FDR - Frequency Domain Reflectometer.pdf

8/12/2019 FDR - Frequency Domain Reflectometer.pdf

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FDR - Frequency Domain Reflectometer

What i s FDR ?he LOCATE mode in the LST-1700 / CLI-1750 provides a means of characterizing the impedance mismatches of anactive cable and associated components to determine potential faults that could disrupt signal transmission. The

ethod of measurement is based on the theory of Frequency Domain Reflectometry, or FDR.

DR is the measure of signal reflections through a medium across frequency. In the case of the LST-1700 / CLI-1750,sweep signal is transmitted over coaxial cable (the medium). A portion of the transmitted sweep signal on the cableill be reflected back to the transmitter (LST-1700) if the load is not a perfect 75 impedance match. The reflectedergy will be the same frequency as the incident (sweep) signal but different in phase. The resulting signal (incidentreflected) will appear as standing waves on a frequency sweep (figure 1). The reflection is such that the peaks of e individual cycles can be translated to distance to the fault (impedance mismatch) through the following equation:

D= (1/f)*c*Vop / 2 = (1/f)(491*Vop) or 491*Vop/f Where D=distance to fault, c=speed of light, Vop=velocity of propagation of the cable, and f = frequency of 1 cycle of

e standing wave.

dditionally, the peak-to-peak value of the reflection determines the magnitude of the fault and is calculated by theuation:

Magnitude Return Loss (dB) = 20*log[(Reflected Peak to Peak)]

Figure 1. Example of standing wavescaused by impedance mismatch.


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Frequency Domain Reflectometer Measurement SequenceThe following measurement sequence is a summary of the 8 critical events that occur withinthe instruments to produce the amplitude versus distance FDR display.

1. Figure 2 is a block diagram of the LST-1700 output configuration. When the relay is in the A

position a reference sweep measurement is performed using an ideal 75 load.2. Again referring to figure 2, the relay is switched to the B position, and a sweep measurement isperformed on the cable under test.3. The ratio (difference in dB) between the reference sweep and measurement sweep is calculated,thus leaving only the reflected response.4. A Fast Fourier Transform (FFT) calculation is performed, extracting the culmination of standingwaves of the reflected response (may include multiple impedance mismatches).5. User programmable cable attenuation is applied to correct for cable loss according to cable type.6. The data from the LST-1700 is transferred to the CLI-1750 via RS232.7. The CLI-1750 applies a distance to fault equation.8. The CLI-1750 displays the measurement in the distance versus amplitude format (figure 3).

Figure 2. LST-1700 ouput configuration Figure 3. Fault location measurement

resistivenetwork

r e s i s t i v e

n e t w o r k

A

BTO CABLE UNER TEST

MEASUREMENTPOINT

resistivenetwork

75

75

r e s i s t i v e

n e t w o r k

FDR Measurement Considerations

Before performing any FDR measurement, two user defined cable parameters are required to properly program

the CLI-1750 and obtain accurate results in the Locate mode. These parameters are Velocity of Propagation(VOP) and the Cable Attenuation Factor @ 50 MHz for the cable under test. Both of these specifications can befound in the coaxial cable specifications available from the cable manufacturer.

Velocity of PropagationThe VOP is a measure of the phase effects in a traveling wave, and is primarily determined by a cables dielectricmaterial surrounding the center conductor. The VOP will be different depending on the size and make of thecoaxial cable you are testing. Common VOP values are .87 for .500 hardline coax, and .82 for drop (RG-6) or subscriber (RG-59) cable. The cable manufacturers specification sheet should be referenced for the precise VOPbefore making FDR measurements.


