Hi Mike,
When you say, “not particularly strongly coupled to each other,” you imply that
there is a third conductor involved (a shield, perhaps?) to which the two wires
are coupled, more strongly than to each other. Is that the case?
If it’s not, then I would argue that the physical structure you’ve described
can be adequately represented, using a 2-port Touchstone model. If it is, then
I argue you need a 4-port model.
In either case, I claim you need either a 2 or a 4 terminal component in your
SPICE style circuit simulation, depending upon whether your differential pair
is driven in a single ended (i.e. - one of the wires connected to local circuit
ground at both ends), or differential (i.e. - neither wire connected to local
circuit ground) fashion, respectively.
I don’t think the "N+1 terminals" option buys you anything, since you’re just
going to connect that extra reference terminal to the “0” net of your netlist
anyway, and that’s exactly what a SPICE simulator will do by default, if you
don’t provide that terminal.
I think what you’re really driving at is whether or not it’s permissible to
short the two local circuit grounds at either end of this mile long
interconnect, by connecting both to the reference terminal (assuming the N+1
configuration) of your Touchstone model component; is that right? Of course,
physically, this is incorrect, since, across that distance, there can be no
true short. However, that doesn’t matter, because voltage across that much
distance is a very ill-defined and meaningless quantity.
I can’t comment on the BIRDs, as I’ve not read them.
-db
On Feb 26, 2018, at 4:36 PM, Mirmak, Michael <michael.mirmak@xxxxxxxxx> wrote:
(apologies if you have heard something like this before – I have discussed it
privately in a couple of contexts)
Imagine that I have a physical interconnect, consisting of a differential
pair of wires not particularly strongly coupled to each other. I want to
model the interconnect using an S-parameter set of at least 4 (four) ports:
two inputs P &N and two outputs P’ & N’.
The problem: the physical pair of wires is approximately one mile in length
from P & N to P’ & N’.
My questions:
1) Can this structure be correctly and unambiguously described using a
circuit with N terminals and an N+1 reference terminal? If not, how many
additional terminals are required?
2) Does the current BIRD189 draft, as written, enable correct and
unambiguous description(s) of this structure?
3) Does BIRD158.7, as written, enable correct and unambiguous
description(s) of this structure?
Thank you!
- MM
