How can I use data from BODE plot and use it on a MATLAB figure window using System Identification Toolbox 7.4.3 (R2011b) ?

I use the following data and MATLAB functions to plot the PSD from the input data:

xn = rand(3001, 1); % Input signal

n = 3001;

NFFT = 2^nextpow2(n)+3; 

Ts = 0.001;

Fs = 1/Ts; % Sample Frequency

h = spectrum.welch('Hann',NFFT/2,0.67); 

Hpsd=psd(h,xn,'Fs',Fs,'ConfLevel',0.95);

plot(Hpsd); % works fine

% Now I would like to edit data plotted as below

H=(10.^(Hpsd/20))^2; 

plot(H)

I am getting an error from last two lines in my code.

Answer 1

[MathWorks Support Team]

The variable 'Hpsd' is not a variable, but, an object.

Hence the PLOT function called on the Object internally calls the Data property of this object and the plot is as expected with units dB/Hz.

In order to extract the data instead of using the PLOT function, use the following approach:

- Get a list of properties of the 'Hpsd' object. This is done with the following step:

get(Hpsd)

- From the property list, collect the values i.e. the data that you would like to use for conversion. The property that I am referring to is 'Data' in the Y-axis and 'Frequency' in the X-axis. This is done in the following step:

y = Hpsd.Data;

x = Hpsd.Frequencies;

- You can use the values 'y' and 'x' to be plotted on a separate MATLAB figure, as done in the following step:

plot(x, 10*log10(y), 'r');