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VOP is required in order to accurately calculate the distance to faults in the locate mode. Specifically, this value isof the distance equation D=(1/f)(c*Vop). The VOP is entered in the LOCATE mode using the numeric keypad, andalways be a value less than 1. The CLI-1750 understands that when you enter a value of less than 1, you are

nging the VOP. Please note that if you enter a value greater than 1, then the unit interprets this as the distanceween markers.

ou do not know the the VOP of the cable under test, it can be calculated automatically using the LOCATE modea known length of sample cable of the same type that you will be testing. The following procedure should be usedalculate VOP with a known length of cable.

culating VOP with a Known Length of Cablet, measure the length of the sample cable from end to end. Then connect one end of the sample cable to the RFt of your LST-1700 and leave the other end unterminated . A peak will appear on the Locate graph at theance where the RF energy is being reflected by the unterminated end of the cable. Place one of the distancekers over this peak. Move the other marker to a distance of zero (left most side of the graph). The distance

ween markers appears in the upper right-hand corner of the display. Now enter the actual length of the samplele that you previously measured. Use the numeric keys followed by the ENTER key. Your CLI-1750 will

ulate the correct velocity of propagation based on the length that you enter.

VOP is entered on thekeypad, or calculatedwith a known length of cable

Distance to the fault iscalculated using theVOP value.

Cable attenuation @ 50 MHzThe second user defined parameter required to achieve accurate FDR fault measurements is the cablecompensation at 50 MHz. Whereas VOP of the cable is concerned with distance to the fault, the cablecompensation value will influence the amplitude of the fault.

As a signal passes through cable, a certain amount of loss is to be expected from 5-750 MHz over the cablelength (figure 4). Cable manufacturers typically characterize cable attenuation in frequency versus distance

as a dB value per 100 feet or 100 meters (figure 5).


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Typical RF Specifications*

RG - 59 RG - 6 RG - 11

Attenuation Attenuation Attenuation Attenuation Attenuation Attenuationin dB per in dB per in dB per in dB per in dB per in dB per

Frequency 100 ft. 100 m 100 ft. 100 m 100 ft. 100m

5 0 .79 2 .59 0.60 1.97 0.38 1.2530 1 .46 4 .79 1.11 3.64 0.70 2.3050 1 .74 5 .71 1.32 4.33 0.87 2.85

108 2 .46 8 .07 1.95 6.40 1.29 4.23216 3 .48 11.41 2.78 9.12 1.85 6.07240 3 .67 12.04 2.93 9.61 1.96 6.43300 4 .11 13.48 3.29 10.79 2.20 7.22325 4 .28 14.04 3.43 11.25 2.29 7.51400 4 .75 15.58 3.81 12.50 2.56 8.40450 5 .04 16.53 4.05 13.28 2.73 8.95500 5 .31 17.42 4.27 14.01 2.88 9.45550 5 .57 18.27 4.49 14.73 3.03 9.94600 5 .83 19.12 4.69 15.38 3.18 10.43750 6 .55 21.48 5.27 17.29 3.47 11.35862 7 .04 23.09 5.66 18.66 3.76 12.32

1000 7 .61 24.96 6.11 20.04 4.10 13.48

Figure 4. Example of cable attenuation versus frequency (per 100 feet)

Frequency Attenuation (dB) per 100 ft. 5 -0.79 30 -1.46 50 -1.74 108 -2.46 216 -3.48 240 -3.67 300 -4.11 325 -4.28 400 -4.75 450 -5.04 500 -5.31 550 -5.57 600 -5.83 750 -6.55 862 -7.04 1000 -7.61

Figure 5. Typical drop cable RF specifications of frequency versus attenuation

-8

-7

-6

-5

-4

-3

-2

-1

0 5

3 0

5 0

1 0 8

2 1 6

2 4 0

3 0 0

3 2 5

4 0 0

4 5 0

5 0 0

5 5 0

6 0 0

7 5 0

8 6 2

1 0 0 0

Referring to figure 4, it can be seen that if a fault (impedance mismatch) is detected in the cable, the amplitude of the fault will be affected by the frequency and distance. For example, figure 4 illustrates the cable loss infrequency per 100 feet. If a fault is detected at 100 feet at a frequency of 450 MHz, then a corresponding loss of approximately 5 dB can be expected. The CLI-1750 applies the cable attenuation factor to the FDR measurement so that the fault amplitude is displayed without cable loss*. Therefore , in order to achieve accuratefault magnitude results in the LOCATE mode, the CLI-1750 applies a cable loss correction factor to the displayedresult. This correction factor is based on the cable attenuation factor @ 50 MHz per 100 feet (meters), and isa required numeric entry by the unit operator. The cable attenuation factor will change depending on the cablesize and manufacturer. The cable manufacturers specifications should be identified prior to performing an FDRmeasurement.**


