GM gain margin: GM = 1/M(PI)

M(PI) = N the amplitude of the curve across the negative real axis

PM phase margin: PM =

= phase at the intersection of the N curve and the unit circle

If GM>1 / PM>0, the system is stable. Bigger is more stable

[Note] : GM is sometimes ambiguous. When GM= infinity, both systems are stable, but the relative stability cannot be determined, as shown in the figure below: (K1 is more stable)

PM has no ambiguous questions!

The notes