Stability analysis of some neutral delay-differential equations with a frequency-domain approach

Two general schemes of neutral delay differential equations are considered. They represent non delayed systems with delayed, nonlinear feedback control. The stability conditions in terms system parameters derived using an approach based on systems, namely, the Nyquist criterion. It is also shown that those results coincide some already found literature, and others here, which classical tools, investigate directly roots characteristic equation. Finally, examples given to illustrate usefulness this approach.