Adjust the phase offset when using data from 'bode'

I'm trying to look at the phase and gain margin of an openloop system. Due to presentation/publications, I had to designa function to plot my amplitude and phase response with good font, font dimensions etc. To do so, I'm using the bode function to extract the data from my state space systems. But I have a problem with a phase offset (value at 0Hz). I know we can remove it from the properties editor avalaible in the classical bode plot but is there a solution to correct it when using this syntax [mag,phase,wout] = bode(sys,w) ?

Answer 1

[Robert U]

Hi Romain Liechti,

if you want - as you write - stick to the bode output you should correct the phase by a reference phase shift.

% create tf
sys = tf(1,[1 0]);

% calculate frequency response using bode
w = 2*pi*logspace(log10(0.1),log10(1e5),1000);
[mag,phase,wout] = bode(sys,w);

mag = squeeze(mag);
phase = squeeze(phase);

% correct phase
w_ref = 2*pi*1; % reference is 1 Hz here, 0 works as well for DC
[~,corrPhase,~] = bode(sys,w_ref);

phase = phase-corrPhase;

Kind regards,

Robert

[Romain Liechti]

Thanks for the answers, that was exactly what I was looking for!

Kind regards,

Romain

Answer 2

[Robert U]

Hi Romain Liechti,

in order to customize the frequency response plot you could use bodeplot() instead of bode(). There, you can match phase as an option.

The method is described in documentation:

https://de.mathworks.com/help/ident/ref/bodeplot.html?s_tid=doc_ta#f0-315364

In difference to bode(), bodeplot() is not suitable to output system analysis data but suits as plotting tool only.

Kind regards,

Robert