Cycle 8 NRAO ALMA Development Study Proposal – ALMA Band 6v2 SIS Mixer-Preamp Development

PI: A. Kerr, NRAO

ABSTRACT 

This proposal will continue the development of the components for a future ALMA Band 6 receiver upgrade – referred to here as Band 6v2. The goals are: (i) to increase the IF bandwidth from the present 4 GHz per sideband per polarization to 12 GHz (4-16 GHz) or 16 GHz (4-20 GHz); (ii) to reduce and flatten the noise temperature across the full IF band; and (iii) to expand the usable RF band from the current 211-275 GHz to 211-280 GHz. While the current ALMA correlator and IF transmission system can only accommodate 4 GHz per sideband per polarization, the new receiver will take advantage of future upgrades enabling increased bandwidth. A number of options are being explored, and a best design will eventually be chosen for the upgrade. The options include the use of SIS junctions with aluminum nitride tunnel barriers (Nb/Al-AlN/Nb) instead of the usual aluminum oxide barrier (Nb/Al-AlOx/Nb) to give increased RF bandwidth; balanced sideband-separating (2SB) mixers instead of the current single-ended 2SB mixers, to reduce LO sideband noise; the use of ferrite isolators or balanced IF amplifiers to flatten the receiver characteristics across the full IF band. It had been planned to use the very-wide-band low-noise cryogenic IF amplifiers made by Low Noise Factory for the B6v2 receivers. However, significant gain variation observed in LNF amplifiers makes them unsuitable for use as IF amplifiers. LNF has been aware of this problem for some time but has so far been unable to find a remedy. Recently, the NRAO CDL has had success using transistors made by Diramics in experimental IF amplifiers for the current Band-6 SIS mixers. It is believed that these devices could be used to make IF amplifiers for 4-16 GHz and probably 4-20 GHz, and it may be possible to make balanced IF amplifiers using the same devices. Continuing this work on IF amplifiers is critical to the success of the B6v2 receiver upgrade, and a study proposal to fund that work is being submitted in parallel with this one. Under a current ALMA study proposal an improved Band-6 LO source with low sideband noise is being developed. Success in that would remove the need for balanced SIS mixers, thereby simplifying the project significantly, and reducing the cost of producing the ~140 mixer-preamps. Because the physical constraints of the ALMA cartridge restrict the configuration of the mixer-preamplifiers, magnets, and orthomode transducer (OMT), all of which must be located near the output of the feed horn but must not protrude into the optical path which traverses the 4-K section of the cartridge, it is important that these components be developed in close collaboration with the optics design team. The essential work on the SIS mixers, the superconducting IF hybrids and components on silicon membranes will be done in close collaboration with the University of Virginia Microfabrication Laboratory (UVML) through separate funding.