MATLAB Programming/Semilog
Note that this page is a copy of the ControlTheoryPro.com page on semilogx/y commands.
Introduction
[edit | edit source]The semilogx or semilogy commands plot data with 1 axis linear and the other axis log scale. The semilogx command plots the data on the x-axis on a log scale. For controls this is particularly useful when manually creating a bode plot.
Bode Plots
[edit | edit source]The MATLAB bode plot is very convenient but when the plot needs to be formatted then the bode command makes this difficult. As a result this author (Gabe 13:30, 20 April 2008 (CDT)) creates bode plots using the following commands
freqVec = logspace(-1, 3, 5000);
[mag, phs] = bode(sys, freqVec * (2*pi));
mag = db(mag(:));
phs = phs(:);
figure;
subplot(2, 1, 1)
semilogx(freqVec, mag)
grid on
title('System Bode Plot')
ylabel('Magnitude (dB)')
subplot(2, 1, 2)
semilogx(freqVec, phs)
grid on
ylabel('Phase (deg)')
xlabel('Frequency (Hz)')