FreeLists / si-list / [SI-LIST] Re: GND is perfect conductor?

["Zabinski, Patrick J." <zabinski.patrick@xxxxxxxx>]

> 1) Does anyone have any information (data would be preferred, vs.
> conjecture) on the effects of using a perfect vs. imperfect conductor
> for ground?

Jeff,

From numerous experiments compared to models, the effects of the ground
plane do have an effect on the signal loss.

We have a few tools we use to generate models from measurement, and
we have a fudge-factor of sorts to the resistance that scales the
signal conductor loss by a certain percentage to account for the ground
loss.  In the general case where you're either microstrip or stripline
and the ground plane is the same metalurgy as the signal conductor,
a 1.4X to 1.8X scaling of resistive loss is need to account for
the ground plane loss (i.e., add 40-80% relative to the signal
conductor alone).

If you have a situation where the ground plane is much lossier than
the signal conductor (e.g., some coax where they use mylar shields
are good examples), this scaling factor will go up.  Where the grounds
are better conductors than the signals, then this scaling goes down.

We've used Ansoft Maxwell 2D, Ansoft HFSS, and Synopsys Hspice's internal
w-element modeling tools, and they appear to appropriately address
the ground plane losses reasonably well.  Many other tools we have
used do not consider ground plane losses.  The best way we've been
able to determine if the tool includes ground plane effects is to
compare measurment to model.  If the modeled losses are too
optimistic by 30-100% while the impedance and velocity are
reasonably accurate, then we generally conclude that the tool does
not consider ground plane loss.

Pat
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