Case Study 1 – Marginal NEXT Pass on a DSX CableAnalyzer

In this case study, the installer was not achieving the level of performance expected when certifying his category 6 permanent links. Using these connectors and cable on a previous installation yielded margins of 5.0 dB and not margins as low as 0.3 dB seen here. With the DSX CableAnalyzer, it is possible to discover why these links were marginal in performance.

When a result is marginal, it is marked with an asterisk. A result marked with an asterisk means that measurements are within the tester’s accuracy uncertainty range and the * notation is required by the cabling standards ANSI/TIA-1152-A and IEC 61935-1 (you are not permitted by the standard to disable marginal reporting).

After the TEST, the installer was presented with the following DSX CableAnalyzer test result.

 

 

It is important to note that this test result for this link is considered a PASS as far as the cabling standards are concerned. However, some cabling specifiers will not accept a marginal pass (PASS*). The most common cause for rejecting a marginal pass is that the end user has been sold "a premium cabling system."  If the cabling system is "premium", you would quite rightly expect to see no marginal passes.

Your DSX CableAnalyzer can help you diagnose the marginal NEXT result. Let's review the process. The most direct way to do this is with the FAULT INFO tab:

 

 

Rarely, FAULT INFO will not identify the cause of the fault. For those situations, here are instructions for how to figure it out on your own. Find out which pair combination of NEXT is causing the FAIL.

 

步骤 1

Find the worst performing pair combination.

• From the "Performance" tab
• Tap "NEXT"
• Tap the Right Arrow of PAIR to identify the worst performing pair combination

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In this example, we see that the 1,2-3,6 pair combination is the cause of the NEXT failing.  Make a note of the failing pair combination.

 

步骤 2

Look at the HDTDX Analyzer trace for this marginal pair.  X for the NEXT failure.
 

• Tap the back arrow in the top left corner 
   
• Tap the FAULT INFO tab 
   
• Tap the HDTDX ANALYZER button 
   
• Tap the Right Arrow next to the PAIR to select the 1,2-3,6 pair combination

 

 

步骤 3

Analyze the trace. What you are seeing is where the crosstalk is happening in the link. The cursor automatically goes to the largest source of crosstalk. In this case, the largest source of crosstalk occurred in the first connector, at 0.0 meters. The two yellow dotted lines represent the ends of the test leads of the DSX CableAnalyzer, so you can assume the lines are the location of your first and last connection.

You will note there is a “Value 49.58” on the screen as well. This is the value of the peak at the place of the cursor. In an ideal world, the value should not exceed 35 units, 30 if the link is less than 15 meters in length. Clearly, that has been exceeded. It looks like the near end connection needs to be re-terminated (assuming the connector is really a Category 6a connector). Before we do that, let’s take a look at the far end of the link for the same pair combination 1,2 – 3,6 on this link by moving the cursor to the near end finding the largest value, in this case “Value: 15.47” at a distance of 13.7 meters. 

 

 

This is a good connection, much less than our limit of 35. This is what you would expect to see on a mated connection. Choose your connector carefully, terminate is carefully, making sure to minimize the untwisting of the pairs and the jacket removal and you can expect to see these small values.

If you have the “Store Plot Data enabled in your DSX CableAnalyzer, these graphs will also be available to analyze in LinkWare PC software.

结论：

The cable looks good. The near end connector needs re-terminating. This conclusion assumes you are using the correct category of connector and that the connector is truly component compliant. How can you tell? If the connectors at each end of the link are the same type, you should see a similar response. If all of your links are marginal and you see an HDTDX response such as this - it may suggest the need to try a different brand of connector.