Making a Distance to Fault Measurement
Making a Distance-to-Fault Measurement
Using the Agilent CSA
White Paper
In wireless communication systems,
cables, connectors, and antennas and
other components cause problems.
The Agilent CSA spectrum analyzer
with the optional stimulus/response
measurement suite can help you
quickly find these problems. Within
the stimulus/ response suite you can
use the return loss and the distanceto-fault (DTF) measurements with
built-in signal source and bridge to
help you determine the severity of
the problems in your communication
The built-in signal source sends
signals through your system and
looking for reflections back to help pin
point fault locations. The bridge separates the input signal and the reflected
signal so they can be measured and
compared. It redirects the returning
signal internally back to the RF input
for analysis. When your system has
minor problems, your return loss could
be 30 dB or higher while a system
with severe faults will show a return
loss close to 0 dB.
The DTF measurement will allow you
to determine how far away from the
Agilent CSA signal source input a fault
is located, up to 300 m away. This
paper will step you through making a
DTF measurement with the Agilent CSA.
Figure 1 shows a screen capture of a
DTF measurement.
Largest fault 6.76 m
away from input
Return loss
Measurement result table
Figure 1. DTF screen capture
Calibration menu
Start/Stop distance
of your cable
Familiarize Yourself with the Agilent CSA
Signal source/RF output
Soft keys
Hard keys
RF input
Figure 2. Agilent CSA front panel overview
Figure 3 shows the hard key front
panel of the Agilent CSA. Hard keys
will be noted in bold text while soft
keys will be noted in italics in this
Figure 3. Hard key front panel
To start using the DTF measurement,
push the following keys:
Mode: Stimulus/response: Distanceto-fault
There are two hard keys that you will
use the most when using the DTF
measurement. They are the Freq
Channel and Mode Preset keys,
which are circled in red in Figure 3.
Mode Preset will reset the instrument
and start you in two-port insertion
loss. To return to the DTF measurement window press Meas and select
Freq channel menu
In DTF you will be concerned with the
Start and Stop distance of your cable.
Push the Freq Range button once to
select Auto as in Figure 5 if you do not
know the frequency range that you
want to use.
Next enter your Start/Stop distance of
the cable you wish to measure. The
maximum distance is 304.8 m with
Auto frequency range selected. If your
start distance is 0 m you will always
show a fault at 0 m. This is caused by
the reflection from the connection at
the RF output. This DC component is
sometimes referred to as the “dead
To calibrate the instrument, you will
press the Calibrate button and walk
through the step-by-step instructions
shown next.
Before you start the calibration
process, you may want to select the
type of cable you are testing as this
will help make your calibration more
accurate. Under Meas Setup you will
find Cable Type, where there is a long
list of radio guide (RG) and base
transceiver station (BTS) cable types.
If you are not using an RG or BTS
cable you can select “custom.” A
sample of the BTS cables in the menu
is shown in Figure 4.
Calibration – the first step
After selecting your Start/Stop
distance and Cable type select the
Calibrate key under the Freq Channel
Calibrating the Agilent CSA will
ensure your measurements are accurate and will save you time in making
duplicate measurements to check your
results. The easy-to-use, step-by-step
guide minimizes the need for training
and helps technicians master the
instruments and get their work done
Figure 5. Frequency channel menu
Figure 4. BTS cables in the menu
Step-by-Step Menu
The step-by-step menu is as follows:
Specify frequency range
You should have already specified the Start/Stop distance under the Freq
Channel menu or selected Auto after entering your Start/Stop distance.
Once confirmed select Continue.
Connect the open
You may connect the open directly to the RF output or use a Type-N cable
connected to an open.
Once confirmed select Continue.
Do not change or remove the connection
Make sure you do not remove the connection until the next screen appears.
The larger the distance you select the longer each step of the calibration will
take. In addition, the more averages you have set the longer this will take. To
turn Averaging off: Meas Setup: Avg Mode: Off.
Connect the short
You may connect the open directly to the RF output or use a Type-N cable
connected to the short.
Once confirmed select Continue.
Connect the load
You may connect the open directly to the RF output or use a Type-N cable
connected to the load.
Once confirmed select Continue.
Now that you have calibrated your instrument, the calibrated frequency range
will appear in the upper left hand corner of the screen.
