Development of Cryogenic IF Low Noise Amplifiers for the ALMA Wideband Sensitivity Upgrade

One of the primary objectives outlined in the ALMA Development Roadmap is to broaden the Intermediate Frequency (IF) bandwidth of the receivers for improved sensitivity. A crucial element in achieving this goal is the Cryogenic Low Noise IF Amplifier (CLNA), which necessitates an upgrade. The current generation of ALMA receivers in the most advanced bands offers two polarizations and two sidebands, delivering an instantaneous IF bandwidth of 8 GHz (per polarization and per sideband in the 4-12 GHz range). Our ambitious aim in facilitating this upgrade was to demonstrate the feasibility of doubling the maximum instantaneous IF bandwidth (4-20 GHz) while preserving or improving the characteristics of existing amplifiers. An additional challenge involved obtaining amplifiers with low input reflection as it was anticipated that the devices used in the current receiver generation to mitigate reflection issues (isolators) would be challenging to obtain. Achieving low reflection was essential for ensuring good rejection of undesired sidebands in 2SB receivers, yet this conflicted with the imperative to obtain very low noise. Another significant constraint was the need for the amplifiers to have low power dissipation, given the limited cooling capacity of the receiver cryostat. The development of these amplifiers has proven successful, meeting all specified requirements. Several demonstration prototypes have been built, and the results will be presented during the upcoming meeting.