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
Current Stage
PPUB - Publication issued
Start Date
25-Jun-2013
Completion Date
25-Jun-2013
Ref Project

Buy Standard

Technical report
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
English language
21 pages
sale 15% off
Preview
sale 15% off
Preview

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)
---------------------- Page: 1 ----------------------
THIS PUBLICATION IS COPYRIGHT PROTECTED
Copyright © 2013 IEC, Geneva, Switzerland

All rights reserved. Unless otherwise specified, 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

either IEC or IEC's member National Committee in the country of the requester.

If you have any questions about IEC copyright or have an enquiry about obtaining additional rights to this publication,

please contact the address below or your local IEC member National Committee for further information.

IEC Central Office Tel.: +41 22 919 02 11
3, rue de Varembé Fax: +41 22 919 03 00
CH-1211 Geneva 20 info@iec.ch
Switzerland www.iec.ch
About the IEC

The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes

International Standards for all electrical, electronic and related technologies.
About IEC publications

The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the

latest edition, a corrigenda or an amendment might have been published.
Useful links:

IEC publications search - www.iec.ch/searchpub Electropedia - www.electropedia.org

The advanced search enables you to find IEC publications The world's leading online dictionary of electronic and

by a variety of criteria (reference number, text, technical electrical terms containing more than 30 000 terms and

committee,…). definitions in English and French, with equivalent terms in

It also gives information on projects, replaced and additional languages. Also known as the International

withdrawn publications. Electrotechnical Vocabulary (IEV) on-line.

IEC Just Published - webstore.iec.ch/justpublished Customer Service Centre - webstore.iec.ch/csc

Stay up to date on all new IEC publications. Just Published If you wish to give us your feedback on this publication

details all new publications released. Available on-line and or need further assistance, please contact the

also once a month by email. Customer Service Centre: csc@iec.ch.
---------------------- Page: 2 ----------------------
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
ICS 33.120.20 ISBN 978-2-83220-918-9

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 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

---------------------- Page: 4 ----------------------
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

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising all

national electrotechnical committees (IEC National Committees). The object of IEC is to promote international co-

operation on all questions concerning standardization in the electrical and electronic fields. To this end and in

addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,

Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their

preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with

may participate in this preparatory work. International, governmental and non-governmental organizations liaising

with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for

Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.

2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international

consensus of opinion on the relevant subjects since each technical committee has representation from all

interested IEC National Committees.

3) IEC Publications have the form of recommendations for international use and are accepted by IEC National

Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC

Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any

misinterpretation by any end user.

4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications

transparently to the maximum extent possible in their national and regional publications. Any divergence between

any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.

5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity

assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any

services carried out by independent certification bodies.

6) All users should ensure that they have the latest edition of this publication.

7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and

members of its technical committees and IEC National Committees for any personal injury, property damage or

other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and expenses

arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publications.

8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is

indispensable for the correct application of this publication.

9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent

rights. IEC shall not be held responsible for identifying any or all such patent rights.

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.
---------------------- Page: 5 ----------------------
– 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.

IMPORTANT – Le logo "colour inside" qui se trouve sur la page de couverture de cette

publication indique qu'elle contient des couleurs qui sont considérées comme utiles à

une bonne compréhension de son contenu. Les utilisateurs devraient, par conséquent,

imprimer cette publication en utilisant une imprimante couleur.
---------------------- Page: 6 ----------------------
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
---------------------- Page: 7 ----------------------
– 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
---------------------- Page: 8 ----------------------
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

open and short circuit impedance;
---------------------- Page: 9 ----------------------
– 8 – TR 61156-1-5  IEC:2013(E)

Z is the (complex) characteristic impedance obtained from a curve fitting of the real and

imaginary part of Z .

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.

Real Part [Ohm]
Imaginary Part [Ohm]
---------------------- Page: 10 ----------------------
TR 61156-1-5 © IEC:2013(E) – 9 –
Input Impedance Operational Return Loss
180 0
160
140
120
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 1615/13
Figure 4 – Input impedance (magnitude) and operational return loss up to 2 GHz
5.2 Parasitic inductance corrected return loss (PRL)

The characteristic impedance of an ideal cable is at high frequencies constant and only has a

real part. Measuring the input impedance Z of a cable in contrast often shows a steady increase

with frequency. The same effect can be seen measuring the return loss. This is mainly due to a

parasitic inductance L in the zone of the connection to the measurement equipment. The

str
correction procedure is based on the equivalent circuit shown in Figure 5.
str
Z Z
i R
Cable under test
IEC 1616/13
Figure 5 – Equivalent circuit for the corrective calculation

The stray inductance L is calculated from the imaginary part of the complex input impedance Z

str i

at specific frequencies. The input impedance is then corrected using the stray inductance. From

the corrected input impedance, a corrected return loss can be calculated.

