Bandslope Effects On Sensitivity In Interferometers With Digital Correlators

In an interferometer with a digital correlator, sensitivity will be reduced by receiver and IF bandpass slopes. Numerical calculations have been carried out to estimate these losses for developing a flatness specification for the ALMA receivers and IF system. For the 3-level digitizer studied here a 2-dB slope across the band gives an efficiency reduction of about 4 % in the frequency channels where the gain is least. There is also an increase of efficiency of a similar magnitude in the high-gain channels. For continuum observations the channels are weighted according to the channel signal-to-noise and averaged, resulting in an efficiency reduction of ~1 %. From these considerations it should be acceptable to have a gain slope of ~2 dB across the digitizer input bandwidth. Since the ALMA digitizer will have a higher number of bits it may be possible to go to 3 dB across the band.