I'm reading about Gain Margin and Phase Margin in a book and having trouble understanding why they calculate the Phase margin like this:
<G(Wcrossover) - (-180) = PM
Why not: <G(Wcrossover) - (180) = PM
Looking at this Nyquist plot, if you take 180 from the phase of G at the Wcrossover, it gives you the Phase Margin.
Where Am I mistaken?