[SI-LIST] Re: s parameters and transient simulation
- From: "Smith, David TQO" <dsmith@xxxxxxx>
- To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 11 Dec 2003 14:30:04 -0800
Steve,
By way of further clarification I have two comments:
You said:
you can't truly know how the voltages at the two
signal nodes are varying with respect to one another,
since you don't know how the voltages at the two
ground reference nodes are varying with respect to
each other.
I say that the voltage between distant nodes is UNDEFINED
--- not simply that we don't know what it is. The voltage
across small "gap ports" is defined only because we can
treat the problem locally as static (i.e. quasi-static). It is
an approximation one can make for "small enough" gaps.
Maxwell told us that Del x E = - dB/dt. It is only when
dB/dt is zero that voltage (i.e. static potential E = del V)
can be defined!
The situation is a slightly different (and better defined) for
extended ports supporting propagating modes but I won't
go into that can of worms here.
Second:
The nodal analysis used for circuit simulation (e.g. SPICE)
is generally understood to be based on the use of node
voltages - all referenced to a single (arbitrary) node.
Now consider importing the results from a 3-port EM
simulaltion to a circuit simulator - say ADS. ADS provides
a 3-port circuit element - all you have to do is place the
symbol on the schematic and link it to the text file
containing the computed S-parameters. The circuit
simulator does the rest.
But there is a conceptual problem - the ADS 3-port
symbol has 4 terminals! The 4-th terminal is called
"the common terminal" in the ADS manual where it
is also suggested that "it is normally grounded".
I have found that most engineers get confused by this.
These three ports are separated in space. For the
EM analysis we understand this. But when we bring
it into the circuit simulator we have this "common
terminal" that has to be hooked up somewhere. But
where?
Further confusion comes because the voltages computed
on the terminals of the 3-port block really have absolutely
nothing to do with this each other! If the engineer wants
to hook up a load across one - it is placed between
the circuit terminal and this "common terminal". Another
get's a short circuit, another a source. All of these
elements are strapped to this "common terminal" on
one end.
So in the EM simulation we can have no communication
between distant ports, but the circuit simulator references
them all to the same "common terminal". In the EM
simulation we put a signal source on one port, a load
on another, and so on. But it the circuit simulator we
are connecting one end of each of these elements to
this common terminal (usually grounded!).
Most engineers I have explained this proceedure to
are initially confused. The key understanding is that
the reference node (ground) in the circuit simulator
is a pure mathematical device (fiction?). It does not
represent a physical point in space or anything of the
sort. The fact that there is an apparent wire (in the
schematic) from this terminal to each of many components
does not imply that there exists a zero-impedance
(or non-zero impedance) current path from one to
the other.
This is why we have to separately enforce the rule
that, in the circuit simulator, components can be
connected between active terminals and this reference,
but never between active terminals. Anything connected
between active (real?) terminals can see a totally
ficticious voltage and we will get unreasonable
results. See my last post on this.
So the engineer has to reconcile the statement
from Kahlil and Steer (quoted by Geoff):
"The essence of the problem is that a global reference node cannot
reasonably be defined for two spatially separated nodes when the
electromagnetic field is transient or alternating."
And the fact that we do indeed connect "everything" up
to a common reference node in the circuit simulator!
Should we be surprised that newcomers to these concepts
are often confused.
Dave
> -----Original Message-----
> From: Steve Corey [SMTP:steven.corey@xxxxxxxxxxxxxx]
> Sent: Thursday, December 11, 2003 9:58 AM
> To: 'si-list@xxxxxxxxxxxxx'
> Subject: [SI-LIST] Re: s parameters and transient simulation
>
> Geoff -- your statements are true about network theory. However, I
> don't see where common ground nodes become a "bad recipe".
>
> When you take a measurement, you connect a reference conductor and a
> signal conductor from your measurement system to the device. By
> connecting the reference conductor to a specific point, you're stating
> that you don't care what the voltage is at that point -- you're only
> interested in the difference between the reference conductor and the
> signal conductor. If you're taking two-port measurements, you can't
> truly know how the voltages at the two signal nodes are varying with
> respect to one another, since you don't know how the voltages at the two
> ground reference nodes are varying with respect to each other.
>
> When you connect a receiver, be it digital or analog, to a port, you are
> roughly making the same statement. You don't care what any of the
> voltages is with respect to some global ground, or some faraway port,
> you care about the difference between the receiver circuit's nodes and
> the local reference. All these voltages are local -- theoretically,
> the power rail, vin+, and vin- at the receiver could all be measured
> against a single local "ground", and those are the same voltage
> differences the device will see.
