'Spice' up your design: predicting electromagnetic fields is no longer a luxury in the high-speed arena. Fortunately, many of today's EM simulators remove the guesswork.

MOST OF TODAY'S designs are in the high-speed regime, where interconnects are no longer transparent to the signals. If the interconnects' electrical properties are not taken into account, the product probably will not work.

But how do we take into account the electrical effects of these interconnects? How do we predict, before we actually build hardware, what the signals will look like after they encounter and interact with the board traces, package leads and connector pins? How do we establish confidence the product will work the first pass, before we build and test it? Simulation is the key ingredient that makes signal integrity predictable.

Fundamentally, it is the electric and magnetic fields of the signals that interact with the interconnects' conductors and dielectrics. The geometry and material properties of the interconnects act as boundary conditions on the equations that describe these fields, Maxwell's equations. In principle, an electromagnetic field solver could be used to predict the output waveforms when electromagnetic waves (the signals) encounter interconnects. In practice, however, except for the smallest and simplest structures, this is not practical. The current generation of tools requires skills and computation resources that relegate this sort of simulation to only selected structures such as short discontinuities.

Instead, we use an approximation that describes all interconnects in terms of just a few simple ideal circuit elements, and then leverage circuit simulators which can solve for the voltages and currents at any nodes throughout the circuit. If you can draw the circuit schematic, a circuit simulator can predict the current and voltage waveforms in both the time or frequency domain at any location.

The first step is translating the physical structure of an interconnect into an equivalent circuit model using combinations of ideal circuit elements. There are four commonly used ideal circuit elements: resistors, capacitors, inductors and lossless transmission lines. But if we limit ourselves to these four elements, two important effects can't be accounted for: crosstalk and loss.

So in order to model coupling, we must also add mutual inductance and a differential pair; i.e., two coupled-lossless transmission lines. To account for loss, we add an ideal lossy transmission line and an ideal lossy differential pair. Combinations of these eight ideal circuit elements can be used to describe virtually all electrical effects from interconnects.

All simulators understand the first five elements. Some simulators understand differential pairs and a few even understand ideal lossy transmission lines. There are three classes of simulators:

Electromagnetic simulators predict the fields directly from Maxwell's equations. These tools are useful when the circuit model approximation is no longer accurate, such as when the physical structure is nonuniform and longer than a tenth of a wavelength or when the transverse dimension is longer than a tenth of a wave.

Circuit simulators rail in two classes, based on the sort of models used to describe the active devices. Behavioral simulators use this models for active devices, or similar formats, while Spice (Simulation Program with Integrated Circuit Emphasis) uses general analytical models of how transistors behave with specific parameter values for each driver.

Spice is an incredibly powerful circuit simulator and in the past five years commercial versions have come available that are easy enough to use that novices can be up and running in a few minutes. A number of free versions can be downloaded from the Web. (1)

In addition to predicting the behavior of a product before it is built, Spice is a powerful learning tool to help feed the intuition of engineers. Building circuit models of transmission line circuits in Spice is as easy as moving icons together on a screen. In seconds, the impact from reflections on termination strategies or stub lengths can be evaluated and multiple iterations evaluated. With such a simple-to-use, powerful tool freely available, there is no longer a reason why Spice isn't on the desk of every engineer who has an interest in signal integrity.

REFERENCES

(1.) To download a free, fully functional evaluation version of Spice, go to spectrum-soft.com/index.shtm and intusoft.com.

Ed.: Many of the details on this and related topics can be found in Bogatin's new took, Signal Integrity--Simplified (Prentice Hall). Additional online lectures and tutorials on getting started with Spice can be viewed at bogent.com.

DR. ERIC BOGATIN is the CTO of Synergetix, responsible for the technology development of new high-performance inter connects. He can be reached at [e-mail].