source: trunk/MagicSoft/TDAS-Extractor/Introduction.tex@ 5635

\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). Because the 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.
% Obviously by doing this, more LONS is integrated and thus the performance of the telescope on the analysis level is degraded. 


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