When I write the following code:
\documentclass{article}
\usepackage{siunitx}
\begin{document}
Given the signal:
\[
v(t)=311\cdot\sin(9\cdot10^5t)
\]
We know that its period is $ T\approx7\si{\micro\second} $
\end{document}
The mu produced by \si{\micro\second} in detail:
Which is pixelated. Does anyone know why this happens?

\usepackage{lmodern}. The problem is theT1encoding when using the default computer modern font. – Steven B. Segletes Mar 22 '18 at 02:34