Cable causes a NEXT failure (DTX CU 202) – By Fluke Networks

Cable causes a NEXT failure (DTX CU 202) – By Fluke Networks


Your DTX CableAnalyzer has the ability to figure out whether a near-end crosstalk failure was caused by a cable or a termination issue. In this example, taken from the field, it was caused by a cable issue. And I’m going to walk you through the steps to prove that it is a cable issue. This is the kind of thing that resolves disputes that can last for weeks and nails it within a matter of minutes. So, with near-end crosstalk highlighted, we see that we have a margin of -0.9 dB. I’m going to hit the ENTER key, and that’s going to give me all six pair combinations. I’m only interested in the pair combinations that gave me that -0.9. So, hit f3 key and we find out that it’s pair 1,2-4,5 that’s shown in the top left-hand corner here. And again, just before, we have this nice smooth line, which is our limit above is good, below is bad. But we see that at 233 megahertz it dipped below the limit line and that’s what’s causing the failure. Alright, so we know it’s 1,2-4,5. Next step, hit the EXIT key, EXIT key and page down to something called HDTDX analyzer. This is that tool I was talking about in previous modules that allows you to look down the cable and see where the crosstalk is happening. I’m going to hit the ENTER key. And, yes, just as before it’s going to give us all 6 pair combinations. I am only interested in that failing pair, 1,2-4,5. I can locate that by just pressing the f3 key. Once more. And that gets me to 1,2-4,5. Now, you may quickly observe that we seem to have a little bit more crosstalk going on in the cable than we would normally expect. Each end seems to be okay suggesting that the terminations are good. Now, what we’re going to do, is we’re going to press the f2 key, which is our zoom key, and I’m going to press the up arrow key until I see 8X in the top right-hand corner, and 12.5 here, and -12.5 here. why have I done that? A general rule of thumb, and it’s not in the standards this is just a diagnostic tool, is that we don’t want to see more than 5% happening in the cable. So, 5% just so happens to be a little bit less than halfway on the screen here. Let’s put in those two 5 markers. Let’s put in the 5% marker across the top, and the 5% marker along the bottom. Now, in reality, it is normal to get one or two events to exceed that 5%. That’s going to happen. It happens in the real world. However, on this occasion here, we have many, many events exceeding the 5% or minus 5% rule. We can categorically say, without any doubt, we have a cable issue. And it doesn’t matter how many times you terminate this cable; it’s not going to get you beyond even a marginal pass. You may be able to get a marginal pass by putting on a better connector, but it’s still going to be right on the edge. And your customer is going to be asking questions. Now, just to put that into perspective, I’m going to hit the f3 key and look at one of the other pairs. That looks just fine. So you can get a sense of scale as to what we’re talking about here. And, so, let’s review the procedure: find out which is the worst-performing pair; go to the HDTDX; look at that worst-performing pair; change the zoom to 8X in the top right-hand corner; and look and see how many events exceed roughly about halfway up the scale and halfway below the scale. And if they exceed three or four, then you want to start getting nervous and, unfortunately, you have a cable issue. And that cable is going to have to be replaced. And that is how you diagnose a cable issue using your DTX CableAnalyzer.

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