[SI-LIST] Re: Diff.Pairs

  • From: Chris Cheng <chris.cheng@xxxxxxxxxxxx>
  • To: "'mchin@xxxxxxxxx'" <mchin@xxxxxxxxx>,"'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 13 Oct 2003 11:52:54 -0700

Michael,
What you are suggesting below is "tight coupling" of differential pair. Lee
was bragging about not needing any coupling for differential pairs. If what
he claims and what you mentioned below is the same, he will be contradicting
himself.

I am still waiting for him to answer my question :

"I would like you to answer a simple question :

If tomorrow you are going into a client's office to consult on designing a
2.4GB/s differential signal system. Will you recommend them to "routed
thousands of differential signal where each member of the pair is on a
different layer". Do you think that is good engineering practice ? Do you
think you can still keep your job as a consultant after making that
statement ?"

Like the famous quote, "I will be waiting until hell freeze over".

-----Original Message-----
From: Michael Chin [mailto:mchin@xxxxxxxxx]
Sent: Sunday, October 12, 2003 10:40 AM
To: scott@xxxxxxxxxxxxx
Cc: bmgman@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff.Pairs


Scott,

I've followed your emails exchanges with Mike Brown
closely.  This subject is a very important item as we
tackle high speed design using differential pairs.
Today, we have differential pair in CMOS logic,
LVDS, and differential HSTL (RLDRAM).

Both of you have argued with very good points and I
appreciated your thoughts.  But in this case, I would
side with what Lee has stated.  He may not have explicitly
stated the stackup in his assumption, but it is commonly
referred as the the broad side coupled differential pair
vs. edge coupled stripline differential pair.

There is no question about the benefits and level of
control from edge coupled differential where both tracks
are routed on the same layer.  But it is also very common
to have a dual stripline layers between two Ground layers
for broadside coupling.  This is mostly used in the backplane
side and have demonstrated success in many designs.

An example of this stackup can be:

=============   Ground plane for image return
                 Core, thickness H
     =====       Diff Layer #1
                 Prepreg
     =====       Diff Layer #2
                 Core, thickness H
=============   Ground plane for image return

I have been to a lot of seminar where SI experts argued
the Pro/Con of edge coupled vs. broadside coupled differential
pair. I have seen good results from both styles on frequency
under 3GHz.

Just my two cents,
Michael Chin
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