ents characterizing LPTV system behavior. This work focuses on the symbolic analysis engine that generates the expressions starting from a block-diagram system description. 4.3.4 Data structures Y11(s) Y12(s) Figure 4.21: Subcircuits consisting of a large number of nodes, like opamps, are replaced by symbolic stamps, representing the subcircuit behavior using (time-varying) multiport parameters SymbolicHTM then expresses the HTM elements of the overall input/output character- istics in terms of these symbols. The extraction of the block diagram from a circuit netlist can be automated using exist- ing techniques for behavioral modeling [Leyn01]. However, in order to keep the result- ing symbolic expressions interpretable, for complex circuits it is necessary to work in an hierarchical way. As illustrated in Fig. 4.21, this means that subcircuits consisting of a large number of nodes, like opamps, are replaced by symbolic stamps, representing the subcircuit, for example, using (time-varying) multiport parameters. Generation of this symbolic stamp can be done separately and is a one-time effort. This procedure is similar to the way how algorithms for traditional symbolic LTI network analysis replace transistors by a stamp containing small-signal parameters like transconduc- tances, resistors and capacitors. With the SymbolicHTM algorithm expressing the system's input/output HTM-elements in terms of the stamp parameters, lengthy and uninterpretable expressions are avoided. This supports hierarchical analysis, where, for example, the detailed dependence of the subcircuit's multiport parameters on com- ponent parameters is extracted afterwards using techniques for classical symbolic LTI network analysis [Fern98, Gie91, Lin91]. Furthermore, before starting to generate symbolic expressions, it is possible to simplify the input block diagram by applying similar techniques as used in LTI network analysis [Dae02, Leyn01]. For example, a = a number of parallel paths can be simplified by eliminating those paths Hm(s) having n