Device plugins
- Implementation
  - AC analysis

Device plugins

Implementation

AC analysis

The AC functions in Gnucap correspond to a combination of the AC and PZ functions in Spice. Actually, they are more like the PZ functions, in the sense that frequency (SIM::jomega) is complex.

void ac_iwant_matrix()

Notify the sparse matrix of what nodes this device uses.

AC analysis uses a single matrix “acx”, which serves as both the nodal admittance matrix and the LU factors. This is likely to change in the future to two matrices like in transient analysis.

This notification is done by invoking the matrix “iwant” call with node pairs that need allocating. This line:

acx.iwant(_n[0].m_(),_n[1].m_());

requests allocation of space for an element stamp connecting _n[0] and _n[1]. This is usually four places in the matrix.

Usually, this is not done directly, but calls another function:

ac_iwant_matrix_passive(), for two-terminal elements like resistors, with a symmetric stamp
ac_iwant_matrix_active(), for controlled sources, with an asymmetric stamp
ac_iwant_matrix_extended(), the general case, allocates all combinations

void ac_begin()

This is called at the beginning of every AC analysis. It should call BASE::ac_begin() before doing anything else. It sets the value of any AC variables that remain constant through a run.

void do_ac()

In most cases, the do_ac functions do the real work, or call the ac_eval function to do it.

If necessary, it evaluates the model. Calling this function more than once on an iteration is harmless, except for the waste of time. Usually, it uses state variables from a previous DC or transient analysis to compute AC values.

Unlike Spice, Gnucap does not actually load the matrix here.

void ac_load()

This function gives the appearance of loading the admittance matrix and current vector with the values calculated in do_ac.

The actual loading is done by one or more of a small group of general functions, depending on whether the element is active, passive, poly, or a source. Only certain patterns can be stamped. Complex devices use a combination of these patterns.

WARNING to model developers: DO NOT stamp the matrix directly!