Positive Im G Plane Negative Gain Margin Phase Margin G Plane h Re R G(joy G(o) Positive Negative Phase Margin Gain Margin Stable System Unstable System 1010 Re Im h 1 GPlane   Positive Gain Margin Positive Phase Margin -1 1 Re Im h 1 GPlane   Negative Gain Margin Negative Phase Margin -1 1 Stable System Unstable System G( j) G( j)