE AND SYMMETRICAL PAIR/QUAD
CABLES FOR DIGITAL COMMUNICATIONS –

Part 1-5: Correction procedures for the measurement
results of return loss and input impedance

FOREWORD
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The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected data
of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC/TR 61156-1-5, which is a technical report, has been prepared by subcommittee SC46C:
Wires and symmetric cables, of IEC technical committee TC46: Cables, wires, waveguides, R.F.
connectors, R.F. and microwave passive components and accessories.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
46C/973/DTR 46C/979/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.

– 4 – TR 61156-1-5  IEC:2013(E)
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 61156 series, published under the general title Multicore and
symmetrical pair/quad cables for digital communications, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until the
stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to
the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

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imprimer cette publication en utilisant une imprimante couleur.

TR 61156-1-5  IEC:2013(E) – 5 –
MULTICORE AND SYMMETRICAL PAIR/QUAD
CABLES FOR DIGITAL COMMUNICATIONS –

Part 1-5: Correction procedures for the measurement
results of return loss and input impedance

1 Scope
This part of IEC 61156 describes correction procedures for the measurement results of return
loss and input impedance.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
IEC 61156-1, Multicore and symmetrical pair/quad cables for digital communications – Part 1:
Generic specification
IEC/TR 61156-1-2, Multicore and symmetrical pair/quad cables for digital communications –
Part 1-2: Electrical transmission characteristics and test methods of symmetrical pair/quad
cables
IEC/TR 62152, Transmission properties of cascaded two-ports or quadripols – Background of
terms and definitions
IEC 62153-1-1, Metallic communication cables test methods – Part 1-1: Electrical –
Measurement of the pulse/step return loss in the frequency domain using the Inverse Discrete
Fourier Transformation
ASTM D4566:1998, Standard Test Methods for Electrical Performance Properties of Insulations
and Jackets for Telecommunications Wire and Cable
3 Acronyms
CUT cable under test
FRL fitted return loss
GRL gated return loss
IFDT Inverse discrete Fourier transformation
OSRL open short return loss
PRL
parasitic inductance corrected return loss
RL return loss
SRL
structural return loss
– 6 – TR 61156-1-5  IEC:2013(E)
4 Return loss measurements
4.1 General
The return loss of a transmission line (cable) can be obtained by different test methods, each
having certain advantages and/or disadvantages and therefore giving not exactly the same
results. At higher frequencies, the measured return loss is strongly influenced by the cable end
preparation (stray inductances and capacitances play an important role) leading to an
underestimated cable performance. Under laboratory conditions, one might be able to minimize
this negative effect; however under general industrial conditions using automated test systems,
one might need mathematical procedures to eliminate the effect of the cable end preparation.
The results of return loss measurements may also depend on the sample length. Therefore, for
balanced cables according to IEC 61156 series, the specified sample length is at least 100 m (if
not specified otherwise).
4.2 Return loss (RL)
The return loss is obtained by measuring the scattering parameter S11 using a test apparatus of
which the test port has the same reference impedance than the cable under test, and where the
far end of the cable is terminated with its reference impedance (See IEC/TR 62152,
IEC 61156-1). The return loss takes into account the structural variations along the cable and the
mismatch between the reference impedance and the (mean) characteristic impedance of the
pair. If the (mean) characteristic impedance of the pair is different from the reference impedance,
one gets, especially at lower frequencies (where the round trip attenuation is low), multiple
reflections that are overlaid to the structural and junction reflections. Therefore, return loss RL is
also referenced as operational return loss.
NOTE The impedance of a homogenous transmission line is a complex quantity where the real part is decreasing with
frequency and tending to an asymptotic value, the so called characteristic impedance and the imaginary part is also
decreasing from negative values to zero. This “normal” behaviour of a transmission line will also generate multiple
reflections.
As an example, Figure 1 shows the operational return loss under different conditions. The blue
line shows the return loss of a pair having a characteristic impedance equal to the reference
impedance but taking into account that the impedance is varying with frequency (see right-hand
graph). The red line shows the return loss of a pair having a characteristic impedance that is
different from the reference impedance (110 Ω vs. 100 Ω). For both lines, we observe periodic
variations that are caused by multiple reflections between the junctions at the near and far end.
The green line shows a simulation of a pair having a frequency independent characteristic
impedance which is equal to the reference impedance.
frequency dependent factor of the characteristic impedance
Return Loss
1,2 0,2
RLRL w/o mismatch
1,16 0,16
5 RLRL w mismatch
RLRL w /o mismatch; ZZcc frequency independent
1,12 0,12
1,08 0,08
1,04 0,04
1 0
25 Real
Imag
0,96 -0,04
0,92 -0,08
0,88 -0,12
0,84 -0,16
0,8 -0,2
0,1 1 10 100
0,1 1 10 100 MHz
MHz
IEC  1612/13
Figure 1 – Return loss with and without junction reflections
dB
TR 61156-1-5  IEC:2013(E) – 7 –
4.3 Open/short return loss (OSRL)
A way to avoid in the measurement of return loss multiple reflections due to a mismatch between
the characteristic impedance (asymptotic value at high frequencies) of the CUT and the
reference impedance is to use a CUT terminated in its nominal impedance and having a very
long test length such that the round trip attenuation of the CUT is at least 40 dB at the lowest
frequency to be measured. For standard LAN cables, this would result in a CUT length of roughly
1 000 m for the lowest frequency of 1 MHz.
Another way (when long CUT length is not available) is to measure the characteristic impedance
(open/short method, see IEC/TR 62152) and to calculate the return loss. As the characteristic
impedance is obtained from the measurement of the open and short circuit impedance, it is
proposed to name such obtained return loss open/short return loss.
This open/short return loss includes the effect of structural variations and the mismatch at the
near end (including the effect due to a frequency-dependent characteristic impedance), but it
does not take into account multiple reflections.
Figure 2 shows the difference between operational return loss and open/short return loss. The
left-hand graph shows the results of a pair having a characteristic impedance which is different
from the reference impedance (110 Ω vs. 100 Ω). The right-hand graph shows the results of a
pair having a characteristic impedance which is equal to the reference impedance (100 Ω). We
recognize that the open/short return loss does not take into account multiple reflections.

