Re: [SI-LIST] : Z Odd-mode, edge-coupled, 50ohms?

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Wed Jan 17 2001 - 11:21:56 PST


We tried several approaches and the most convenient (and working) solution
was - use broadside coupling. When entering hi-dens. BGA it yields uniform
and manageable routing; most tools will have an instrument to utilize
broadside; when entering the ball-field you can "neck" the "Y" just a notch
for a short distance to keep the clearance - it did not seem to affect SI
significantly.

Mike.

cadpro2k@dacafe.com wrote:

> Hi all,
>
> Then... Could someone please tell me (any guidance is appreciated) in
> layman's terms how I "might" route the differential traces, #1 using
> microstrip; and #2 doing it stripline. I'm working a board with GETEK
> material, it has a couple .1mm BGAs and many, many, many differential
> pairs. I've done some preliminary calcs (using Polar Instr. CITS
> software), and come up with .008" lines with .008 spacing and .024"
> spacing between the pairs, routing on the microstrip layers. I'd like to
> go smaller, and possibly decrease the dielectric to the plane layer, but
> I've heard there "might be" crosstalk problems. Anyone?
>
> I'm especially interested in routing these on stripline layers, but I've
> heard that's not a good practice either. Help?
>
> "This is just the beginning..."
>
> Thanks for all your help.
>
> Mitch
> Sr PCB Designer
> San Diego, CA
>
> (P.S. I've got most all the articles I need, from Lee Ritchey's to
> Mentor's App note, to Eric Bogatin's stuff to UltraCad's stuff, etc.
> etc., but I need something in PCB layout terms. Thanks)
>
> ---------Included Message----------
> > Date: Wed, 17 Jan 2001 09:58:42 -0800
> > From: "Neffody Kraskoff" <Neffody.Kraskoff@Plexus.com>
> > Reply-To: "Neffody Kraskoff" <Neffody.Kraskoff@Plexus.com>
> > To: "'cadpro2k@dacafe.com'" <cadpro2k@dacafe.com>
> > Subject: RE: [SI-LIST] : Z Odd-mode, edge-coupled, 50ohms?
> >
> > Hi Mitch,
> >
> > What type of driver technology are you using, is it LVDS? If so
> then LVDS
> > traces are 100ohm differential.
> >
> > If the 100ohm terminating resistor connects to the pos and neg
> signals of
> > your differential pair like so:
> >
> > -------------DIFF_sigP---------.---load pin (device)
> > |
> > /
> > \ 100ohm R
> > /
> > |
> > -------------DIFF_sigN---------.---load pin (device)
> >
> > Then your required trace impedence must be 100ohms differential
> (matches the
> > resistor value).
> >
> > Its extremely unusual to have 50ohm single-ended lines on the
> same trace
> > layer with 50ohm differential. Usually, its 100ohm differential
> and 50 ohm
> > single ended.
> (snip)
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