ALMA Band 7 cartridge: status report

The ALMA front end is designed for ten frequency bands, spanning all atmospheric windows from 31 GHz up to 950 GHz. The electronics, as well as part or all of the optics, for each individual band, fits inside a field-replaceable cartridge, and the ten cartridges are housed in a respectable (1m diameter) cryostat. The initial budget allows the implementation of four bands (3mm, 1.3mm, 0.9mm, 0.45mm).

IRAM has been contracted by the ALMA project to perform the development and detailed design of the band 7 (275 - 373 GHz) cartridge, and the production of a pre-series of eight cartridges. This work has started Januray 2003, and should be completed until December 2005.

The band 7 cartridges implements a dual-polarization, sideband separating frontend; the IF band being 4 - 8 GHz, this means that for, e.g., an LO frequency of 338 GHz, the sky frequency ranges 330 - 334 GHz (lower sideband) and 342 - 346 GHz (upper sideband) are available on separate IF outputs, and that the cartridge delivers four IF outputs for a total of 16 GHz bandwidth.

The ``blank'' cartridge (Rutherford Appleton Laboratories) is a fiberglass/metal tubular structure inside which four main subsystems are mounted:

• the cold optics, comprising three elliptical mirrors, a polarization - diplexing grid, and two corrugated horns;
• two sideband separating mixer assemblies, each comprising:
  • a quadrature 3 dB signal hybrid;
  • an in - phase LO power splitter;
  • two -17 dB LO injection couplers;
  • two DSB mixer units
• four cryogenic IF amplifiers (Yebes Observatory)
• the LO subsystem, including two cryogenic frequency triplers (Virginia Diodes, Inc.)

Figure 1: A view of the upper part of the prototype cartridge, comprising the cold optics and two 2SB assemblies. The two horns are staring into two elliptical mirrors machined into the roof of the assembly, while a third mirror, partly hidden at the right, couples the cartridge to the telescope optics. The polarization diplexing grid (round frame) can also be seen.

Figure 2: Recent results of mixer characterization. Left: noise temperature (uncorrected) versus LO frequency, for DSB unit mixers (lower two curves), and for a 2SB assembly (upper two curves, for the LSB and USB channels); the horizontal bar is the ALMA noise specification. Right: image band rejection versus LO frequency for the LSB and USB channels; the ALMA specification is 10 dB minimum.

The development of the optics, mixers, superconducting junctions, and global cartridge configuration, has resulted in a prototype cartridge (figure 1) that, in the tests performed so far, meets the ALMA technical specifications. Figure 2 shows the noise and image rejection of recently produced DSB and 2SB (sideband-separating) mixers. The band 7 project recently underwent a Preliminary Design Review, and the response of the review board to the technical achievements seems to have been positive; the formal review report has not yet been finalized.

Besides the undersigned, members of the IRAM receiver and mechanical groups, too numerous to be all listed, have contributed to the results reported here through their work or advice.

M.Carter, S.Claude (now at HIA), A.L.Fontana, S.Halleguen, B.Lazareff, S.Mahieu, D.Maier, F.Mattiocco, P.Serres, K.Schuster and the SIS group.