The excess space is due to two factors
center adds vertical space; use \centering instead;
- two consecutive floats are separated by the amount of
\floatsep.
Here's how you can reduce it, still maintaining separate floats.
\documentclass{article}
\usepackage{siunitx}
\sisetup{separate-uncertainty}
\begin{document}
\begin{table}[!htb]
\centering
\caption{Properties of the magnetic Resolver}\label{tab:resvprop}
\begin{tabular}{|l|c|}
\hline
\multicolumn{1}{|c|}{\textbf{Parameter}} & \textbf{Value}\
\hline
Maximum amplitude of the excitation voltage & \SI{9.0}{\volt} \
Excitation frequency & \SI{10}{\kilo\hertz} \
Number of Pole pairs & 4 \
\hline
\end{tabular}
\end{table}
\begin{table}[!htb]
\centering
\caption{Properties of the Reference Encoder}\label{tab:Encprop}
\begin{tabular}{|l|c|}
\hline
\multicolumn{1}{|c|}{\textbf{Parameter}} & \textbf{Value}\
\hline
Pulses per Revolution/Line count & 1024 \
Output signals & Sine, Cosine \
Output signal amplitude & \SI{1.0}{\volt} \
Accuracy & \SI{\pm 0.0176}{\degree} \
Power supply & DC \SI{5\pm 0.5}{\volt} \
\hline
\end{tabular}
\end{table}
\end{document}

I used siunitx in order to get correct output for the measurements with their units. I also changed the alignment of the first columns to left.
You might prefer to use a single table environment, though. Here I also modify the table codes for better appearance. But you can take the idea and keep the caged tables, if you prefer.
\documentclass{article}
\usepackage{siunitx}
\usepackage{booktabs}
\sisetup{separate-uncertainty}
\AtBeginEnvironment{table}{%
\setlength{\belowcaptionskip}{\medskipamount}
\setlength{\abovecaptionskip}{0pt}%
}
\begin{document}
\begin{table}[!htb]
\centering
\caption{Properties of the magnetic Resolver}\label{tab:resvprop}
\begin{tabular}{@{}lc@{}}
\toprule
\multicolumn{1}{c}{Parameter} & Value \
\midrule
Maximum amplitude of the excitation voltage & \SI{9.0}{\volt} \
Excitation frequency & \SI{10}{\kilo\hertz} \
Number of Pole pairs & 4 \
\bottomrule
\end{tabular}
\bigskip
\caption{Properties of the Reference Encoder}\label{tab:Encprop}
\begin{tabular}{@{}lc@{}}
\toprule
\multicolumn{1}{c}{Parameter} & Value\
\midrule
Pulses per Revolution/Line count & 1024 \
Output signals & Sine, Cosine \
Output signal amplitude & \SI{1.0}{\volt} \
Accuracy & \SI{\pm 0.0176}{\degree} \
Power supply & DC \SI{5\pm 0.5}{\volt} \
\bottomrule
\end{tabular}
\end{table}
\end{document}

centerenvironments, use\centeringrather than the environment form in floats. – David Carlisle Mar 30 '21 at 13:18\vspace{-1em}between the twotableenvironments? – Erwann Mar 30 '21 at 15:34