Return Paths and Image Currents
SPICE does not know about return paths and image currents. When the
frequency of interest was a few megahertz, this was no big deal. When
frequencies got up to a few hundred megahertz, it became a big deal. To
get good correlation between simulations and measurements, it became
necessary to explicitly model the return paths.
SPICE provides a node, zero, that is ground. At low frequencies,
this is fine. It makes little difference that node zero at this end of
the board is at precisely the same potential as node zero at that end
of the board. It makes little difference that the signal into node zero
at this end of the board sees absolutely no time delay in getting to
that end of the board, as shown in Figure
7.7 below.
At low frequencies, the distance from this end to that end of a
board were small enough that the timing differences were imperceptible.
They were inconsequential. It takes about two nanoseconds, maybe a
little less, for a signal to cross a typical baseboard in a personal
computer.
The original personal computers had clock cycles that were over 100
times longer than this. Now cycle times are approaching an order of
magnitude smaller than this, and the time required to cross a board is
very significant. Even the time required for the signal to traverse a
package and pin can be significant.
 |
| Figure
7.7. SPICE's Ideal World |
To accommodate the reality that return paths are a part of the
interconnect circuit, the return paths must be modeled in SPICE just as
the signal path must be modeled. Unfortunately, the return path is
often less conspicuous than is the signal path.
As an example of this dilemma, consider an integrated circuit housed
in a package with multiple ground pins. The ground pins may be
distributed throughout the pin field. Some may be much nearer the
signal pin than others.
Some may connect through paths inside the silicon or the package
that are not made public to the board designer. It has never been easy
to generate a really good SPICE model of the return paths for many real
circuits.
And now things are going to get even more complicated. Microwave
signals respond to capacitances and inductances that are small enough
to be nearly immeasurable. At microwave frequencies, components that
were intended to be capacitors can look like inductors. Short stubs can
look inductive at some frequencies and capacitive at others. SPICE
programs can directly handle some effects, others it cannot.
Features such as plane-splits and vias cannot be directly modeled in
SPICE and so must be simulated in field solvers and then converted into
SPICE-compatible formats—usually L-C equivalents.
That is not to say that you have to throw your SPICE simulator away
when you encounter these features. No, what it means is that you may
need another tool to help you generate L-C equivalent models for these
features.
Next in Part 2: Differential
transmission lines and receivers
Dennis Miller has worked in electronics since 1963. His early
engineering interests and education centered on control theory and
numerical analysis. Now his interests are signal integrity and
numerical analysis. Since joining Intel
Corp. in 1991, he has been instrumental in the development of
Infiniband technology and similar high speed signaling technologies.
This series of articles is based on material from Designing High
Speed Interconnect Circuits," by Dennis Miller, used here with the
permission of Intel Press which holds all copyrights. It can be
purchased on-line.
