Description
Driven by the requirements of astronomy (e.g., HERSCHEL HIFI and ALMA) and aeronomy instruments (e.g., EOS MLS, STEAM-R), sub-millimetre heterodyne radiometer technology has markedly improved in the last few years.
Heterodyne mixers using planar Schottky diode technology have been demonstrated at all frequencies up to at least 2,500 GHz in a laboratory environment.
The advantage of Schottky diodes is the fact that they can work at temperatures that can readily be achieved by passive cooling. At frequencies above 150 GHz, Schottky diodes are a common choice for non-cryogenic detectors (mixers) and sources (frequency multipliers). In addition, Schottky diodes are robust enough to work in hostile environments and have been space qualified. These factors make Schottky diodes the preferred choice for several applications.
A drawback of Schottky based sub-systems is that they traditionally require relatively high local oscillator (LO) power. The conventional way of generating LO power at sub-mm wave frequencies consists of a multiplier chain of 2 or 3 frequency multipliers in cascade driven by a Gunn oscillator. The amount of LO power generated is determined by the output power available from the Gunn oscillator and the frequency conversion efficiency of the multipliers, i.e. by the performance of the varactor devices at their respective operating frequencies.
State of the art performance of the complete front-end can therefore only be achieved if both the mixer and multiplier chain elements are optimized together (either as separate units or even intimately integrated in a single block).
The operational frequency bands for Science missions carrying planetary atmosphere characterisation instruments are moving towards higher frequencies (1 to a few THz), the technology for which was not readily available in Europe at the start of the activity. The activity focused efforts on ensuring European State of the Art capability at frequencies close to 1 THz and demonstrate that the performance meets the requirements for future missions. The activity also had to demonstrate appropriate TRL level for potential inclusion on JUICE.