Figures 6a-6d. Step-by-step menu
Making a Distance-to-Fault
After calibrating your instrument, connect the cable you want to test to the
RF output on the Agilent CSA. You will
notice that the top four faults over the
distance that you selected are shown
in severity order with yellow markers.
These are fault indicators and can be
turned On/Off under the View/
Display menu. Even when these are
turned off, you will see the return loss,
distance of the fault from the RF output, and voltage standing wave ratio
(VSWR) of those top four faults as
shown in Figure 7. VSWR measures
the impedance mismatch between the
transmission line and a load; the higher
the VSWR, the greater the mismatch.
Figure 7. Distance-to-fault measurement, fault indicators
Again, if your start distance is 0 m you
will see a fault at 0 m that corresponds
to your connection at the RF output.
This is shown as fault two in Figure 1.
In order to determine the severity of
the any other faults on screen you can
also use markers to see the distance
and return loss of up-to-four other
faults at one time as shown in Figure 7.
Press Marker to use up-to-four markers and their associated delta marker.
How to Pick the Best Cable
Matches and Connections
It is important that your connections
are solid so that you have little loss
between cables. When choosing
between different connectors and
matching cables the Agilent CSA will
help you make the best connection
Figure 8 shows the DTF measurement
with two 6-feet RG-214 Type-N cables
connected with a Type-N female-tofemale connector. You can see the
fault at 6-feet is 32.3 dB. The higher
the return loss, the better the connection/cable match.
Figure 8. Good connector and matching of cables
Figure 9 shows the DTF measurement
with the same two cables as shown
above with a less than perfect connection. The return loss at 6-feet, fault
#2, is now 20 dB. This measurement
can be used to pick the best cable
matches as well. If you have determined a good cable and connector you
will be able to find the best matching
cable using this same technique of
comparing the return loss at a known
connection. You can compare these
different cables/matches by using the
following technique of comparing two
Comparing Two Traces
Faults in cables and connections can
occur due to weather, erosion over
time, construction damage, and many
other reasons. The Agilent CSA allows
you to save and name traces so that
you can recall them at a later date. It
is a good idea to make a DTF measurement and save that trace on new
or newly repaired systems so that you
can compare them later when doing
maintenance check ups.
Figure 9. Less than perfect connection between two similar cables
When saving a trace you can either
name it yourself or the Agilent CSA
will name it automatically. To name
your trace, make sure you select Ask
under the following menu: Save:
Name: Filename: Ask.
To save your measurement press
Save: Type: Trace: Device: Internal
(or USB): Save Now. See menu in
Figure 10a.
When recalling a saved file you can
choose whether to view it as trace 1
or trace 2. The fault indicators and
table will only show on trace 1; however, you can use markers on either
trace 1 or trace 2.
To Recall a saved trace push the following: Recall: Type: Trace: Device:
Internal (or USB): Destination: Trace 1
or 2: Recall Now. On your Agilent CSA,
trace 1 will be in yellow and trace 2
will be in blue as in Figure 11.
Figure 10a - 10b. Save and recall menus
To see the change in severity from the
saved trace to the new trace press
Marker and push Marker Trace to
select trace 2. Select the Normal
marker and then use the knob to scroll
to the fault you want to evaluate. In
Figure 11 you can see the marker
readout for the old #3 fault is 29.8 dB
while the current reading in the Fault
Indicator Table for fault #3 is 21.4 dB.
This difference of 8.4 dB may indicate
a connection that has degraded, and
may be loose, dirty or broken.
Your communication system needs to
be reliable, so you need to be sure
your cables and connections are
clean, tight, and in good condition.
The Agilent CSA spectrum analyzer
with built-in bridge and signal source
makes it easy to measure distance-tofault and return loss, so you know the
condition of the cables when you
install them and during routine maintenance, and you can quickly find the
problem when a repair is needed. Test
results can be stored and recalled so
you can monitor your system and
identify potential problems before they
become big problems. The Agilent
CSA has outstanding RF performance
in an easy-to-use, portable package so
you can be confident in the measurements. And, the built in step-by-step
calibration and test set-up graphics
minimize the learning curve on a new
Figure 11. Comparing two traces
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Revised: 11/08/06
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© Agilent Technologies, Inc. 2006, 2007
Printed in USA, March 1, 2007
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