Phase Margin

Topics: Feedback Control Systems

The phase margin answers the following question:

How much phase lag can be inserted before the closed-loop system reaches the verge of stability?

Phase margin is measured on the Bode plot at the unity-gain crossover frequency, i.e., the frequency where $|L(j \omega)| = 1$:

$$PM = 180 \degree + \angle L(j \omega) + n 360\degree \in (-180 \degree, 180 \degree)$$

References

1. https://ocw.mit.edu/courses/16-06-principles-of-automatic-control-fall-2012/abb629fb7c2e395ee8c6df58d4754329_MIT16_06F12_Lecture_23.pdf