Return Loss Return Loss
0 0
RL RLRL w /o mismatch
RL w mismatch
5 5
OSOSRLRL w/o mismatch
OSOSRLRL w mismatch
10 10
15 15
20 20
25 25
30 30
35 35
40 40
45 45
50 50
0,1 1 10 100 0,1 1 10 100
MHz MHz
IEC  1613/13
Figure 2 – Return loss and open/short return loss
4.4 Structural return loss (SRL)
The structural return loss is the return loss where only structural variations along the cable are
taken into account. The mismatch effects at the input and output of the transmission line
(including the effect due to a frequency-dependent characteristic impedance) have been
eliminated (see IEC/TR 62152). The structural return loss cannot be measured directly but is
calculated from the measurement of the characteristic impedance (open/short method).
Z − Z
CM C
SRL= 20⋅ log (1)
Z + Z
CM C
where
Z is the (complex) mean characteristic impedance obtained from the measurement of the
CM
open and short circuit impedance;
dB
dB
– 8 – TR 61156-1-5  IEC:2013(E)
Z is the (complex) characteristic impedance obtained from a curve fitting of the real and
C
imaginary part of Z .
CM
The left-hand graph of Figure 3 shows the operational return loss, open/short return loss and
structural return loss of a CUT having a characteristic impedance of 110 Ω. We clearly see the
differences between them. The operational return loss takes into account all effects (structural
variations, mismatch effects at the input and output). The open/short return loss does not take
into account mismatch effects at the output (i.e. no multiple reflections). Whereas the structural
return loss only takes into account structural variations along the cable.
The right-hand graph shows the real and imaginary part of the mean characteristic impedance
(obtained from the measurement of the open and short circuit impedance) and it’s fitting.

Return Loss Mean Characteristic Impedance
0 140 80
RLRL
OSOSRLRL 130 70
SSRLRL
120 60
110 50
100 40
Re(Zos)
fitted Re(Zos)
40 90 30
Im(Zos)
fitted Im(Zos)
80 20
70 10
60 0
50 -10
80 40 -20
0,1 1 10 100 0,1 1 10 100
MHz MHz
IEC  1614/13
Figure 3 – Return loss, open short return loss and structural return loss
5 Correction procedures
5.1 General
From the transmission line theory we know that the characteristic impedance is decreasing with
frequency and approaching an asymptotic value (assuming dielectrics having an almost
frequency-independent dielectric constant). Therefore an asymptotic value is assumed in the
described correction procedures.
However measurements at higher frequencies often show an increase of the characteristic
impedance. This is due to the cable end preparation and related stray inductances and
capacitances. In fact we can consider the CUT as a cascade of 3 transmission lines, the two
cable ends and the cable, or even 5 lines taking into account the test fixtures.
Figure 4 shows the results of an S/FTP cable obtained with an automated test system. On the
left-hand side we see the input impedance (magnitude) and its fitting and on the right-hand side
the operational return loss. One can recognize the above-described effect of increasing
impedance and related decreasing return loss. Thus correction procedures are needed.
dB
Real Part [Ohm]
Imaginary Part [Ohm]
TR 61156-1-5 © IEC:2013(E) – 9 –
Input Impedance Operational Return Loss
180 0
100 25
20 50
0 200 400 600 800 1000 1200 1400 1600 1800 2000 0 200 400 600 800 1000 1200 1400 1600 1800 2000
MHz MHz
IEC
...