Nyquist Contour

The Nyquist contour is a closed contour in the complex plane used to apply the argument principle to the open right half-plane. It is obtained as the boundary of a right half-disk and then taking the radius to infinity. The contour runs along the imaginary axis and closes through the right half-plane. If an open-loop transfer function has poles on the imaginary axis, the contour is indented around them so that no pole lies on the contour. Its orientation must be stated because reversing it reverses all winding numbers.