Page "Bode plot" Paragraph 95

Another equivalent measure of proximity to instability is the phase margin.

The Bode magnitude plot locates the frequency where the magnitude of | βA < sub > OL </ sub >| reaches unity, denoted here as frequency f < sub > 0dB </ sub >.

Using this frequency, the Bode phase plot finds the phase of βA < sub > OL </ sub >.

If the phase of βA < sub > OL </ sub >( f </ sub >< sub > 0dB </ sub >) > − 180 °, the instability condition cannot be met at any frequency ( because its magnitude is going to be < 1 when f = f < sub > 180 </ sub >), and the distance of the phase at f < sub > 0dB </ sub > in degrees above − 180 ° is called the phase margin.