MATLAB Loglog Command

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MATLAB Loglog Command
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MATLAB MATLAB's semilog Command
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1 Introduction

The loglog command plots both x and y data sets on a log scale while the plot command plots both axes on linear scales and the semilogx/y command plots 1 axis on a linear scale and the other axis on a log scale. Other than the scale of the axes, the 3 plotting commands are identical in most ways.

Within the realm of controls, the loglog command is used to plot PSDs.

2 Basic Usage

The basic usage of the plot, semilogx/y, and loglog command are identical. Below is an example of how a PSD would be plotted

 >> Fs = 1000;                             % Sample Rate of 1 kHz
 >> t = 0:(1/Fs):1000;                  % Time vector
 >> x = sin(pi*t);                          % Sine wave based on time vector, note pi is MATLAB defined constant
 >> [Pxx, f] = pwelch(x, [], [], [], Fs);
 >> loglog(f, Pxx)
 >> grid on
 >> xlabel('Frequency (Hz)')
 >> ylabel('Magnitude (units^2/Hz)')
 >> title('PSD of Sine Wave')

Note that PSD is a tricky to calculate correctly. Most calculations using default arguments will produce a decent PSD but different data vectors will have different problems. Choosing the correct values for the various inputs is an art.

3 See Also