[SI-LIST] Re: IEEE1394/Ethernet Routing Guidelines
- From: Ravinder.Ajmani@xxxxxxxxxxxxxx
- To: wdowsley@xxxxxxxxx
- Date: Thu, 31 May 2007 08:42:46 -0700
Bill,
While I agree with almost all of your recommendations, I do not understand
the need to match the length of differential pairs to within 1 mil. Will
you please give your reasoning for this.
Thanks.
Regards, Ravinder
Server PCB Development
Hitachi Global Storage Technologies
Email: Ravinder.Ajmani@xxxxxxxxxxxxxx
Bill Owsley <wdowsley@xxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
05/30/2007 08:13 PM
Please respond to
wdowsley@xxxxxxxxx
To
Sam.Charles@xxxxxxxx, jeff.loyer@xxxxxxxxx
cc
si-list@xxxxxxxxxxxxx, si-list-bounce@xxxxxxxxxxxxx, weirsi@xxxxxxxxxx
Subject
[SI-LIST] Re: IEEE1394/Ethernet Routing Guidelines
I went below to check what Mr.Voorhies asked for;
"...have had a very difficult time finding exact numbers for trace
spacing/individual trace impedence and so on."
And now I don't find in any of the suggestions (or in Mr.Voorhies note)
anything about copper weight, dielectric constant at what frequency, trace
width, distance above or between reference planes, length matching, number
of vias across how many layers, crosstalk coupling factor and it's effect
on even/odd mode impedance, pre-emphasis to compensate for longer lengths
at higher frequencies, various "cute tricks" to emulate differential
signals (one I really like, nearly eliminates most of the above),
differential drivers or complementary drivers or current steering drivers,
differential to common mode conversion factor, terminations, decoupling,
and so on. All the books/sources referenced have all the conflicting
information that Mr. Voorhies has already discovered.
For us, immersed in the culture (or lack of it on my part) the references
are like preaching to the choir, we nod our collective heads in agreement
that those are indeed good sources and Mr. Voorhies says to himself, Hey I
can catch lightning in a jar, but I still don't WTF they just said, thus
the reason for his request. Everyday we work with digit heads, okay,
digital guys, who connect to the dots and then look to the SI and EMC guys
to make the board work. ps. we work well together cuz the only dots I
connect are in my granddaughters coloring book.
So given the dearth of details here's one answer (of many) he can use;
50 ohm surface traces (no vias), spaced 5 times the distance to the
reference plane, no breaks in the plane below the traces, matched lengths
to within 1 mil. (my layout guys can do that in about 30 seconds all day,
another "cute trick"), and all added parts are absolute mirrow symmetric
(the parts might need vias).
Sam.Charles@xxxxxxxx wrote:
I second that.
"Loyer, Jeff"
Sent by: To
si-list-bounce@xxxxxxxxxxxxx
cc
05/30/2007 02:24 PM Subject
[SI-LIST] Re: IEEE1394/Ethernet Routing Guidelines
Please respond to
jeff.loyer@xxxxxxxxx
THANK YOU!!!
A response absent of sarcasm and/or scorn, and with some useful
information. A pleasant change from the recent postings on the forum.
Thanks Steve,
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of steve weir
Sent: Wednesday, May 30, 2007 1:39 PM
To: voorhies@xxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: IEEE1394/Ethernet Routing Guidelines
Randolph, I suggest any of the following four books would be helpful to
y=3D
ou:
Signal Integrity Simplified, Bogatin
High Speed Signal Propagation, Johnson and Graham
Digital Techniques for High Speed Design, Granberg
Right the First Time Vol II, Ritchey
Lee Ritchey self publishes so if you want Right the First Time you will
=3D
need to go to www.speedingedge.com. Your alternative is to rely on
PHY=3D20
manufacturer guidelines.
The basic guide that I can offer you is this:
* Loosely coupled pairs are easier to manage than tightly coupled=3D20
pairs. If you don't specifically need tight coupling for density=3D20
reasons, you will be less likely to go wrong with loosely coupled pairs.
=3D
* The crude approximation for a loosely coupled pair is Zodd =3D3D
2*Zeven.=3D
=3D20
Ie two loosely coupled 50 ohm traces will have about a 100ohm odd
mode=3D20
impedance.=3D20
* Maintaining a uniform impedance and matched electrical length will
do=3D20
much to get you were you need to go. If you are a tiny bit sloppy,
it=3D20
shouldn't matter for a student project. Your project doesn't have =
to=3D20
pass FCC part B, and it is unlikely that you have to support =
maximal=3D20
length links.
Eric Bogatin's book has some impedance nomographs. However, there are a
=3D
number of free calculators around some better than others. Google
for=3D20
impedance calculator and take your pick.=3D20
Good luck.
Steve.
Randolph Voorhies wrote:
> Hi Everyone,
> I'm brand new not just to this list, but to the world of PCB
product=3D
ion in general. I'm currently a masters student at USC in robotics and
h=3D
ave been producing some relatively simple PCBs for the past few months
us=3D
ing Eagle. I'm now working on a fairly complex project that involves
crea=3D
ting a custom baseboard for an XTX module (xtx-standard.org). My formal
=3D
training is in digital electronics, computer architecture, and computer
s=3D
cience, so when it comes to signal integrity I'm pretty much at a loss.
=3D
Does anyone know where to find specific routing guidlines for
differentia=3D
l pair lines such as firewire and ethernet? I have read all sorts of
conf=3D
licting advice (differential impedence is crucial vs. differential
impede=3D
nce couldn't matter less, etc..), and have had a very difficult time
find=3D
ing exact numbers for trace spacing/individual trace impedence and so
on.=3D
If anyone with experience has any advice that has worked for them it
wou=3D
ld be very very very much appreciated.
>
> Thanks so much,
> Rand Voorhies
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