Development of 664GHz sub-harmonic mixers

Due to demand from Earth and astronomical sensing applications a sub-harmonic Schottky diode based mixer is designed and optimised to work at a receiver frequency of 664GHz. Practical work is undertaken to lower the junction capacitance of the diodes by fabricating them with smaller anode sizes by using electron beam lithography instead of ultra-violet photolithography. The diode topology is optimised focusing on the diode fingers themselves to improve performance at this frequency by studying the component electrical characteristics of the diode and then optimising the inductance of the diode fingers. This is the major novel activity in this body of work and shows that subtle changes in the design can have a large positive effect on the performance of the diodes at these ultra-high frequencies. The optimised diodes are used as the basis of a sub-harmonic mixer design which is designed to work at a centre frequency of 664GHz. After initial difficulties and a re-design the conversion loss of the mixer is measured at 15-16dB with a noise temperature of 9000K which whilst below the target performance is still a significant achievement. The optimised diodes in a separate mixer also operating at 664GHz are shown to give a performance of 11dB conversion loss and around 3500K noise temperature. The diodes and this second mixer (designed by RPG) are now available commercially.