Two different instruments that are often described as “works like radar” can test copper and fiber cabling. For copper, it is a time-domain reflectometer (TDR). For fiber, it is an optical time-domain reflectometer (OTDR). These instruments are alike in some ways but different in others and are equally tricky to use. How well do you know these gadgets? Answers and explanations are in red.
1. Both TDRs and OTDRs measure the length of a cable. What information is needed to make this measurement accurately?
A. Impedance of the cable or attenuation of the fiber
B. Speed of the test signal in the cable
C. Attenuation of the copper pair or fiber
D. Connection loss
The TDR or OTDR calculates the distance by measuring time and multiplying it by the speed of the test signal in the copper cable or optical fiber.
2. Reflections seen in traces from either TDRs or OTDRs come from mismatches in the media being tested (impedance in copper or indices of refraction in fiber).
Any change in impedance in a copper cable (e.g. going from 50 to 75Ω coax) or index of refraction in fiber (worst case is a connection with air between the connectors) causes a reflection of the test signal.
3. If a TDR trace has a reflected pulse from the cable end of the same polarity as the test pulse, it indicates that the end of the cable is/has _______.
C. Low impedance
D. High crosstalk
Same polarity return means opens, opposite polarity means shorted.
4. Besides opens and shorts, a TDR can also find _______.
A. Mismatched coax, e.g., 50 cable connected to 75 cable
B. Bad connections or terminations in coax or twisted-pair cable
C. Water-soaked cable
D. All of the above
The knowledgeable user can interpret the TDR trace to find all these kinds of faults.
5. TDRs are also good at finding _______, a problem with inexpensive copper twisted-pair wire-map testers.
A. Crossed pairs
B. Split pairs
C. Reversed pairs
D. Shorts and opens
Inexpensive wiremappers do not detect split pairs, but the TDR detects them as impedance mismatches.
6. Although both instruments work like radar, OTDRs are able to measure fiber _______ from the backscattered light.
The backscatter trace of a fiber allows measuring fiber attenuation between two points.
7. Length measurements by the OTDR are about _______ shorter than the actual cable because the fiber is loosely wound in the cable for protection from tension during pulling.
A. 1–2 percent
B. 2–5 percent
C. 5–10 percent
D. >10 percent
Cable has about 1-2 percent excess fiber to prevent stress during pulling, so the actual cable length will be that much shorter than the OTDR measurement of fiber length.
8. OTDRs should always use launch cables to _______.
A. Allow recovery from overloading caused by the test pulse
B. Avoid damaging the OTDR launch and receive connectors from dirt or high usage
C. Enable the OTDR to test the connector on the cable under test
D. All of the above
All these are good reasons to use a launch cable (sometimes called a “pulse suppressor cable”) when making OTDR measurements.
9. High-reflectance events, usually connectors, cause “ghosts” in a short fiber optic cable.
Big reflected signals can bounce back and forth in a short cable, causing ghosts on the OTDR trace.
10. The most important issue in testing properly with either a TDR or OTDR is _______.
A. Being properly trained and familiar with the instrument
B. Using the proper adapters or launch cables
C. Setting the range correctly
D. Ensuring the batteries are fully charged
Both TDRs and OTDRs are complicated instruments that require training to understand how to set them up properly for a test and interpret test results. Most problems with these instruments come from mistakes made by inexperienced or untrained users.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.