[SI-LIST] Re: s parameters and transient simulation
- From: "Smith, David TQO" <dsmith@xxxxxxx>
- To: gstokes@xxxxxxxxx,"'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 8 Dec 2003 17:39:53 -0800
There is a lenghtier followup paper, posted here (reference 7):
http://vision.lakeheadu.ca/publications
These papers provide the basic understanding you need to
use S-parameter matricies in a circuit simulator. You have
to avoid making circuits that allow communications (i.e.
current flow) between spacially separated ports. (EM
simulations use ports, Circuit simulation uses nodes
or terminals).
Yes, the results can be confusing if you don't keep in
mind that the circuit voltages are not absolute - even
though they seem to be in the circuit simulator. They
are just the local voltages at the EM ports. Thus, you
might apply a positive voltage (power supply) on one
port and the circuit simulator will compute a negative
voltage on one of the other ports. This is not a real
negative voltage with respect to the circuit simulator
"ground", it is just the (differential) voltage at the
EM simulation port.
Here's the simple case I'm thinking of. It's a
microstrip with a horizontal "gap port" in the
middle:
+ port 2 -
port 1 + --------| |--------------------- + port 3
- ------------------------------------- -
In the EM simulator, we compute a 3x3 S-parameter
matrix relating the ports. We can convert this
matrix to a Z-matrix. In the circuit simulator
it might look like this:
V_2
|
|
----------------
| |
V_1 -----| 3x3 Z-matrix |------- V_3
| |
----------------
|
|
--- <-- circuit ground
-
Put a short on terminal 1, leave terminal 2 open,
and apply a volt to terminal 3, and get a negative
volt at terminal 2. Other circuit elements should
only be connected between a terminal and circuit
ground, never between two spacially separated
terminals. This will insure that there are not
issues with about comparing voltages at spacially
separated locations. The voltages are just across
the little gaps used for EM ports.
This is a bit counter to how we are accustomed to
thinking about circuit voltages - the voltage that
appears to be between a terminal and circuit ground
is really an analog (or stand-in) of the voltage
across the EM port. I think you should understand
this principle completely and keep it firmly in
mind to correctly interpret the results of the
circuit analysis.
You can follow up in some of the other references at
the web site to get a better understanding.
Please forgive me if this discussion was off-topic
or out-of-phase. I didn't follow the earlier discussion
list that you mentioned in this mailing and I am
making a few assumptions.
Dave Smith
> -----Original Message-----
> From: Geoff Stokes [SMTP:gstokes@xxxxxxxxx]
> Sent: Monday, December 08, 2003 8:45 AM
> To: 'si-list@xxxxxxxxxxxxx'
> Subject: [SI-LIST] s parameters and transient simulation
>
> 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
>
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>
>
>
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