Hello SI-Listers,
I am describing a physical model with s-parameter, where I have a group of
ports, call them A-group, referencing a local plane. There is then another
group of ports, B-group, referencing another local plane. There are general
paths between A-group and B-group which is what I am interested to describe by
s-parameter. An important detail is that all B-group connections, including
returns, are ports which are referenced to B-plane as opposed to conventional
method where a port represents both forward and return paths. (This is done
intentionally for reasons that are not as relevant here.)Suppose the planes are
interconnected through a conductive path, with conductivity perhaps few orders
of magnitude higher than conductivity of local planes. Logically this is
similar to a split plane scenario except there is a weakly conductive path
connecting the planes, to ensures a predictable conductive path from A-plane
to B-plane at DC. The s-parameter is solved with an explicit conductive solve
at 0Hz, presumably correctly.Â
I then use this parameter in spice, where I am careful to properly connect
circuits to ports such that there is no improper cross-plane interaction. What
I mean by that is, if A-group ports are referenced to same spice node as a
reference for s-parameter, then connectivity for B-Â ports referenced only
with respect to each other. In this way my intention is allow currents to flow
through through immediate paths between A-and B- group with (theoretically)
no/negligible DC currents to flow through path between planes. Now, I energize
a set of A- ports with known voltage source. This will energize the circuit on
B- side. What I expect is voltages on any B-side node to remain within levels
of A- side (since B- circuitry is drawing and not sourcing, and as measured
both relative to B-side and A-side). What I may often see on B- side is the
entire group is shifted *outside* of the range of the A-group voltage,
typically below the reference level of A-group. However, without looking
across A- to B- boundaries, each A- or B- side locally is behaving as expected.
Note that this entire discussion is at 0Hz as computed by spice DC operating
point for the complete circuit.Â
I don't think all this is some illegal use of s-parameters -- this is just
simply calling out a different location as port reference. I am wondering if
any one has some insight into what could be causing this. More importantly is
there a method to detect this condition? It seems satisfying passivity is not
sufficient here.
Best regards,Lenny Rayzman
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