Did you know your DTX CableAnalyzer
can diagnose a return loss problem
caused by a cable issue?
In the next few minutes,
I'm going to walk you through the simple
processes in dealing with that.
But, before we dive into this,
whenever you see a return loss issue,
the first thing you actually want
to go and do is check out resistance.
So I'm going to move the cursor
up to resistance, and press the ENTER key.
Now you've got your four pairs,
which are the individual four pairs
within the cable and they're more than likely
going to be a little bit different
than each other and that's because
the different twists in the cable.
But they should all be roughly
about the same.
If you see one pair here
which is substantially different,
say by a couple ohms,
5 ohms or 10 ohms, than the other pairs,
we know we have a DC contact
resistance issue somewhere, and that is,
of course, our return loss.
Well, in this example here
our resistance figures are all
roughly about the same.
So I hit the EXIT key.
And we'll go back down to return loss.
And hit the ENTER key.
So, we're seeing all four pairs
of return loss, and again we're only
interested in the one that failed.
So, press the f3 key,
And that automatically goes
to pair 7,8, which is our
worst-performing pair.
Again, as with previous traces,
you have this smooth line here,
which is our limit.
Above it is our return loss measurement,
which is where we want it to be.
We want it to be above, not below, the line.
Now, what you'll see is, at the lower
frequencies, it actually dropped below
the line. Rule of thumb:
if you see a return loss failure
happen at the lower frequencies,
it's very likely
that you have a cable issue,
assuming that resistance is good
and we checked resistance
and resistance was good.
So, first off, my DTX CableAnalyzer
is telling me "hey, I think
you've got a cable issue going on."
Alright, so, 7,8 let's keep that in mind.
We're going to hit the EXIT key,
and the EXIT key to return to the home page.
F3, 'Page Down'.
And we're not going to go to TDX this time,
X is crosstalk. We're going to go to HDTDR.
High-Definition Time-Domain Reflectometer.
R being return loss.
and we're going to hit the ENTER key.
Now we're looking at all four pairs
on the screen here, and, just as before,
we have two vertical red lines,
indicating our first connection
and our last connection.
We're only interested in that pair that
caused us the problem, which is 7,8.
So I'm going to hit the f3 key
until I see 7,8.
Now, it looks nice and smooth.
There doesn't appear
to be any issues going on.
What you have to do with the HDTDR
almost every time, there are exceptions,
is press the f2 key
and zoom in.
And we're going to do that
by pressing the up arrow key
going to keep pressing the up arrow key
until we see 64X up here
in the top right-hand corner.
Why did I do that?
As a rule of thumb,
again, this is not in the standards,
we don't want to see more than 0.8%
anywhere in the cabling system.
And using this scale here
where I got 1.5% on the left and -1.5,
0.8 represents roughly about
halfway up the scale.
Let's put that 0.8% line in now,
and also the -0.8% line in there.
Now, you are going to get
one or two events that probably
drop out to that 0.8 or -0.8%.
But what we're looking at is
multiple significant events.
And what we see here,
is on the left-hand side, we have
many events exceeding that,
and on the right-hand side,
we also have many events
exceeding that 0.8, -0.8% allowance.
Interesting enough, when we look
in the middle of the cable
everything looks good.
Now, this is a rather unique trace.
It is not that unique, it can happen.
The reason why I say it's rather unique
is because it's one of the few traces where
not only can we say it's a cable issue
we can say the installer did this to the cable,
which is rather controversial. But
if the first part of the cable here is
clearly exceeding our 0.8, -0.8 allowance,
and then it's nice
and then it goes excessive again
on the 0.8, -0.8,
you could easily conclude that this cable
was good when it was installed
but, unfortunately,
whatever happened on site
the cable was treated roughly,
probably where it exited the cable run.
If we press the f3 key
we also see on the other pair
that we see a similar thing.
And on this one
we see a similar thing as well.
And on this pair, not so bad.
But the bottom line is,
when we go back to this pair 7,8,
it's very clear that we have a cable issue
and, unfortunately, whoever installed
that cable created that problem.
this cable will have to be removed
and replaced, unfortunately.
And that is how you diagnose
a return loss issue
in your DTX CableAnalyzer
using the HDTDR feature.
And to recap how you do that:
find out which pair it is, go to HDTDR,
change the zoom to 64X,
and look to see if you got
multiple points exceeding 0.8% or -0.8%.