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About Basebands, SideBands and
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The bandwidth of a baseband is fixed at 2 GHz, but data is only taken within one or more spectral windows.These are defined using the Tunable Filter Bank (TFB), a piece of hardware that allows up to 32 spectral windows to be placed within each baseband. Each spectral window is built up from filters that have a bandwidth of 31.25 or 62.5 kHz, the smaller value only being available when Twice Nyquist sampling i.e. oversampling, is utilised. In this way, one or more spectral windows with a variety of bandwidths can be placedindependently in each baseband. The rules for doing so are complicated, but the OT tries to guide you through the restrictions. Up to 8192 spectral channels can be placed across the spectral windows in each baseband, depending on various things, but including the number of polarisation products that are being calculated. This number also depends on the number of correlation bits and on whether the data are being oversampled. The maximum bandwidth of a spectral window cannot be greater than the baseband bandwidth and the choices for the resolution are restricted. The ALMA correlator is therefore extremely flexible and can be configured in a large number of ways. For example, a 1-GHz wide spectral window with 4096 channels can be combined with one with a 125-MHz bandwidth, also with 4096 channels, but delivering much higher spectral resolution. The central frequency of each can be set independently.
In order to determine the continuum level in a spectral window, a maximum of ten spectral average regions can be specified. For each of these spectral average regions, a start channel number and the number of channels over which the average is to be calculated should be specified.
The spectral specification is defined in the SpectralSpec form in the
Instrument Setup
folder in the
Resources
folder of the SBs. Below, an overview of the various panels in the SpectralSpec is given.
Next:
Instrument Setup
Up:
About Basebands, SideBands and
Previous:
Upper and Lower Sideband
Contents
Index
The ALMA OT Team, 2011 May 20