Agilent Technologies 8712ET User manual

Option 100 Fault Location and Structural
Return Loss Measurements
Agilent Technologies 8712ET/ES and 8714ET/ES
RF Network Analyzers
User’s Guide Supplement
Part No. 08714-90020
Printed in USA
Print Date: June 2000
Supersedes: October 1999
© Copyright 1998-2000 Agilent Technologies, Inc.

ii
Notice
The information contained in this document is subject to change without
notice. Agilent Technologies makes no warranty of any kind with regard
tothis material, including but not limited to, the implied warranties of
merchantability and fitness for a particular purpose. Agilent
Technologies shall not be liable for errors contained herein or for
incidental or consequential damages in connection with the furnishing,
performance, or use of this material.
Safety Information
Refer to your analyzer’s User’s Guide for safety, warranty, and assistance
information.
Key Conventions
This manual uses the following conventions:
: This represents a key physically located on the
analyzer (a “hardkey”).
: This indicates a “softkey”—a key whose label is determined
by the instrument’s firmware, and is displayed on the right side of the
instrument’s screen next to the eight unlabeled keys.
FRONT PANEL KEY
Softkey

iii
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iv
Book Overview
This book documents the use of Option 100 (fault location and structural
return loss (SRL) measurement capability) with the following network
analyzers:
• Agilent Technologies 8712ET/ES
• Agilent Technologies 8714ET/ES
Fault location and SRL measurements can help you test and
troubleshoot 50 ohm or 75 ohm transmission lines. Both fault location
and SRL measurements are used to identify damaged cables.
Fault location is best used to identify single faults greater than −40 dB.
SRL measurements are typically made before a cable has been installed
and are best used to identify many small evenly-spaced imperfections
that may be too small for a fault location measurement to detect.
The following illustration depicts a cable being measured simultaneously
in fault location and SRL modes. The upper display shows a fault
location measurement in the distance domain. The bottom display shows
a structural return loss measurement.

v
Agilent 8712ET/ES and 8714ET/ES
Network Analyzer
Documentation Map
The CDROM provides the contents of all of the documents
listed below.
The User’s Guide shows how to make measurements,
explains commonly-used features, and tells you how to get
the most performance from the analyzer.
The LAN Interface User’s Guide Supplement shows
how to use a local area network (LAN) for programming and
remote operation of the analyzer.
The Automating Measurements User’s Guide
Supplement provides information on how to configure and
control test systems for automation of test processes.
The Programmer’s Guide provides programming
information including GPIB and SCPI command references,
as well as short programming examples.
The Example Programs Guide provides a tutorial
introduction using BASIC programming examples to
demonstrate the remote operation of the analyzer.

vi
The Service Guide provides the information needed to
adjust, troubleshoot, repair, and verify analyzer conformance
to published specifications.
The HP Instrument BASIC User’s Handbook describes
programming and interfacing techniques using
HP Instrument BASIC, and includes a language reference.
The HP Instrument BASIC User’s Handbook
Supplement shows how to use HP Instrument BASIC to
program the analyzer.
The Option 100 Fault Location and Structural Return
Loss Measurements User’s Guide Supplement provides
theory and measurement examples for making fault location
and SRL measurements. (Shipped only with Option 100
analyzers.)
The CATV Quick Start Guide provides abbreviated
instructions for testing the quality of coaxial cables.
(Shipped only with Option 100 analyzers.)
The Cellular Antenna Quick Start Guide provides
abbreviated instructions for verifying the performance of
cellular antenna systems. (Shipped only with Option 100
analyzers.)

Contents
vii
1. Introduction and Measurement Theory
Fault Location Measurement Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2
How the Analyzer Converts Frequency Data to Distance Data . . . . . . . . . . . . . . . . 1-4
Start/Stop Distance and Frequency Span Explanation . . . . . . . . . . . . . . . . . . . . . . . 1-5
Cable Impedance and Structural Return Loss Measurement Theory . . . . . . . . . . . . . 1-6
Cable Impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-6
SRL and Periodic Cable Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-10
SRL and Discrete Cable Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-14
Techniques for Removing Connector Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-14
Measurement Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-18
2. Cable Preparation
Cable Preparation Problems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3
Bent Cable. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3
Deformed Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5
Contaminated Dielectric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7
Damaged Outer Conductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9
Non-Flush Cut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11
Recommended Tools and Cables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-13
Recommended Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-13
Recommended Cables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-13
3. Making Fault Location Measurements
How to Make and Interpret Fault Location Measurements . . . . . . . . . . . . . . . . . . . . . 3-3
1. Select Fault Location as the Type of Measurement . . . . . . . . . . . . . . . . . . . . . . . . 3-4
2. Enter the Measurement Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5
3. Calibrate the Analyzer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6
4. Connect the Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9
5. Interpret the Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10
Example Fault Location Measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14
Example 1: Identify Mismatches Expressed as Return Loss. . . . . . . . . . . . . . . . . . 3-14
Example 2: Identify Mismatches as the Magnitude of Reflection Coefficient. . . . . 3-16
Example 3: Identify Mismatches at Greater Distances . . . . . . . . . . . . . . . . . . . . . . 3-17
4. Making SRL Measurements
How to Make and Interpret SRL Measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3

