Re: [SI-LIST] : Broadside Coupled Traces

Ron Miller (rmiller@Brocade.COM)
Thu, 22 Apr 1999 10:42:14 -0700

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In theory broadside traces have the advantage of relying less on the ground for
impedance and could work with no groundplane at all, so they could go across a
pcb and various planes with impunity.

In practice, since we are limited to about 3 or mils minimum trace widths by the
board houses, the increased height for 50 ohms(100 ohms differential) starts to take up a lot of the thickness available in the board. Also, the increased height between ground
planes makes cross coupling worse between pairs. In order to reduce the coupling
between pairs on the same layer you should figure that spacing =3 X Height(ground to ground plane) will give about 40 db or .01 X voltage coupling at the worst frequency
considered.

With differential traces on the same layer this spacing is relatively easy to get. With
broadside coupled lines you have 3 sandwiched layers of dielectric, and the top
and bottom dielectric must be 2 or 3 times thicker. Then the spacing between pairs
goes up as a factor of about 5.

So, with broadside coupled lines you will get a reduction in density to about 1/10 of what
you get with standard differential traces on the same layer.

Ron Miller

mannand@wlgore.com wrote:

> Can anyone outline the advantages and disadvantages of using broadside coupled
> vs. edge coupled differential traces? Is either one better from a signal
> integrity perspective ( less lossy? lower crosstalk?). Is it easier to route
> broadside coupled traces in high density applications? And what are the issues
> board manufactures need to deal with such as tolerances, trace registration,
> impedance control, number of layers, etc.? Any insight you can provide would be
> helpful. Thanks!
>
> **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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<!doctype html public "-//w3c//dtd html 4.0 transitional//en"> In theory broadside traces have the advantage of relying less on the ground for
impedance and could work with no groundplane at all, so they could go across a
pcb and various planes with impunity.

In practice, since we are limited to about 3 or mils minimum trace widths by the
board houses, the increased height for 50 ohms(100 ohms differential) starts to take up a lot of the thickness available in the board.  Also, the increased height between ground
planes makes cross coupling worse between pairs.   In order to reduce the coupling
between pairs on the same layer you should figure that  spacing =3 X Height(ground to ground plane) will give about 40 db or .01 X voltage coupling at the worst frequency
considered.

With differential traces on the same layer this spacing is relatively easy to get.  With
broadside coupled lines you have 3 sandwiched layers of dielectric, and the top
and bottom dielectric must be 2 or 3 times thicker.  Then the spacing between pairs
goes up as a factor of about 5.

So, with broadside coupled lines you will get a reduction in density to about 1/10 of what
you get with standard differential traces on the same layer.

Ron Miller

mannand@wlgore.com wrote:

Can anyone outline the advantages and disadvantages of using broadside coupled
vs. edge coupled differential traces?  Is either one better from a signal
integrity perspective ( less lossy?  lower crosstalk?).  Is it easier to route
broadside coupled traces in high density applications?  And what are the issues
board manufactures need to deal with such as tolerances, trace registration,
impedance control, number of layers, etc.?  Any insight you can provide would be
helpful.  Thanks!

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

-- 
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017                 
     ==========0000-(_)0000=========== 
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net
  --------------E10EA27A3F3C8CFEA262BC99-- **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****