Nyquist plot

A Nyquist plot is a type of graph used in signal processing in which the magnitude and phase of a frequency response are plotted on orthogonal axes. This plot combines the two types of Bode plot - magnitude and phase - on a single graph, with frequency as a parameter along the curve. It is useful for assessing the stability of a system with feedback.

The Nyquist plot shows the amplification/attenuation and phase-shift of the signal in the complex plane. The phase-shift of a signal with frequency ω is represented by the argument and the magnitude is represented by the length of a vector from the origin in the direction described by the argument.

The Nyquist plot is very useful in looking at the stability of an open negative-feedback-system. If the magnitude function of a frequency that is phase-shifted 180° is greater than or equal to unity then the closed system will be unstable.

The Nyquist plot is named after Harry Nyquist, a former engineer at Bell Laboratories.