PI Model
The Pi Model may be used for circuits that are electrically small and can represented by a simple Pi topology (see Topology Used for Pi Model Output). The results are generally valid only over a narrow frequency band. The model may be saved in PSpice or Cadence Spectre format.
The Pi model Spice generation capabilities are intended for any circuit which is small with respect to the wavelength at the highest frequency of excitation. Typically, 1/20th wavelength is an appropriate limit. (If a circuit is too large, you can often split it into two or more circuits and analyze each separately.) This limitation is due to the circuit theory limitations of modeling a circuit with just a few lumped elements. The Sonnet electromagnetic analysis is not intrinsically limited in this fashion.
The model generated by the analysis includes any lumped elements (including mutual inductors) between any ports of the circuit layout. Lumped elements from any port to ground are also included. The synthesis capability does not allow internal nodes (nodes which are not connected to a port in the layout) with the single exception of the internal node required to specify a resistor in series with an inductor.
Any circuit which requires internal nodes for an accurate model should be split into several parts so that the required points become nodes. Internal ports without ground reference give incorrect results. Any internal ports should be carefully specified and checked for reasonable results.