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HDTDR - DTX CableAnalyzer

Editor's note: Although this appears technical, it is worth understanding as it can save your company $1,000s / €1,000s.

Are you looking to troubleshoot Return Loss problems? The following videos are available. Although titled "Category 6", the rules in these videos also apply to Category 5e, 6A, Class D, E and EA links.
 

High Definition Time Domain Reflectometer


Here we see an HDTDR trace taken from a Category 5e permanent link which had a Return Loss margin of 2.0 dB. It is a pass, but for this particular connector and cable vendor we expected to see larger margins. The HDTDR trace suggests that both the connectors and cable are the issue.

When looking at an event, there are two types to consider. It can be either a bipolar or unipolar response.


Bipolar response

This is an event that contains a positive and negative characteristic.
Bipolar Response


Unipolar response

This is an event thats either a positive or negative characteristic.
Unipolar Response


What you need to look for are responses greater than 3 % at the location of the connectors. If the response is a unipolar response, this task is simple. Move the cursor on the HDTDR trace to the event and read the value off the scale.

If the event is a bipolar response, note the two peak values. In the example below, we see that the event rises to 8.1 % and then falls to -7.1 %.

 

HDTDR Raise to 8.1%

 


Our goal is for less than 3 %.

 

  • Turn the negative number into a positive one, so -7.1 % becomes 7.1 %.
  • Take the smaller of the two away from the larger.
  • This gives us 1.0 % = 8.1 % - 7.1 % for the first connection.
This connection is not a significant contributor to the return loss in the link we tested. Lets look at the remote connection:

 

HDTDR Less Than 3%

 


Our goal is for less than 3 %.

 

  • Turn the negative number into a positive one, so -8.5 % becomes 8.5 %.
  • Take the smaller of the two away from the larger.
  • This gives us 2.5 % = 8.5 % - 6.0 % for the remote connection.

 

This indicates that the remote connection is not as good as the main connection. A visual inspection would be advisable. Inconsistent termination techniques are a common problem. More so when John terminates the main connector and Barry terminates the remote connector. In this case, we may wish to monitor Barry's workmanship.

Given this, one would assume that the cable must be the primary cause of concern. In this case it is, but that remote connection is not helping.

Return loss diagnosis is a matter of deduction. If the connectors look good, then it must be the cable causing the issue.

If there are lots of small impedance mismatches in the cable, they are unlikely to show up on the HDTDR. Since there are many of them, their combined 'power' causes a reduction in Return Loss performance.

In the traces above, we are fortunate enough to be able to see lots of little events along the cable. This should not be the case. It is not unusual to see one or two events, but when they are this many in number, the alarm bells should start ringing.

Lets look at the frequency response for the cable pair above:

Return Loss Pair 3-6


We can read a little into this trace as well.

In general:

 

  • RL issues at high frequencies tend to be caused by connectors
  • RL issues at low frequencies (below 50 MHz) tend to be caused by impedance mismatches in the cable

 

Unfortunately, this example appears unclear. Ignoring the effect of the 3 dB rule (the grey part of the limit line) for a moment and moving the cursor to the left indicates that at 38 MHz, the margin is 2.3 dB. Almost the same as the margin at 90.8 MHz. That suggests a combination of connection(s) and cable issues.

This is why the HDTDR should always be analyzed when return loss issues arise. In addition, you are not guessing as to which end needs re-terminating.

If you truly understand this feature, you can avoid a potential commercial loss for your installation.

For this reason, always store the plot data. Fluke Networks helps users interpret such traces. Once you have done a few, it is very easy to use.