A Nyquist plot is used in automatic control and signal processing for assessing the stability of a system with feedback. It is represented by a graph in polar coordinates in which the gain and phase of a frequency response are plotted. The plot of these phasor quantities shows the phase as the angle and the magnitude as the distance from the origin. This plot combines the two types of Bode plot — magnitude and phase — on a single graph, with frequency as a parameter along the curve. The Nyquist plot is named after Harry Nyquist, a former engineer at Bell Laboratories.
The high frequency response is at the origin. The plot provides information on the poles and zeros of the transfer function1 (eg. from the angle at which the curve approaches the origin).
Assessment of the stability of a closed-loop negative feedback system is done by applying the Nyquist stability criterion to the Nyquist plot of the open-loop system (i.e. the same system without its feedback loop). This method is easily applicable even for systems with delays which may appear difficult to analyze by means of other methods.
Nyquist and related plots are classic methods of assessing stability, but have been supplemented or supplanted by computer-based mathematical tools in recent years. Such plots remain a convenient method for an engineer to get an intuitive feel for a circuit.