viii
Contents
1. Select SRL as the Type of Measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-3
2. Calibrate the Analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-4
3. Connect the Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-8
4. Determine the Connector Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-8
5. Perform the SRL Cable Scan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-16
6. Interpret the SRL Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-19
5. Making Simultaneous Fault Location and SRL Measurements
How to Make Simultaneous Measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-4
6. Making Impedance Measurements
How to Make Impedance Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-4
7. Characterizing and Verifying Antenna Systems
Antenna Feedline System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-3
Potential Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-5
Transmission Lines/Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-5
Connectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-6
Typical Measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-7
Before Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-8
Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-8
Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-9
Installation and Maintenance Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-10
8. Key Reference
Alphabetical Key Reference. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-3
9. Characteristics
Overview of Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-2
Frequency Range Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-2
Phase Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-2
General Performance Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-3
SRL Measurement Uncertainty vs System Directivity . . . . . . . . . . . . . . . . . . . . . . . . .9-4
SRL Measurement Uncertainty vs Connector Fault . . . . . . . . . . . . . . . . . . . . . . . . . . .9-8
Minimizing Measurement Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-8

Contents
ix
Fault Location Distance Range and Resolution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-12
Range. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-12
Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-12
Typical Distance Data in Feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-13
Distance Data in Meters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-16
A. Cable Loss and Velocity Factors
Cable Loss and Velocity Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A-2

1-1
1 Introduction and Measurement
Theory

1-2 Option 100 Fault Location and SRL
Introduction and Measurement Theory
Fault Location Measurement Theory
Fault Location Measurement Theory
This section describes basic fault location measurement theory, how the
analyzer converts frequency-domain data to distance-domain data, and
the relationship between start distance, stop distance and frequency
span.
Fault location measurements are designed to quickly and easily locate
faults, or discontinuities, in either 50 ohm or 75 ohm transmission lines.
Refer to Figure 1-1 for the following discussion.
The network analyzer has an RF signal source that produces an incident
signal that is used as a stimulus to locate and measure discontinuities in
your transmission line or cable. Each fault or discontinuity responds by
reflecting a portion of the incident signal and transmitting the remaining
signal.
The analyzer measures the frequency response of the cable and then
transforms the frequency data to distance data.
Figure 1-1 Fault Response to an RF Signal

Option 100 Fault Location and SRL 1-3
Introduction and Measurement Theory
Fault Location Measurement Theory
Typically, fault location measurement results are expressed in one of four
ways:
Format Select Description
Return Loss
(RL) The number of dB that the reflected signal is below the incident signal. Its relationship to the
reflection coefficient (ρ) is described by the
following formula:
RL = −20 log ρ.
Reflection
Coefficient (ρ)The ratio of the reflected voltage wave to the incident voltage wave.
Standing
Wave Ratio
(SWR)
Any two waves traveling in opposite directions
(the incident and reflected for example) cause a
“standing wave” to be formed on the transmission
line. SWR is defined as the maximum voltage
over the minimum voltage of the standing wave.
SWR can also be mathematically derived from
the reflection coefficient (ρ) with the following
formula:
Impedance
Magnitude The magnitude of the complex impedance at each measurement point. See “How to Make
Impedance Measurements” on page 6-4 for
information on making impedance
measurements.
FORMAT
Log Mag
Lin Mag
SWR
SWR =1 + ρ
1 − ρ
−−−−−−−
Impedance Magnitude
ImpedanceMagnitude Z Z2
real Z2 imaginary
+==

1-4 Option 100 Fault Location and SRL
Introduction and Measurement Theory
Fault Location Measurement Theory
How the Analyzer Converts Frequency Data
to Distance Data
Fault-location measurements are single-ended measurements, meaning
that only one end of a cable under test need be connected to the
analyzer's RF OUT (or PORT 1) test port.
This type of measurement is generally called a reflection measurement
and typically displays a response commonly known as return loss.
The analyzer performs swept-frequency measurements of return loss
versus frequency, then uses the Fourier transform to convert the
response-versus-frequency to a response-versus-distance. The analyzer's
internal computer makes the calculation by using either the inverse
discrete Fourier transform (inverse FFT) technique or the chirp-Z
Fourier transform technique.
The Fourier transform technique is essentially a process of adding the
signals measured by the analyzer in the frequency domain and
combining them to create the fault-location response in the time domain.
The resulting measurement is an error-corrected fault-location response
of the cable under test.

