There is a rather strange rule that
applies to Near-End Crosstalk if you're
testing to any of the ISO limits or
European Norm limits, called the 4 dB rule.
When we tap on the screen here, for next.
Now I'll go to the worst-performing pair.
We see that part of the limit line is black,
and part of it is red.
Where the limit line is black,
any crosstalk measurement made in that region
is ignored under the 4 dB rule.
What is the 4 DB rule?
If the insertion loss
of the pair is less than 4 dB,
then that near-end crosstalk
measurement is ignored.
Let's see if we can find
roughly where it turns from black
to red on our limit line here
and see what frequency
that happens at.
It's pretty close. Getting there.
Move it across one more,
I think it's more like over here somewhere.
About...
I would say there.
There. 188 1/2 megahertz.
Remember that number.
Now you'll notice that a little bit
after that it goes red
and then black and then red again.
And that's because of something called
insertion loss deviation.
It's not exactly smooth the insertion loss.
Particularly when you're doing
high frequency testing.
So,
this is for crosstalk on 1,2 3,6
and I said the 4 dB rule
applies based on the insertion loss.
Well, does it apply on the insertion loss
of pair 1,2 or pair 3,6?
it's pair 3,6, that's how it's defined
in the standards.
Let's go and look at the
insertion loss on pair 3,6.
So I'm at pair 3,6
and I'm going to move this
to my 188 1/2 megahertz.
I'm going to zoom in again.
We'll zoom in quite excessively.
Okay, so, our insertion loss here
is showing 3.9 dB, but,
the moment I move it forward
it changes to 4 dB.
So what we can say here
is 188 1/2 megahertz and backwards
we're actually going to ignore
all those crosstalk measurements.
Now, there's an interesting
quirk with this 4 dB rule.
Let me go and look at another test result.
So when we look at the result here,
we see that crosstalk has a margin of 3.3 dB,
but you'll notice there's an "i" next to it,
not a tick mark.
And that's because, if the insertion loss
never reaches 4 dB,
then you end up ignoring the entire
near-end crosstalk measurement.
When we look at this near-end crosstalk
we see that the entire limit line is black,
so the entire measurement is ignored.
If we go and look at the insertion loss,
we can see here that it reported
the worst case as 2.4 dB,
not even close to 4 dB,
so it's ignoring
all of the crosstalk measurements.
Now, there is another interesting quirk.
Let's look at this result here.
The margin is -3.9 dB for near-end crosstalk,
so that should be a fail, correct?
Because minus is below the test limit,
but the DSX is showing an "i" next to it,
not an x.
Why? Because the
insertion loss on this particular link,
quite short at 3.3 meters,
never actually reached 4 dB.
Let's look at them.
Indeed, my worst case is 2.2 dB.
It didn't reach 4 dB.
So, as a result,
my entire limit line is black and
I ignore the entire near-end crosstalk
measurement across the entire frequency range.
So even though
it dips below the limit line here,
this is actually considered a pass
under the 4 dB rule, which, again,
is not found in TIA but it is found
in ISO and EN standards.