## 1.1 What is it?

Gnucap is a general purpose mixed analog and digital circuit simulator. It performs nonlinear dc and transient analyses, fourier analysis, and ac analysis linearized at an operating point. It is fully interactive and command driven. It can also be run in batch mode. The output is produced as it simulates. Spice compatible models for the MOSFET (levels 1-7) and diode are included in this release.
Since it is fully interactive, it is possible to make changes and re-simulate quickly. This makes Gnucap ideal for experimenting with circuits as you might do in an iterative design or testing design principles as you might do in a course on circuits.
In batch mode it is mostly Spice compatible, so it is often possible to use the same file for both Gnucap and Spice.
The analog simulation is based on traditional nodal analysis with iteration by Newton's method and LU decomposition. An event queue and incremental matrix update speed up the solution considerably for large circuits and provide some of the benefits of relaxation methods but without the drawbacks.
It also has digital devices for true mixed mode simulation. The digital devices may be implemented as either analog subcircuits or as true digital models. The simulator will automatically determine which to use. Networks of digital devices are simulated as digital, with no conversions to analog between gates. This results in digital circuits being simulated faster than on a typical analog simulator, even with behavioral models.
Gnucap also has a simple behavioral modeling language that allows simple behavioral descriptions of most components including capacitors and inductors.
Gnucap is an ongoing research project. It is being released in a preliminary phase in hopes that it will be useful and that others will use it as a thrust or base for their research.