[SI-LIST] : guard traces

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From: Mark Gailus (Mark_Gailus@notes.teradyne.com)
Date: Fri Jun 23 2000 - 10:01:07 PDT


Just to further emphasize some useful things that have already been
pointed out
by other si-list subscribers:

A two dimensional field solver does not by itself include the effects of
the
vias that connect a guard trace to ground. The 2D solver can typically
treat
the guard trace as one of the following:

 (1) as an "ideal" ground which is more or less the same as connecting
the guard
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
connecting
it to ground anywhere and in addition cutting it into many short
disconnected
portions so that it does not carry any significant total current in the
end-to-end direction,

  or (3) as another "signal" trace.

Case (1) is a reasonable approximation if the portions of the guard
trace
between vias are well below any resonant frequencies (see below), case
(3)
allows a more detailed transmission line circuit model of the actual
situation
to be created (also see below) if this seems necessary.

A "guard trace" between two signal traces can definitely INCREASE
coupling
between the signals at frequencies for which the guard line resonates.
The
resonant frequencies for a portion of the guard line are those for which
the
distance between successive ground vias is a multiple of a quarter
wavelength.
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
inch)
spacing between ground vias, and 714 MHz for a 10 cm ( 4 inch) spacing
between
vias. ( Of course each portion also resonates at approximate odd
multiples of
the lowest frequency, depending upon other details like via inductance.
) So
the spacing and number of ground vias desired will depend upon the
frequency
content of your signal, your tolerance for crosstalk, etc.

 As has been pointed out, these resonant coupling effects can be
simulated in
any SPICE or other circuit simulators by using a two-dimensional solver
to
derive a coupled line model for the portion of interconnect between a
pair of
ground vias, WHILE TREATING THE GUARD TRACES AS YOU WOULD ANY OTHER
SIGNAL LINE,
and then hooking up these models with appropriate via-to-ground models
in the
right places (as a first cut just a low resistance or possibly a small
inductance to ground is good).

I hope this is helpful.

Best regards,

Mark Gailus
mark_gailus@notes.teradyne.com

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