Carson,
nice anecdote :-)
it really depends on how good does it need to be. If you simulate a
differential via (youri for instance has many simulation videos online) then
you find that each via that you add will reduce radiation and crosstalk (if you
simulate two differential vias). On a densly packed electronics card you will
have tons of GND vias all over the board, and these will already act as return
vias, so it might turn out that no extra return current vias are needed. But
designing that way forces you to check for GND vias every time you place a
signal via. Most EDA tools have no rule to do this check automatically, thus
there is a good chance that you miss something and that can be a reason to fail
EMC. That’s why I recommend to place at least two return vias at every
differential signal via.
BR
Gert
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Carson Au
Sent: Wednesday, May 31, 2017 9:52 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Return Path Vias
The perennial debate... on SI-List.
SERDES differential channels 10Gbps+. For layer transitions, is it OK to use
just one return path via (both sides of the differential via sharing one GND
return path via, symmetrical/balanced)? Is it OK to use no return path vias at
all (Lee Ritchey advocates for this)?
Is there literature comparing 2x, 1x and NO return path vias?
I can imagine that the common mode conversion would be an issue. But how much
of an issue?
Anecdotal stories that one method "just works" is not sufficient proof (i.e.
for the no return vias case) I have been able to get a 10Gbps+ channel working
with only one side of the differential connected - with a BER better than
10e-15 (ran overnight with no bit-errors)!! Yet none would say this channel is
well designed.
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