Wide Bandwidth Considerations for ALMA Band 2
  One of the main considerations in the ALMA Development Roadmap for the future
of operations beyond 2030 is to at least double its on-sky instantaneous
bandwidth capabilities. Thanks to the technological innovations of the past two
decades, we can now produce wider bandwidth receivers than were foreseen at the
time of the original ALMA specifications. In several cases, the band edges set
by technology at that time are also no longer relevant. In this memo, we look
into the scientific advantages of beginning with Band 2 when implementing such
wideband technologies. The Band 2 receiver system will be the last of the
original ALMA bands, completing ALMA's coverage of the atmospheric windows from
35-950 GHz, and is not yet covered by any other ALMA receiver. New receiver
designs covering and significantly extending the original ALMA Band 2 frequency
range (67-90 GHz) can now implement these technologies. We explore the
scientific and operational advantages of a receiver covering the full 67-116
GHz atmospheric window. In addition to technological goals, the ALMA
Development Roadmap provides 3 new key science drivers for ALMA, to probe: 1)
the Origins of Galaxies, 2) the Origins of Chemical Complexity, and 3) the
Origins of Planets. In this memo, we describe how the wide RF Band 2 system can
help achieve these goals, enabling several high-profile science programmes to
be executed uniquely or more effectively than with separate systems, requiring
an overall much lower array time and achieving more consistent calibration
accuracy: contiguous broad-band spectral surveys, measurements of deuterated
line ratios, and more generally fractionation studies, improved continuum
measurements (also necessary for reliable line flux measurements), simultaneous
broad-band observations of transient phenomena, and improved bandwidth for 3 mm
very long baseline interferometry (VLBI).
