[alma_na] [almanews] ALMA Memo 407 Released

[Stacy Oliver]

ALMA Memo # 407

Bandslope Effects On Sensitivity In Interferometers With Digital Correlators

James W Lamb (Caltech)

2002/01/15

Keywords: Digitizers, samplers, correlators, interferometers, sensitivity

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 %.