VDV Works Virtual Hands-On Training Fiber Optics Testing: Using OTDRs   Part of the VDV Academy Fiber Optic Training Programs
Home Read About OTDRs  Dos & Don'ts  Tools  Setup Take Data Viewing the trace Measure Loss Modify Test Parameters Compare Traces Troubleshooting Quiz Quiz - Answers More Useful References  Understanding OTDRs "Virtual Hands-On" Termination "Virtual Hands-On" Testing	Check Your Answers 1. Identify the features shown in the OTDR trace:   A1. Splice (between low loss cable and high loss cable). A2. Gainer as a result of a splice between fibers with low and high scattering coefficients. A3. End of cable with large back reflection. See Viewing the Trace   2.What two parameters are measured by the OTDR ? (They are the X and Y Axes of the OTDR display.) Distance and loss See viewing the trace   3. List two effects that will occur if a user increases the width of the OTDR pulse from the low range to the high range in the table below. The Amount of backscattered light sent back to the OTDR will increase, The resolution between two events will decrease. See read about OTDRs 4. Besides measuring splice loss between the markers, most OTDRs can measure splice loss with another method. How do they do it? Least squares analysis using curve fitting. See Modifying Parameters 5. In the trace below, where is the "dead zone" and how should the effect of the dead zone be accounted for? The dead zone is the length of fiber nearest the OTDR that is affected by the recovery of the receiver from the laser test pulse. It appears to the far left of the trace. Here you can see the saturated reflection peak and the long baseline recovery. The "dead zone" can be overcome with a "launch cable" sometimes called a "pulse suppressor." 6. Which of the reflections in the trace below is a "ghost"? How do you know? All of the events to the right of the marker are ghosts. (Look at multiples of distances to saturated reflections at real events.) See Troubleshooting 7. What is the effect of different backscatter coefficients on either side of a splice? Backscatter differences causes losses to be measured inaccurately - too high or low - or even show gains. 8. What is the instrument used to find faults too close for the OTDR to see? Visual fault locator. See Tools 9. What does the OTDR actually measure the length of and what problem does that cause? The OTDR measures the length of the fiber, not the cable.The fiber is longer than the cable, so it the cable is pulled with high tension, the fiber will not be affected.  When locating faults, the actual cable should be 1% shorter than the fiber, so you have to dig closer than you thought. See read about OTDRs 10. How can you determine which of the event(s) in the trace below have overloaded the OTDR? The flat peak on reflective events shows the OTDR is overloaded. See troubleshooting 11. There are a number of fluids that work well as index matching fluids to reduce reflections. Which two can you buy almost anywhere? Mineral oil and petroleum jelly. See Ghosts/Troubleshooting 12. What information makes cable plant testing much easier? Complete documentation. See Home
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