AgilentMaking a Distance-to-Fault MeasurementUsing the Agilent CSA

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Introduction

In wireless communication systems,cables, connectors, and antennas andother components cause problems.The Agilent CSA spectrum analyzerwith the optional stimulus/responsemeasurement suite can help youquickly find these problems. Withinthe stimulus/ response suite you canuse the return loss and the distance-to-fault (DTF) measurements withbuilt-in signal source and bridge tohelp you determine the severity of the problems in your communicationsystem.

The built-in signal source sends signals through your system and looking for reflections back to help pinpoint fault locations. The bridge sepa-rates the input signal and the reflectedsignal so they can be measured andcompared. It redirects the returningsignal internally back to the RF inputfor analysis. When your system hasminor problems, your return loss couldbe 30 dB or higher while a systemwith severe faults will show a returnloss close to 0 dB.

The DTF measurement will allow youto determine how far away from theAgilent CSA signal source input a faultis located, up to 300 m away. Thispaper will step you through making aDTF measurement with the Agilent CSA.Figure 1 shows a screen capture of aDTF measurement.

Figure 1. DTF screen capture

Return loss

Measurement result table

Calibration menu

Largest fault 6.76 m away from input

Start/Stop distanceof your cable

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Figure 3 shows the hard key frontpanel of the Agilent CSA. Hard keyswill be noted in bold text while softkeys will be noted in italics in thisdocument.

Familiarize Yourself with the Agilent CSA

Figure 2. Agilent CSA front panel overview

Signal source/RF output

On/Off Help RF input

Soft keys Hard keys

Figure 3. Hard key front panel

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Demonstration

To start using the DTF measurement,push the following keys:

Mode: Stimulus/response: Distance-to-fault

There are two hard keys that you willuse the most when using the DTFmeasurement. They are the FreqChannel and Mode Preset keys,which are circled in red in Figure 3.Mode Preset will reset the instrumentand start you in two-port insertionloss. To return to the DTF measure-ment window press Meas and selectDistance-to-fault.

Freq channel menuIn DTF you will be concerned with theStart and Stop distance of your cable.Push the Freq Range button once toselect Auto as in Figure 5 if you do notknow the frequency range that youwant to use.

Next enter your Start/Stop distance ofthe cable you wish to measure. Themaximum distance is 304.8 m withAuto frequency range selected. If yourstart distance is 0 m you will alwaysshow a fault at 0 m. This is caused bythe reflection from the connection atthe RF output. This DC component issometimes referred to as the “deadzone.”

To calibrate the instrument, you willpress the Calibrate button and walkthrough the step-by-step instructionsshown next.

Before you start the calibrationprocess, you may want to select thetype of cable you are testing as thiswill help make your calibration moreaccurate. Under Meas Setup you willfind Cable Type, where there is a longlist of radio guide (RG) and base transceiver station (BTS) cable types.If you are not using an RG or BTScable you can select “custom.” A sample of the BTS cables in the menuis shown in Figure 4.

Calibration – the first stepAfter selecting your Start/Stopdistance and Cable type select theCalibrate key under the Freq Channelbutton.

Calibrating the Agilent CSA willensure your measurements are accu-rate and will save you time in makingduplicate measurements to check yourresults. The easy-to-use, step-by-stepguide minimizes the need for trainingand helps technicians master theinstruments and get their work doneefficiently.

Figure 5. Frequency channel menu

Figure 4. BTS cables in the menu

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Step-by-Step Menu

The step-by-step menu is as follows:

Specify frequency rangeYou should have already specified the Start/Stop distance under the FreqChannel menu or selected Auto after entering your Start/Stop distance.

Once confirmed select Continue.

Connect the openYou 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 connectionMake 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 willtake. In addition, the more averages you have set the longer this will take. Toturn Averaging off: Meas Setup: Avg Mode: Off.

Connect the shortYou 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 loadYou 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 rangewill appear in the upper left hand corner of the screen.

