Why is the gain margin returned by the MARGIN function in the Control Systems Toolbox different than the gain margin shown in the associated bode plot?

When I compare the gain margin returned by the MARGIN command to that shown in the Bode plot produced by the same command, I notice that they are not the same.

The gain margin Gm returned by the MARGIN command:

[Gm,Pm,Wg,Wp] = margin(sys)

is unitless. The gain margin shown in the bode plot produced by the command:

margin(sys)

has units of dB.

Answer 1

[MathWorks Support Team]

This enhancement has been incorporated in Release 2006a (R2006a). For previous product releases, read below for any possible workarounds:

To convert the gain margin Gm to have units in dB, use the following code:

Gm_dB = 20*log10(Gm)