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 assessing the stability of a negative feedback system. An application of Cauchy's Argument principle to the open loop transfer function, the Nyquist plot identifies singularities in the semicircle of infinite radius in the right half of the s-plane, the presence of which indicates instability in the feedback system.
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.
[edit] See also
- Nyquist stability criterion
- Bode plot
- Transfer function
- Frequency response
- Nichols plot
- Cauchy's Argument Principle
[edit] External links
- The Nyquist Plot
- The Barkhausen Stability Criterion — a simple and intuitive yet completely incorrect method of assessing the stability of a feedback system.
