## Introduction: Log Frequency Scale in Octave

Say you are defining a filter in GNU Octave. You need the frequency on the horizontal axis to be logarithmic instead of linear. This is done using the **semilogx()** function. In the next step you can see a high pass filter (HP) that demonstrates just that and generates the above image.

## Step 1: Semilogx()

freq = [20:100:20000];

xc = 1 ./ (2 * pi * 0.000000016 * freq); z = sqrt(xc.^2 + 1000^2); vout = 10.0 * 1000 ./ z; semilogx(freq, vout, ‘r-’); grid on; xlabel(‘freq’); ylabel(‘volts’); legend(‘HP filter’);

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