Nyquist Plots

Nyquist plots trace the complex-valued frequency response G(jω) of a linear system in the real-imaginary plane as frequency sweeps from −∞ to +∞. They are used in control systems engineering to assess closed-loop stability using the Nyquist stability criterion: the number of clockwise encirclements of the critical point (−1, 0) equals the number of unstable closed-loop poles. Gain and phase margins can be read directly from the plot.