>
> When you drop a multiport network into SPICE, you have to be aware that
> you're making the same assumptions. Your simulation needs to be set up
> -- the same way your circuit design is -- so that the behavior doesn't
> depend on the difference between reference voltages of distant ports.
> This is the same partitioning into "groups" that is outlined in the MTT
> paper you referenced. The paper does little more than to cast the logic
> outlined above into modified nodal analysis (MNA) matrix notation which
> casts all local references to zero volts by zeroing out their
> rows/columns in the MNA matrix.
>
> The bottom line is, if you can't know the voltage between two faraway
> points, you need to make sure that you don't care what it is -- by good
> design, good measurement setup, good simulation setup. It should be
> reassuring that they all have to comply with the same set of limitations.
>
> -- Steve
>
> -------------------------------------------
> Steven D. Corey, Ph.D.
> Time Domain Analysis Systems, Inc.
> "The Interconnect Analysis Company."
> http://www.tdasystems.com
>
> email: steven.corey@xxxxxxxxxxxxxx
> phone: (503) 246-2272
> fax: (503) 246-2282
> -------------------------------------------
>
>
> Geoff Stokes wrote:
> > Hi Ray
> >
> > With reference to your posting earlier this year regarding n-ports etc.,
> > here is a thought on simulation of interconnects at high frequencies
> where
> > the concept of common voltage reference nodes seems to become a bad
> recipe,
> > thinking particularly of RF modelling of IC packages.
> >
> > As Khalil and Steer (paper cited below) have pointed out, the voltage
> > between two points is undefined in general. This is an aspect of field
> > theory which becomes relevant when the frequency is high enough that the
> > phase delay between two points in a structure is a significant
> proportion of
> > the wavelength. The significant proportion of course depends upon the
> > application, so we can't define a specific threshold frequency even for
> a
> > specific mechanical dimension. In analog or mixed-mode circuit designs,
> > relatively small values of couplings or impedance may be significant,
> but
> > such values might be ignored in a purely digital circuit. In addition,
> for
> > a correct DC simulation of the operating point and power supply
> currents,
> > together with broad band accuracy, the effect of internal inductance and
> > frequency dependent resistance
> > (both arising from skin effect and providing several percent effects)
> will
> > need to be included.
> >
> > In an earlier posting, Ege Engin wrote this helpful comment:
> >
> > "If an S parameter matrix is implemented in a circuit simulator, it
> > actually divides the rest of the circuit (all the other linear and
> > non-linear elements) into groups, that are only coupled to each other by
> > means of this S parameter matrix (due to the fact that an S parameter
> > matrix represents a distributed circuit). Since the voltage drops
> > between the local reference nodes in various groups are undefined, they
> > can be connected to each other in an arbitrary manner."
> >
> > I would just add that in practice, from the s-parameters obtained by
> > electromagnetic simulation or measurement, we have to formulate a
> polynomial
> > or lumped-element solution to feed into the nodal transient circuit
> > simulator (Spice or Spectre). Ege Engin's final sentence would then
> apply
> > to the interconnection of the extracted n-port model with the chip
> > schematic.
> > In Khalil and Steer, "Circuit Theory for Spatially Distributed Microwave
> > Circuits" (IEEE Transactions on Microwave Theory and Techniques, Vol. 46
> No.
> > 10, October 1998), we find:
> >
> > "The essence of the problem is that a global reference node cannot
> > reasonably be defined for two spatially separated nodes when the
> > electromagnetic field is transient or alternating. In this situation,
> the
> > electric field is nonconservative and the voltage between any two points
> is
> > dependent on the path of integration and, hence, voltage is undefined.
> This
> > includes the situation of two separated points on an ideal conductor."
> >
> > So we see that each port requires its own separate local return pin in
> order
> > to describe the distributed structure with sufficient accuracy over the
> > required frequency range. Two or more ports can only use a common
> ground if
> > they are physically close enough to one another (for the specific case).
> >
> > Finally, we make the arbitrary (?) decision to join the local ground(s)
> to
> > the common ground and hope it's OK. From the network theory it seems
> OK,
> > but is a little hard to swallow.
> >
> > Any comments?
> >
> > Best wishes,
> >
> > Geoff
> >
> > ______________________________________________
> >
> > Geoff Stokes
> > Applications Engineer, Signal Management Group
> >
> > Zetex plc
> > Lansdowne Road, Chadderton, Oldham, OL9 9TY, UK
> > Tel direct: +44-161-622-4857 Switchboard: +44-161-622-4444
> > Fax: +44-161-622-4469
> > http://www.zetex.com <http://www.zetex.com/>
> > e-mail: gstokes@xxxxxxxxx
> >
> >
> >
> >
> > _________________________________________________________
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