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From: bkemp@lexmark.com
Date: Tue Jun 20 2000 - 09:03:35 PDT


I have gotten numbers using xfx that indicate guard traces do work. More
importantly, I have seen positive effects on real boards. It is not important
to via to ground every "X" intervals of distance. It is critical that the
"guard traces" are grounded at the source and receiver. It is also necessary
for the guard traces to via whenever the signal does. For instance, if you have
a clock trace that vias from layer 1 to layer 4 and is routed as
ground-signal-ground (GSG), the ground (guard) trace should via from layer 1 to
layer 4 very closely.

See below for some quick results. Compare numbers in bold for effects on
crosstalk.

Brandon Kemp
EMC/SI Engineer
Lexmark International, Inc.

Trace width = 6 mils
Edge to edge separation = 6 mils
Height above ground plane = 7 mils
Er = 4.5

1.
          S S
          ___ ___
     ___________________________GP
     Signals separated by 6 mil

 Configuration Name: SS_GP_7mil Conductors: 2
#INTEGRAL solution#
Conductor index: 0 name: $$GND$$
Conductor index: 1 name: 1
Conductor index: 2 name: 2

 i j Lij Cij Ze Zo Se So Fwdx Rvsx
from to (nh/in) (pf/in) (ohms) (ohms) (ns/ft)(ns/ft) (s/s) (v/v)
--------------------------------------------------------------------------
  1 1 9.791 2.176 68.57 - 1.71 - - -
  1 2 2.735 0.334 82.47 53.02 1.82 1.60 0.132 0.217
  2 2 9.791 2.176 68.56 - 1.71 - - -
;

2.
          S S
          ___ ___
     _____________________________GP
     Signals separated by 18 mil

Configuration Name: S_S_GP_7mil Conductors: 2
#INTEGRAL solution#
Conductor index: 0 name: $$GND$$
Conductor index: 1 name: 1
Conductor index: 2 name: 2

 i j Lij Cij Ze Zo Se So Fwdx Rvsx
from to (nh/in) (pf/in) (ohms) (ohms) (ns/ft)(ns/ft) (s/s) (v/v)
--------------------------------------------------------------------------
  1 1 9.842 2.117 68.27 - 1.73 - - -
  1 2 0.928 0.061 72.37 63.97 1.79 1.67 0.066 0.062
  2 2 9.842 2.117 68.28 - 1.73 - - -
;

3.
     G S G S
     ___ ___ ___ ___
     _____________________________GP
     Signals separated by 18 mils, one trace routed GSG

Configuration Name: GSGS_GP_7mil Conductors: 2
#INTEGRAL solution#
Conductor index: 0 name: $$GND$$
Conductor index: 1 name: 1
Conductor index: 2 name: 2
XFX V6.5.0 Report 16 Jun 13:55 2000 Setup File =
F:\bkemp\xfx\Scarecrow\Clocks\clks.xfx

 i j Lij Cij Ze Zo Se So Fwdx Rvsx
from to (nh/in) (pf/in) (ohms) (ohms) (ns/ft)(ns/ft) (s/s) (v/v)
--------------------------------------------------------------------------
  1 1 8.206 2.253 60.35 - 1.63 - - -
  1 2 0.237 0.022 61.51 59.18 1.65 1.62 0.019 0.019
  2 2 8.928 2.184 63.94 - 1.68 - - -
;

.xfx
CONFIG SS_GP_7mil e: 1.00
INTEGRAL
DIELECTRIC e: 4.50 HORIZ_PLANE y: 0.0 7.0
CONDUCTOR 1 RECTANGLE x: 0.0 6.0 y: 7.0 9.0
CONDUCTOR 2 RECTANGLE x: 12.0 18.0 y: 7.0 9.0
GROUND HORIZ_PLANE y: -1.5 0.0;

CONFIG S_S_GP_7mil e: 1.00
INTEGRAL
DIELECTRIC e: 4.50 HORIZ_PLANE y: 0.0 7.0
CONDUCTOR 1 RECTANGLE x: 0.0 6.0 y: 7.0 9.0
CONDUCTOR 2 RECTANGLE x: 24.0 30.0 y: 7.0 9.0
GROUND HORIZ_PLANE y: -1.5 0.0
;

CONFIG GSGS_GP_7mil e: 1.00
INTEGRAL
DIELECTRIC e: 4.50 HORIZ_PLANE y: 0.0 7.0
GROUND RECTANGLE x: -12.0 -6.0 y: 7.0 9.0
CONDUCTOR 1 RECTANGLE x: 0.0 6.0 y: 7.0 9.0
GROUND RECTANGLE x: 12.0 18.0 y: 7.0 9.0
CONDUCTOR 2 RECTANGLE x: 24.0 30.0 y: 7.0 9.0
GROUND HORIZ_PLANE y: -1.5 0.0
;

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