\documentclass[border=9pt]{standalone}
\input{figure_preamble.tex}
\begin{document}
\begin{tikzpicture}

\def\ssep{1.3cm}
\def\bsep{2.5cm}

\tikzstyle{lnode} = [fig/module,minimum width=3.0cm,minimum height=0.6cm,inner sep=2pt]

\begin{scope}[scale=0.6]

\draw (-5.5,-5.5) rectangle (5.5,5.5);

\foreach \x  in {-4,-3,...,4}{
    \foreach \y  in {-4,-3,...,4}{
        \node [circle,minimum size=1pt,inner sep=0,fill=black] (point\x\y) at (\x,\y) {};
        \pgfmathparse{or(\x != 0, \y != 0) ? 1 : 0}
        \ifdim\pgfmathresult pt=1pt
            \draw [-{Stealth[scale=1.0]}] (\x,\y) -- ++({-0.2*\y}, {0.2*\x});
        \fi
    }
}

\node [fig/caption,anchor=center] (caption) at (0,-7) {\small{(a) 向量场 $f(\mathbf{x}) = \begin{bmatrix} -x_2 \\ x_1 \end{bmatrix}$}};

\end{scope}

\begin{scope}[scale=0.6,xshift=14cm]

\draw (-5.5,-5.5) rectangle (5.5,5.5);

\foreach \x  in {-4,-3,...,4}{
    \foreach \y  in {-4,-3,...,4}{
        \node [circle,minimum size=1pt,inner sep=0,fill=black] (point\x\y) at (\x,\y) {};
        \pgfmathparse{or(\x != 0, \y != 0) ? 1 : 0}
        \ifdim\pgfmathresult pt=1pt
            \draw [-{Stealth[scale=1.0]}] (\x,\y) -- ++({0.2*(sin(\x) + sin(\y))}, {0.2*(sin(\x) - sin(\y))});
        \fi
    }
}

\node [fig/caption,anchor=center] (caption) at (0,-7) {\small{(b) 向量场 $f(\mathbf{x}) = \begin{bmatrix} \sin(x_1) + \sin(x_2) \\ \sin(x_1) - \sin(x_2) \end{bmatrix}$}};

\end{scope}

\end{tikzpicture}
\end{document}
