Any lumped network obeys three basic laws: Kirchhoff’s voltage law (KVL), Kirchhoff’s current law (KCL), and the elements’ laws (branch characteristics). For filter applications, we write the frequencydomain instead of the time-domain network equations. Three general methods for writing network equations are described in Chapter 2.1. They are the node equations, the loop equations, and the hybrid equations. This section outlines another method, the signal flow graph (SFG) method of characterizing a linear network.