Level 1 handles arbitrary RLCG configurations using the Pade approximation approach. A Pade approximation is used as a rational function approximation to the transfer function in the Laplace domain, which has a trivial inverse transformation to the time domain. Further, separability avoids the need to perform a complex convolution at each time point. The model is very fast and accurate enough for most purposes.
Level 2 handles RLC configurations using a full numerical convolution, equivalent to the LTRA model. It does not allow a G element, and is much slower than the Pade approximation approach, however it may be more accurate. Level 2 supports the following types of lines: RLC (uniform transmission line with series loss only), RC (uniform RC line), LC (lossless transmission line), and RG (distributed series resistance and parallel conductance only).