Hi Tesla,
The effect of the common mode on eye can be roughly separated into three
quantifiable and inter-related areas.
1. Common to differential mode transformation and the other way around, also
observed as skew. Any discontinuity for the common mode immediately affect
the mode transformation. For instance, if strip differential traces turn
left, it causes the mode transformation. It should be compensated with the
mirror right turn. Though, if you put a discontinuity between the turns
(vias or capacitors), the compensation will not happen. See more on the mode
transformation at app notes #2009_01 and #2013_06 at
http://www.simberian.com/AppNotes.php This will affect the eye.
2. Common mode localization. Common mode coupling to parallel plane
structures at the non-localized differential vias for instance may cause
unwanted coupling between PDN and the link path for the differential signal,
taking into account the mode transformation. Worst of all, the behavior of
un-localized vias is not predictable at high frequencies (requires whole
board analysis, that is not possible with sufficient accuracy for practical
cases). This will also affect the eye.
3. Common mode termination. Poor common mode termination may turn a
microstrip link path into antenna with possible EMI consequences. Even with
good termination, discontinuities such as vias or capacitors can case common
mode resonances with EMI consequences. Unterminated common mode can also
affect the eye through the mode transformation, especially for the
microstrip links.
Best regards,
Yuriy
Yuriy Shlepnev, Ph.D.
President, Simberian Inc.
3030 S Torrey Pines Dr. Las Vegas, NV 89146, USA
Office +1-702-876-2882; Fax +1-702-482-7903
Cell +1-206-409-2368; Virtual +1-408-627-7706
Skype: shlepnev
www.simberian.com
Simbeor - Accurate, Fast, Easy and Affordable Electromagnetic Signal
Integrity Software
2010 and 2011 DesignVision Award Winner
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Tesla
Sent: Wednesday, April 1, 2015 1:47 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Common mode perfornace of Serdes Channel
Hi Expert
In the case of optimizing channel IL and RL, most time we only consider the
differential mode of channel. For example, Using larger antipad for via and
void the plane beneath the AC capacitor to improve the differential IL,
while degrading the common mode performance.
My question is that the common mode performan relly do not affect the Eye?
Can we always let the common mode perfomance of channel be?
Thanks and Best Regards.
Tesla------------------------------------------------------------------
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