\documentclass[12pt,border=5pt,tikz,class=scrartcl,multi=false]{standalone}
\usepackage{times}
\usepackage[T1]{fontenc}
\usepackage[latin1]{inputenc}
\usepackage{mathtools,amssymb,siunitx}
\usepackage{pgfplots}
\pgfplotsset{compat=1.13}
\usepackage{textcomp}
\begin{document}
\begin{tikzpicture}[
	>=stealth',
	block/.style={draw,rectangle,minimum width=2cm,align=center,fill=blue!30,inner sep=10pt},
]
	\node[block] (a1) {Phase\\frequency\\detector};
	\node[block] at ($(a1)+(4,0)$) (a2) {charge\\pump};
	\node[block] at ($(a2)+(4,0)$) (a3) {Low-pass\\filter};
	\node[block] at ($(a3)+(5,0)$) (a4) {Bias\\generator};
	\node[block] at ($(a4)+(4,0)$) (a5) {Voltage\\controlled\\oscillator};
	\node[block] at ($(a5)+(4,0)$) (a6) {Output\\converter};
	\node[block] at ($(a3)!.5!(a4)-(0,3)$) (a7) {\textdiv\ N\\\small (optional)};
	\node[fit=(a4)(a5)(a6),draw,dashed,very thick,rectangle,fill=blue!30,opacity=.4,text opacity=1,inner sep=10pt,label=below:{\Large VCO}] {};
	\draw[->,ultra thick] ($(a1.160)-(2,0)$) node [anchor=east] (fi) {\Large $\mbox{F}_{\mbox{I} }$} -- (a1.160);
	\draw[->,ultra thick] (a1.20) -- node[above] {\Large U} (a1.20 -| a2.west);
	\draw[->,ultra thick] (a1.-20) -- node[below] {\Large D} (a1.-20 -| a2.west);
	\draw[->,ultra thick] (a2) -- (a3);
	\draw[->,ultra thick] (a3) -- (a4);
	\draw[->,ultra thick] (a4) -- (a5);
	\draw[->,ultra thick] (a5) -- (a6);
	\draw[->,ultra thick] (a6) -- ++(3,0) node [anchor=west] {\Large $\mbox{F}_{\mbox{o} }$};
	\draw[->,ultra thick] ($(a6)+(2,0)$) |- (a7);
	\draw[->,ultra thick] (a7) -- (a7 -| fi) |- (a1.200);
\end{tikzpicture}
\end{document}