Create bode plot from state space in dB units?

Hi everyone,

I have a big SS (1 input, 1 output, 186 states), and I try to create the Bode plot. This all works if I use the following code:

h_bode = bodeoptions;
h_bode.Xlim = [1 2000];
h_bode.FreqUnits = 'Hz';
h_bode.MagUnits = 'abs';
h_bode.MagScale = 'log';
h_bode.PhaseWrapping = 'on';
h_bode.phaseVisible = 'on';
h_bode.Grid = 'on';
wmin = log10(0.001)*2*pi;
wmax = log10(2000)*2*pi;
npoints = 10000;
figure('name','SS Bode Plant P(s)');
w = logspace(wmin,wmax,npoints);
h_bode.MagUnits = 'abs';
bodeplot(P,w,h_bode);
set(findobj('Type','line'),'Linewidth',2)
title('Bode Diagram: Plant P(s) (dp2np)');

However, when I use "

h_bode.MagUnits = 'dB';

it gives the warning 'negative data ignored'.

Do I need to do with the State Space model before giving it as input for 'bodeplot'? Or am I missing something?

Thanks in advance,

Thijs

Answer 1

[Craig]

The issue is setting the MagScale to log and using dB for units. Any magnitude less than one will be negative in dB and thus can not be shown in log scale axes. I have created a bug report to better handle this case. If you change the MagScale to 'linear' with MagUnits 'dB' should solve the issue you are seeing.

Answer 2

[Francesco]

I was doing something similar but had a lot of problems with the bode function. My advice is to use the bode function to get the magnitude, phase and frequency values and then make your own plot in the units you prefer.