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.
2 Bode Plots
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)')