Figures 6a-6d. Step-by-step menu

a

b

c

d

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Making a Distance-to-FaultMeasurementAfter calibrating your instrument, con-nect the cable you want to test to theRF output on the Agilent CSA. You willnotice that the top four faults over thedistance that you selected are shownin severity order with yellow markers.These are fault indicators and can beturned On/Off under the View/Display menu. Even when these areturned off, you will see the return loss,distance of the fault from the RF out-put, and voltage standing wave ratio(VSWR) of those top four faults asshown in Figure 7. VSWR measuresthe impedance mismatch between thetransmission line and a load; the higherthe VSWR, the greater the mismatch.

Again, if your start distance is 0 m youwill see a fault at 0 m that correspondsto your connection at the RF output.This is shown as fault two in Figure 1.

In order to determine the severity ofthe any other faults on screen you canalso use markers to see the distanceand return loss of up-to-four otherfaults at one time as shown in Figure 7.Press Marker to use up-to-four mark-ers and their associated delta marker.

How to Pick the Best CableMatches and ConnectionsIt is important that your connectionsare solid so that you have little lossbetween cables. When choosingbetween different connectors andmatching cables the Agilent CSA willhelp you make the best connectionpossible.

Figure 8 shows the DTF measurementwith two 6-feet RG-214 Type-N cablesconnected with a Type-N female-to-female connector. You can see thefault at 6-feet is 32.3 dB. The higherthe return loss, the better the connec-tion/cable match.

Figure 7. Distance-to-fault measurement, fault indicators

Figure 8. Good connector and matching of cables

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Figure 9 shows the DTF measurementwith the same two cables as shownabove with a less than perfect con-nection. The return loss at 6-feet, fault#2, is now 20 dB. This measurementcan be used to pick the best cablematches as well. If you have deter-mined a good cable and connector youwill be able to find the best matchingcable using this same technique ofcomparing the return loss at a knownconnection. You can compare thesedifferent cables/matches by using thefollowing technique of comparing twotraces.

Comparing Two TracesFaults in cables and connections canoccur due to weather, erosion overtime, construction damage, and manyother reasons. The Agilent CSA allowsyou to save and name traces so thatyou can recall them at a later date. Itis a good idea to make a DTF mea-surement and save that trace on newor newly repaired systems so that youcan compare them later when doingmaintenance check ups.

When saving a trace you can eithername it yourself or the Agilent CSAwill name it automatically. To nameyour trace, make sure you select Askunder the following menu: Save:Name: Filename: Ask.

To save your measurement pressSave: Type: Trace: Device: Internal (or USB): Save Now. See menu inFigure 10a.

When recalling a saved file you canchoose whether to view it as trace 1or trace 2. The fault indicators andtable will only show on trace 1; how-ever, you can use markers on eithertrace 1 or trace 2.

To Recall a saved trace push the fol-lowing: Recall: Type: Trace: Device:Internal (or USB): Destination: Trace 1or 2: Recall Now. On your Agilent CSA,trace 1 will be in yellow and trace 2will be in blue as in Figure 11.

Figure 9. Less than perfect connection between two similar cables

Figure 10a - 10b. Save and recall menus

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To see the change in severity from thesaved trace to the new trace pressMarker and push Marker Trace toselect trace 2. Select the Normalmarker and then use the knob to scrollto the fault you want to evaluate. InFigure 11 you can see the markerreadout for the old #3 fault is 29.8 dBwhile the current reading in the FaultIndicator Table for fault #3 is 21.4 dB.This difference of 8.4 dB may indicatea connection that has degraded, andmay be loose, dirty or broken.

Conclusion

Your communication system needs tobe reliable, so you need to be sureyour cables and connections areclean, tight, and in good condition.The Agilent CSA spectrum analyzerwith built-in bridge and signal sourcemakes it easy to measure distance-to-fault and return loss, so you know thecondition of the cables when youinstall them and during routine main-tenance, and you can quickly find theproblem when a repair is needed. Testresults can be stored and recalled soyou can monitor your system andidentify potential problems before theybecome big problems. The AgilentCSA has outstanding RF performancein an easy-to-use, portable package soyou can be confident in the measure-ments. And, the built in step-by-stepcalibration and test set-up graphicsminimize the learning curve on a newinstrument.

Figure 11. Comparing two traces

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© Agilent Technologies, Inc. 2006, 2007

Printed in USA, March 1, 2007

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