What to do when SPICE models run out of gas: Part 1

But it will be necessary to answer three questions in relation to any design in this realm: (1) When are SPICE (Simulation Program with IC Emphasis) simulations valid? (2) What does a simulation tool need to yield good results? (3) What do you do with problems that are outside the capabilities of SPICE simulators?

This series of articles explores modeling alternatives and issues that cannot be directly resolved by a SPICE simulator, even though it is not yet time to abandon SPICE.

Enhance your ability with SPICE tools by learning how to cast problems in terms that SPICE can simulate. The slow field solvers may be correct for some structures, but they can also be used to generate models that are useful in SPICE simulations.

The underlying reality is that a simple problem, such as 10 inches of differential trace with a connector in the middle, can be modeled in seconds with SPICE, and can take hours in a 3D full-wave field solver. Real engineers need to be productive; you cannot afford slow tools in cases that don't mandate such.

Time Domain Analysis
Signal integrity engineers have good reasons for preferring to work in the time domain. There is no reason to abandon the time domain now, but there is good reason to add the frequency domain to your areas of competence. 

When functions are expressed as a function of time, they are said to be in the time domain. Examples are such things as voltage, v(t), or current, i(t). Similarly, an oscilloscope waveform is almost always a time-domain presentation.

In SPICE, time-domain analysis is performed by the .tran statement. This statement tells the simulator to observe the circuit for some specified amount of time. The simulator is usually initialized at the beginning of this time period with a voltage step or a pulse.

SPICE
Whether recognized or not, SPICE simulators are at the heart of many or most circuit simulators. So the material that is about to be described can be of use to you if you use numerous other circuit simulators in addition to SPICE.

A critical requirement of any simulator that qualifies it for use at microwave frequencies is that the simulator absolutely must have the capability of modeling transmission lines with frequency-dependent loss. Frequency-dependent loss (Figure 7.1 below)  is so pervasive at microwave frequencies that any tool without this capability will be of little use.

Figure 7.1. A Plot of Frequency-Dependent Loss

Unmodelable Features
Real interconnect circuits have numerous features that circuit simulators simply don't know how to deal with. The presence of such features does not render the simulator useless; rather, it usually means some other tool is needed to translate the feature into language the circuit simulator understands. Such an approach applies to things such as corners (Figure 7.2 below), end effects, bends in edge-coupled pairs, and vias (Figure 7.3 below).

Figure 7.2. A Corner in an Edge-Coupled Pair

Figure 7.3. A Via

Other features are random in nature and deviate from the ideal characteristics that SPICE presumes. These features, such as roughness and etching variations, are modeled in SPICE only when the simulation deck is intentionally designed to include such characteristics.

In some instances, the major loss mechanism is radiation. SPICE simulators do not know about radiation. Finally, there are solutions to Maxwell's equations that do not conform to circuit theory, and, in instances where these higher order modes become significant, the accuracy of circuit simulators decreases. Following in this article are several examples of such features and how they can be accommodated.