Do circa 1 THz radio telescope front end amplifiers actually down convert before amplifying? At what frequency does amplification become untenable?

Question

A discussion of how astronomers view where "radio" ends and "infrared" begins under What does the celestial sphere look like in thermal IR? led me to wonder just how high of a frequency is currently electronically amplified in radio telescopes before it is instead down-converted via some local oscillator and mixer.

Just for example ALMA has ten different frequency bands listed at ESO's ALMA Receiver Bands.

The table says that the lowest two bands (35–50 and 65–90 GHz) use High-electron-mobility transistor receiver technology, while the top eight use "SIS".

The top band (Band 10) is listed as 0.3–0.4 mm or 787–950 GHz with a Noise Temperature Specification of 344 Kelvin.

But I don't know how to find out for each of these bands how the front ends work; whether they amplify or mix and down-convert first. So I'd like to ask:

Question: Do circa 1 THz radio telescope front end amplifiers actually down convert before amplifying? At what frequency does amplification become untenable for radio astronomy applications?

Related and possibly helpful:

• Highest frequency that's been imaged by a radio telescope?
• Has optical interferometry been done at radio frequency using heterodyning with a laser in a nonlinear material? (yes!)
• How does ALMA produce stable, mutually coherent ~THz local oscillators for all of their dishes?
• Is there any work underway to push the long baseline capabilities of the Event Horizon Telescope to sub-millimeter wavelengths?

Answer 1

An article in Astronomy & Astrophysics 1 (A. M. Baryshev et al., A&A 577, A129 (2015)) titled “The ALMA Band 9 receiver Design, construction, characterization, first light” covers this.

The relevant quote from the Methods section is:

The ALMA Band 9 receiver units (so-called “cartridges”), which are installed in the telescope’s front end, have been designed to detect and down-convert two orthogonal linear polarization components of the light collected by the ALMA antennas. The light entering the front end is refocused with a compact arrangement of mirrors, which is fully contained within the cartridge. The arrangement contains a grid to separate the polarizations and two beam splitters to combine each resulting beam with a local oscillator signal. The combined beams are fed into independent double-sideband mixers, each with a corrugated feedhorn coupling the radiation by way of a waveguide with backshort cavity into an impedance-tuned superconductor-insulator-superconductor (SIS) junction that performs the heterodyne down-conversion. Finally, the generated intermediate frequency (IF) signals are amplified by cryogenic and room-temperature HEMT amplifiers and exported to the telescope’s IF back end for further processing and, finally, correlation.

So, that covers band 9, not quite 10.

A recent article Poster/paper from SPIE. I don’t currently have access to the full paper, but the abstract includes

A new 790 – 940 GHz heterodyne receiver, ASTE Band 10, was installed in October 2019 on ASTE (Atacama Submillimeter Telescope Experiment), a 10 m submillimeter telescope near of the ALMA site in Chile. An ALMA Band 10 prototype receiver was upgraded with SIS mixers employing high-Jc junctions.

which to me implies a very similar architecture. Note that this particular receiver is actually installed on a telescope near ALMA, intended to be focused as a test bed for Band 10 experiments (most ALMA time is spent on other bands).

As for what has (likely) been installed on ALMA, an article in IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011, p606-611 by Yasunori Fujii et al. discusses the prototype they developed. It also used a superconductor-insulator-superconductor (SIS) mixer on the front end to directly downconvert to a 4-12GHz intermediate frequency.