Re: [SI-LIST] : differential pairs

Ron Poon (rpoon@cisco.com)
Wed, 17 Sep 1997 16:56:02 -0700

Joachim,

The board thickness will dictate the use of edge-side coupling.

Because of strong coupling between two broadside signal lines,
they need to be separated considerably to achieve 50 to 60 ohm
differential impedance normally used. That makes the whole
board quite thick. I have seen clever ways to overcome this
difficulty. One of these uses a plane cutout method. Imagine
two regular striplines with their common ground plane
separation cut out so the striplines face each other to form
the broadside coupled pair. That will work if you don't have
too many differential pairs, and you're careful about cut planes.

----------------------------

-----

-------- --------- common ground plane with cut

-----

----------------------------

(the two signal lines and the cut may not line up if
you don't have fixed font).

Joachim Mueller wrote:
>
> Hi,
> I would like to get some opinions on how to layout differential pairs for
> high speed (1 Gbit/s) data signals. I am not quite sure if it is preferable
> to use broad side or edge side coupled differential pairs?
>
> For data signals at that speed loss effects needs to be considered too. What
> is a minimum trace width to avoid significant skin effect losses?
>
> edge side coupled: broad side coupled:
>
> ----------------------- -------------------------
>
> --- --- ---
>
> ---------------------- ---
>
> -------------------------
>

-- 
Ron Poon               			Cisco Systems
Phone: (408) 527-8243			250 W. Tasman Drive
Fax:   (408) 527-8254			San Jose CA 95134-1706