[Tex/LaTex] Figure caption problem

Question

Is there anybody who can guess what is wrong with this figure caption?

\caption{Graphical representation of the simple approximation proposed to determine the linewidth of a laser from its frequency noise PSD $S_{\delta\nu}(f)$, calculated from the surface \emph{A} of the slow modulation area. The shadowed areas on this schematized frequency noise spectrum represent the surface \emph{A} that encloses all spectral components for which $S_{\delta\nu}(f)$ exceeds the $\beta$\nobreakdash-separation line $S_{\delta\nu}(f) = \frac{8\ln2}{\pi ^2}f$ (dashed line).}

If I don't comment it out, I get these error messages:

inputenc: Keyboard character used is undefined(inputenc) in inputencoding `utf8'. ...) = \frac{8\ln2}{\pi ^2}f$ (dashed line).}

Incomplete \iffalse; all text was ignored after line 71

Thanx,
Richard

Answer 1

Once I copied your code to my editor, a strange character revealed itself right after the comma in

PSD $S_{\delta\nu}(f)$,

that spurious character has to be removed; also \nobreakdash needs to be \protected (this wouldn't be necessary of you plan to use the optional argument for \caption).

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{figure}
\caption{Graphical representation of the simple approximation proposed to determine the linewidth of a laser from its frequency noise PSD $S_{\delta\nu}(f)$, calculated from the surface \emph{A} of the slow modulation area. The shadowed areas on this schematized frequency noise spectrum represent the surface \emph{A} that encloses all spectral components for which $S_{\delta\nu}(f)$ exceeds the $\beta$\protect\nobreakdash-
separation line $S_{\delta\nu}(f) = \frac{8\ln2}{\pi ^2}f$ (dashed line).}
\end{figure}

\end{document}

Note:

Unfortunately, after formatting the code both in the original question and in my answer, the spurious character disappeared, but it was there.