For this example, consider the following SISO feedback loop where the system contains multiple gain crossover or phase crossover frequencies, which leads to multiple gain or phase margin values:

Create the transfer function.

G = tf(20,[1 7]) * tf([1 3.2 7.2],[1 -1.2 0.8]) * tf([1 -8 400],[1 33 700]);

Use the allmargin command to compute all stability margins.

m = allmargin(G)

m = struct with fields:
     GainMargin: [0.3408 3.3920]
    GMFrequency: [1.9421 16.4807]
    PhaseMargin: 68.1178
    PMFrequency: 7.0762
    DelayMargin: 0.1680
    DMFrequency: 7.0762
         Stable: 1

Note that gain margins are expressed as gain ratios and not in decibels (dB). Use mag2db to convert the values to dB.

GainMargins_dB = mag2db(m.GainMargin)

GainMargins_dB = 1×2

   -9.3510   10.6091