The Nichols chart is used to determine the peak response (most unstable point) from an open loop transfer function plot, as well as the crossover frequency, phase margin and gain margin.

Fig. 4. Nyquist chart created from the Bode plot data in Fig. 1 shows the phase margin, gain margin, and stability margin. Three points are indicated on the Nyquist chart; phase margin, gain ...

To perform the measurements, you inject a small AC signal into the power supply’s control-loop circuit and measure the loop’s gain and phase shift. By measuring the gain and phase, you can plot them ...

For this, the amplitude response and the phase response are determined from the transfer function of the system and plotted as a graph with the gain and the phase as a function of frequency.