Cryogenic 3 mm Schottky diode mixer receiver

The best cooled Schottky diode mixer receivers at mm and submm wavelengths use specially fabricated "cryogenic" diodes. Parameters of these diodes permit one to realize very low noise temperatures when cryocooled, the noise temperature of the UVA2P9-600 diode being less than 50 K at 20 K). Preliminary investigation at 20 K of the I-V characteristics of the available diodes has shown them possessing the noise temperature theoretical limit of about 110 K. This is considerably higher than the noise of the above-mentioned "cryogenic" diode UVA2P9-600. With the purpose of compensating this high level of Schottky diode noise, we have focused our efforts on the reduction of the mixer conversion loss. Our receiver design incorporates a full-height waveguide mixer mount employing no input guide impedance transformer, thus reducing the waveguide channel and backshort losses. The necessary input impedance transformation in this mixer mount is realized immediately at the Schottky diode location. The DSB receiver noise temperature with a full-height waveguide mixer at room temperature is 270 K. For cryogenic operation an overmoded waveguide and linear taper to rectangular waveguide are added to the receiver input circuit.