Describing functions, circle criteria and multiple-loop feedback systems

Two graphical methods used for studying the stability of nonlinear systems-the circle criterion and the descibing function-are compared and shown to be complementary in that each tends to compensate for the deficiencies of the other. A means of increasing the reliabilty of the describing-function method is given. The describing-function method is then extended to a fairly wide class of multivariable systems. The usual advantages to system designers of the describing function are retained in the suggested to produce qualitative changes from the behavior with the interactions removed.