Comparison of Double-Sideband and two-Single-Sideband Systems

We compare a Double-Sideband (DSB) system with 4 IFs of 2-GHz bandwidth each (4 GHz times two polarizations = 8 GHz total) and the two-Single-Sideband (2SB) system with 8 IFs of 2-GHz bandwidth each (4 GHz times two polarizations times separated two sidebands = 16 GHz total) that has been proposed for the baseline ALMA. We compare the sensitivity of these systems by adopting the state-of-art receiver temperatures and, for the DSB system, the conventional process of sideband separation in correlation by switching of first LO phase by p /2 or by fringe rotation. Under a good atmospheric condition (25-percentile atmosphere at Chajnantor), the sensitivity ratio (DSB/2SB) is 0.83 at 110 GHz, 0.98 at 225 GHz, 0.83 at 650 GHz and 0.86 at 950 GHz, in both continuum and spectral line observations. In general, the advantage of the 2SB system in sensitivity over the much simpler DSB system is only marginal. In reality, many of the observations at millimeter wavelengths may be executed under conditions that make submillimeter observations difficult. Through the 75-percentile atmosphere (225 GHz opacity of 0.12), the sensitivity ratio (DSB/2SB) would be 0.76 and 0.86 at 110 GHz and 225 GHz, respectively. In imaging extended sources, ALMA will rely on accurate single-dish data taken with the 12-m dishes. Unlike the 2SB system, the DSB system requires an accurate calibration of the sideband gain ratio for single-dish observing. This may be possible by an interferometry of a continuum source with known spectral indices, although its feasibility and the achievable accuracy need to be examined. These considerations coupled with the currently limited availability of 2SB mixers for the ALMA bands and the simplicity of the DSB system makes the choice of the DSB system a viable option for the initial ALMA system.