XAUI experts: Another XAUI issue that crossed my mind - help
Hi XAUI people
Here is another problem that came across my mind yesterday.
I was looking at Mysticom's eye diagram simulations and all see quite
realistic to me with the given set of parameters. However, I assume that
this has be done with a 50 % transition density signal. (i.e. overall DC
balanced)
I wonder if anybody have taken this example into consideration and if my
assumptions are valid:
I have been told that the worst case relative DC voltage (unbalance) could
be 0.3. I.e. let's assume a 8B10B character (I don't know if it is a valid
character, but I guess one can find a similar valid example): 01000 11000
If this character is repeated infinitely (or at least characters with the 3
to 7 mark/space ratio) and when the signal is AC coupled (as defined by the
XAUI channel), this will cause the differential signal to look like this:
_ __
== === ===
==_===__===
00100011000
ie. the differential swing after the AC coupling has a reduced swing for the
'0' symbols. In other words: the '0' has a single-ended relative amplitude
of -0.3 ant the '1' has +0.7 (ie. relative to the DC level of the signal and
due to the AC coupling). Now the complementary(inverted) signal has the same
properties ie. its DC values is -.3 and the amplitude of the inverted '0' is
+0.3 and the amplitude of the '1' is -0.7. So the differential AC coupled
signal swing at the input of the CDR (or LIA) is 0.6 for the '0' and 1.4
for '1' symbols. Hey, that is quite amplitude modulation in relation to a DC
balanced or DC coupled signal.
This is all due to the AC coupling. For DC coupling the amplitude of all
signals are 0.5 and the differential swing is 1 for both symbols.
What is wrong with my conclusion (if it is wrong - I don't think so).
Benny
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GIGA, an Intel company
Benny Christensen, M.Sc.E.E, Ph.D.
Mileparken 22, DK-2740 Skovlunde, Denmark
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