\documentclass{article}
\pagestyle{empty}

\usepackage{tikz}
\usetikzlibrary{shapes,shapes.geometric,arrows}
\begin{document}

\input{tikz_styles.tex}

\tikzstyle{mix} = [draw, circle, node distance=1cm, minimum size=0.7cm]
\tikzstyle{input} = [coordinate]

\begin{tikzpicture}[auto, thick, node distance=1.5cm, >=stealth']

  \node [input, name=input] {};
  \node [sum, right of=input] (sum) {};
  \node [simpleblock, right of=sum] (kd) {$K_d$};
  \node [block, right of=kd] (F) {$F[z]$};
  \node [coordinate, right of=F, node distance=1cm] (foo) {};
  \node [block, below of=kd] (N) {$N[z]$};

  \draw [->] (input) -- (sum);
  \draw [->] (sum) -- (kd);
  \draw [->] (kd) -- (F);
  \draw [-] (F) -- (foo);
  \draw [->] (foo) |- (N);
  \draw [->] (N) -| (sum);

  \node [below left of=sum, node distance=0.5cm] {$-$};

  \draw [-,thin] (sum.north) -- (sum.south);
  \draw [-,thin] (sum.east) -- (sum.west);

\end{tikzpicture}

\end{document}

