While there are strictly
analog electronics circuit simulators, popular simulators often include both analog and event-driven digital simulation capabilities, and are known as mixed-mode or
mixed-signal simulators if they can simulate both simultaneously. An entire mixed
signal analysis can be driven from one integrated schematic. All the digital models in mixed-mode simulators provide accurate specification of propagation time and rise/fall time delays. The event-driven
algorithm provided by mixed-mode simulators is general-purpose and supports non-digital types of data. For example, elements can use real or integer values to simulate DSP functions or sampled data filters. Because the event-driven algorithm is faster than the standard SPICE matrix solution, simulation time is greatly reduced for circuits that use event-driven models in place of analog models. Mixed-mode simulation is handled on three levels: with primitive digital elements that use timing models and the built-in 12 or 16 state digital logic simulator, with subcircuit models that use the actual transistor topology of the
integrated circuit, and finally, with inline
Boolean logic expressions. Exact representations are used mainly in the analysis of
transmission line and
signal integrity problems where a close inspection of an IC’s I/O characteristics is needed. Boolean logic expressions are delay-less functions that are used to provide efficient logic signal processing in an analog environment. These two modeling techniques use SPICE to solve a problem while the third method, digital primitives, uses mixed mode capability. Each of these methods has its merits and target applications. In fact, many simulations (particularly those which use A/D technology) call for the combination of all three approaches. No one approach alone is sufficient. Another type of simulation used mainly for
power electronics represent
piecewise linear algorithms. These algorithms use an analog (linear) simulation until a
power electronic switch changes its state. At this time a new analog model is calculated to be used for the next simulation period. This methodology both enhances simulation speed and stability significantly. Another approach that dramatically shortens the simulation time of switch-mode converters is based on average behavioral circuits that apply the switched inductor model. In this approach, the simulation is carried out on the average signals, voltages, and currents, and hence runs much faster. Another advantage of this approach is that it can carry both time domain and frequency domain simulations. The latter can be used to extract the transfer functions in open and closed loop. ==Complexities==