# [SI-LIST] Re: Concept of Voltage

• To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
• Date: Tue, 19 Jul 2011 21:36:33 +0000

```Yuriy,

Thanks for making this point about proper pin/port connection; this is indeed a
source of many problems.
Circuit simulators, including SPICE and e.g. ADMS (ELDO) are flexible enough
and allow not only assignments for ports, but also individual assignments for
"pins" (pos and neg terminals of every port), leaving the user a possibility
for misuse. Same with T-line.

Of course, to ensure proper use, all local references could be connected to a
common ground, but they also could be connected together to another node, not
necessarily global ground. However in some cases this requirement may become
too restrictive.

There is a theory of linear multiports, where distinction is made between
arbitrary N-ports and so called "2N ports" or better to say 2N terminal models.
Complete N port model typically is an N+1 terminal model, with N independent
port voltages (differences between terminal potentials) are independent. This
model (N ports and N+1 terminals) allows any type of external connections
without the risk of violating the condition of 'regularity'. (I may be mistaken
about the exact term, but this is a literal translation).

With "2N terminal" models, we typically have N ports and 2N terminals. Here, we
are very restricted in a way we can connect this model to external circuits.
Most of practical models we deal with belong to this second type. Single T-line
is a 2x2 terminal model with 2 ports. Although 4 terminal model may create 3
ports maximum, this is not a legitimate way for T-line because it was created
as 2 port only (or in other words, the model does not reflect the relationships
other than between the currents and voltages or A/B for these two ports, hence,
we cannot take from this model more info than was put into it during
measurement). For example, if we connect a resistor between side A, positive
and side B positive terminals, this would result non-physical solution since
then the model is actually used as 3 port, and by itself cannot enforce
"regularity" since it was not designed this way.

Unfortunately, there is no way up to now to specify legitimate connectivity for
a given S-parameter model inside the Touchstone file. We see the number of
ports but, we do not have an info on how these ports should be created by
selecting the pairs of external nodes.

From: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Concept of Voltage
Date: Tue, 19 Jul 2011 09:20:08 -0700

Shawn,

You have described a perfect example of what should never be done with
multiports described with S-parameters (or equivalent t-line models). It is
typical model misuse (garbage in - garbage out).
Formally, each single port in a multiport has 2 terminals (or nodes) -
signal and local reference. There is no information on the potentials of
each terminal - only information on the difference of potentials or voltage.
Moreover, as I pointed earlier, the voltage may be defined not as the
difference of potentials (or line integral of electric field), but as a
projection of the electric field on the electric field of a wave-guiding
eigen-mode. Current flowing out of the reference terminal must be exactly
opposite to the current flowing into signal terminal. Again, current may be
defined either locally or as a projection of magnetic field on the
eigen-mode field. The projection definition must be used even for MTL with
quasi-TEM modes at high frequencies.

The only possible way to use a port is to connect it to another port that is
defined identically. Lumped components can be connected to local or lumped
ports defined in electromagnetic analysis. Wave-ports can be connected only
with the identically defined wave ports. An exception is quasi-TEM
transmission lines at lower frequencies. Such ports can be connected with
the lumped or local ports as long as the cross section stays much smaller
than the wavelength (due to similarity of the voltage definition for MTL and
lumped element). In cases when distance between strips becomes comparable
with the wave-length, connection of a quasi-TEM t-line port with a lumped
port becomes ambiguous and erroneous.

Considering use of multiport models in a SPICE circuit simulator, the only
possible and legitimate use of the local reference terminal is to connect it
to a common reference node (global ground). The signal terminal must be
connected to a signal terminal of the identically defined port. In case of
lumped port, the lumped element must be connected between the signal
terminal and the global reference node. If a lumped element contains more
than two nodes, there must be a separate lumped port (possibly with common
reference) constructed to connect each signal node of the lumped element.
There are no restrictions in the circuit analysis as long as one follows the
port construction and connection rules and build or collect a sufficient
number of multiport models for each interconnect component. Not only
quasi-TEM, but optical or substrate integrated waveguides for instance can
be analyzed with a SPICE solver without any problems.

Best regards,
Yuriy

Yuriy Shlepnev
www.simberian.com

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