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I'm trying to plot simple Bode plots using pgfplots. After much research, I discarded the package Bodegraph due to its bad documentation and a lot of troubles with the gnuplot. So, I'm using an axis environment with the logarithmic x-axis activated. The next code is sample example of a tikzpicture that shows the problem.

\begin{tikzpicture} \begin{axis}[ width=0.8\linewidth, height = 3cm, xmode = log, axis x line*=box, axis y line*=box, xmin=1e-2, xmax=1e2,% range for the x axis ymin=-100, ymax = 100, scale only axis , ytick distance=40, xmajorgrids = true, ymajorgrids = true, title = Fase, ylabel = $\phi$, xlabel = $\omega$ ] \addplot[smooth, blue, ultra thick, mark=none, domain=1e-2:1e2, samples=400, trig format plots = deg] {atan((2*0.1*(x/2))/(1-(x/2)^2))}; \end{axis} \end{tikzpicture}

The problem is that I get the following wrong plot: The bad result

What I'm expecting to get is something like: What should be plotted

Please, notice that I'm using the trig format plots=true to set degrees for the trigonometric functions. Can anybody help me out? This is just the phase angle plot of a second order system.

Thanks in advance

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    Use the restrict y to domain=[min,max] keyword. You need to have at least one point outside the clip region to interpolate correctly. See tex.stackexchange.com/questions/324646/… Commented Jun 27, 2017 at 19:29
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    Why is that wrong/unexpected? You divide by 1-x/2 in argument to atan, so near x=2 you're approaching the limit of infinity, and arctan(+/- inf) is +/-90 degrees. (I have no idea what a phase angle plot of a second order system is.) Commented Jun 27, 2017 at 20:24
  • Use atan2 not atan Commented Jun 27, 2017 at 23:30
  • Yes, I'm trying to use atan2 but I get several errors while using atan2 with pgfplots. Commented Jun 28, 2017 at 7:53
  • It seems like atan2 is undefined so a default value 1 is substituted. It raise errors because 1 is not a floating number. Commented Jul 2, 2017 at 14:19

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The plot you show is correct I think, in the sense that it correctly displays the formula you entered. As mentioned in a comment, you divide by 1-x/2 in the argument to atan, and the limit of atan(x) as x approaches infinity is 90 degrees.

percusse mentions in a another comment that you want to use atan2 instead. atan2 takes two values, atan2(y,x). I have no idea exactly what kind of expression you're supposed to use, but as an example (atan2(1,x)-45)*2 gives you something similar to what you expect, though with a less steep curve in the middle part. I'm not familiar with these things though, so I cannot help you find a more appropriate formula.

enter image description here

\documentclass{standalone} \usepackage{pgfplots} \begin{document} \begin{tikzpicture} \begin{axis}[ width=0.8\linewidth, height = 3cm, xmode = log, axis x line*=box, axis y line*=box, xmin=1e-2, xmax=1e2,% range for the x axis ymin=-100, ymax = 100, scale only axis , ytick distance=40, xmajorgrids = true, ymajorgrids = true, title = Fase, ylabel = $\phi$, xlabel = $\omega$ ] \addplot[smooth, blue, ultra thick, mark=none, domain=1e-2:1e2, samples=400, trig format plots = deg] {(atan2(1,x)-45)*2}; \end{axis} \end{tikzpicture} \end{document}
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