New-Tech Magazine Europe | Dec 2015 Digital edition

Figure 3. Shown here is a FieldFox DTF measurement for two connected coaxial cables terminated in a 50-ohm load.

Figure 4. The top left-most and right-most images, A and B, respectively, depict damage in a coaxial cable. The bottom image, 2C, is the result of a TDR measurement of the damaged cable using the FieldFox TDR option. FieldFox can identify various types of discontinuities in TDR mode, including: R > Z0, R < Z0, inductive, and capacitive

calculated (e.g., 13.8 m - 4 m = 9.8 m). The noticeable drop in measured amplitude to the right of the 50- ohm load signifies the end of the cable. As this reflection measurement represents two-way signal paths, FieldFox properly adjusts the marker values and x-axis formatting to the appropriate one-way lengths. Now, consider a coaxial cable that has been damaged in two areas (Figures 4A and 4B). Fault A is a bend in the cable that has exceeded the manufacturer’s specification for minimum bend radius of 1-inch. The bend at Fault A is well below this radius, creating an undesired reflection from this part of the cable. Fault B is a cut through the outer conductor of the cable. The braided shield has been partially removed, exposing the inner dielectric of the coax. Both faults can be examined using the DTF and TDR modes on FieldFox; however, only the TDR measurement will characterize fault type. Figure 4C shows the measurement for the damaged cable with FieldFox in TDR mode. As can be seen from the TDR response, the cable impedance is generally 50-ohms across most of the time-domain response until

time-domain techniques. FieldFox’s comprehensive suite of cable test measurements, including the new TDR and ERTA options provide the ideal solution for testing any cable system in the field. FieldFox’s DTF and TDR time-domain measurements identify fault locations and causes in coaxial cables, while its bandpass measurement finds the physical location of faults in a waveguide. Using FieldFox, today’s engineers and technicians now have a faster, easier way to test cable systems in the field. About the Author Tom Hoppin is an application consultant on contract to Keysight Technologies. Tom started his career as an electronic aviation technician in the U.S. Marine Corps. He joined Hewlett-Packard after his service ended in 1973. Through the years he has held a number of engineering and management roles at HP/Agilent and now Keysight, focused on test system design and spectrum analysis. Tom retired in 2009. He has since returned to Keysight as an application specialist for its RF and Microwave Handheld Analyzers.

a discontinuity is encountered. Discontinuities occur at the input connector, the bend at fault A, the cut at fault B, and the 50-ohm termination at the end. Of all discontinuities on the cable, the cut at fault B has the largest mismatch. This is demonstrated by the magnitude of the associated peak. The cut on the TDR response has a single peak in the positive direction, indicating an inductive mismatch. This is typical for cuts in the outer conductor of a coaxial cable. Generally speaking, if the cable terminated in a load with a resistance smaller than the characteristic impedance, the TDR response would show a step in the negative direction. If the load resistance were larger than the characteristic impedance, the TDR response would show a step in the positive direction. Summary Cable measurement in the field can be tricky. Determining that a cable is faulty is just the first step in the process. Engineers and technicians then need to identify the fault’s physical location and its cause. This is accomplished using various

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