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To enter the correct cable compensation value, select the NAVIGATOR key followed by the CONFIGURE icon. Fromhe CONFIGURE mode, select TRANSMITTER followed by LOCATE mode. The user may turn the cableompensation on or off, and the cable compensation may be entered.

*Note1: The cable compensation factor accounts for the loss of the cable only. If passive devices such as splittersare installed in the path of the cable, the detected fault amplitude will be reduced by the amount of the passive deviceoss x 2.

EXAMPLE: An FDR measurement is being performed on a residence that includes a two way splitter. A two wayplitter will typically have 3.5 dB loss on each output leg. If a fault is detected after the splitter, then the amplitude of he fault will actually be 7 dB lower than the displayed dBrl value (see figure 5).

Example: If the cable loss is known to be 1.6 dB at 55 MHz, then the loss at 50 MHz is calculated as follows:Cable attenuation at 50 MHz = 50

55 x 1 .6 = 1 . 5 3 d B

**Note2: If the cable attenuation is not known at 50 MHz, but attenuation is known at another frequency, then thecorrect attenuation can be calculated using the following cable loss ratio formula

Cable Loss Ratio = f f

1

2

2-way splitter 3.5 dB loss

on eachoutput port

75 Ohmtermination

1

3

4

1

2

2

3

4

FDR measurement begins

3.5 dB loss from splitter on forward sweep

Fault detected at open cable

Additional 3.5 dB loss added to FDR measurement

5

5 Cable compensation is accounted for in the CLI-1750, but displayed open fault will be 7 dB low due to loss of splitter

open

Figure 5. Splitter loss must be considered when performing FDR measurements


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Using the CLI-1750 Locate Mode

The CLI-1750 Locate mode helps you detect impedance mismatches within the home wiring.

The measurement will display both the distance and magnitude of the fault, and help determinethe passive devices, if any, that are installed in the network under test. The purpose of thisdocument is to serve as a quick start manual for the inexperienced operator, providing step bystep instructions on user interface and how to interpret the measurement display. Practice andhands-on experience should bring the novice user up to the pro level with minimal supervision.

Getting StartedThe Locate measurement is performed with the CLT-1700 and CLI-1750 linked via RS-232. TheLST-1700 performs the FDR (frequency domain reflectometry) measurement, and the CLI-1750displays the results. The following procedure will get you started performing the locatemeasurement.

NOTE: Prior to performing Locate measurements, you should be familiar with two importantparameters of the cable you will be testing. Specifically, you should know the cables Velocity of Propagation and Cable Attenuation @ 50 MHz (per 100 feet or meters) . These twospecifications are critical, as the CLI-1750 relies on this user entered information to calculate thedistance and magnitude of the cable fault. For more information, please see the previoussections on FDR, VOP and cable attenuation.

STEP 1. Connect the 1700/1750 and RS-232cable. The cable or cable network under test isconnected to the LST-1700 (figure 1). For purposes of training and demonstration, at leastone of the cables in your network under testshould be unterminated. The Locate graphsused in this instruction were taken from thesample network in figure 1.

STEP 2. From the CLI-1750s NAVIGATORmode, select CONFIGURE / TRANSMITTER /LOCATE MODE. The LOCATE MODE

configuration screen will allow you to enter resolution (MEDIUM is always a good place tostart) and cable attenuation factor (the defaultvalue is 1.5 dB). These are the only 2parameters that need to be programmed in thismode. The harmonic filter should be turned on,and should remain on for most commonmeasurements.

