[SI-LIST] Re: N-port model limitations in simulators
- From: "Muranyi, Arpad" <arpad.muranyi@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 25 Apr 2003 10:16:21 -0700
I would like someone to give us a definition of "port".
The reason I am asking for this is because I think there
is an important detail that makes things confusing in these
responses. Is a port just one node, *assuming* a universal,
global reference (as in SPICE node 0)? Or does a port
consist of two nodes, neither one of which is GND (node 0)
between which the measurements are done with respect to
each other? The first one could also be called single=20
ended, and the second one differential. There may be far
reaching implications depending on how we define "port".
Arpad
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-----Original Message-----
From: Marek Schmidt-Szalowski
[mailto:marek.schmidt-szalowski@xxxxxxxxxxx]
Sent: Friday, April 25, 2003 2:02 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: N-port model limitations in simulators
Dear All,
1. N-port scattering parameters do not say anything about
voltages between ground terminals of different ports. In
particular, they do not say whether you have a common ground
terminal or not. Thus, in my eyes, S-parameters give only a
*partial* description of a multiport device. =20
2. Although both twoports and fourpoles have four terminals there
is no general one-to-one relationship between twoports and
fourpoles. In case of a twoport we define 2 voltages and 2
(balanced) currents. In case of a fourpole we define 4
voltages and 4 currents (only 3 voltages and 3 currents are
independent). Thus, the fourpole formalism gives more
information on the device than twoport S-parameters. In
particular, it is possible to define a common terminal by
zeroing one of the voltages.
3. Generally speaking, there is no global ground node in a
distributed circuit. A good microwave circuit simulator
defines a local ground node for each cluster of lumped
components. Some simulators automatically connect the local
ground nodes one with each other. Otherwise, you must connect
them explicitly. You must do it cautiously or simulation
results will be completely wrong.
4. There are cases when the partial description of fourpole
offered by twoport S-parameters is not enough for a circuit
simulation. Imagine a lumped twoport A (obviously having a
well-defined ground node) to which you connect a distributed
twoport B, e.g. long coaxial cable. In this case twoport B
does not divide the circuit in two parts with local ground
nodes. The current at both ports of the cable are in general
no longer balanced. For instance the current though the inner
connector can be much larger then in the outer one. However,
S-parameters fail to describe this kind of behavior.
Assume, that you have a lossless cable whose length is 1
lambda. If you replace it with a 10 times longer cable you
will have still the same S-parameters but twoport A will see a
completely different connection between its ports.=20
Or remove the outer connector and leave the inner one.=20
S-matrix is now {{1,0},{0,1}}, thus no transmission. However,
if twoport A gives an alternative return-current path it will
see some transmission.=20
5. If you simulator implements only a common-ground-node
S-parameter component but also a four-terminal transformer you
may still simulate generic S-parameters (S-parameter with
local ground nodes). In order to de-couple the ground nodes
use an S-parameter component you have and connect a 1:1
transformer to one of its ports.
6. The above issues are mere consequences of a circuit theory.=20
The type of propagation mode (TEM, waveguide etc.) is here
irrelevant.
with kind regards,
Marek
M.Schmidt-Szalowski
Philips Semiconductors
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