MATLAB Programming/Bode Plot
Introduction[edit | edit source]
This article is on the topic of creating Bode plots in MATLAB. The quick answer is use the bode command. However, the bode command has several options and the plots generated by the bode command are not easily reformatted. The default formatting of most MATLAB plots is good for analysis but less than ideal for dropping into Word and PowerPoint documents or even this website. As a result this article presents an alternative that requires more lines of code but offers the full formatting flexibility of the generic plot command.
MATLAB's Bode Command[edit | edit source]
The basic bode command is as follows
The bode command will automatically call gcf which will put the bode plot on the current figure. If no figure exists then one is created by gcf.
>> freqVec = logspace(-1, 3, 5000); >> bode(LTI_SYS, freqVec * (2*pi))
- freqVec is a vector of 5000 frequencies, in Hz, spaced evenly on a log scale from 10-1 to 103
- pi is a MATLAB constant equal to the value of and in this case it is used to convert freqVec to rad/sec as it is passed to the bode command
In order to get the magnitude and phase at each frequency point the bode command must be called with output arguments such as
>> [mag, phase] = bode(LTI_SYS);
>> [mag, phase] = bode(LTI_SYS, freqVec * (2*pi));
- mag is the magnitude (not in dB) at each point in freqVec
- phase is the phase (in degrees) at each point in freqVec
The mag and phase variables must come out as 3D arrays. Assuming LTI_SYS is SISO then the commands below will convert mag and phase into the vectors you would expect
>> mag = mag(:); >> phase = phase(:); >> mag = db(mag); % to get the magnitude in 20log dB
Issues with the bode command[edit | edit source]
The main issue with the bode command is reformatting of the plot. The bode command appears to use a normal semilogx plot and then apply patches ro something similar to the figure. This can lead to odd behavior when attempting to create multi-line titles, reformat line widths or font sizes, etc. The normal relationship of axes to figure is just not quite present.