Re: [SI-LIST] : Broadside Coupled Traces

D. C. Sessions (dc.sessions@vlsi.com)
Thu, 22 Apr 1999 16:37:11 -0700

"Dr. Edward P. Sayre" wrote:
> At 12:25 PM 4/22/99 -0400, you wrote:
> >I believe that I can address the PCB fabrication issues.
> >
> >Edge-Coupled Stripline:
> >
> >This differential impedance model consists of two traces of equal width
> >centered between two planes. It represents the most controllable
> >differential impedance model. The symmetry of the edge-coupled stripline in
> >all three axes results in maximum predictability. Considering that it is
> >shielded on both sides by planes, it is the most noise immune differential
> >model.
>
> All differential transmission systems are basically immune to common mode
> effects not just or especially edge coupled systems.

Not necessarily. The receivers are relatively immune to common-mode voltage
noise, but if the two lines have different common-mode impedances then any
common-mode currents (eg capacitively coupled noise) will go through mode
conversion and show up at the receiver as differential noise.

It's a secondary effect, but a very real one in some cases (as I found to
my grief with some metro-area cabling once upon a time.)

-- 
D. C. Sessions
dc.sessions@vlsi.com

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