Re: FW: [SI-LIST] : Broadside v edge coupled striplines

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From: DORIN OPREA (dorin.oprea@alcatel.com)
Date: Mon Jan 22 2001 - 06:31:08 PST


Broadside or edge coupled traces ? Mainly the choice is determined by the design
requirements.
I see the following case in which one would prefer broadside coupled:
- signals require common mode immunity in which case single landed impedance is
about 60 ohms
- skin effect losses are critical (minimum 0.005" trace width)
- minimum 2 signals layers are needed for routing;
plane-signal-plane-signal-plane stackup would require more stackup thickness
than plane-signal-signal-plane (allows also higher routing density)
- mainly the signals are running in only one direction (backplane)
- you may have have to connect a small pitch connector
So long the broad side coupled traces are manufactured on a core construction
the PCB vendors appear not have any issue with the trace alignment.

Dorin

"Heard, Chris" wrote:

> Keeping Broadside pairs on Cores only is not practical in dense designs. A
> 20 layer construction only has 3 transmission line constructions available
> for Broadside routing. Broadside 50 ohm constructions always end up thicker
> than edge-coupled, which drives thickness up and total available layer count
> in a given mechanical design down. Increased thickness makes manufacturing
> engineers angry and less layers makes pcb layout folks angry.
>
> If you don't have too many diff pairs, either way works. If your design has
> a large quantity of diff pairs, broadside will make you crazy.
>
> Broadside on backplanes is a bad choice for similar reasons. Cores aren't
> available at every construction, increased thickness drives up plated
> through hole capacitance which dominates connector signal integrity. Wide
> lines with low skin effect are difficult to use because achieving 100 ohms
> differentially requires thicker construction...and on it goes.
>
> Chris
>
> -----Original Message-----
> From: Scott McMorrow [mailto:scott@vasthorizons.com]
> Sent: Saturday, January 20, 2001 2:17 AM
> To: Ron Miller
> Cc: Aubrey_Sparkman@dell.com; ldmiller@rhapsodynetworks.com;
> ribrooks@nortelnetworks.com; SI-LIST@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Broadside v edge coupled striplines
>
> Ron,
>
> I would agree. Just trying to dispel the blanket statement that
> broadside should be avoided due to manufacturing issues. There
> are ways to engineer a board and reduce the tolerance issues
> with broadside traces. Trace density is, of course, a seperate
> issue.
>
> regards,
>
> scott
>
> --
> Scott McMorrow
> Principal Engineer
> SiQual, Signal Quality Engineering
> 18735 SW Boones Ferry Road
> Tualatin, OR 97062-3090
> (503) 885-1231
> http://www.siqual.com
>
> Ron Miller wrote:
>
> > Hi scott
> >
> > Broadside lines require more real estate than edge coupled
> > lines. If you have the real estate that is good. We do not.
> >
> > If you disagree the contention that broadside requires more
> > real estate please e-mail me and I will fill in the details.
> >
> > ron Miller
> >
> > -----Original Message-----
> > From: Scott McMorrow [mailto:scott@vasthorizons.com]
> > Sent: Friday, January 19, 2001 10:00 AM
> > To: Aubrey_Sparkman@Dell.com
> > Cc: ldmiller@rhapsodynetworks.com; ribrooks@nortelnetworks.com;
> > SI-LIST@silab.eng.sun.com
> > Subject: Re: [SI-LIST] : Broadside v edge coupled striplines
> >
> > Aubrey,
> >
> > The manufacturing issues can be reduced by constructing the
> > broadside pair on core material and keeping the spacing between
> > the pairs small compared to the spacing to the planes. Then the
> > separation between the pair is well controlled and the fields are
> > well contained between the pairs.
> >
> > Another solution which works quite well is to use CPW or
> > grounded CPW construction for diff pairs on outer layers.
> >
> > regards,
> >
> > scott
> >
> > --
> > Scott McMorrow
> > Principal Engineer
> > SiQual, Signal Quality Engineering
> > 18735 SW Boones Ferry Road
> > Tualatin, OR 97062-3090
> > (503) 885-1231
> > http://www.siqual.com
> >
> > Aubrey_Sparkman@Dell.com wrote:
> >
> > > Thanks for that correction. Isn't the difference is really in
> > > manufacturing, not physics? An EDGE-coupled diff pair is more uniform
> > > because the pattern is etched in the same process. The
> BROADSICE-coupled
> > > diff pair is etched at two different times and additionally has to be
> > > mechanically aligned for lamination. This adds two additional error
> terms
> > > to the accuracy of your BROADSICE-coupled diff pair that the
> EDGE-coupled
> > > diff pair does not have. So IMHO, you should really have a packing
> density
> > > problem before you consider using BROADSICE-coupled traces.
> > >
> > > But if you are doing work for Compaq or Sun, you should use
> > > BROADSICE-coupled diff pairs whenever possible. :-)
> > >
> > > Aubrey Sparkman
> > > Signal Integrity
> > > Aubrey_Sparkman@Dell.com
> > > (512) 723-3592
> > >
> > >
> > >
> >
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