Distance to Fault (DTF) Measurements
What happens if your LONG cable breaks somewhere underground? Or in a wall?
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Today, we’re going to discuss how to find the fault in a cable using Keysight’s FieldFox handheld analyzer. There are two important measurements for finding out the length of a cable, verifying cable performance, and determining if failures or discontinuities exist in your transmission line. Both of these measurements are useful in cases where you can’t visually inspect the entirety of your cable because it’s extremely long, like cables on an airplane or cruise ship.
The first is a Distance-To-Fault or DTF measurement. A DTF measurement performs a line sweep and displays the return loss in decibels versus distance. In layman’s terms, it injects a frequency sweep into the cable, and reads back how much power gets lost along the cable.
It also tells you the length of your cable, given that you know or can calculate the velocity factor of the cable. For this measurement, I have an 8 m-long cable and a 0.6 m-long cable with an adapter connecting the two and a short on the end.
I’ve centered the display around the largest reflection point which represents the end of the transmission line. You can see here that a marker on the peak gives me a total distance reading of 8.576 m. When we measure it with a tape measure, we get the same value. Measuring it physically was also a lot of work, and in many cases it’s not possible because the cable is buried in a wall or the ground.
These two larger bumps on the screen represent the reflection of the signal at the connection point between the two cables. Looking at this display, I noticed there’s a reflection reading here that’s a little concerning to me. It looks like there’s a fault near the end of my transmission line.
So, I’m going to switch to a Time Domain Reflectometry or TDR measurement to get a little more information about this fault. A TDR measurement displays the voltage reflections along the transmission line in Ohms. I’ve placed a marker at the first set of peaks, and you can see that the distance to that point is 7.97 m which is where the adapter is located. This peak on the far right represents the short at the end of the cable. But that spike we saw on the DTF measurement before can also be seen here right before the end of the cable. Because the signal increases, we know that this fault is inductive, which can be useful when determining what is wrong with the cable and how to approach the problem.
So, I’ll place a marker near the fault. We can see that it’s located around 8.46 m down the cable. Let’s go see if we can figure out what the problem is! We’re just going to walk the length of the cable instead of measure because my measuring tape maxes out at 5 ft and I don’t want to go through that again.
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