Option 100 Fault Location and SRL 1-5
Introduction and Measurement Theory
Fault Location Measurement Theory
Start/Stop Distance and Frequency Span
Explanation
When the analyzer is set up for a fault location measurement, you can
determine the center frequency (when in band pass mode — see “2. Enter
the Measurement Parameters” on page 3-5 for more information on band
pass mode), and start and stop distances for the measurement. The
distance range (start distance −stop distance) determines the frequency
span, which in turn determines the start and stop frequencies.
NOTE In band pass mode (as opposed to low pass mode), you can select center
frequency. Changes to distance range do not affect the user-chosen center
frequency. Refer to “2. Enter the Measurement Parameters” on page 3-5
for more information about band pass and low pass mode.
The analyzer will attempt to set the frequency span to the setting
required for the distance range. The maximum setting for the frequency
span cannot exceed the analyzer's frequency capability. For instance, the
start frequency cannot be lower than the analyzer's low frequency limit,
and the stop frequency cannot be higher than the analyzer's high
frequency limit.
When the distance range requires a span that exceeds the analyzer's
capability, the frequency span is set to the maximum available setting
and a chirp-Z transform is done on the frequency domain data to provide
the response in the distance domain.
The chirp-Z transform creates a distance domain response with arbitrary
start and stop distances for a given frequency span. Because the chirp-Z
transform requires more processing, sweep update rates will slow down
in this mode. The analyzer will generally attempt to set frequency span
to an allowable setting for the requested distance range before using the
chirp-Z transform.

1-6 Option 100 Fault Location and SRL
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
Cable Impedance and Structural Return
Loss Measurement Theory
The SRL feature is designed to measure cable impedance and structural
return loss. Cable impedance is the ratio of voltage to current of a signal
traveling in one direction down the cable. Structural return loss is the
ratio of incident signal to reflected signal in a cable, referenced to the
cable's impedance.
The network analyzer uses a synthesized RF signal source to produce an
incident signal as a stimulus. A reflection measurement is made and
then used to compute the cable impedance. The structural return loss
measurement is displayed referenced to the measured cable impedance.
For CATV cable, the cable is measured from 5 MHz to 1000 MHz at
narrow frequency resolutions down to 125 kHz. The analyzer will
automatically scan the cable, then report the worst-case responses.
Cable Impedance
The analyzer automatically computes the cable impedance (Z). However,
if you wish to turn off this “auto Z” function and input your own value of
impedance, you can. See “Connector Model for Short Cables” on
page 4-13.
In coaxial cable, the value of the impedance will depend upon the ratio of
the inner and outer conductor diameters, and the dielectric constant of
the material between the inner and outer conductors. The cable
impedance will also be affected by changes in conductivity. These
changes are a natural consequence of RF currents that flow near the
surface of a conductor. This effect is known as the “skin effect.” Also, the
construction of the cable can change along the length of the cable, with
differences in conductor thickness, dielectric material and outer
conductor diameter changing due to limitations in manufacturing. Thus
the cable impedance may vary along the length of the cable.
The extent to which manufacturing imperfections degrade cable
performance is characterized by a specification called structural return
loss (SRL). SRL is the ratio of incident signal to reflected signal in a
cable. This definition implies a known incident and reflected signal. In
practice, the SRL is loosely defined as the reflection coefficient of a cable
referenced to the cable's impedance. The reflection seen at the input of a

Option 100 Fault Location and SRL 1-7
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
cable, which contributes to SRL, is the sum of all the tiny reflections
along the length of the cable. In terms of cable impedance, the SRL can
be defined mathematically as:
Equation 1
Zin is the impedance seen at the input of the cable, and Zcable is the
nominal cable impedance.
Cable impedance is a specification that is defined only at a discrete point
along the cable, and at a discrete frequency. However, when commonly
referred to, the impedance of the cable is some average of the impedance
over the frequency of interest. Structural return loss, on the other hand,
is the cumulative result of reflections along a cable as seen from the
input of the cable. The above definitions need to be expressed in a more
rigorous form in order to apply a measurement methodology.
Defining Cable Impedance
Following are three common methods of defining cable impedance.
Although all three methods may be commonly used in your industry,
your network analyzer uses the third method (Z-average normalization)
to define cable impedance.
ZSRL ω() Zin ω()Zcable
–
Zin ω() Zcable +
--------------------------------------=

