RE: [SI-LIST] : Broadside v edge coupled striplines

About this list Date view Thread view Subject view Author view

From: Ron Miller (rmiller@brocade.com)
Date: Sun Jan 21 2001 - 14:28:06 PST


Our goal is typically to get:
1. as much trace density as possible into a given PCB
2. good impedance tolerances(low reflections)
3 as much isolation or low crosstalk as possible

Limitations
1. Lower dielectric constant materials cost more money. We are up
        against a limit here, buying the best fibreglass/epoxy material
        available, but the next step up is teflon which increases material
        costs four-fold, doubles fabrication costs and makes assembly and
        rework very difficult and expensive.
2. Narrower traces and spacing below 4 mils costs a lot more. We are presently getting the best we can at 4 mils and to get below that
        size would drive the FAB houses to the very expensive processes that
        would multiply costs several times.

Broadside vs edge coupled
1. On stripline layers between DC planes, coupling
        between pairs is proportional to the distance between
        the planes and inversely proportional to spacing from
        pair to pair. So, for a given amount of coupling between
        pairs, if you increase the distance between the DC planes
        you open up the field lines and must seperate the pairs
        further away from each other. Broadside coupled lines require
        three dielectric layers increasing the distance between the
        DC planes forcing us to seperate the pairs farther apart
        for the same amount of crosstalk.

2. Broadside coupling increases the coupling between the lines
        which is good for reducing coupling to other pairs, but decreases
        the differential impedance between the lines. This means
        for a given total impedance the impedance to ground-planes
        must be higher, which means that the dielectric thicknesses
        must be increased.

SUMMARY:
So, 2 above drives the thickness apart for impedance control while
1 above drives the thickness further apart.

The net result is that the thickness goes up by a factor of about 50
percent while the spacing required is increased by a factor of about
200 percent.

The total space required for broadside coupled lines, all things being
equal is about 4 times that required for edge coupled lines.

Good luck

Ron Miller

-----Original Message-----
From: Michael Khusid [mailto:mkhusid@sitaranetworks.com]
Sent: Sunday, January 21, 2001 9:26 AM
To: 'Ron Miller'
Subject: RE: [SI-LIST] : Broadside v edge coupled striplines

Ron,

I totally agree with you, but I often find myself arguing with people trying
to prove my point.

I would greatly appreciate if you could forward me your arguments, just to
simplify my own life :)

Thanks,
Mike Khusid

> -----Original Message-----
> From: Ron Miller [mailto:rmiller@brocade.com]
> Sent: Friday, January 19, 2001 8:18 PM
> To: 'Scott McMorrow'; 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
>
>
> 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
> >
> >
> >
>
>
>
>
> **** To unsubscribe from si-list or si-list-digest: send e-mail to
> majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
> si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
> si-list archives are accessible at http://www.qsl.net/wb6tpu
> ****
>
>
> **** To unsubscribe from si-list or si-list-digest: send e-mail to
> majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
> si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
> si-list archives are accessible at http://www.qsl.net/wb6tpu
> ****
>

**** To unsubscribe from si-list or si-list-digest: send e-mail to
majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
si-list archives are accessible at http://www.qsl.net/wb6tpu
****


About this list Date view Thread view Subject view Author view

This archive was generated by hypermail 2b29 : Tue May 08 2001 - 14:30:41 PDT