source: trunk/MagicSoft/TDAS-Extractor/Introduction.tex@ 5598

\section{Introduction}

\begin{itemize}
\item{Short description of the MAGIC read-out system}
\end{itemize}

The MAGIC read-out scheme, including the PMT camera, the analog-optical link, the majority trigger logic and FADCs, is schematically shown in figure \ref{fig:MAGIC_read-out_scheme}. The used PMTs provide a very fast response to the input light signal. The response of the PMTs to sub-ns input light pulses shows a FWHM of 1.0 - 1.2 ns and rise and fall times of 600 and 700 ps correspondingly \cite{Magic-PMT}. By modulating VCSEL type laser diodes in amplitude the ultra fast analogue signals from the PMTs are transferred via 162m long, 50/125 $\mu$m diameter optical fibers to the counting house \cite{MAGIC-analog-link-2}. After transforming the light back to an electrical signal, the original PMT pulse has a FWHM of about 2.2 ns and rise and fall times of about 1ns. % was 2.2 ns

%an analog optical link \ci
te{MAGIC-analog-link-2} to the counting house.


\begin{figure}[h!]
\begin{center}
\includegraphics[width=\textwidth]{Magic_readout_scheme1.eps}
\end{center}
\caption[Current MAGIC read-out scheme.]{Current MAGIC read-out scheme: the analog PMT signals are transferred via an analog optical link to the counting house where after the trigger decision the signals are digitized by using a 300 MHz FADCs system and written to the hard disk of a DAQ PC.} \label{fig:MAGIC_read-out_scheme}
\end{figure}

 

%After modulating VCSEL type laser diodes, after traveling through 162m of multi-mode graded index fiber of 50/125 $\mu$m diameter and.


In order to sample this pulse shape with the used 300 MSamples/s FADC system, the pulse is stretched to a FWHM $>6$ ns (the original pulse is folded with a stretching function of 6ns). Obviously by doing this, more LONS is integrated and thus the performance of the telescope on the analysis level is degraded. 

Because the current MAGIC FADCs have a resolution of 8 bit only, the signals are split into two branches with different gains by a factor of 10. One branch is delayed by 50 ns and then both branches are multiplexed and consecutively read-out by one FADC. % The maximum sustained trigger rate could be 1 kHz. The FADCs feature a FIFO memory which allows a significantly higher short-time rate.


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