[SI-LIST] Re: Diff.Pairs

  • From: "Moran, Brian P" <brian.p.moran@xxxxxxxxx>
  • To: <leeritchey@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 9 Oct 2003 16:38:08 -0700

Lee,

When this topic first came up I knew it was going to be one of those
religious discussions, exposing the philosophical rift regarding
diff pair routing within the SI community. Last time this happened I =
think
was when you commented on grounding philosophy. Anyway its been =
interesting
reading. I was anyways in the camp that said implement diff pairs as =
loosely
coupled complimentary signals and then you don't have to worry about
differences in common mode and diff mode termination impedance, or
propagation velocity variance between single ended data and differential
strobes. The last few years, however, I had almost been converted into
the tighly coupled camp when you had to speak up and get everyone all
riled up. All I know is this has been a good opportunity to stash away
some very good argumnets for future consideration. Thanks for stirring
things up a bit. A have to say that I'm still favoring tightly coupled
these days, but I also make sure to have contiguous reference plane
return paths. Contrary to your statemnt, however, I'm seeing better edge
rates with tightly coupled pairs, but that may be a function of
lower Zdiff and not the tighter coupling itself. I can't say I fully
understand the phenomenon and I have not done any experiments comparing
apples and apples. I may do so when I get a chance.  =20

Brian P. Moran
Signal Integrity Engineer
Intel Corporation
brian.p.moran@xxxxxxxxx


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris Cheng
Sent: Thursday, October 09, 2003 2:26 PM
To: 'leeritchey@xxxxxxxxxxxxx'; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff.Pairs


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 ?

We all cheat in some of our design, when spacing is tight we might =
change
the minimum separation between trace from x mils to y mils. And most of =
the
time we can get away with it because everything being worst case and =
fail
your margin is very rare. But that doesn't mean it is good design =
practice
or something you should brag about.=20

This is a public discussion forum and people are welcome to share =
whatever
ridiculous idea they have as long as they themselves believe in it and
actually practicing it. I just want to make sure you "talk the talk and =
walk
the walk".

-----Original Message-----
From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
Sent: Thursday, October 09, 2003 9:30 AM
To: Doug Brooks; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff.Pairs

..........................This is
a very common termination for 2.4 GB/S signal links.=20

It is time to stop representing differential signals as needing to be
tightly coupled to each other in order to operate properly.  It is =
simply
not so.  I have routed thousands of differential signal where each =
member
of the pair is on a different layer.  If this were not possible, 1 mm =
pitch
BGAs with differential signals would be un routable.  There are tens of
thousands of such parts being shipped every month on PCBs where they are
routed apart from each other.

This is all described in my recently published book, "Right the First =
Time,
A Practical Handbook on High Speed PCB and System Design".  It is also
covered in Howard Johnson's new book whose title escapes me at the =
moment..

Lee


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:    =20
                //www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages=20
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
 =20

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                //www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

Other related posts: