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

["Ingraham, Andrew" <a.ingraham@xxxxxxxx>]

> Forgive me, but I don't understand how your models are constructed???
> My assumptions would be that your ground plane is part of the structure
> in the model and not connected to "0" an ideal ground???  Is it
> commonplace
> to analyze point to point or multidrop in this manner???

True, an actual ground plane or cable shield is not an ideal ground.  In the
real world, there is no such thing as ground.  It does not exist.  Every
point we call "ground" is relative, and no two grounds are the same.

But, most simulators including SPICE rely on a reference node (node 0) and
tend to use it everywhere.  It's often rather difficult to construct or
modify a device model to be truly independent of node 0; there may be hidden
connections everywhere (capacitance to bulk/substrate, etc.).  Plus, many
models from device vendors come encrypted and cannot be separated from node
0 by the end-user.

Consequently, we need to construct and use models for things like
transmission lines, that can be referenced to node 0.  Given certain
limitations, it is possible to do this when creating the transmission line
model, by folding the return path's effects on the primary mode signal, into
the signal path, thus making the model appear as if the return path is ideal
node 0.

Actually, what you're constructing is a 2-port network where one node on
both ports CAN be connected to node 0.  That is what SPICE's single line
model is.  If it accurately represents the normal TEM mode, then it is
usually OK for analyzing signals going from point A to point B and
vice-versa; but it does ignore some effects such as ground noise across a
system, hidden current paths, higher order modes, etc.

SPICE does not require you to connect the reference nodes on both ends of a
transmission line to ideal node 0.  But, if you simply lift the reference
node from node 0 on one end (along with all the circuitry attached to it),
it won't tell you anything about ground noise, ground impedance, etc.

It's possible to construct more complicated models that include differences
between return paths and "ideal" ground; but I think you need to know in
advance exactly what other effects you are looking for and construct your
models around those effects.  SPICE has no concept of three or even two
dimensions in physical space.

Regards,
Andy




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