I doubt it has to do as much with electromagnetics physics as physical
limitations. For multi-bit busses, we are forced to minimize the spacing
between adjacent signals travelling in the same direction, else that group
is enormously wide and not routable. Therefore, we simulate, and find
solution spaces for, that group with fairly tight routing. On the other
hand there's only one instance of the adjacent signals' spacing, and they
can be placed far enough away (it probably doesn't matter whether they are
TX or RX) so that their influence is insignificant and doesn't have to be
simulated.
SATA is probably just following the same convention they set with PCIe and
other wider bus groups.
That's my guess.
Jeff Loyer
Signal and Power Integrity Product Manager, Altium
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Richard Allred
Sent: Tuesday, December 22, 2015 7:14 PM
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: a question about the spacing differences between TX
to TX distance and TX to RX distance
I think the reason for the difference in the spacing is two fold. One,
NEXT is usually stronger than FEXT since it isn't attenuated as much by
the channel and two, the xtalk phase relation.
For two DQ signals in the same byte group, they are always going the same
direction and their edge transitions have a somewhat fixed phase relation
to each other. The crosstalk will always be in the same relative position
in the victim eye.This is far end xtalk (FEXT).
For two signals that go opposite directions, the phase relation of the
edge transitions are not fixed. This means that this near end crosstalk
can attack any position in the eye and so on average is worse than a fixed
phase relation.
If you allow me, I would say it is because:
FEXT is proportional to (Kc - Kl) while NEXT is proportional to Kc+Kl
where Kc=Capacitive coupling coeficient and Kl=Inductive coupling
coeficient.
Even more, NEXT prevails during all agressor signal travel so I understad
it is more sensitive than FEXT.
Lets see what more experienced people say.
Ivan Perino
On 22 dic 2015, at 22:25, Carson Au <carsona@xxxxxxxxxxxx> wrote:other defects.
NEXT and FEXT.
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On 23 December 2015 at 12:17, jun zhang <zhangjun5960@xxxxxxxxx> wrote:
I have a question about the spacing differences between TX to TX
distance and TX to RX distance. For example, in Intel PDG, TX to RX
distance requirement is stricter than TX to TX requirement. For
example, for SATA-Gen3, TX to TX: 7h for microstrip; TX to RX: 9h
for microstrip
thinkh is the height of the mcrostrip layer.
If FEXT for TX to TX is -20db and NEXT for TX to RX is also -20db, I
------------------------------------------------------------------their noise levels are the same. Then why does Intel give stricter
requirements for the distance between TX to RX?
--
best wishes,
Jun Zhang
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