sinc.tex

\documentclass[tikz]{standalone}
\usetikzlibrary{math}
\tikzmath{
  function sinc(\x) {
    if  abs(\x) < .001 then { % (|x| < .001) ~ (x = 0)
      return 1;
    } else {
      return sin(\x r)/\x;
    };
  };
}
\begin{document}
  \begin{tikzpicture}
    \draw[help lines] (-3.5,-1.5) grid (3.5,1.5);
    \begin{scope}[very thick, domain=-pi:pi, samples=100, smooth]
      \draw[blue] plot (\x,{sin(5*\x r)});
      \draw[red]  plot (\x,{sinc(5*\x)});
    \end{scope}
  \end{tikzpicture}
\end{document}