25-FeetRG59

Vop=0.82

35-FeetRG59

Vop=0.82

25-FeetRG59

Vop=0.82

50-FeetRG59

Vop=0.82

RS-232

75Feet2-waysplitter

Event #2Splitter 75 feet

Event#3Unterminated

Cable100feet

Event#4Unterminated

Cable110feet

Figure 1. Sample cable configuration


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STEP 3. From the Locate Mode configurescreen or Navigator screen, select the LOCATEicon ! . You will now switch immediately to theLocate Mode and the measurement will begin.

" Dont be alarmed when you hear the clickingnoise, thats simply the LST-1700 relayswitching from a reference signal to theactive test signal.

STEP 4. The first screen that you see will besimilar to figure 2. Now is time to enter theVelocity of Propagation for the cable under test(the default value is .82) To enter the VOP,simply press the CLI-1750 decimal point hard

key followed by the numerical value of the VOP.The VOP will change in the upper right handcorner of the Locate mode screen.

"""" Any value entered on the keypad 1 isassumed to be the distance betweenmarkers A and B. Dont do this unless yourecalculating the VOP of test cable with a

known cable length (see calculating VOP inthe previous section).

STEP 5. Referring again to figure 2, pressFunction (green key) followed by the Scale (#3)icon $ . This action will auto scale the display.Note that the initial starting distance is atmaximum (about 2600 feet). At this point youshould observe any event from 0 to 2600 feet(depending on the distance of the fault, someevents may be compressed at the extreme leftside of the display).

Figure 2.

Figure 3.

Figure 4.

STEP 6. Toggle the A/B marker select key toenable marker B (figure 3). Using the right arrowkey position marker B to the furthest event onthe display (figure 4). Now press the zoomicon % and press the plus soft key & until theevent is at the middle of the display. You maynote at this point that the single event is actuallytwo or more events (figure 5). You may wish torepeat the marker B position / zoom sequenceto further center the event in the middle of the

display.


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STEP 7. Once the event has been center positioned and the peak of the event can beseen, the horizontal amplitude marker can beused to determine the magnitude of the event.Simply use the up and down arrows to positionthe dotted horizontal markers on the peak of theevent, and its amplitude will be displayed in theupper left hand corner of the display (figure 6).Similarly, marker B can be positioned on theevent, and the amplitude will be read on themarker B measurement line (figure 6).

Figure 5.

Figure 6.

Figure 7.

STEP 8. At this point, you may want to select ahigher resolution. This is accomplished bypressing the second function key (green key)followed by the up arrow. Note the resolutionicon ' in the top left corner of the screen. Eachhighlighted bar corresponds to the resolutionsetting being used (low, medium, high or ultra).

" The most accurate distance and amplitudemeasurements are obtained using thehighest resolution setting. Since themeasurement time is longest using the ultrasetting, you may want to use the mediumsetting to find the events, and high or ultrato make the final distance and amplitudemeasurements. High resolution isrecommended when testing cable lengths of > 1000 feet (305 meters).


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REFERENCE - ICONS AND SOFT KEYS

1 STEP 3 LOCATE MODE ICONS

Locate icon in Navigator Mode Locate Mode Icon in Configure Screen

2 STEP 5 - AUTO SCALE

PRESS FUNCTION / SCALE TO AUTO ADJUSTLOCATE DISPLAY

3 STEP 6 - ZOOM ICONS

ZOOM SELECTINCREASE / DECREASE ZOOM RANGE

4&


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5 STEP 8 - RESOLUTION ICONS

RESOLUTION ICON INDICATES LOW,MEDIUM , HIGH OR ULTRA RESOLUTIONSETTING. RESOLUTION CAN BECHANGED BY PRESSING THEFUNCTION KEY AND UP / DOWN

ARROWS.

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FDR - Frequency Domain Reflectometer.pdf

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