\begin{tikzpicture}[->, >=stealth, shorten >= 0pt, draw=black!50, node distance=3.5cm, font=\sffamily]
	\tikzstyle{node}=[circle,fill=black,minimum size=2pt,inner sep=0pt]
	\tikzstyle{adder}=[draw=black, circle,minimum size=20pt,inner sep=0pt]
	\tikzstyle{block}=[draw=black,rectangle,fill=none,minimum size=1.5cm, inner sep=0pt]
	\tikzstyle{annot} = []
	
	\node[node] (xc) {};
	\node[adder, right=1.5cm of xc] (add) {\Large $\times$};
	\node[node, above=1cm of add] (st) {};
	\node[block, right of=add, text width=3cm, align=center] (b1) {\small Impulse train to sequence converter};
	\coordinate[right=1.5cm of b1] (yc) {};
	
	\path (xc) edge (add);
	\path (add) edge (b1);
	\path (b1) edge (yc);
	\path (st) edge (add);
	
	\node[text width = 5cm, align=center] at ($(st.center) + (20mm, 3mm)$) {$s(\omega) = \displaystyle \frac{2\pi}{N}\sum_{k=-\infty}^\infty\delta\Big(\omega-k\frac{2\pi}{N}\Big)$};
	\node[above = 0mm of xc, text width = 2cm, align=center] {$X(e^{j\omega})$};
	\node[text width = 4cm, align=center] at ($(yc)+(7mm, 3mm)$) {$X[k] = X(e^{j\omega})\bigg|_{\omega = k\frac{2\pi}{N}}$}; 
	\node[text width = 3cm, align=center] at ($(add.east)!0.5!(b1.west) + (0,3mm)$) {$X_s(e^{j\omega})$}; 
	\draw[dashed] ($(add.west) - (0.25cm, 1cm)$) rectangle ($(b1.east) + (0.25cm, 2.5cm)$) {};
	\node[below] at ($(add.west)!0.5!(b1.east) - (0, 1.2cm)$) {Sampling the DTFT};
\end{tikzpicture}