Nyquist plot
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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 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 real use of Nyquist plot is in that the stability of the system can be easily predicted by plotting its open loop polar plot along a path that goes along the jω axis and along a semicircle of infinite radius in the positive (real part) half of the s-plane. At the same time the system stability and characteristics can be improved by modifying the plot graphically. Nyquist and related plots are classic methods of assessing stability; they 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.