From: Mark Gailus ([email protected])
Date: Fri Jun 23 2000 - 10:01:07 PDT
Just to further emphasize some useful things that have already been
by other si-list subscribers:
A two dimensional field solver does not by itself include the effects of
vias that connect a guard trace to ground. The 2D solver can typically
the guard trace as one of the following:
(1) as an "ideal" ground which is more or less the same as connecting
trace "continuously" to the ground plane(s) along its entire length,
(2) as a "floating" conductor which is more or less the same as not
it to ground anywhere and in addition cutting it into many short
portions so that it does not carry any significant total current in the
or (3) as another "signal" trace.
Case (1) is a reasonable approximation if the portions of the guard
between vias are well below any resonant frequencies (see below), case
allows a more detailed transmission line circuit model of the actual
to be created (also see below) if this seems necessary.
A "guard trace" between two signal traces can definitely INCREASE
between the signals at frequencies for which the guard line resonates.
resonant frequencies for a portion of the guard line are those for which
distance between successive ground vias is a multiple of a quarter
For example, a guard line on an inner (stripline) layer of an FR-4
board with a
delay of ~69 picoseconds per centimeter, would have its lowest resonant
frequency at 7.14 GHz ( = (0.5) (1cm) / (70ps/cm) ) for a 1cm ( 0.4
spacing between ground vias, and 714 MHz for a 10 cm ( 4 inch) spacing
vias. ( Of course each portion also resonates at approximate odd
the lowest frequency, depending upon other details like via inductance.
the spacing and number of ground vias desired will depend upon the
content of your signal, your tolerance for crosstalk, etc.
As has been pointed out, these resonant coupling effects can be
any SPICE or other circuit simulators by using a two-dimensional solver
derive a coupled line model for the portion of interconnect between a
ground vias, WHILE TREATING THE GUARD TRACES AS YOU WOULD ANY OTHER
and then hooking up these models with appropriate via-to-ground models
right places (as a first cut just a low resistance or possibly a small
inductance to ground is good).
I hope this is helpful.
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