Chapter 11

# 11.2 Definition of Phase Margin

A corollary to the Gain Margin can also be defined, describing the so-called Phase Margin. Let the crossover frequency be defined as [latex]\omega_{cp}[/latex], the frequency at which the gain plot (in dB) crosses over 0 dB line, then let Phase Margin be defined as:

[latex]\Phi_{m} =180^{0}+\angle GH(\omega_{cp})[/latex] |
Equation 11‑4 |

As shown in Figure 11‑5, if Phase Margin is found above [latex]-180^{0}[/latex] line, then it is considered positive, [latex]\Phi_{m} > 0[/latex], and is associated with the stable system, where Gain margin is also positive. [latex]\Phi_{m} = 0^{0}[/latex] is associated with a marginally stable system, where gain margin is also equal to zero. If Phase Margin is found below [latex]-180^{0}[/latex] line, then it is considered negative,[latex]\Phi_{m}< 0[/latex], and is associated with the unstable system, where Gain margin is also negative.