Locating cable faults with the time domain reflectometer

Opens, shorts or less-severe impedance discontinuities have a way of showing up on cables in strange places_places you might never suspect. These can occur on coaxial transmission lines or twisted-pair lines. Such opens, shorts or other impedance discontinuities are called faults. The location of faults cannot be determined with simple ohmmeters. Even the existence of certain faults cannot be determined with an ohmmeter. After all, we can’t begin to correct a fault unless we know where it is or at least have some idea where to start looking for it.

Time domain reflectomer The basic principle of the time domain reflectometer (TDR) is fairly simple. See Figure 1 below. A pulse is generated and sent down the line. If the line is terminated in a load impedance that is equal to the characteristic impedance of the line (ZO = ZL), no reflected wave or echo will be returning to the generator or source. Let Pi represent the incident pulse generated by the pulse generator and Pr represent the echo or pulse reflected by the impedance discontinuity. By measuring the time difference between the incident pulse (Pi) and the echo or reflected pulse (Pr), it is possible to determine the distance to the fault that produced the echo pulse. Calculations must include the velocity factor of the cable.

If a transmission cable has a velocity factor of 1.0, then a pulse will propagate down the line at 0.982 foot per nanosecond (0.982ft/ns). If the velocity factor of a transmission cable is 0.66 and the time difference between Pi and Pr is 430ns as measured on the time base of the oscilloscope, then the distance traveled by the echo pulse is: 0.66 * 0.982 * 430 = 278.69. Thus the echo pulse has traveled 278.69 feet in 430ns. To get the distance from the source to the fault, we must divide this figure (278.69) by 2 to get 139.35 feet, the distance that the echo must travel back up the line to reach the point of measurement on the scope. The general formula for this equation is:

D = 0.982VT/2

where D = distance to fault in feet V = velocity factor T = time between pulses in nanoseconds.

Cablemate Analyst reflectometer The Cablemate Analyst reflectometer from Advanced Electronic Applications (a division of Tempo Research, Vista, CA) is a low-priced, feature-packed TDR in a small, hand-held unit. All of the features of this unit cannot be described in this space; however, some of the more essential features will be presented. Although it is not necessary, in most cases, to use an oscilloscope with this instrument, the manufacturer has made it convenient to do so. The use of an oscilloscope makes it possible to see minor reflections that might not be picked up by the instrument.

Using the oscilloscope The oscilloscope is connected to the reflectometer exactly as shown in Figure 1. When using coax cable, a BNC Tee connector is needed. The oscilloscope connects to one side of the Tee connector and the transmission line to the other side. When twisted pair cable is used, the twisted pair connects to the binding posts, and the oscilloscope to the BNC jack. In this case a 50V cable is connected to the reflectometer. Figure 2 on page 8 shows several waveforms for various load impedances. The TDR will provide the distance to the fault if you enter the time delay between the incident and echo pulse. For example, if you examine the timebase of the scope and determine that the time between the incident pulse and the echo pulse is 400ns, just enter the number 400 into the reflectometer, and it will return the distance of 130 feet. If you use an oscilloscope, make sure that the time base is accurate. Otherwise, your distance calculations will be inaccurate. Usually, we are looking for the distance to a major fault_open or short circuit. For this application an oscilloscope is not needed. The oscilloscope is only needed for locating minor reflections or for observing the echo waveforms.

Selecting the transmission line As many as 96 transmission line types can be stored in the reflectometer for instant recall. The unit comes preloaded with many lines. You can add transmission lines by setting up the characteristic impedance, velocity factor and then storing under a name you choose. A BNC jack is used for connecting coaxial cables, and binding posts are used for connecting twisted-pair lines. The binding posts apparently connect through a balun that provides a 4:1 impedance change. To test a 600V balanced line, the line would connect to the binding posts, and the impedance would be set to 150V so that when passing through the balun, the impedance would be transformed to 600V.

LCD screen An LCD provides an actual plot display along with other vital information. The echo is plotted, and a cursor is provided to enable an examination of the echo(s). Upon placing the cursor over the echo, the information is provided on the screen. Information such as distance-to-fault and return loss of echo (an indication of the severity of the mismatch) is provided.

General information The reflectometer can locate faults from 16 feet to 2,000 feet (5 meters to 600 meters). Faults less than 10 feet apart can be resolved on the display. Accuracy is rated at 6(1%12′). For example, at 200 feet the accuracy would be 64′. Characteristic impedance for coaxial cable is 45V to 150V in 7V steps. For balance line (two-wire cable), the characteristic impedance range is 180V to 600V in 28V steps. The velocity factor range is 30% to 99% in 1% steps. The output pulse is 15V with a pulse duration of 25ns. The unit can run with supplied ac power supply or a battery pack (8 AA cells). Battery life is about 12 to 14 hours. Cable types and plots along with modes, settings and calibration data are stored in non-volatile memory. A serial port is provided for connection to a computer for use with optional software. The unit weighs slightly over 1.5 pounds including batteries.

Summary The CableMate TDR is user-friendly. In an hour or so one can become sufficiently familiar with the unit to make useful measurements with it. The menu is easy to understand, and condensed instructions are printed on the back of the unit. Such an instrument is useful for making reference measurements on transmission lines for future maintenance reference, as well as for determining line length, velocity factor and fault locations. For further information on this instrument, contact Advanced Electronic Applications (Division of Tempo Research) 1221 Liberty Way, Vista, CA 92803. Phone: 760-598-9677. Until next time_stay tuned!

Kinley, a certified electronics technician, is regional communications manager, South Carolina Forestry Commission, Spartanburg, SC. He is a member of the Radio Club of America. He is the author of Standard Radio Communications Manual: With Instrumentation and Testing Techniques, which is available for direct purchase. Write to 204 Tanglewylde Drive, Spartanburg, SC 29301. Kinley’s email address is [email protected].