\typeout{------- backends --------}

\subsection{Backends}

\label{ss:backends}

The following five spectral line backends are available which can be
individually connected to any EMIR band and to HERA.


{\bf The 1 MHz filterbank} consists of 4 units. Each unit has 256
channels with 1~MHz spacing and can be connected to different or
the same receivers giving bandwidths
between 256 MHz and 1024 MHz. The maximum bandwidth is available
for only one receiver, naturally one having a 1 GHz wide IF bandwidth.
Connection  of the filterbank in the 1~GHz mode presently excludes the
use of  any other  backend with  the same receiver.

Other configurations of the 1 MHz filterbank include
a setup in 2 units of 512 MHz connected to two different receivers, or 4 units
of 256 MHz width connected to up to four (not necessarily) different receivers.
Each unit can be shifted in steps of 32 MHz relative to the center frequency
of the connected receiver.

{\bf VESPA}, the versatile spectrometric  and polarimetric array,
can be connected either to HERA or to  a subset of 4 single pixel receivers,
or to a pair of single pixel receivers for polarimetry.
The many VESPA configurations and user modes are summarized in a
\htmladdnormallink{Newsletter contribution}{\www/IRAMFR/ARN/dec02/node6.html}
\cite{vespasummary} and in a
\htmladdnormallink{user guide}{\wwwes/otherDocuments/manuals/vespa_ug.ps},
but are best visualised on a demonstration program which can be downloaded
from \htmladdnormallink{our web page}{\wwwfr/vespaconf.py}
at URL \wwwfr/veleta.htm.
Connected to a set of 4 single pixel receivers, VESPA typically provides up to
12\,000 spectral channels (on average 3\,000 per receiver).
Up to 18\,000 channels are possible in special
configurations.  Nominal spectral resolutions range from 3.3 kHz
to 1.25 MHz. Nominal bandwidths are in the range  10 --- 512 MHz.
When VESPA is connected to HERA, up to 18\,000 spectral channels can be used
with the following typical combinations
of nominal resolution (kHz) and maximum bandwidth (MHz):
20/40, 40/80, 80/160, 320/320, 1250/640.

{\bf The  4 MHz filterbank} consists of nine units.
Each unit has 256 channels (spacing of 4 MHz, spectral resolution at 3 dB
is 6.2 MHz) and thus covers a total bandwidth of 1 GHz.
The 9 units are designed for connection to HERA, but a subset of 4 units
can also be connected to the backend distribution box which feeds
the  single  pixel spectral line receivers. All these receivers have a 1~GHz RF
bandwidth except for A100 and B100 (500 MHz only).
%At the present time, a 4 MHz filterbank cannot be used simultaneously with
%the autocorrelator or the 100 kHz filterbank on the same receiver.

The {\bf wideband autocorrelator WILMA} consists of 18 units. They can
be connected to the 18 detectors of HERA. Each unit provides 512
spectral channels, spaced
out by 2 MHz and thus covering a total bandwidth of 1 GHz. Each band is sliced
into two 500 MHz subbands which are digitized  with 2~bit/1~GHz samplers.
An informative technical overview of the architecture is
available at URL
\htmladdnormallink{../IRAMFR/\-TA/backend/veleta/\-wilma/index.htm}
{/IRAMFR/TA/backend/veleta/wilma/index.htm}.
%
%Note that WILMA cannot presently  be connected to any of  the
%single pixel receivers.
%\item The 500 channel AOS: ({\em Under repair})\\
%Bandwidth 500 MHz; actual spectral
%resolution 1.5 MHz. Using the AOS with the 3 mm SIS receiver results
%in higher noise at the band edges, so the combination 3 mm SIS + AOS
%is not recommended.