1-8 Option 100 Fault Location and SRL
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
Method 1. One definition of cable impedance is that impedance which
results in minimum measured values for SRL reflections over the
frequency of interest. This is equivalent to measuring a cable with a
return loss bridge that can vary its reference impedance. The value of
reference impedance that results in minimum reflection, where
minimum must now be defined in some sense, is the cable impedance.
Mathematically, this is equivalent to finding a cable impedance such
that:
Equation 2
where ρis some mean reflection coefficient. Thus, cable impedance and
SRL are somewhat inter-related: the value of SRL depends upon the
cable impedance, and the cable impedance is chosen to give a minimum
SRL value.
Method 2. An alternate definition of cable impedance is the average
impedance presented at the input of the cable over a desired span. This
can be represented as:
Equation 3
The value found for Zavg would be substituted for Zcable in Equation 1 to
obtain the structural return loss from the cable impedance
measurement.
∂ρωZcable (, )[]
∂Zcable
()
------------------------------------0=
Zavg Fmin FmaxZin F()Fd
∫
Fmax Fmin –()
---------------------------------------------------=

Option 100 Fault Location and SRL 1-9
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
Method 3 (Z-average normalization). The mathematics for the
Z-average normalization as performed by the analyzer are shown below.
Equation 4
Z0= system impedance, 50 or 75 Ω
Equation 5
Equation 6
In Equation 4, ρ(ω) is the reflection coefficient from the analyzer
measured at each frequency and Zin(ω) is the impedance of the cable for
that measured reflection coefficient.
The calculation of Zcable, described in Equation 5, is the Z-average
impedance of the cable over the number of frequency points (N). The
default frequency range is approximately 5 MHz to 200 MHz. This
frequency range is chosen because mismatch effects of the input
connector are small. High quality connectors must be used if the average
impedance is calculated over a wider span. The frequency range for this
calculation can be modified by using the softkey
in the connector model menu to change the cable impedance cutoff
frequency.
Equation 6 is the structural return loss for the cable. This calculation
can be done by the analyzer or an external computer.
Zin ω() Z01ρω()+()
1ρω()–()
--------------------------
×=
Zcable
Zin ωn ()
n1=
N
∑
N
---------------------------------=
ρSRL ω() Zin ω()Zcable
–
Zin ω() Zcable +
--------------------------------------=
Z cutoff Frequency

1-10 Option 100 Fault Location and SRL
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
SRL and Periodic Cable Faults
SRL is the measure of the reflection of incident energy that is caused by
imperfections or disturbances (bumps) in the cable which are distributed
throughout the cable length. These bumps may take the form of a small
dent, or a change in diameter of the cable. These bumps are caused by
periodic effects on the cable while in the manufacturing process. For
example, consider a turn-around wheel with a rough spot on a bearing.
The rough spot can cause a slight tug for each rotation of the wheel. As
the cable is passed around the wheel, a small imperfection can be created
periodically corresponding to the tug from the bad bearing.
Each of these small variations within the cable causes a small amount of
energy to reflect back to the source due to the non-uniformity of the cable
diameter. Each bump reflects so little energy that it is too small to
observe with fault location techniques. However, reflections from the
individual bumps can sum up and reflect enough energy to be detected as
SRL. As the bumps get larger and larger, or as more of them are present,
the SRL values will also increase. The energy reflected by these bumps
can appear in the return loss measurement as a reflection spike at the
frequency that corresponds to the spacing of the bumps. The spacing
between the bumps is one half the wavelength of the reflection spike and
is described by equations 7 and 8.
Equation 7
Equation 8
The wavelength/2 spacing corresponds to the frequency at which down
and back reflections will add coherently (in-phase). The reflections
produce a very narrow response on the analyzer display that is directly
related to the spacing of the bumps. The amount of reflected energy is
observed as return loss. When this return loss measurement is
normalized to the cable impedance, the return loss becomes structural
return loss.
wavelength c
f
---
≈
cspeed of light =f frequency=
wavelength
2
------------------------------ spacing between the bumps =

Option 100 Fault Location and SRL 1-11
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
Figure 1-2 diagrams reflections from bumps in a cable. We can combine
the energy reflected by each bump in a cable and make a few basic
assumptions, to mathematically describe SRL by the series shown in
Equation 9.
Equation 9
Vref = reflected Voltage
Vin = incident Voltage
L = cable loss
Γ= reflection coefficient of the bumps
The bumps are assumed to be uniform in reflection and spaced by a
wavelength/2 separation.
Figure 1-2 Periodic Bumps in a Cable
Vref VinLΓL[]VinL1Γ–()LΓLL[]VinL1Γ–()L1Γ–()LΓLLL[]…++ +=