The calculation of the corrected input impedance and the return loss data is done in the following

steps:

For frequencies higher than 30 MHz up to the half of the maximum measurement frequency f ,

max

the value of stray inductance is calculated for every measurement frequency point:

Im Z
(2)
L =
str
2πf
where
f is the frequency, in Hz, and 30 MHz ≤ f ≤ f /2;
max
L is the stray inductance;
str
Z is the measured input impedance.
Then the mean value L and the standard deviation L of the stray inductance are
str,m stad
calculated:
Ohm
---------------------- Page: 11 ----------------------
– 10 – TR 61156-1-5  IEC:2013(E)
L = L (3)
str,m str,n
n=1
where
L is the stray inductance at the frequency corresponding with n;
str,n
N is the number of frequency points between 30 MHz and f /2.
max
L = (L − L ) (4)
stad ∑ str,n str,m
n=1

The corrected input impedance Z is calculated for every measurement frequency point:

i,corr
Z = Z − j2πfL (5)
i,corr i str,m

The corrected return loss RL is calculated for every measurement frequency point:

corr
Z − Z
i,corr R
RL = 20log (6)
corr
Z + Z
i,corr R
where
Z is the reference impedance of the system.

To ensure the calculation is valid, the mean value stray inductance, the standard deviation of the

stray inductance and the ratio between these two values shall be below the following limits:

L 24 nH
str,m
0 nH
str,m
L 8 nH
stad
L / L 0,8
stad str,m

NOTE These conditions have been found by experimentations. Values outside these specified limits indicate that the

measurement error is not only due to a stray inductance but other effects. For example a negative value of L

str,m

indicates a capacitive effect. For high frequencies or when the sample preparation is not well done one can no longer

assume a concentrated stray inductance but has to take into account the line transformation effect of the cable ends

(see also fitted return loss procedure).
5.3 Gated return loss (GRL)

A way to eliminate mathematically the effects of the test fixtures and sample preparation is to

use gating. The operational return loss is measured in the frequency domain and the results are

converted to the time domain using an inverse discrete Fourier transformation (IDFT), see also

IEC 62153-1-1. In the time domain one can observe the reflection peaks at the cable ends. The

gate is set on the near end side just after the peak and on the right hand side just before the

peak. After activating the gate, the peaks are eliminated and transforming back into the

frequency domain results in the gated return loss.

The IFDT described in IEC 62153-1-1 describes the time domain low pass mode which has some

specific requirements for the frequency range. The measured frequencies must be set so that the

start frequency is equal to the frequency step between two successive frequencies. The

advantage of the time domain low pass mode is that it allows determining the location and type

of default.
---------------------- Page: 12 ----------------------
TR 61156-1-5  IEC:2013(E) – 11 –

However, in general, the frequency step between two successive frequencies is different from

the start frequency. In this case, one has to use the time domain band pass mode. It allows

identifying the location of the default but does not indicate whether the default is capacitive,

inductive or resistive.

The used frequency range and number of measurement points results in the resolution and a

maximum length that can be measured.
1 n− 1
CUT = ⋅ c ⋅ v (7)
max 0 r
2 f − f
stop start
1 c ⋅ v
0 r
∆x= (8)
2 f − f
stop start

NOTE The factor ½ is due to the reflection measurement where the signal is propagating forward to the default

location and then backward to the input.
where
∆x is the minimum resolution (physically in the cable);
n is the number of points in the frequency sweep;
CUT is the maximum physical length contained in the results;
max
c is the speed of light in free space;
v is the relative propagation constant;
f is the lowest frequency of the measurement;
start
f is the highest frequency of the measurement.
stop

Let’s consider an S/FTP cable having a relative propagation constant of 70 % (“worst case”). If

the measurement is done in the frequency range from 1 MHz to 2 000 MHz using 1601 frequency

points (maximum value available for common network analysers), we obtain:
CUT =84 m ∆x=53 mm
max

This means that only samples with a length of up to 84 m can be measured and corrected by

gating.

The graphs of Figure 6 show the measured and gating corrected results for an S/FTP cable in

the frequency range up to 2 000 MHz. The upper two graphs show the two set points for the

gating. After the gate is turned on, the values left respectively right from the set points are set to

zero. The lower two graphs show the return loss and input impedance (magnitude) before and

after gating correction. After gating, the impedance is well centred around a constant value and

the return loss has significantly improved.
---------------------- Page: 13 ----------------------
– 12 – TR 61156-1-5  IEC:2013(E)
Magnitude of S11 in time domain Magnitude of S11 in
...

Questions, Comments and Discussion

Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.